Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Episode 12 ppt

These steels offer excellent resistance to pitting and crevice corrosion, significantly better resistance to chloride SCC than the austenitic stainless steels, good toughness, and yield

(a) ASTM designations are the same as ACI designations

(b) Rem Fe in all compositions Manganese content: 0.35 to 0.65% for HA, 1% for HC, 1.5% for HD, and 2% for the other alloys Phosphorus and sulfur contents: 0.04% (max) for all but HP-50WZ Molybdenum is intentionally added only to HA, which has 0.90 to 1.20% Mo; maximum for other alloys is set at 0.5% Mo HH also contains 0.2% N (max)

(c) Also contains 4 to 6% W, 0.1 to 1.0% Zr, and 0.035% S (max) and P (max)

The three principal categories of H-type cast steels, based on composition, are:

Cast Stainless Steels

Revised by Malcolm Blair, Steel Founders' Society of America

Composition and Microstructure

As shown in Table 1, cast stainless steels can also be classified on the basis of microstructure Structures may be austenitic, ferritic, martensitic, or ferritic-austenitic (duplex)

The structure of a particular grade is primarily determined by composition Chromium, molybdenum, and silicon promote the formation of ferrite (magnetic), while carbon, nickel, nitrogen, and manganese favor the formation of austenite (nonmagnetic) For example, a cast extra-low-carbon grade such as 0.03% C (max) cannot be completely nonmagnetic unless it contains 12 to 15% Ni The wrought grades of these alloys normally contain about 13% Ni They are made fully austenitic to improve rolling and forging characteristics

Chromium (a ferrite and martensite promoter), nickel, and carbon (austenite promoters) are particularly important in determining microstructure (see the section "Ferrite Control " in this article) In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic

Ferrite in Cast Austenitic Stainless Steels. Cast austenitic alloys usually have from 5 to 20% ferrite distributed in discontinuous pools throughout the matrix, the percent of ferrite depending on the nickel, chromium, and carbon contents (see the section "Ferrite Control" ) The presence of ferrite in austenite may be beneficial or detrimental, depending on the application

Ferrite is beneficial and intentionally present in various corrosion-resistant cast steels (see some of the CF grades in

Table 1, for example) to improve weldability and to maximize corrosion resistance in specific environments Ferrite is also used for strengthening duplex alloys The section "Austenitic-Ferritic (Duplex) Alloys" in this article gives further information

Ferrite can be beneficial in terms of weldability because fully austenitic stainless steels are susceptible to a weldability problem known as hot cracking, or microfissuring The intergranular cracking occurs in the weld deposit and/or in the weld heat-affected zone and can be avoided if the composition of the filler metal is controlled to produce about 4% ferrite

in the austenitic weld deposit Duplex CF grade alloy castings are immune to this problem

The presence of ferrite in duplex CF alloys improves the resistance to stress-corrosion cracking (SCC) and generally to intergranular attack In the case of SCC, the presence of ferrite pools in the austenite matrix is thought to block or make more difficult the propagation of cracks In the case of intergranular corrosion, ferrite is helpful in sensitized castings because it promotes the preferential precipitation of carbides in the ferrite phase rather than at the austenite grain boundaries, where they would increase susceptibility to intergranular attack The presence of ferrite also places additional grain boundaries in the austenite matrix, and there is evidence that intergranular attack is arrested at austenite-ferrite boundaries It is important to note, however, that not all studies have shown ferrite to be unconditionally beneficial to the general corrosion resistance of cast stainless steels Some solutions attack the austenite phase in heat-treated alloys, whereas others attack the ferrite For instance, calcium chloride solutions attack the austenite On the other hand, a 10° Baumé cornstarch solution, acidified to a pH of 1.8 with sulfuric acid and heated to a temperature of 135 °C (275 °F), attacks the ferrite Whether corrosion resistance is improved by ferrite and to what degree depends on the specific alloy composition, the heat treatment, and the service conditions (environment and stress state)

Ferrite can be detrimental in some applications One concern may be the reduced toughness from ferrite, although

this is not a major concern, given the extremely high toughness of the austenite matrix A much greater concern is for applications that require exposure to elevated temperatures, usually 315 °C (600 °F) and higher, where the metallurgical changes associated with the ferrite can be severe and detrimental In applications requiring that these steels be heated in the range from 425 to 650 °C (800 to 1200 °F), carbide precipitation occurs at the edges of the ferrite pools in preference

to the austenite grain boundaries When the steel is heated above 540 °C (1000 °F), the ferrite pools transform to a χ or σ phase If these pools are distributed in such a way that a continuous network is formed, embrittlement or a network of corrosion penetration may result Also, if the amount of ferrite is too great, the ferrite may form continuous stringers where corrosion can take place, producing a condition similar to grain boundary attack

In the lower end of this temperature range, the reductions in toughness observed have been attributed to carbide precipitation or reactions associated with 475 °C (885 °F) embrittlement The 475 °C embrittlement is caused by the precipitation of an intermetallic phase will a composition of approximately 80Cr-20Fe The name derives from the fact that this embrittlement is most severe and rapid when it occurs at approximately 475 °C (885 °F)

At 540 °C (1000 °F) and above, the ferrite phase may transform to a complex iron-chromium-nickel-molybdenum intermetallic compound known as σ phase, which reduces toughness, corrosion resistance, and creep ductility The extent

of the reduction increases with time and temperature to about 815 °C (1500 °F) and may persist to 925 °C (1700 °F) In extreme cases, Charpy V-notch energy at room temperature may be reduced 95% from its initial value (Ref 1, 2)

At temperatures above 540 °C (1000 °F), austenite also has better creep resistance than ferrite The weaker ferrite phase may lend better plasticity to the alloy, but after long exposure at temperatures in the 540 to 760 °C (1000 to 1400 °F) range, it may transform to σ or χ phase, which reduces resistance to impact In some instances, the alloy is deliberately aged to form the σ or χ phase and thus increase strength Austenite can transform directly to σ or χ without going through the ferrite phase

In weld deposits, the presence of σ or χ phase is extremely detrimental to ductility When welding for service at room temperature or up to 540 °C (1000 °F), 4 to 10% ferrite may be present and will greatly reduce the tendency toward weld cracking However, for service at temperatures between 540 and 815 °C (1000 and 1500 °F), the amount of ferrite in the weld must be reduced to less than 5% to avoid embrittlement from excessive σ or χ phase

Ferrite Control. From the preceding discussion, it is apparent that ferrite in predominantly austenitic cast stainless steels can offer property advantages in some steels (notably the CF alloys) and disadvantages in other cases (primarily at elevated temperatures) The underlying causes for the dependence of ferrite content on composition are found in the phase equilibria for the iron-chromium-nickel system These phase equilibria have been exhaustively documented and related to commercial stainless steels

The major elemental components of cast stainless steels are in competition to promote austenite or ferrite phases in the alloy microstructure Chromium, silicon, molybdenum, and niobium promote the presence of ferrite in the alloy microstructure; nickel, carbon, nitrogen, and manganese promote the presence of austenite By balancing the contents of ferrite-and austenite-forming elements within the specified ranges for the elements in a given alloy, it is possible to control the amount of ferrite present in the austenite matrix The alloy can usually be made fully austenitic or with ferrite contents up to 30% or more in the austenite matrix

The relationship between composition and microstructure in cast stainless steels permits the foundryman to predict and control the ferrite content of an alloy, as well as its resultant properties, by adjusting the composition of the alloy This is accomplished with the Schoefer constitution diagram for cast chromium-nickel alloys (Fig 2) This diagram was derived from an earlier diagram developed by Schaeffler for stainless steel weld metal (Ref 1) The use of Fig 2 requires that all ferrite-stabilizing elements in the composition be converted into chromium equivalents and that all austenite-stabilizing elements be converted into nickel equivalents by means of empirically derived coefficients representing the ferritizing or austenitizing power of each element A composition ratio is then obtained from the total chromium equivalent, Cre, and nickel equivalent, Nie, calculated for the alloy composition by:

Fig 2 Schoefer diagram for estimating the ferrite content of steel castings in the composition range of 16 to

26% Cr, 6 to 14% Ni, 4% Mo (max), 1% Nb (max), 0.2% C (max), 0.19% N (max), 2% Mn (max), and 2% Si (max) Dashed lines denote scatter bands caused by the uncertainty of the chemical analysis of individual elements See text for equations used to calculate Cre and Nie Source: Ref 1

The Schoefer diagram possesses obvious utility for casting users and the foundryman It is helpful for estimating or predicting ferrite content if the alloy composition is known and for setting nominal values for individual elements when calculating the furnace charge for an alloy in which a specified ferrite range is desired

Limits of Ferrite Control. Although ferrite content can be estimated and controlled on the basis of alloy composition

only, there are limits to the accuracy with which this can be done The reasons for this are many First, there is an unavoidable degree of uncertainly in the chemical analysis of an alloy (note the scatter band in Fig 2) Second, in addition to composition, the ferrite content depends on thermal history, although to a lesser extent Third, ferrite contents

at different locations in individual castings can vary considerably, depending on section size, ferrite orientation, presence

of alloying-element segregation, and other factors

Both the foundryman and the user of stainless steel castings should recognize that the factors mentioned above place significant limits on the degree to which ferrite content (either as ferrite number or ferrite percentage) can be specified and controlled In general, the accuracy of ferrite measurement and the precision of ferrite control diminish as the ferrite number increases As a working rule, it is suggested that the ±6 about the mean or desired ferrite number be viewed as a limit of ferrite control under ordinary circumstances, with ±3 possible under ideal circumstances

References cited in this section

1 M Prager, Cast High Alloy Metallurgy, in Steel Casting Metallurgy, J Svoboda, Ed., Steel Founders'

Society of America, 1984, p 221-245

2 C.E Bates and L.T Tillery, Atlas of Cast Corrosion-Resistant Alloy Microstructures, Steel Founders' Society

of America, 1985

Cast Stainless Steels

Revised by Malcolm Blair, Steel Founders' Society of America

Heat Treatment

The heat treatment of stainless steel castings is very similar in purpose and procedure to the thermal processing of

comparable wrought materials (see the article "Heat Treating of Stainless Steels" in Heat Treating, Volume 4 of ASM

Handbook However, some differences warrant separate consideration here

Homogenization. Alloy segregation and dendritic structures may occur in castings and may be particularly pronounced

in heavy sections Because castings are not subjected to the high-temperature mechanical reduction and soaking treatments involved in the mill processing of wrought alloys, it is frequently necessary to homogenize some alloys at temperatures above 1095 °C (2000 °F) to promote uniformity of chemical composition and microstructure The full annealing of martensitic castings results in recrystallization and maximum softness, but it is less effective than homogenization in eliminating segregation Homogenization is a common procedure in the heat treatment of precipitation-hardening castings

Sensitization and Solution Annealing of Austenitic and Duplex Alloys. When austenitic or duplex (ferrite in

austenite matrix) stainless steels are heated in or cooled slowly through a temperature range of about 425 to 870 °C (800

to 1600 °F), chromium-rich carbides form at grain boundaries in austenitic alloys and at ferrite-austenite interfaces in duplex alloys These carbides deplete the surrounding matrix of chromium, thus diminishing the corrosion resistance of the alloy In small amounts, these carbides may lead to localized pitting in the alloy, but if the chromium-depleted zones are extensive throughout the alloy or heat-affected zone (HAZ) of a weld, the alloy may disintegrate intergranularly in some environments

An alloy in this condition of reduced corrosion resistance due to the formation of chromium carbides is said to be sensitized, a situation that is most pronounced for austenitic alloys In austenitic structures, the complex chromium carbides precipitate preferentially along the grain boundaries This microstructure is susceptible to intergranular corrosion, especially in oxidizing solutions In partially ferritic alloys, carbides tend to precipitate in the discontinuous carbide pools; thus, these alloys are less susceptible to intergranular attack

Solution annealing of austenitic and duplex stainless steels makes these alloys less susceptible to intergranular attack by ensuring the complete solution of the carbides in the matrix Depending on the specific alloy in question, temperatures between 1040 and 1205 °C (1900 and 2200 °F) will ensure the complete solution of all carbides and phases, such as σ and

χ, that sometimes form in highly alloyed stainless steels Alloys containing relatively high total alloy content, particularly high molybdenum content, often require the higher solution treatment temperature Water quenching from the temperature range of 1040 to 1205 °C (1900 to 2200 °F) normally completes the solution treatment Solution-annealing procedures for all austenitic alloys require holding for a sufficient amount of time to accomplish the complete solution of carbides and quenching at a rate fast enough to prevent reprecipitation of the carbides, particularly while cooling through the range of 870 to 540 °C (1600 to 1000 °F)

A two-step heat-treating procedure can be applied to the niobium-containing CF-8C alloy The first treatment consists of solution annealing This is followed by a stabilizing treatment at 870 to 925 °C (1600 to 1700 °F), which precipitates niobium carbides, prevents the formation of damaging chromium carbides, and provides maximum resistance to intergranular attack

Because of their low carbon content, CF-3 and CF-3M as-cast do not contain enough chromium carbide to cause selective intergranular attack; therefore, these alloys can be used in some environments in this condition However, for maximum corrosion resistance, these grades require solution annealing

If the usual quenching treatment is difficult or impossible, holding for 24 to 48 h at 870 to 980 °C (1600 to 1800 °F) and air cooling is helpful for improving the resistance of castings to intergranular corrosion However, except for alloys of

very low carbon content and castings with thin sections, this treatment fails to produce material with as good a resistance

to intergranular corrosion as properly quench-annealed material

Cast Stainless Steels

Revised by Malcolm Blair, Steel Founders' Society of America

Corrosion-Resistant Steel Castings

As previously mentioned, various high-alloy steel castings are classified as corrosion resistant (Table 1) These resistant cast steels are widely used in chemical processing and power-generating equipment that requires corrosion resistance in aqueous or liquid-vapor environments at temperatures normally below 315 °C (600 °F) These alloys are also used in special applications with temperatures up to 650 °C (1200 °F)

corrosion-Compositions

The chemical compositions of various corrosion-resistant cast steels are given in Table 1 These cast steels are specified

in the ASTM standards listed in Table 1

Straight chromium stainless steels contain 10 to 30% Cr and little or no nickel Although about two-thirds of the corrosion-resistant steel castings produced in the United States are of grades that contain 18 to 22% Cr and 8 to 12% Ni, the straight chromium compositions are also produced in considerable quantity, particularly the steel with 11.5 to 14.0%

Cr Corrosion resistance improves as chromium content is increased In general, intergranular corrosion is less of a concern in the straight chromium alloys (which are typically ferritic), especially those containing 25% Cr or more This is attributed to the high bulk chromium contents and the rapid diffusion rates of chromium in ferrite

Iron-chromium-nickel alloys have found wide acceptance and constitute about 60% of total production of high-alloy castings They generally are austenitic with some ferrite The most popular alloys of this type are CF-8 and CF-8M these alloys are nominally 18-8 stainless steels are the cast counterparts of wrought types 304 and 316, respectively The carbon content of each is maintained at 0.08% (max)

Effects of Molybdenum on Corrosion Resistance Alloys 3M and 8M are modifications of 3 and

CF-8 containing 2 to 3% Mo to enhance general corrosion resistance Their passivity under weakly oxidizing conditions is more stable than that of CF-3 and CF-8 The addition of 2 to 3% Mo increases resistance to corrosion by seawater and improves resistance to many chloride-bearing environments The presence of 2 to 3% Mo also improves crevice corrosion and pitting resistance compared to the CF-8 and CF-3 alloys The CF-8M and CF-3M alloys have good resistance to such corrosive media as sulfurous and acetic acids and are more resistant to pitting by mild chlorides These alloys are suitable for use in flowing seawater, but will pit under stagnant conditions

Alloy CG-8M is slightly more highly alloyed than the CF-8M alloys, with the primary addition being increased molybdenum (3 to 4%) The increased amount of molybdenum provides superior corrosion resistance to halide-bearing media and reducing acids, particularly H2SO3 and H2SO4 solutions The high molybdenum content, however, renders CG-8M generally unsuitable in highly oxidizing environments

Molybdenum-bearing alloys are generally not as resistant to highly oxidizing environments (this is particularly true for boiling HNO3), but for weakly oxidizing environments and reducing environments, Mo-bearing alloys are generally superior Molybdenum may also produce detrimental catalytic reactions For example, the residual molybdenum in CF-8 alloy must be held below 0.5% in the presence of hydrazine

Effects of Chromium, Carbon, and Silicon on Corrosion Resistance In alloys of the CF type, the effects of

composition on rates of general corrosion attack have been studied, and certain definite relationships have been established Through the use of the Huey test (five 48 h periods of exposure to boiling 65% nitric acid, as described in practice C of ASTM A 262), it has been shown that, in this standardized environment, carbide-free quench-annealed alloys of various nickel, chromium, silicon, carbon, and manganese contents have corrosion rates directly related to these contents

Figure 3 shows the influences on corrosion rate exerted by various elements in a 19Cr-9Ni casting alloy Variations in nickel, manganese, and nitrogen contents for the ranges shown have relatively slight influences, but variations in chromium, carbon, and silicon have marked effects The relationship between composition and corrosion rate for properly heat-treated CF alloys in boiling 65% nitric acid is summarized in the nomograph presented in Fig 4

Fig 3 Effects of various elements in a 19Cr-9Ni casting alloy on corrosion rate in boiling 65% nitric acid Data

were determined for solution-annealed and quenched specimens Composition of base alloy was 19Cr, 9Ni, 0.09C, 0.8Mn, 1.0Si, 0.04P (max), 0.03S (max), 0.06N

Fig 4 Nomograph for determining corrosion rate in boiling 65% nitric acid for solution-annealed and quenched

type CF casting alloys

Iron-Nickel-Chromium Alloys. For some types of service, extensive use is made of iron-nickel-chromium alloys that contain more nickel than chromium Most important among this group is alloy CN-7M, which has a nominal composition

of 28% Ni, 20% Cr, 3.5% Cu, 2.5% Mo, and 0.07% C (max) In effect, this alloy is made by adding 20% Ni and 3.5% Cu

to alloy CF-8M, which greatly improves resistance to hot, concentrated, weakly oxidizing solutions such as sulfuric acid and also improves resistance to severely oxidizing media Alloys of this type can withstand all concentrations of sulfuric acid at temperatures up to 65 °C (150 °F) and many concentrations up to 80 °C (175 °F) They are widely used in nitric-hydrofluoric pickling solutions; phosphoric acid; cold dilute hydrochloric acid; hot acetic acid; strong, hot caustic solutions; brines; and many complex plating solutions and rayon spin baths

Results of in-plant corrosion testing of CF-8, CF-8M, and CN-7M alloys are shown in Table 3 These tests give the specific effect of molybdenum on 19Cr-9Ni alloys in reducing selective attack and pitting, and the overall corrosion rate computed from loss in weight The higher nickel plus copper and molybdenum in the CN-7M alloy reduces the rate of corrosion to a rate lower than that of the CF-8M alloy

Table 3 Results of in-plant corrosion testing of CF-8, CF-8M, and CN-7M alloys

Temperature

of solution,

Metal loss on surface

Type and composition of

examination

Remarks

CF-8 665 26.2 Very heavy etch(a)

8M

CF-28 1.1 Light tarnish(b)

Neutralizer after formation

of ammonium sulfate:

ammonium sulfate plus

small excess of sulfuric

acid, ammonia vapor, and

steam

100 212

7M

CN-18 0.7 Bright

CF-8M was installed for low corrosion tolerance equipment in this service and performed satisfactorily

CF-8 385 15.2 Very heavy etch(a)

8M

CN-CF-8 in service showed excessive corrosion rate plus heavy concentration cell attack

CF-8 685 27.0 Heavy etch

8M

CN-50 2.0 Light etch

CF-8M had too high a corrosion rate in service for good valve life, although suitable for equipment

of greater corrosion tolerance CN-7M was installed in this service

99 to 100% fuming nitric

acid

20 68 CF-8 245 9.6 Moderate etch CF-8 was satisfactory except for

low-tolerance equipment such as valves CN-7M valves performed

7M

CN-79 3.1 Light etch acid

8M

CF-345 13.5 Moderate etch

valves CN-7M valves performed satisfactorily in service

8M 2.5 0.1 Bright Saturated solution of

CF-sodium chloride plus 15%

sodium sulfate; pH of 4.5

60 140

CF-8 240 9.5 Concentration cell

corrosion at various small areas of specimen

CF-8M was installed for valves in service

(a) Concentration cell attack under insulating washer

(b) Slight concentration cell attack under insulating washer

Corrosion From Chlorine. The influence of contaminants is one of the most important considerations in selecting an alloy for a particular process application Ferric chloride in relatively small amounts, for example, will cause concentration cell corrosion and pitting The buildup of corrosion products in a chloride solution may increase the iron concentration to a level high enough to be destructive Thus, chlorine salts, wet chlorine gas, and unstable chlorinated organic compounds cannot be handled by any of the iron-base alloys, creating a need for nickel-base alloys

Martensitic grades include Alloys CA-15, CA-40, CA-15M, and CA-6NM The CA-15 alloy contains the minimum amount of chromium necessary to make it essentially rustproof It has good resistance to atmospheric corrosion, as well as

to many organic media in relatively mild service A higher-carbon modification of CA-15, CA-40 can be heat treated to higher strength and hardness levels Alloy CA-15M is a molybdenum-containing modification of CA-15 that provides improved elevated-temperature strength Alloy CA-6NM is an iron-chromium-nickel-molybdenum alloy of low carbon content

Austenitic grades include CH-20, CK-20 and CN-7M The CH-20 and CK-20 alloys are high-chromium, high carbon, wholly austenitic compositions in which the chromium content exceeds the nickel content The more highly alloyed CN-7M, as described earlier in the section "Iron-Nickel-Chromium Alloys," has excellent corrosion resistance in many environments and is often used in sulfuric acid environments The CN-7MS alloy has a corrosion resistance similar to that

of CN-7M The CN-7MS alloy has outstanding resistance to corrosion from high-strength (>90%) nitric acid

Ferritic grades include CB-30 and CC-50 Alloy CB-30 is practically nonhardenable by heat treatment As this alloy is normally made, the balance among the elements in the composition results in a wholly ferritic structure similar to wrought AISI type 442 stainless steel Alloy CC-50 has substantially more chromium than CB-30 and has relatively high resistance to localized corrosion in many environments

Austenitic-ferritic (duplex) alloys include CE-30, CF-3, CF-3A, CF-8, CF-8A, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and CG-8M The microstructures of these alloys usually contain 5 to 40% ferrite, depending on the particular grade and the balance among the ferrite-promoting and austenite-promoting elements in the chemical

composition (see the section "Ferrite Control" in this article) Duplex alloys offer superior strength, corrosion resistance, and weldability

The use of duplex cast steels has focused primarily on the CF grades, particularly by the power generation industry Strengthening in the cast CF grade alloys is limited essentially to that which can be gained by incorporating ferrite into the austenite matrix phase These alloys cannot be strengthened by thermal treatment, as can the cast martensitic alloys, not by hot or cold working, as can the wrought austenitic alloys Strengthening by carbide precipitation is also out of the question because of the detrimental effect of carbides on corrosion resistance in most aqueous environments Thus, the alloys are effectively strengthened by balancing the alloy composition to produce a duplex microstructure consisting of ferrite (up to 40% by volume) distributed in an austenite matrix It has been shown that the incorporation of ferrite into 19Cr-9NI cast steels improves yield and tensile strengths without substantial loss of ductility or impact toughness at temperatures below 425 °C (800 °F) The magnitude of this strengthening effect for CF-8 and CF-8M alloys at room temperature is shown in Fig 5 Table 4 shows the effect of ferrite content on the tensile properties of 19Cr-9Ni alloys at room temperature and at 355 °C (670 °F) Table 5 shows the effect of ferrite content on impact toughness

Table 4 Effect of ferrite content on tensile properties of 19Cr-9Ni alloys

Tensile strength Yield strength at

0.2% offset Ferrite content, %

Table 5 Charpy V-notch impact energy, ferrite content, and Cr e /Ni e ratio of duplex cast steels

Alloy Charpy V-notch

energy

Ferrite content, % Cr e /Ni e ratio(c)

(c) See Eq 1 and 2 for formulas to compute Cr e and Ni e

Fig 5 Yield strength and tensile strength versus percentage of ferrite for CF-8 and CF-8M alloys Curves are

mean values for 277 heats of CF-8 and 62 heats of CF-8M Source: Ref 3

Other duplex alloys of interest include CD-4MCu and Ferralium Alloy CD-4MCu is the most highly alloyed duplex alloy Ferralium was developed by Langley Alloys and is essentially CD-4MCu with about 0.15% N added With high levels of ferrite (about 40 to 50%) and low nickel, the duplex alloys have better resistance to stress-corrosion cracking (SCC) than CF-3M Alloy CD-4MCu, which contains no nitrogen and has a relatively low molybdenum content, has only

slightly better resistance to localized corrosion than CF-3M Ferralium which has nitrogen and slightly higher molybdenum than CD-4MCu, exhibits better localized corrosion resistance than either CF-3M or CD-4MCu

Improvements in stainless steel production practices (for example, electron beam refining, vacuum and argon-oxygen decarburization, and vacuum induction melting) have also created a second generation of duplex stainless steels These steels offer excellent resistance to pitting and crevice corrosion, significantly better resistance to chloride SCC than the austenitic stainless steels, good toughness, and yield strengths two to three higher than those of type 304 or 316 stainless steels

First-generation duplex stainless steels, for example, AISI type 329 and CD-4MCu, have been in use for many years The need for improvement in the weldability and corrosion resistance of these alloys resulted in the second-generation alloys, which are characterized by the addition of nitrogen as an alloying element

Second-generation duplex stainless steels are usually about a fifty-fifty blend of ferrite and austenite The new duplex alloys combine the near immunity to chloride SCC of the ferritic grades with the toughness and ease of fabrication of the austenitics Among the second-generation duplexes, Alloy 2205 seems to have become the general-purpose stainless Table 6 lists the nominal compositions of first- and second-generation duplex alloys

Table 6 Nominal compositions of first- and second-generation duplex stainless steels

Composition, %(a) UNS designation Common name

(a) All compositions contain balance of iron

Precipitation-Hardening Alloys. Corrosion-resistant alloys capable of being hardened by low-temperature treatment

to obtain improved mechanical properties are usually duplex-structure alloys with much more chromium than nickel The addition of copper enables these alloys to be strengthened by precipitation hardening These alloys are significantly higher

in strength than the other corrosion-resistant alloys even without hardening

The alloys CB-7Cu-1 and CB-7Cu-2 have corrosion resistances between those of CA-15 and CF-18 They are widely used for structural components requiring moderate corrosion resistance, as well as for components requiring resistance to erosion and wear

The alloy CD-4MCu is widely used in many applications where its good corrosion resistance (which often equals or even exceeds that of CF-8M) and excellent resistance to erosion make it the most desirable alloy The steel CD-4MCu has outstanding resistance to nitric acid and mixtures of nitric acid and organic acids, as well as excellent resistance to a wide range of corrosive chemical process conditions This alloy is normally used in the solution-annealed condition, but it can

be precipitation hardened for carefully selected applications when lower corrosion resistance can be tolerated and when there is no potential for stress-corrosion cracking

Corrosion Characteristics

Table 7 compares the general corrosion resistance of the C-type (corrosion-resistant in liquid service) cast steels

Additional information on the corrosion resistance of cast steels is contained below and in Corrosion, Volume 13 of ASM

Handbook, formerly 9th Edition Metals Handbook

Table 7 Summary of applications for various corrosion-resistant cast steels

CA-15 Widely used in mildly corrosive environments; hardenable; good erosion resistance

CA-40 Similar to CA-15 at higher strength level

CA-6NM Improved properties over CA-15, especially improved resistance to cavitation

CA-6N Outstanding combinations of strength, toughness, and weldability with moderately good corrosion

resistance

CB-30 Improved performance in oxidizing environments compared to CA-15; excellent resistance to corrosion

by nitric acid, alkaline solutions, and many organic chemicals

CB-7Cu-1 Hardenable with good corrosion resistance

CB-7Cu-2 Superior combination of strength, toughness, and weldability with moderately good corrosion resistance

CC-50 Used in highly oxidizing media (hot HNO 3 , acid mine waters)

CD-4MCu Similar to CF-8 in corrosion resistance, but higher strength, hardness, and stress-corrosion cracking

resistance; excellent resistance to environments involving abrasion or erosion-corrosion; usefully

employed in handling both oxidizing and reducing corrodents

CE-30 Similar to CC-50, but Ni imparts higher strength and toughness levels A grade available with controlled

ferrite

CF types: most widely used corrosion-resistant alloys at ambient and cryogenic temperatures

M variations: enhanced resistance to halogen ion and reducing acids

C and F variations: used where application does not permit postweld heat treat

CF-3, CF-8, CF-20,

3M, 8M,

CF-8C, CF-16F

A grades available with controlled ferrite

CG-8M Greater resistance to pitting and corrosion in reducing media than CF-8M; not suitable for nitric acids or

other strongly oxidizing environments

CH-20 Superior to CF-8 in specialized chemical and paper application in resistance to hot H 2 SO 3 , organic acids,

and dilute H 2 SO 4 ; the high nickel and chromium contents also make this alloy less susceptible to intergranular corrosion after exposure to carbide-precipitating temperatures

CK-20 Improved corrosion resistance compared to CH-20

CN-7M Highly resistant to H 2 SO 4 , H 3 PO 4 , H 2 SO 3 salts, and seawater Good resistance to hot chloride salt

solutions, nitric acid, and many reducing chemicals

General Corrosion of Martensitic Alloys. The martensitic grades include CA-15, CA-15M, CA-6NM, CA-6NM-B, CA-40, CB-7Cu1, and CB-7Cu-2 These alloys are generally used in applications requiring high strength and some corrosion resistance

Alloy CA-15 typically exhibits a microstructure of martensite and ferrite This alloy contains the minimum amount of chromium to be considered a stainless steel (11 to 14% Cr) and as such may not be used in aggressive environments It does, however exhibit good atmospheric-corrosion resistance and it resists staining by many organic environments Alloy CA-15M may contain slightly more molybdenum than CA-15 (up to 1% Mo) and therefore may have improved general corrosion resistance in relatively mild environments Alloy CA-6NM is similar to CA-15M except that it contains more nickel and molybdenum, thereby improving its general corrosion resistance Alloy CA-6NM-B is a lower-carbon version

of this alloy The lower strength level promotes resistance to sulfide stress cracking Alloy CA-40 is a higher-strength version of CA-15 and it, too, exhibits excellent atmospheric-corrosion resistance after a normalize and temper heat treatment Microstructurally, the CB-7Cu alloys usually consist of mixed martensite and ferrite and, because of the increased chromium and nickel levels compared to the other martensitic alloys, they offer improved corrosion resistance

to seawater and some mild acids These alloys also have good atmospheric-corrosion resistance The CB-7Cu alloys are hardenable and offer the possibility of increased strength and improved corrosion resistance among the martensitic alloys

General Corrosion of Ferritic Alloys. Alloys CB-30 and CC-50 are higher-carbon and higher-chromium alloys than are the CA alloys mentioned above Each alloy is predominantly ferritic, although a small amount of martensite may be found in CB-30 Alloy CB-30 contains 18 to 21% Cr and is used in chemical processing and oil refining applications The chromium content is sufficient to have good corrosion resistance to many acids, including nitric acid (HNO3)

General Corrosion of Austenitic and Duplex Alloys. Alloy CF-8 may be fully austenitic, but it more commonly contains some residual ferrite (3 to 30%) in an austenite matrix In the solution-annealed condition, this alloy has excellent resistance to a wide variety of acids It is particularly resistant to highly oxidizing acids, such as boiling HNO3 The duplex nature of the microstructure of this alloy imparts additional resistance to SCC compared to its wholly

austenitic counterparts Alloy CF-3 is a reduced-carbon version of CF-8 with essentially identical corrosion resistance except that CF-3 is much less susceptible to sensitization For applications in which the corrosion resistance of the weld HAZ may be critical, CF-3 is a common material selection

Alloys CF-8A and CF-3A contain more ferrite than their CF-8 and CF-3 counterparts Because the higher ferrite content

is achieved by increasing the chromium/nickel equivalent ratio, the CF-8A and CF-3A alloys may have slightly higher chromium or slightly lower nickel contents than the low-ferrite equivalents In general, the corrosion resistance is very similar, but the strength increases with ferrite content Because of the high ferrite content, service should be restricted to temperatures below 400 °C (750 °F) because of the possibility of severe embrittlement Alloy CF-8C is the niobium-stabilized grade of the CF-8 alloy class This alloy contains small amounts of niobium, which tend to form carbides preferentially over chromium carbides and improve intergranular corrosion resistance in applications involving relatively high service temperatures

Alloy CF-16F is a selenium-bearing free-machining grade of cast stainless steel Because CF-16F nominally contains 19% Cr and 10% Ni, it has adequate corrosion resistance to a wide range of corrosive materials but the large number of selenide inclusions makes surface deterioration and pitting definite possibilities

Alloy CE-30 is a nominally 27Cr-9Ni alloy that normally contains 10 to 20% ferrite in an austenite matrix The high carbon, high ferrite content provides relatively high strength The high chromium content and duplex structure act to minimize corrosion because of the formation of chromium carbides in the microstructure This particular alloy is known for good resistance to sulfurous acid and sulfuric acid and is used extensively in the pulp and paper industry (see the

article "Corrosion in the Pulp and Paper Industry" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition

Metals Handbook)

Alloy CD-4MCu is the most highly alloyed material in this group of alloys, and a microstructure containing approximately equal amounts of ferrite and austenite is common The low carbon content and high chromium content render the alloy relatively immune to intergranular corrosion High chromium and molybdenum provide a high degree of localized corrosion resistance (crevices and pitting), and the duplex microstructure provides SCC resistance in many environments This alloy can be precipitation hardened to provide strength and is also relatively resistant to abrasion and erosion-corrosion

Fully Austenitic Alloys. Alloys CH-10 and CH-20 are fully austenitic and contain 22 to 26% Cr and 12 to 15% Ni The high chromium content minimizes the tendency toward the formation of chromium-depleted zones due to sensitization These alloys are used for handling paper pulp solutions and are known for good resistance to dilute H2SO4

and HNO3

Alloy CK-20 contains 23 to 27% Cr and 19 to 22% Ni and is less susceptible than CH-20 to intergranular corrosion attack

in many acids after brief exposures to the chromium carbide formation temperature range Maximum corrosion resistance

is achieved by solution treatment Alloy CK-20 possesses good corrosion resistance to many acids and, because of its fully austenitic structure, can be used at relatively high temperatures

Alloy CN-7M exhibits excellent corrosion resistance in a wide variety of environments and is often used for H2SO4

service Relatively high resistance to intergranular corrosion and SCC make this alloy attractive for many applications Although CN-7M is relatively highly alloyed, its fully austenitic structure may lead to SCC susceptibility for some environments and stress states

Alloy CF-20 is a fully austenitic, relatively high-strength corrosion-resistant alloy The 19% Cr content provides resistance to many types of oxidizing acids, but the high carbon content makes it imperative that this alloy be used in the solution-treated condition for environments known to cause intergranular corrosion

Intergranular Corrosion. Ferritic alloys may also be sensitized by the formation of extensive chromium carbide networks, but because of the high bulk chromium content and rapid diffusion rates of chromium in ferrite, the formation

of carbides can be tolerated if the alloy has been slowly cooled from a solutionizing temperature of 780 to 900 °C (1435

to 1650 °F) The slow cooling allows replenishment of the chromium adjacent to the carbides Martensitic alloys normally

do not contain sufficient bulk chromium to be used in applications in which intergranular corrosion is likely to be a concern

Austenitic and duplex stainless steels use solution annealing for the prevention or reduction of intergranular corrosion (see "Sensitization and Solution Annealing of Austenitic and Duplex Alloys" in this article) Failure to solution treat a particular alloy or an improper solution treatment may seriously compromise the observed corrosion resistance in service

If solution treatment of the alloy after casting and/or welding is impractical or impossible, the metallurgist has several tools from which to choose to minimize potential intergranular corrosion problems The low-carbon grades CF-3 and CF-3M are commonly used when heat treatment is impractical or as a solution to the sensitization incurred during welding The low carbon content, that is, 0.03% C (max), of these alloys precludes the formation of an extensive number of chromium carbides In addition, these alloys normally contain 3 to 30% ferrite in an austenitic matrix By virtue of rapid carbide precipitation kinetics at ferrite/austenite interfaces compared to austenite/austenite interfaces, carbide precipitation is confined to ferrite-austenite boundaries in alloys containing a minimum of about 3 to 5% ferrite (Ref 4, 5)

If the ferrite network is discontinuous in the austenite matrix (depending on the amount, size, and distribution of ferrite pools), extensive intergranular corrosion will not be a problem in most of the environments to which these alloys will be subjected

The niobium-modified grade of 18-8, known as CF-89C, is produced for similar applications in which heat treatment is impractical Niobium-containing alloys that have been heated to sensitizing temperatures around 650 °C (1200 °F) are not susceptible to intergranular corrosion However, they are more susceptible to overall corrosion when tested in nitric acid, compared to the niobium-free, quench-annealed alloys of the same nickel, chromium, and carbon contents Addition of niobium to molybdenum-containing type CF alloys has also been found unsatisfactory for castings When both niobium and molybdenum are present, the ferrite phase tends to form as an interconnected network and is especially likely to transform into the brittle σ phase As a result, castings in the as-cast condition become embrittled and have a tendency to crack

When the niobium-bearing grade CF-8C is in the as-cast condition, most of its carbon is in the form of niobium carbide, precluding chromium carbide precipitation in the critical temperature range from 425 to 870 °C (800 to 1600 °F) and particularly from 565 to 650 °C (1050 to 1200 °F) The alloy CF-8C is solution treated at 1120 °C (2050 °F), quenched to room temperature, and then reheated to 870 to 925 °C (1600 to 1700) °F), at which temperature precipitation of niobium carbide occurs An alternative method is solution treating at 1120 °C (2050 °F), cooling to the 870 to 925 °C (1600 to

1700 °F) range, and then holding at this temperature before cooling to room temperature For maximum corrosion resistance, it is recommended that this alloy be solution treated before being stabilized

Weld crack sensitivity of CF alloys containing niobium (CF-8C) is more pronounced in the fully austenitic grade Cracking may be alleviated through the introduction into the weld deposit of a small amount of ferrite, usually between 4 and 10% However, appreciable amounts of ferrite in niobium-bearing corrosion-resistant steels will transform, at least partly, to the σ or χ phase upon heating to between 540 and 925 °C (1000 and 1700 °F)

Stress-Corrosion Cracking. The SCC of cast stainless steels has been investigated for only a limited number of environments, heat treatments, and test conditions From the limited information available, the following generalizations apply

First, SCC resistance seems to improve as the composition is adjusted to provide increasingly greater amounts of ferrite in

an austenitic matrix This trend continues to a certain level, apparently near 50% ferrite (Fig 6) Second, a lower nickel content tends to improve SCC resistance in cast duplex alloys, possibly because of its effect on ferrite content (Ref 6) Third, ferrite appears to be involved in a keying action in discouraging SCC At low and medium stress levels, the ferrite tends to block the propagation of stress-corrosion cracks This may be due to a change in composition and/or crystal structure across the austenite/ferrite boundary As the stress level increases, crack propagation may change from austenite/ferrite boundaries to transgranular propagation (Ref 6, 7) Finally, reducing the carbon content of cast stainless alloys, thereby reducing the susceptibility to sensitization, improves SCC resistance This is also true for wrought alloys

Fig 6 Stress required to produce stress-corrosion cracking in several corrosion-resistant cast steels with

varying amounts of ferrite

Mechanical Properties of Corrosion-Resistant Cast Steels

The importance of mechanical properties in the selection of corrosion-resistant cast steels is established by the casting application The paramount basis for alloy selection is normally the resistance of the alloy to the specific corrosive media

or environment of interest The mechanical properties of the alloy are usually, but not always, secondary considerations in these applications

Room-Temperature Mechanical Properties. Representative room-temperature tensile properties, hardness, and Charpy impact values for corrosion-resistant cast steels are given in Fig 7 These properties are representative of the alloys rather than the specification requirements Minimum specified mechanical properties for these alloys are given in ASTM standards A 351, A 743, A 744, and A 747 A wide range of mechanical properties are attainable depending on the selection of alloy composition and heat treatment Tensile strengths ranging from 475 to 1310 MPa (69 to 190 ksi) and hardness from 130 to 400 HB are available among the cast corrosion-resistant alloys Similarly, wide ranges exist in yield strength, elongation, and impact toughness

Alloy Heat treatment

CA-15 (a) AC from 980 °C (1800 °F), T at 790 °C (1450 °F)

(b) AC from 980 °C (1800 °F), T at 650 °C (1200 °F)

Fig 7 Mechanical properties of various cast corrosion-resistant steels at room temperature (a) Tensile

strength (b) 0.2% offset yield strength (b) 0.2% offset yield strength (c) Charpy keyhole impact energy (d) Brinell hardness (e) Elongation Also given are the heat treatments used for test materials: AC, air cool; FC, furnace cool; WQ, water quench; A, anneal; T, temper

The straight chromium steels (CA-15, CA-40, CB-30, and CC-50) possess either martensitic or ferritic microstructures in the end-use condition (Table 1) The CA-15 and CA-40 alloys, which contain nominally 12% Cr, are hardenable through heat treatment by means of the martensite transformation and are of the selected as much or more for their high strength

as for their comparatively modest corrosion resistance

The higher-chromium CB-30 and CC-50 alloys, on the other hand, are fully ferritic alloys that are not hardenable by heat treatment These alloys are generally used in the annealed condition and exhibit moderate tensile properties and hardness Like most ferritic alloys, CB-30 and CC-50 possess limited impact toughness, especially at low temperatures

Three chromium-nickel alloys, CA-6NM, CB-7Cu, and CD-4M Cu, are exceptional in their response to heat treatment and in the resultant mechanical properties Alloy CA-6NM is balanced compositionally for martensitic hardening response This alloy was developed as an alternative to CA-15 and has improved impact toughness and weldability The CB-7Cu and CD-4MCu alloys both contain copper and can be strengthened by age hardening These alloys are initially solution heat treated and then cooled rapidly (usually by quenching in oil or water); thus, the phases that would normally precipitate at slow cooling rates cannot form The casting is then heated to an intermediate aging temperature at which the precipitation reaction can occur under controlled conditions until the desired combination of strength and other properties

is achieved The CB-7Cu alloy possesses a martensitic matrix, while the CD-4MCu alloy possesses a duplex microstructure, consisting of approximately 40% austenite in a ferritic matrix Alloy CB-7 Cu is applied in the aged condition to obtain the benefit of its excellent combination of strength and corrosion resistance, but alloy CD-4MCu is seldom applied in the aged condition because of its relatively low resistance to SCC in this condition compared to its superior corrosion resistance in the solution-annealed condition

The CE, CF, CG, CH, CN, and CK alloys are essentially not hardenable by heat treatment To ensure maximum corrosion resistance, however, it is necessary that castings of these grades receive a high-temperature solution anneal (see

"Sensitization and Solution Annealing of Austenitic and Duplex Alloys" in this article) By virtue of their microstructures, which are fully austenitic or duplex without significant carbide precipitation, the alloys exhibit generally excellent impact toughness at low temperatures The tensile strength range represented by these alloys typically extends from 475 to 670 MPa (69 to 97 ksi) As indicated earlier in the section "Austenitic-Ferritic (Duplex) Alloys" in this article, the alloys with duplex structures can be strengthened by balancing the composition for higher ferrite levels (Fig 5) The tensile and yield strengths of CF alloys with a ferrite number of 35 are typically 150 MPa (22 ksi) higher than those of fully austenitic alloys Tensile ductility (Table 4) and impact toughness (Table 5) are lowered with increasing ferrite content

Effects from High Temperatures. Cast corrosion-resistant high-alloy steels are used extensively at moderately

elevated temperatures (up to 650 °C, or 1200 °F) Elevated-temperature properties are important selection criteria for these applications Table 8 gives the tensile properties of a corrosion-resistant cast steel at various test temperatures In addition, mechanical properties after long-term exposure at elevated temperatures are increasingly considered because of the aging effect that these exposures may have For example, cast alloys CF-8C, CF-8M, CE-30A, and CA-15 are currently used in high-pressure service at temperatures up to 540 °C (1000 °F) in sulfurous acid environments in the petro-chemical industry Other uses are in the power-generating industry at temperatures up to 565 °C (1050 °F)

Table 8 Short-time tensile properties of peripheral-welded cylinders of CF-8 alloy

Cylinders were 38 mm (11

2 in.) thick; specimens were machined with longitudinal axes perpendicular to welded seam and with seam

at middle of gage length

Modulus of elasticity (a)

GPa 10 6 psi

Location of

final rupture

(a) Values of proportional limit and modulus of elasticity at elevated temperatures are apparent values because creep occurs

(b) Separately cast from same heat as cylinders

Room-temperature properties after exposure to elevated service temperatures may differ from those in the as-heat-treated condition because of the microstructural changes that may take place at the service temperature Microstructural changes

in iron-nickel-chromium-(molybdenum) alloys may involve the formation of carbides and such phases as σ, χ, and η (Laves) The extent to which these phases form depends on the composition, as well as the time at elevated temperature

The martensitic alloys CA-15 and CA-6NM are subject to minor changes in mechanical properties and SCC resistance in NaCl and polythionic acid environments upon exposure for 3000 h at up to 565 °C (1050 °F) In CF-type chromium-nickel-(molybdenum) steels, only negligible changes in ferrite content occur during 10,000 h exposure at 400 °C (750 °F) and during 3000 h exposure at 425 °C (800 °F) Carbide precipitation, however, does occur at these temperatures, and noticeable Charpy V-notch energy losses have been reported

Above 425 °C (800 °F), microstructural changes in chromium-nickel-(molybdenum) alloys take place at an increased rate Carbides and phase form rapidly at 650 °C (1200 °F) at the expense of ferrite Tensile ductility and Charpy V-

notch impact energy (Fig 8) are prone to significant losses under these conditions Density changes, resulting in contraction, have been reported as a result of these high-temperature exposures

Fig 8 Charpy V-notch impact energy of three corrosion-resistant cast steels at room temperature after aging at

594 °C (1100 °F) Source: Ref 8

Fatigue Properties and Corrosion Fatigue. The resistance of cast stainless steels to fatigue depends on a sizable number of material, design, and environmental factors For example, design factors of importance include the stress distribution within the casting (residual and applied stresses), the location and severity of stress concentrators (surface integrity), and the environment and service temperatures Material factors of importance include strength and microstructure It is generally found that fatigue strength increases with the tensile strength of a material Both fatigue strength and tensile strength usually increase with decreasing temperature Under equivalent conditions of stress, stress concentration, and strength, evidence suggests that austenitic materials are less notch sensitive than martensitic or ferritic materials

Corrosion fatigue is highly specific to the environment and alloy The martensitic materials are degraded the most in both absolute and relative terms If left to corrode freely in seawater, they have very little resistance to corrosion fatigue This

is remarkable in view of their very high strength and fatigue resistance in air

Properties can be protected if suitable cathodic protection is applied However, because these materials are susceptible to hydrogen embrittlement, cathodic protection must be carefully applied Too large a protective potential will lead to catastrophic hydrogen stress cracking

Austenitic materials are also severely degraded in corrosion fatigue strength under conditions conducive to pitting, such

as in seawater However, they are easily cathodically protected without fear of hydrogen embrittlement and perform well

in fresh waters The corrosion fatigue behavior of duplex alloys has not been widely studied

References cited in this section

3 F Beck, E.A Schoefer, E Flowers, and M Fontana, New Cast High Strength Alloy Grades by Structure

Control, in Advances in the Technology of Stainless Steels and Related Alloys, STP 369, American Society

for Testing and Materials, 1965, p 159-174

4 T.M Devine, Mechanism of Intergranular Corrosion and Pitting Corrosion of Austenitic and Duplex 308

Stainless Steel, J Electrochem Soc., Vol 126 (No 3), 1979, p 374

5 E.E Stansbury, C.D Lundin, and S.J Pawel, Sensitization Behavior of Cast Stainless Steels Subjected to

Simulated Weld Repair, in Proceedings of the 38th SFSA Technical and Operating Conference, Steel

Founders' Society of America, 1983, p 223

6 S Shimodaira et al., Mechanisms of Transgranular Stress Corrosion Cracking of Duplex and Ferrite Stainless Steels, in Stress Corrosion Cracking and Hydrogen Embrittlement in Iron Base Alloys, NACE

Reference Book 5, National Association of Corrosion Engineers, 1977

7 P.L Andersen and D.J Duquette, The Effect of Cl- Concentration and Applied Potential on the SCC

Behavior of Type 304 Stainless Steel in Deaerated High Temperature Water, Corrosion, Vol 36 (No 2),

1980, p 85-93

8 S.B Shendye, "Effect of Long Term Elevated Temperature Exposure on the Mechanical Properties and Weldability of Cast Duplex Steels," Master's thesis, Oregon Graduate Center, 1985

Cast Stainless Steels

Revised by Malcolm Blair, Steel Founders' Society of America

Heat-Resistant Cast Steels

As previously mentioned, castings are classified as heat resistant if they are capable of sustained operation while exposed, either continuously or intermittently, to operating temperatures that result in metal temperatures in excess of 650 °C (1200

°F) Cast steels for this type of service include chromium (straight chromium), chromium-nickel, and nickel-chromium alloys In applications of heat-resistant alloys, considerations include:

iron-• Resistance to corrosion at elevated temperatures

• Stability (resistance to warping, cracking, or thermal fatigue)

• Creep strength (resistance to plastic flow)

Table 9 briefly compares the various H-type grades of heat-resistant steel castings in terms of general corrosion resistance and creep values

Table 9 General corrosion characteristics of heat-resistant cast steels and typical limiting creep stress values at indicated temperatures

Creep test temperature

Limiting creep stress

(0.0001%/h) Alloy Corrosion characteristics

°C °F MPa ksi

HA Good oxidation resistance to 650 °C (1200 °F); widely used in oil refining industry 650 1200 21.5 3.1

HC Good sulfur and oxidation resistance up to 1095 °C (2000 °F); minimal mechanical

properties; used in applications where strength is not a consideration or for moderate load

bearing up to 650 °C (1200 °F)

870 1600 5.15 0.75

HD Excellent oxidation and sulfur resistance plus weldability 980 1800 6.2 0.9

HE Higher temperature and sulfur resistance capabilities than HD 980 1800 9.5 1.4

HF Excellent general corrosion resistance to 815 °C (1500 °F) with moderate mechanical

properties

870 1600 27 3.9

HH(a) High strength; oxidation resistant to 1090 °C (2000 °F); most widely used 980 1800 7.5

(type I) 14.5 (type II)

1.1 (type I) 2.1 (type II)

HK Because of its high temperature strength, widely used for stressed parts in structural

applications up to 1150 °C (2100 °F); offers good resistance to corrosion by hot gases,

including sulfur-bearing gases, in both oxidizing and reducing conditions (although HC,

HE, and HI are more resistant in oxidizing gases); used in air, ammonia, hydrogen, and

molten neutral salts; widely used for tubes and furnace parts

1040 1900 9.5 1.4

HL Improved sulfur resistance compared to HK; especially useful where excessive scaling must

be avoided

980 1800 15 2.2

HN Very high strength at high temperatures; resistant to oxidizing and reducing flue gases 1040 1900 11 1.6

HP Resistant to both oxidizing and carburizing atmospheres at high temperatures 980 1800 19 2.8

HP-50WZ

Improved creep rupture strength at 1090 °C (2000 °F) and above compared to HP 1090 2000 4.8 0.7

HT Widely used in thermal shock applications; corrosion resistant in air, oxidizing and

reducing flue gases, carburizing gases, salts, and molten metals; performs satisfactorily up

to 1150 °C (2100 °F) in oxidizing atmospheres and up to 1095 °C (2000 °F) in reducing

atmospheres, provided that limiting creep stress values are not exceeded

980 1800 14 2.0

HU Higher hot strength than HT and often selected for its superior corrosion resistance 980 1800 15 2.2

HW High hot strength and electrical resistivity; performs satisfactorily to 1120 °C (2050 °F) in

strongly oxidizing atmospheres and up to 1040 °C (1900 °F) in oxidizing or reducing

products of combustion that do not contain sulfur; resistant to some salts and molten metals

980 1800 9.5 1.4

HX Resistant to hot-gas corrosion under cycling conditions without cracking or warping;

corrosion resistant in air, carburizing gases, combustion gases, flue gases, hydrogen,

980 1800 11 1.6

molten cyanide, molten lead, and molten neutral salts at temperatures up to 1150 °C

(2100 °F)

(a) Two grades: type I (ferrite in austenite) and type II (wholly austenitic), per ASTM A 447

Commercial applications of heat-resistant castings include metal treatment furnaces, gas turbines, aircraft engines, military equipment, oil refinery furnaces, cement mill equipment, petrochemical furnaces, chemical process equipment, power plant equipment, steel mill equipment, turbochargers, and equipment used in manufacturing glass and synthetic rubber Alloys of the iron-chromium and iron-chromium-nickel groups are of the greatest commercial importance

General Properties

General corrosion and creep properties of heat-resistant steel castings are compared in Table 8 The compositions of these heat-resistant cast steels are given in Table 2 These heat-resistant cast steels resemble corrosion-resistant cast steels (Table 1) except for their higher carbon content, which imparts greater strength at elevated temperatures Typical tensile properties of heat-resistant cast steels at room temperature are given in Table 10 and (at elevated temperatures) in Table

11

Table 10 Typical room-temperature properties of ACI heat-resistant casting alloys

Tensile strength Yield strength Alloy Condition

(a) Aging treatment: 24 h at 760 °C (1400 °F), furnace cool

(b) Aging treatment: 24 h at 760 °C (1400 °F), air cool

(c) Aging treatment: 48 h at 980 °C (1800 °F), air cool

(d) Aging treatment: 48 h at 980 °C (1800 °F), furnace cool

Table 11 Representative short-term tensile properties of cast heat-resistant alloys at elevated temperatures

Property at indicated temperature

Ultimate tensile strength

Yield strength at 0.2% offset

Ultimate tensile strength

Yield strength at 0.2% offset

Ultimate tensile strength

Yield strength at 0.2% offset Alloy

HT 240 35 180 26 10 130 19 103 15 24 76 11 55 8 28

HU 275 40 135 19.5 20 69 10 43 6.2 28

(a) In this instance, test temperature was 540 °C (1000 °F)

(b) Test temperature was 590 °C (1100 °F)

(c) Type I and II per ASTM A 447

(d) Test temperature was 650 °C (1200 °F)

Iron-chromium alloys contain 10 to 30% Cr and little or no nickel These alloys are useful chiefly for resistance to oxidation; they have low strength at elevated temperatures Use of these alloys is restricted to conditions, either oxidizing

or reducing, that involve low static loads and uniform heating Chromium content depends on anticipated service temperature

Iron-chromium-nickel alloys contain more than 13% Cr and more than 7% Ni (always more chromium than nickel) These austenitic alloys are ordinarily used under oxidizing or reducing conditions similar to those withstood by the ferritic iron-chromium alloys, but in service they have greater strength and ductility than the straight chromium alloys They are used, therefore, to withstand greater loads and moderate changes of temperature These alloys also are used in the presence of oxidizing and reducing gases that are high in sulfur content

Iron-nickel-chromium alloys contain more than 25% Ni and more than 10% Cr (always more nickel than chromium) These austenitic alloys are used for withstanding reduction as well as oxidizing atmospheres, except where sulfur content is appreciable (In atmospheres containing 0.05% or more hydrogen sulfide, for example, iron-chromium-nickel alloys are recommended.) In contrast with iron-chromium-nickel alloys, iron-nickel-chromium alloys do not carburize rapidly or become brittle and do not take up nitrogen in nitriding atmospheres These characteristics become enhanced as nickel content is increased, and in carburizing and nitriding atmospheres casting life increases with nickel content Austenitic iron-nickel-chromium alloys are used extensively under conditions of severe temperature fluctuations such as those encountered by fixtures used in quenching and by parts that are not heated uniformly or that are heated and cooled intermittently In addition, these alloys have characteristics that make them suitable for electrical resistance heating elements

Metallurgical Structures

The structures of chromium-nickel and nickel-chromium cast steels must be wholly austenitic, or mostly austenitic with some ferrite, if these alloys are to be used for heat-resistant service Depending on the chromium and nickel content (see the section "Composition and Microstructure" in this article), the structures of these iron-base alloys can be austenitic (stable), ferritic (stable, but also soft, weak, and ductile) or martensitic (unstable); therefore, chromium and nickel levels should be selected to achieve good strength at elevated temperatures combined with resistance to carburization and hot-gas corrosion

A fine dispersion of carbides or intermetallic compounds in an austenitic matrix increases high-temperature strength considerably For this reason, heat-resistant cast steels are higher in carbon content than are corrosion-resistant alloys of comparable chromium and nickel content By holding at temperatures where carbon diffusion is rapid (such as above

1200 °C) and then rapidly cooling, a high and uniform carbon content is established, and up to about 0.20% C is retained

in the austenite Some chromium carbides are present in the structures of alloys with carbon contents greater than 0.20%, regardless of solution treatment, as described in the section "Sensitization and Solution Annealing of Austenitic and Duplex Alloys" in this article

Castings develop considerable segregation as they freeze In standard grades, either in the as-cast condition or after rapid cooling from a temperature near the melting point, much of the carbon is in supersaturated solid solution Subsequent reheating precipitates excess carbides The lower the reheating temperature, the slower the reaction and the finer the precipitated carbides Fine carbides increase creep strength and decrease ductility Intermetallic compounds such as

Ni3Al, if present, have a similar effect

Reheating material containing precipitated carbides in the range between 980 and 1200 °C (1800 and 2200 °F) will agglomerate and spheroidize the carbides, which reduces creep strength and increases ductility Above 1100 °C (2000

°F), so many of the fine carbides are dissolved or spheroidized that this strengthening mechanism loses its importance For service above 1100 °C (2000 °F), certain proprietary alloys of the iron-nickel-chromium type have been developed Alloys for this service contain tungsten to form tungsten carbides, which are more stable than chromium carbides at these temperatures

Aging at a low temperature, such as 760 °C (1400 °F), where a fine, uniformly dispersed carbide precipitate will form, confers a high level of strength that is retained at temperatures up to those at which agglomeration changes the character

of the carbide dispersion (overaging temperatures) Solution heat treatment or quench annealing, followed by aging, is the treatment generally employed to attain maximum creep strength

Ductility is usually reduced when strengthening occurs; but in some alloys the strengthening treatment correct an unfavorable grain-boundary network of brittle carbides, and both properties benefit However, such treatment is costly and may warp castings excessively Hence, this treatment is applied to heat-resistant castings only for the small percentage of applications for which the need for prenium performance justifies the high cost

Carbide networks at grain boundaries are generally undesirable in iron-base heat-resistant alloys Grain-boundary networks usually occur in very-high-carbon alloys or in alloys that have cooled slowly through the high-temperature ranges in which excess carbon in the austenite is rejected as grain-boundary networks rather than as dispersed particles These networks confer brittleness in proportion to their continuity

Carbide networks also provide paths for selective attack in some atmospheres and in certain molten salts Therefore, it is advisable in some salt bath applications to sacrifice the high-temperature strength imparted by high carbon content and gain resistance to intergranular corrosion by specifying that carbon content be no greater than 0.08%

Straight Chromium Heat-Resistant Castings

Iron-chromium alloys, also known as straight chromium alloys, contain either 9 or 28% Cr HC and HD alloys are included among the straight chromium alloys, although they contain low levels of nickel

HA alloy (9Cr-1Mo), a heat treatable material, contains enough chromium to provide good resistance to oxidation at temperatures up to about 650 °C (1200 °F) The 1% molybdenum is present to provide increased strength HA alloy castings are widely used in oil refinery service A higher-chromium modification of this alloy (12 to 14% Cr) is widely used in the glass industry

HA alloy has a structure that is essentially ferritic; carbides are present in pearlitic areas or as agglomerated particles, depending on prior heat treatment Hardening of the alloy occurs upon cooling in air from temperatures above 815 °C (1500 °F) In the normalized and tempered condition, the alloy exhibits satisfactory toughness throughout its useful temperature range

HC alloy (28% Cr) resists oxidation and the effects of high-sulfur flue gases at temperatures up to 1100 °C (2000 °F) It

is used for applications in which strength is not a consideration, or in which only moderate loads are involved, at temperatures of about 650 °C (1200 °F) It is also used where appreciable nickel cannot be tolerated, as in very-high-sulfur atmospheres, or where nickel may act as an undesirable catalyst and destroy hydrocarbons by causing them to crack

HC alloy is ferritic at all temperatures Its ductility and impact strength are very low at room temperature and its creep strength is very low at elevated temperatures unless some nickel is present In a variation of HC alloy that contains more than 2% Ni, substantial improvement in all three of these properties is obtained by increasing the nitrogen content to 0.15% or more

HC alloy becomes embrittled when heated for prolonged periods at temperatures between 400 and 550 °C (750 and 1025

°F), and it shows low resistance to impact The alloys is magnetic and has a low coefficient of thermal expansion, comparable to that of carbon steel It has about eight times the electrical resistivity and about half the thermal conductivity of carbon steel Its thermal conductivity, however, is roughly double the value for austenitic iron-chromium-nickel alloys

HD alloy (28Cr-5Ni) is very similar in general properties to HC, except that its nickel content gives it somewhat greater

strength at high temperatures The high chromium content of this alloy makes it suitable for use in high-sulfur atmospheres

HD alloy has a two-phase, ferrite-plus-austenite structure that is not hardenable by conventional heat treatment Long exposure at 700 to 900 °C (1300 to 1650 °F), however, may result in considerable hardening and severe loss of room-temperature ductility through the formation of σ phase Ductility may be restored by heating uniformly to 980 °C (1800

°F) or higher and then cooling rapidly to below 650 °C (1200 °F)

Iron-Chromium-Nickel Heat-Resistant Castings

Heat-resistant ferrous alloys in which the chromium content exceeds the nickel content are made in compositions ranging from 20Cr-10Ni to 30Cr-20Ni

HE alloy (28Cr-10Ni) has excellent resistance to corrosion at elevated temperatures Because of its higher chromium content, it can be used at higher temperatures than HF alloy and is suitable for applications up to 1100 °C (2000 °F) This alloy is stronger and more ductile at room temperature than the straight chromium alloys

In the as-cast condition, HE alloy has a two-phase, austenite-plus-ferrite structure containing carbides HE castings cannot

be hardened by heat treatment; however, as with HD castings, long exposure to temperatures near 815 °C (1500 °F) will promote formation of σ phase and consequent embrittlement of the alloy at room temperature The ductility of this alloy can be improved somewhat by quenching from about 1100 °C (2000 °F)

Castings of HE alloy have good machining and welding properties Thermal expansion is about 50% greater than that of either carbon steel or the Fe-Cr alloy HC Thermal conductivity is much lower than for HD or HC, but electrical resistivity is about the same HE alloy is weakly magnetic

HE alloy (20Cr-10Ni) is the cast version of 18-8 stainless steel, which is widely used for its outstanding resistance to corrosion HF alloy is suitable for use at temperatures up to 870 °C (1600 °F) When this alloy is used for resistance to oxidation at elevated temperatures, it is not necessary to keep the carbon content at the low level specified for corrosion-resistant castings Molybdenum, tungsten, niobium, and titanium are sometimes added to the basic HF composition to improve elevated-temperature strength

In the as-cast condition, HF alloy has an austenitic matrix that contains interdendritic eutectic carbides and, occasionally,

a lamellar constituent presumed to consist of alternating platelets of austenite and carbide or carbonitride Exposure at service temperatures usually promotes precipitation of finely dispersed carbides, which increases room-temperature strength and causes some loss of ductility If improperly balanced, as-cast HF may be partly ferritic HF is susceptible to embrittlement due to σ-phase formation after long exposure at 760 to 815 °C (1400 to 1500 °F)

HH Alloy (26Cr-12Ni) Alloys of this nominal composition comprise about one-third of the total production of iron-base heat-resistant castings Alloy HH is basically austenitic and holds considerable carbon in solid solution, but carbides, ferrite (soft, ductile, and magnetic) and (hard, brittle, and nonmagnetic) may also be present in the microstructure The amounts of the various structural constituents present depend on composition and thermal history In fact, two distinct grades of material can be obtained within the stated chemical compositional range of the type alloy HH These grades are defined as type I (partially ferritic) and type II (wholly austenitic) in ASTM A 447

The partially ferritic (type I) alloy HH is adapted to operating conditions that are subject to changes in temperature level and applied stress A plastic extension in the weaker, ductile ferrite under changing load tends to occur more readily than

in the stronger austenitic phase, thereby reducing unit stresses and stress concentrations and permitting rapid adjustment

to suddenly applied overloads without cracking Near 870 °C (1600 °F), the partially ferritic alloys tend to embrittle from the development of σ phase, while close to 760 °C (1400 °F), carbide precipitation may cause comparable loss of ductility Such possible embrittlement suggests that 930 to 1090 °C (1700 to 2000 °F) is the best service temperature range, but this is not critical for steady temperature conditions in the absence of unusual thermal or mechanical stresses

To achieve maximum strength at elevated temperatures, the HH alloy must be wholly austenitic Where load and temperature conditions are comparatively constant, the wholly austenitic (type II) alloy HH provides the highest creep strength and permits the use of maximum design stress The stable austenitic alloy is also favored for cyclic temperature service that might induce σ-phase formation in the partially ferritic type When HH alloy is heated to between 650 and

870 °C (1200 and 1600 °F), a loss in ductility may be produced by either of two changes within the alloy: precipitation of carbides or transformation of ferrite to σ When the composition is balanced so that the structure is wholly austenitic, only carbide precipitation normally occurs In partly ferritic alloys, both carbides and σ phase may form

The wholly austenitic (type II) HH alloy is used extensively in high-temperature applications because of its combination

of relatively high strength and oxidation resistance at temperatures up to 1100 °C (2000 °F) Typical tensile properties and impact toughness of the type II HH alloy at elevated temperatures are shown in Fig 9(a) The HH alloy (type I or II)

is seldom used for carburizing applications because of embrittlement from carbon absorption High silicon content (over 1.5%) will fortify the alloy against carburization under mild conditions, but will promote ferrite formation and possible embrittlement

Fig 9 Effect of short-term elevated-temperature exposure on the tensile properties of wholly austenitic (type

II) HH cast steel (a) and of five other heat-resistant cast steels: (b) HF cast steel, (c) HK-40 cast steel, (d) HN cast steel, (e) HP cast steel, and (f) HT cast steel Long-term elevated-temperature exposure reduces the strengthening effects between 500 to 750 °C (900 to 1400 °F) in (c), (d), and (e) Tensile properties of alloy

HT in (f) include extrapolated data (dotted lines) below 750 °C but should be similar to alloy HN in terms of yield and tensile strengths Source: Ref 9

For the wholly austenitic (type II) HH alloy, composition balance is critical in achieving the desired austenitic microstructure (see "Composition and Microstructure" in this article) An imbalance of higher levels of ferrite-promoting elements compared to levels of austenite-promoting elements may result in substantial amounts of ferrite which improves ductility, but decreases strength at high temperatures If a balance is maintained between ferrite-promoting elements (such

as chromium and silicon) and austenite-promoting elements (such as nickel, carbon, and nitrogen), the desired austenitic structure can be obtained In commercial HH alloy castings, with the usual carbon, nitrogen, manganese, and silicon contents, the ratio of chromium to nickel necessary for a stable austenitic structure is expressed by the inequality:

Before HH alloy is selected as a material for heat-resistant castings, it is advisable to consider the relationship between chemical composition and operating-temperature range For castings that are to be exposed continuously at temperatures appreciably above 870 °C (1600 °F), there is little danger of severe embrittlement from either the precipitation of carbide

or the formation of σ phase, and composition should be 0.50% C (max) (0.35 to 0.40% preferred), 10 to 12% Ni, and 24

to 27% Cr On the other hand, castings to be used at temperatures from 650 to 870 °C (1200 to 1600 °F) should have compositions of 0.40% C (max), 11 to 14% Ni, and 23 to 27% Cr For applications involving either of these temperature ranges, that is, 650 to 870 °C (1200 to 1600 °F), or appreciably above 870 °C (1600 °F), composition should be balanced

to provide an austenitic structure For service from 650 to 870 °C (1200 to 1600 °F), for example, a combination of 11%

Ni and 27% Cr is likely to produce σ phase and its associated embrittlement, which occurs most rapidly around 870 °C (1600 °F) It is preferable, therefore, to avoid using the maximum chromium content with the minimum nickel content

Short-time tensile testing of fully austenitic HH alloys shows that tensile strength and elongation depend on carbon and nitrogen contents For maximum creep strength, HH alloy should be fully austenitic in structure (Fig 10) In design of load-carrying castings, data concerning creep stresses should be used with an understanding of the limitations of such data An extrapolated limiting creep stress for 1% elongation in 10,000 h cannot necessarily be sustained for that length of time without structural damage Stress-rupture testing is a valuable adjunct to creep testing and a useful aid in selecting section sizes to obtain appropriate levels of design stress

Fig 10 Creep strength of heat-resistant alloy castings (HT curve is included in both graphs for ease of

comparison) Source: Ref 10

Because HH alloys of wholly austenitic structure have greater strength at high temperatures than partly ferritic alloys of similar composition, measurement of ferrite content is recommended Although a ratio calculated from Eq 3 that is less than 1.7 indicates wholly austenitic material, ratios greater than 1.7 do not constitute quantitative indications of ferrite content It is possible, however, to measure ferrite content by magnetic analysis after quenching from about 1100 °C (2000 °F) The magnetic permeability of HH alloys increases with ferrite content This measurement of magnetic permeability, preferably after holding 24 h at 1100 °C (2000 °F) and then quenching in water, can be related to creep strength, which also depends on structure

HH alloys are often evaluated by measuring percentage elongation in room-temperature tension testing of specimens that have been held 24 h at 760 °C (1400 °F) Such a test may be misleading because there is a natural tendency for engineers

to favor compositions that exhibit the greatest elongation after this particular heat treatment High ductility values are often measured for alloys that have low creep resistance, but, conversely, low ductility values do not necessarily connote high creep resistance

HI alloy (28Cr-15Ni) is similar to HH but contains more nickel and chromium The higher chromium content makes HI more resistant to oxidation than HH, and the additional nickel serves to maintain good strength at high temperatures Exhibiting adequate strength, ductility, and corrosion resistance, this alloy has been used extensively for retorts operating with an internal vacuum at a continuous temperature of 1175 °C (2150 °F) It has an essentially austenitic structure that contains carbides and that, depending on the exact composition balance, may or may not contain small amounts of ferrite Service at 760 to 870 °C (1400 to 1600 °F) results in precipitation of finely dispersed carbides, which increases strength and decreases ductility at room temperature At service temperatures above 1100 °C (2000 °F), however, carbides remain

in solution, and room-temperature ductility is not impaired

HY alloy (26Cr-20Ni) is somewhat similar to wholly austenitic HH alloy in general characteristics and mechanical properties Although less resistant to oxidizing gases than HC, HE or HI (Table 12), HK alloy contains enough chromium

to ensure good resistance to corrosion by hot gases, including sulfur-bearing gases, under both oxidizing and reducing conditions The high nickel content of this alloy helps make it one of the strongest heat-resistant casting alloys at temperatures above 1040 °C (1900 °F) Accordingly, HK alloy castings are widely used for stressed parts in structural applications at temperatures up to 1150 °C (2100 °F) As normally produced, HK is a stable austenitic alloy over its entire range of service temperatures The as-cast microstructure consists of an austenitic matrix containing relatively large carbides in the forms of either scattered islands or networks After the alloy has been exposed to service temperature, fine, granular carbides precipitate within the grains of austenite and, if the temperature is high enough, undergo subsequent agglomeration These fine, dispersed carbides contribute to creep strength A lamellar constituent that resembles pearlite, but that is presumed to be carbide or carbonitride platelets in austenite, is also frequently observed in HK alloy

Table 12 Approximate rates of corrosion for ACI heat-resistant casting alloys in air and in flue gas

Oxidation rate in air, mm/yr Corrosion rate, mm/yr, at 980 °C (1800 °F)

in flue gas with sulfur content of:

1090 °C (2000 °F)

Oxidizing Reducing Oxidizing Reducing

HB 0.63- 6.25- 12.5- 2.5+ 12.5 6.25- 12.5

HC 0.25 1.25 1.25 0.63- 0.63+ 0.63 0.63-

Unbalanced compositions are possible within the standard composition range for HK alloy, and hence some ferrite may

be present in the austenitic matrix Ferrite will transform to brittle σ phase if the alloy is held for more than a short time at about 815 °C (1500 °F), with consequent embrittlement upon cooling to room temperature Direct transformation of austenite to σ phase can occur in HK alloy in the range of 760 to 870 °C (1400 to 1600 °F), particularly at lower carbon levels (0.20 to 0.30%) The presence of σ phase can cause considerable scatter in property values at intermediate temperatures

The minimum creep rate and average rupture life of HK are strongly influenced by variations in carbon content Under the same conditions of temperature and load, alloys with higher carbon content have lower creep rates and longer lives than lower-carbon compositions Room-temperature properties after aging at elevated temperatures are affected also: The higher the carbon, the lower the residual ductility For these reason, three grades of HK alloys with carbon ranges narrower than the standard HK alloy in Table 2 are recognized: HK-30, HK-40, and HK-50 In these designations, the number indicates the midpoint of a 0.10% C range HK-40 (Table 2) is widely used for high-temperature processing equipment in the petroleum and petro-chemical industries

Figure 9(c) shows the effect of short-term temperature exposure on an HK-40 alloy Figure 11 indicates the statistical spread in room-temperature mechanical properties obtained for an HK alloy These data were obtained in a single foundry and are based on 183 heats of the same alloy

Fig 11 Statistical spread in mechanical properties of HK alloy Data are for 183 heats of HK alloy produced in a

single foundry Tests were performed at room temperature on as-cast material

HL alloy (30Cr-20Ni) is similar to HK; its higher chromium content gives it greater resistance to corrosion by hot gases, particularly those containing appreciable amounts of sulfur Because essentially equivalent high-temperature strength can

be obtained with either HK or HL, the superior corrosion resistance of HL makes it especially useful for service in which excessive scaling must be avoided The as-cast and aged microstructures of HL alloy, as well as its physical properties and fabricating characteristics, are similar to those of HK

Iron-Nickel-Chromium Heat-Resistant Castings

Iron-nickel-chromium alloys generally have more stable structures than those of iron-base alloys in which chromium is the predominant alloying element There is no evidence of an embrittling phase change in iron-nickel-chromium alloys that would impair their ability to withstand prolonged service at elevated temperature Experimental data indicate that composition limits are not critical; therefore, the production of castings from these alloys does not require the close composition control necessary for making castings from iron-chromium-nickel alloys

The following general observations should be considered in the selection of iron-nickel-chromium alloys:

• As nickel content is increased, the ability of the alloy to absorb carbon from a carburizing atmosphere decreases

• As nickel content is increased, tensile strength at elevated temperatures decreases somewhat, but resistance to thermal shock and thermal fatigue increases

• As chromium content is increased, resistance to oxidation and to corrosion in chemical environments increases

• As carbon content is increased, tensile strength at elevated temperatures increases

• As silicon content is increased, tensile strength at elevated temperatures decreases, but resistance to carburization increases somewhat

HN alloy (25Ni-20Cr) contains enough chromium for good high-temperature corrosion resistance HN has mechanical properties somewhat similar to those of the much more widely used HT alloy, but has better ductility (see Fig 9d and 9f for a comparison of HN and HT tensile properties above 750 °C, or 1400 °F) It is used for highly stressed components in the temperature range of 980 to 1100 °C (1800 to 2000 °F) In several specialized applications (notably, brazing fixtures),

it has given satisfactory service at temperatures from 1100 to 1150 °C (2000 to 2100 °F) HN alloy is austenitic at all temperatures: Its composition limits lie well within the stable austenite field In the as-cast condition it contains carbide areas, and additional fine carbides precipitate with aging HN is not susceptible to phase formation, and increases in its carbon content are not especially detrimental to ductility

HP, HT, HU, HW, and HX alloys make up about one-third of the total production of heat-resistant alloy castings When used for fixtures and trays for heat treating furnaces, which are subjected to rapid heating and cooling, these five high-nickel alloys have exhibited excellent service life Because these compositions are not as readily carburized as iron-chromium-nickel alloys, they are used extensively for parts of carburizing furnaces Because they form an adherent scale that does not flake off, castings of these alloys are also used in enameling applications in which loose scale would be detrimental

Four of these high-nickel alloys (HT, HU, HW, and HX) also exhibit good corrosion resistance with molten salts and metal They have excellent corrosion resistance to tempering and to cyaniding salts and fair resistance to neutral salts, with proper control With molten metal, these alloy exhibit excellent resistance to molten lead, good resistance to molten tin to 345 °C (650 °F), and good resistance to molten cadmium to 410 °C (775 °F) The alloys have poor resistance to antimony, babbitt, soft solder, and similar metal In many respects, there are no sharp lines of demarcation among the HP,

HT, HU, HW, and HX alloys with respect to service applications

HP alloy (35Ni-26Cr) is related to HN and HT alloys, but is higher in alloy content It contains the same amount of

chromium but more nickel than HK, and the same amount of nickel but more chromium than HT This combination of elements makes HP resistant to both oxidizing and carburizing atmospheres at high temperatures It has creep-rupture properties that are comparable to, or better than, those of HK-40 and HN alloys (Fig 12)

Fig 12 Stress-rupture properties of several heat-resistant alloy castings (a) 10,000 h rupture stress (b)

100,000 h rupture stress Source: Ref 10

HP alloy is austenitic at all temperatures, and is not susceptible to -phase formation Its microstructure consists of massive primary carbides in an austenitic matrix; in addition, fine secondary carbides are precipitated within the austenite grains upon exposure to elevated temperatures This precipitation of carbides is responsible for the strengthening between

500 and 750 °C (900 and 1400 °F) in Fig.(e) 9 This strengthening, which is reduced after long-term exposure at high temperatures, also occurs for the cast stainless steels shown in Fig (c) 9 and (d) 9

HT alloy (35Ni-17Cr) contains nearly equal amounts of iron and alloying elements Its high nickel content enables it to

resist the thermal shock of rapid heating and cooling In addition, HT is resistant to high-temperature oxidation and carburization and has good strength at the temperatures ordinarily used for heat treating steel Except in high-sulfur gases, and provided that limiting creep-stress values are not exceeded, it performs satisfactorily in oxidizing atmospheres at temperatures up to 1150 °C (2100 °F) and in reducing atmospheres at temperatures up to 1100 °C (2000 °F)

HT alloy is widely used for highly stressed parts in general heat-resistant applications It has an austenitic structure containing carbides in amounts that vary with carbon content and thermal history In the as-cast condition, it has large carbide areas at interdendritic boundaries; but fine carbides precipitate within the grains after exposure to service temperature, causing a decrease in room-temperature ductility Increases in carbon content may decrease the high-temperature ductility of the alloy A silicon content above about 1.6% provides additional protection against carburization, but at some sacrifice in elevated-temperature strength HT can be made still more resistant to thermal shock

by the addition of up to 2% niobium

HU alloy (39Ni-18Cr) is similar to HT, but its higher chromium and nickel contents give it greater resistance to

corrosion by either oxidizing or reducing hot gases, including those that contain sulfur in amounts up to 2.3 g/m3 (see Table 12) Its high-temperature strength and resistance to carburization are essentially the same as those of HT and thus its superior corrosion resistance makes it especially well suited for severe service involving high stress and/or rapid thermal cycling, in combination with an aggressive environment

HW alloy (60Ni-12Cr) is especially well suited for applications in which wide and/or rapid fluctuations in temperature

are encountered In addition, HW exhibits excellent resistance to carburization and high-temperature oxidation HW alloy has good strength at steel-treating temperatures, although it is not as strong as HT HW performs satisfactorily at temperatures up to about 1120 °C (2050 °F) in strongly oxidizing atmospheres and up to 1040 °C (1900 °F) in oxidizing

or reducing products of combustion, provided that sulfur is not present in the gas The generally adherent nature of its oxide scale makes HW suitable for enameling furnace service, where even small flakes of dislodged scale could ruin the work in process

HW alloy is widely used for intricate heat-treating fixtures that are quenched with the load and for many other applications (such as furnace retorts and muffles) that involve thermal shock, steep temperature gradients, and high stresses Its structure is austenitic and contains carbides in amounts that vary with carbon content and thermal history In the as-cast condition, the microstructure consists of a continuous interdendritic network of elongated eutectic carbides Upon prolonged exposure at service temperatures, the austenitic matrix becomes uniformly peppered with small carbide particles except in the immediate vicinity of eutectic carbides This change in structure is accompanied by an increase in room-temperature strength, but there is no change in ductility

HX alloy (66Ni-17Cr) is similar to HW, but contains more nickel and chromium Its higher chromimum content gives it

substantially better resistance to corrosion by hot gases (ever sulfur-bearing gases), which permits it to be used in severe service applications at temperatures up to 1150 °C (2100 °F) However, it has been reported that HX alloy decarburized rapidly at temperatures from 1100 to 1150 °C (2000 to 2100 °F) High-temperatures strength (Table 11), resistance to thermal fatigue, and resistance to carburization are essentially the same as for HW; hence HX is suitable for the same general applications in which corrosion must be minimized The as-cast and aged microstructure of HX, as well as its mechanical properties and fabricating characteristics, are similar to those of HW

Properties of Heat-Resistant Alloys