Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Episode 7 pps

1 Effect of carbon on the hardness of martensite structures The hardenability of steel is governed almost entirely by the chemical composition carbon and alloy content at the austenitizi

Fig 1 Effect of carbon on the hardness of martensite structures

The hardenability of steel is governed almost entirely by the chemical composition (carbon and alloy content) at the austenitizing temperature and the austenite grain size at the moment of quenching In some cases, the chemical composition of the austenite may not be the same as that determined by chemical analysis, because some carbide may be undissolved at the austenitizing temperature Such carbides would be reflected in the chemical analysis, but because the carbides are undissolved in the austenite, neither their carbon nor alloy content can contribute to hardenability In addition, by nucleating transformation products, undissolved carbides can actively decrease hardenability This is especially important in high-carbon (0.50 to 1.10%) and alloy carburizing steels, which may contain excess carbides at the austenitizing temperature Consequently, such factors as austenitizing temperature, time at temperature, and prior microstructure are sometimes very important variables when determining the basic hardenability of a specific steel composition Certain ingot casting and hot reduction practices may also develop localized or periodic inhomogeneities within a given heat, further complicating hardenability measurements The effects of all these variables are discussed in this article

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Hardenability Testing

The hardenability of a steel is best assessed by studying the hardening response of the steel to cooling in a standardized configuration in which a variety of cooling rates can be easily and consistently reproduced from one test to another

The Jominy end-quench test fulfills the cooling rate requirements of hardenability testing of a broad range of alloy

steels The test specimen, a 25.4 mm (1.000 in.) diam bar 102 mm (4 in.) in length, is water quenched on one end face The bar from which the specimen is made must be normalized before the test specimen in machined The test involves heating the test specimen to the proper austenitizing temperature and then transferring it to a quenching fixture so designed that the specimen is held vertically 12.7 mm (0.5 in.) above an opening through which a column of water can be directed against the bottom face of the specimen (Fig 2a) While the bottom end is being quenched by the column of water, the opposite end is cooling slowly in air, and intermediate positions along the specimen are cooling at intermediate rates After the specimen has been quenched, parallel flats 180° apart are ground 0.38 mm (0.015 in.) deep on the cylindrical surface Rockwell C hardness is measured at intervals of 1

16in (1.6 mm) for alloy steels and 1

32in (0.8 mm) for carbon steels, starting from the water-quenched end A typical plot of these hardness values and their positions on the test bar, as shown in Fig 2(b), indicates the relation between hardness and cooling rate, which in effect is the hardenability of the steel Figure 2(b) also shows the cooling rate for the designated test positions Details of the standard test method are available in ASTM A 255 and SAE J406

Fig 2 Jominy end-quench apparatus (a) and method for presenting end-quench hardenability data (b)

The Carburized Hardenability Test It is often necessary to determine the hardenability of the high-carbon case

regions of carburized steels Such information is important in controlling carburizing and quenching practice and in determining the ability of a specific steel to meet the microstructural and case depth requirements of the carburized component manufactured from the steel As a general rule, adequate core hardenability does not ensure adequate case hardenability, especially when it is required to reheat for hardening after carburizing rather than to quench directly from the carburizing furnace Two factors are responsible for this fact The first is that equal alloying additions do not have the same effect on the hardenability of all carbon levels of alloyed steels The second factor (as noted earlier) is that the high-carbon case regions do not always achieve full solution of alloy and carbides, as is normally achieved in the austenite of the low-carbon core region, prior to quenching Accordingly, direct measurements of case hardenability are very important whenever a carburizing steel must be selected for a specific application

Measurements of case hardenability are performed as follows A standard end-quench bar is pack carburized for 9 h at

925 °C (1700 °F) and end quenched in the usual manner A comparison bar is simultaneously carburized in the same pack

to determine carbon penetration Successive layers are removed from it and analyzed chemically to determine the carbon content at various depths When a carbon-penetration curve is established, depths to various carbon levels can be determined in the Jominy bar, assuming that the distribution of carbon in the end-quench specimen is the same as in the carbon gradient bar Longitudinal flats are then carefully ground to various depths on the end-quench bar (usually to carbon concentrations of 1.1, 1.0, 0.9, or 0.8%, and in some cases to as low as 0.6%), and hardenability is determined at these carbon levels by hardness traverses In grinding, care must be exercised to avoid overheating and tempering, and in

conducting hardness surveys, similar concern must be shown to ensure that the hardness level corresponds to a single carbon level by remaining in the exact center of the flat Rockwell A hardness readings are preferable to Rockwell C readings because they minimize the depth of indentor penetration into softer subsurface layers Rockwell A values are converted into Rockwell C values for plotting, as illustrated in Fig 3, which shows the curves of carburized hardenability

of an EX19 steel In the higher-carbon layers of carburized specimens, the hardness will be influenced by the presence of retained austenite Therefore, it is often useful to evaluate the microstructure/depth relationship by metallographically polishing and etching the ground flats The Jominy distance to some chosen level of nonmartensitic transformation product can then be used as a measure of hardenability

Fig 3 Carburized hardenability, EX19 steel Composition: 0.18 to 0.23% C, 0.90 to 1.20% Mn, 0.40 to 0.60%

Cr, 0.08 to 0.15% Mo, 0.0005% B (min)

The case hardenability of steels that are carburized and then reheated for hardening at temperatures below 925 °C (1700

°F), such as 8620, 4817, and 9310, can also be determined by using a modification of this technique The carburized quench specimens and companion gradient bars are oil quenched together from carburizing, but are then reheated in an atmosphere furnace to the desired austenitizing temperature for a total of 55 to 60 min, which should ensure at least 30 to

end-35 min at temperature The hardenability specimen is then end quenched, and the carbon gradient bar is oil quenched and tempered to facilitate machining for carbon gradient determination, as described above It is recommended that case hardenability tests be performed on no fewer than two test specimens A more detailed description of the case hardenability measurement technique appears in SAE J406

Air Hardenability Test Occasionally, the hardening performance either of a steel cooled at a rate slower than that

applied to the end-quench bar or of steels of very high hardenability must be determined An air hardenability test method described in Ref 1 can be employed for this purpose In this test, a machined and partially threaded round test specimen, 25.4 mm (1.000 in.) in diameter and 254 mm (10 in.) long, is inserted to a depth of 152 mm (6 in.) in a hole drilled in a bar 152 mm (6 in.) in diameter and 381 mm (15 in.) long, thus leaving 102 mm (4 in.) of the test bar length exposed (Fig 4) A second test specimen can be inserted at the opposite end of the bar holder to serve as a duplicate With both test bars securely in place, the assembly is heated to the proper austenitizing temperature, after which it is transferred to a convenient location for cooling in still air This cooling procedure results in very slow and ever decreasing cooling rates

along the length of the test bars Hardness is then measured at discrete intervals along each test bar and plotted against distance from the exposed end on charts specifically designed for this purpose

Fig 4 Dimensions (given in inches) of components in air hardenability test setup

Continuous-Cooling-Transformation Diagrams The use of continuous-cooling-transformation diagrams

determined dilatometrically, for example, can also be helpful in evaluating the cooling behavior of high-hardenability steels

Reference cited in this section

1 C.F Jatczak, Effect of Microstructure and Cooling Rate on Secondary Hardening of Cr-Mo-V Steels, Trans ASM, Vol 58, 1965, p 195

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Low-Hardenability Steels

In plain carbon and very low-alloy steels, the cooling rate at even the 1.6 mm ( 1

16 in.) position on a standard Jominy bar may not be fast enough to produce full hardening Therefore, this test lacks discrimination between these steels Tests that are more suited to very low hardenability steels include the hot-brine test and the surface-area-center (SAC) test

In the hot-brine test proposed by Grange, coupons (Fig 5) are quenched in brine maintained at a series of different

temperatures As shown in Fig 6, the resulting hardnesses provide a very sensitive test of hardenability

Fig 5 Hot-brine hardenability test specimen (a) Specimen dimensions (b) Method of locating hardness

impressions after heat treatment Dimensions given in millimeters Source: Ref 2

Fig 6 Typical results of the hot-brine hardenability test Steel composition: 0.18% C, 0.81% Mn, 0.17% Si,

and 1.08% Ni Austenitized at 845 °C (1550 °F) Grain size: 5 to 7 RT, room temperature Source: Ref 2

In the SAC test, a 25.4 mm (1.000 in.) round bar is normalized by cooling in air and then reaustenitized for water

quenching Hardnesses are measured on a specimen cut from the center of the 100 mm (4 in.) length Hardness is determined on the surface, the center, and at 1.6 mm ( 1

16 in.) intervals from surface to center An area hardness is then computed as the sum of the average hardness in each interval × 1

16 (Fig 7) The resulting set of three-digit numbers, for example, SAC No 63-52-42, indicates a surface hardness of 63 HRC, a Rockwell-inch area of 52, and a center hardness

of 42 HRC Testing details are given in SAE J406

Fig 7 Surface-area-center estimation of area

Reference cited in this section

2 R.A Grange, Estimating the Hardenability of Carbon Steels, Metall Trans., Vol 4, Oct 1973, p 2231

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Calculation of Hardenability

The hardenability of a steel is primarily a function of the composition (carbon, alloying elements, and residuals) and the grain size of the austenite at the instant of quenching If this relationship can be determined quantitatively, it should be possible to predict the hardenability of a steel through a relatively simple calculation

Such a technique was published by Grossmann in 1942, based on his observation that hardenability could be expressed as the product of a series of composition-related multiplying factors (Ref 3) The result of the calculation is an estimate of

DI, the ideal critical diameter of the steel The multiplying-factor principle is still used today in several hardenability calculation techniques (see the article "High-Strength Structural and High-Strength Low-Alloy Steels" in this Volume for examples of multiplying factors for quench and tempered low-alloy steels) Other researchers have developed methods based on regression equations and on calculation from thermodynamic and kinetic first principles To date, none of the hardenability prediction methods has proved to be universally applicable to all steel types; that is, different predictors are more suited to steels of given alloying systems, carbon contents, and hardenability levels In addition, it is often necessary

to fine-tune the predictions based on the characteristics (residuals, melt practice, and so on) of a particular steel producer Some excellent discussions of current thinking on this subject are available in Ref 4 and 5 Properly used, hardenability calculations can provide a valuable tool for designing cost-effective alternative steels, for deciding the disposition of heats

in the mill prior to rolling, and possibly for replacing the costly and time-consuming measurement of hardenability

References cited in this section

3 M.A Grossmann, Hardenability Calculated from Chemical Composition, Trans AIME, Vol 150, 1942, p 227

4 D.V Doane and J.S Kirkaldy, Ed., Hardenability Concepts With Applications to Steel, The Metallurgical

Society, 1978

5 C.S Siebert, D.V Doane, and D.H Breen, The Hardenability of Steels, American Society for Metals, 1977

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Effect of Carbon Content

Carbon has a dual effect in hardenable alloy steels: It controls maximum attainable hardness and contributes substantially

to hardenability The latter effect is enhanced by the quality and type of alloying elements present It might be concluded, therefore, that increasing the carbon content is the least expensive approach to improving hardenability This is true to a degree, but several factors weigh against the use of large amounts of carbon:

• High carbon content generally decreases toughness at room and subzero temperatures

• It produces harder and more abrasive microstructures in the annealed conditions, which makes cold shearing, sawing, machining, and other forms of cold processing more difficult

• It makes the steel more susceptible to hot shortness in hot working

• It makes the steel more prone to cracking and distortion in heat treatment Because of these disadvantages, more than 0.60% C is seldom used in steels for machine parts, except for springs and bearings, and steels with 0.50 to 0.60% C are used less frequently than those containing less than 0.50%

C

Figure 8 shows the differences between minimum hardenability curves for six series of steels In each series, alloy content

is essentially constant, and the effect of carbon content on hardenability can be observed over a range from 0.15 to 0.60% The hardness effect is shown by the vertical distance between the curves at any position on the end-quench specimen, that

is, for any cooling rate This effect varies significantly, depending on the type and amounts of alloying elements For example, referring to Fig 8 (d) to (f), an increase in carbon content from 0.35 to 0.50% in each of the three series of steels causes hardness increases (in Rockwell C points) at four different end-quench positions, as shown below:

Distance from quenched surface, in

Series

1

16

416

816

1216

41xxH 8 10 17 20

51xxH 8 13 9 8

86xxH 8 12 18 12

Fig 8 Effect of carbon content on the minimum end-quench hardenability of six series of alloy H-steels The

number adjacent to each curve indicates the carbon content of the steel, to be inserted in place of xx in alloy

designation

The hardenability effect of carbon content is read on the horizontal axis in Fig 8 If the inflection points of the curves are used to approximate the position of 50% martensite transformation, the effect of carbon content on hardenability in 8650 versus 8630 steel can be expressed as + 4

16; that is, the inflection point is moved from the 5

16position to the 9

16position Similarly, with nominal carbon contents of 0.35 and 0.50%, the hardenability effect of carbon is seen to be less ( 2

16) in

51xx series steels and more ( 6

16) in 41xx steels

Considering the combined hardening and hardenability effects in terms of quenching speed, the cooling rate (or quenching speed) required to produce 45 HRC is affected more by 0.15% C with certain combinations of alloying

elements than it is by other combinations For example, in a steel containing 0.75 Cr and 0.15 Mo (a 41xxH series steel,

for example), increasing the carbon content by 0.15% lowers the required or critical cooling rate to obtain 45 HRC from

25 to 4.6 °C (45 to 8.3 °F) per second, while in a steel containing 0.75% Cr and no molybdenum (51xxH series), the same

increase in carbon content lowers the cooling rate from 47 to 21 °C (85 to 37 °F) per second

The practical significance of the effect of carbon and alloy contents on cooling rate is considerable In a 51 mm (2 in.) diam bar of 4150 steel, a hardness of 45 HRC can be obtained at half-radius using an oil quench without agitation In a

4135 steel bar of the same diameter, to obtain the same hardness at half-radius would require a strongly agitated water quench Comparing 32 mm (11

4 in.) diam bars of 5135 and 5150 steel, an agitated water quench will produce a hardness

of 45 HRC at half-radius in the 5135 bar; the identical condition can be obtained in the 5150 bar using an oil quench with moderate agitation Thus, an increase or decrease in carbon content or an alloying addition, such as 0.15% Mo, affects the results obtained both in terms of the quenching severity required and the section size in which the desired results can be obtained

Figure 9 shows how steels are rated on the basis of ideal critical diameter by expressing the effect of carbon and alloy content on the section size that will harden to 50% martensite at the center, assuming an ideal quench An ideal quench is defined as one that removes heat from the surface of the steel as fast as it is delivered to the surface In general, the relation between hardness and carbon content that is important in practice is obscured in this rating method because the steel is rated to a constant microstructure Hardness decreases continuously with lower carbon contents

Fig 9 Effect of carbon content on ideal critical diameter, calculated for the minimum chemical composition of

each grade

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Alloying Elements

The most important function of the alloying elements in heat-treatable steel is to increase hardenability Increased hardenability makes possible the hardening of larger sections and the use of an oil rather than a water quench to minimize distortion and to avoid quench cracking

When the standard alloy steels are considered, it is found that, for practical purposes, all compositions develop the same tensile properties when quenched to martensite and tempered to the same hardness below 50 HRC However, it should not

be inferred that all tempered martensites of the same hardness are alike in all respects For example, plain carbon martensites have lower reduction-in-area values than alloy martensites A further difference, sometimes important, is that fully quenched alloy steels require, for the same hardness levels, higher tempering temperatures than carbon steels This difference in tempering temperature may serve to reduce the residual stress level in finished parts The stress reduction could be an advantage or a disadvantage, depending on whether a controlled compressive stress is desired in the part Although tensile properties may not differ significantly from one alloy steel to another, considerable differences may exist

in fracture toughness and low-temperature impact properties In general, steels with a higher nickel content, such as 4320,

3310, and 4340, offer much greater toughness at a given hardness level In some applications, the toughness factor rather than hardenability may dictate steel selection, but hardenability is still important, because steels that can be fully quenched to 100% martensite are much tougher than those that cannot

Usually, the least expensive means of increasing hardenability at a given carbon content is by increasing the manganese content Chromium and molybdenum, already referred to as increasing hardenability, are also among the most economical elements per unit of increased hardenability Nickel is the most expensive per unit, but is warranted when toughness is a primary consideration

Important synergistic effects, not yet fully defined, can also occur when combinations of alloying elements are used in place of single elements Some examples of known synergistic combinations are nickel plus manganese, molybdenum plus nickel, and silicon plus manganese

Boron Another potent and economical alloying element is boron, which markedly increases hardenability when added to

a fully deoxidized steel The effects of boron on hardenability are unique in several respects:

• A very small amount of boron (about 0.001%) has a powerful effect on hardenability

• The effect of boron on hardenability is much less in high-carbon than in low-carbon steels

• Nitrogen and deoxidizers influence the effectiveness of boron

• High-temperature treatment reduces the hardenability effect of boron

Recommended austenitizing temperatures for boron H-steels are given with the H-bands

Figure 10 illustrates the very small amount of boron required for an optimum hardenability effect when appropriate protection of the boron is afforded by additions of titanium or zirconium In carburizing steels, the effect of boron on case hardenability may be completely lost if nitrogen is abundant in the carburizing atmosphere The cost of boron is usually much less than that of other alloying elements having approximately the same hardenability effect

Fig 10 Influence of effective boron content (βeff) on the hardenability of an 8620 type steel βeff = 0.002)-Ti/5-Zr/15] ≥0 Source: Ref 5

B-[(N-Reference cited in this section

5 C.S Siebert, D.V Doane, and D.H Breen, The Hardenability of Steels, American Society for Metals, 1977

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Effect of Grain Size

The hardenability of a carbon steel may increase as much as 50% with an increase in austenite grain size from ASTM 8 (6

to 10) to ASTM 3 (1 to 4) The effect becomes more pronounced if the carbon content is increased at the same time When the danger of quench cracking is remote (no abrupt changes in section thickness) and engineering considerations permit, it may sometimes appear to be more practical to use a coarser-grain steel rather than a fine-grain or more expensive alloy steel to obtain hardenability However, this is not recommended, because the use of coarser-grain steels usually involves a serious sacrifice in notch toughness and may lead to other difficulties

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Variations Within Heats

Segregation of carbon, manganese, and other elements always occurs during ingot pouring and solidification As a result, the hardenability of the steel in these segregated portions will differ from that in the remainder of the ingot In general, specimens taken from the top of the ingot have higher hardenability than steel from the middle, and specimens from the bottom of the ingot will have lower hardenability than steel from the middle This gradual increase in hardenability from the bottom of the first ingot to the top of the last ingot is illustrated in Fig 11(a) for 16 heats of 1035 carbon steel The hardenability spread for 8 heats of 1035 steel containing 0.05 to 0.12% Mo, plotted in Fig 11(b), shows a similar trend Comparison between Fig 11(a) and 11(b) shows the effect of molybdenum on hardenability

Fig 11 Effect of test location of (a) 1035 steel and (b) 1035 steel with 0.05 to 0.12% Mo on SAC (Rockwell-in.)

hardenability

The same effect is observed in alloy steels End-quench hardenability test results for one heat of 4028 steel (Fig 12) show higher hardenability for a cast bar taken from the top of the last ingot of the heat than for a specimen from the melting floor and labeled cast end-quench specimen The latter was taken from about the middle of the heat After the heat of steel

was rolled, the hardenability was slightly lower, as shown by the curve representing results on eight end-quench specimens Data for 465 heats of ten other steels are summarized in Fig 13

Fig 12 Variation of hardenability within a heat of 4028 steel

Fig 13 Variation in hardenability from first to last ingot in heat for several carbon and alloy steels

Effect of Hot Working Processing variables, such as the amount of hot working and the location of the test specimen

in the semifinished section, have an effect on hardenability A 330 mm (13 in.) square bloom of 1330 steel was forged progressively to bar sizes of 305, 255, 205, and 150 mm (12, 10, 8, and 6 in.) in diameter Each bar size was evaluated by tests on end-quench specimens cut from five locations (center, quarter-radius, half-radius, three-quarter-radius, and just below the surface) Data in Fig 14 show that the variation in hardenability narrows as the bar size is decreased by hot work

Fig 14 Effect of hot working and location of test bars on end-quench hardenability of 1330 steel A 330 mm

(13 in.) bloom was progressively forged to bars of the diameters shown

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Determining Hardenability Requirements

The basic information needed to specify a steel with adequate hardenability includes:

• The as-quenched hardness required prior to tempering to final hardness that will produce the best resisting microstructure

stress-• The depth below the surface to which this hardness must extend

• The quenching medium that should be used in hardening

As-Quenched Hardness The Iron and Steel Technical Committee of the Society of Automotive Engineers (SAE)

War Engineering Board approved and issued the relation shown in Fig 15(a) as a recommendation for as-quenched hardness as a function of the hardness desired after tempering Figure 15(a) does not specify the degree of hardening (percentage martensite) preferred in obtaining the as-quenched hardnesses indicated It is possible, as shown in Fig 15(b),

to select steels that will produce these hardnesses with less than 90% martensite

Fig 15 Curves for steel selection based on hardness (a) Minimum as-quenched hardness to produce various

final hardnesses after tempering (b) Dependence of as-quenched hardness on percentages of martensite and carbon

To ensure optimum properties, common practice is to select the steel with the lowest carbon content that will produce the indicated as-quenched hardness using the quenching medium available (or one that can be made available) Following this procedure, the structures possessing the indicated hardnesses would be fully hardened; that is, they would contain more than 90% martensite, which is a common and practical definition of full hardening and the one employed by the SAE committee For components subjected to bending in service, it is considered adequate to have 90% martensite at the three-quarter-radius location To ensure this, hardness levels are specified at half-radius

Depth of Hardening The depth and percentage of martensite to which parts are hardened may affect their

serviceability, but it always affects the hardenability required and therefore the cost In parts less highly stressed in bending, hardening to 80% martensite at three-quarter-radius of the part as finished may be sufficient; in other parts, even less depth may be required the latter include principally those parts designed for low deflection under load, in which even the exterior regions are only moderately stressed In contrast, some parts loaded principally in tension and others operating at high hardness levels, such as springs of all types, are usually hardened more nearly through the section In automobile leaf springs, the leaves are designed with a low section modulus in the direction of loading The allowable deflection is large, and most of the cross section is highly stressed

In general, hardening need be no deeper than is required to provide the strength to sustain the load at a given depth below the surface Therefore, parts designed to resist only surface wear, pure bending, or rolling contact often do not justify the cost of providing the hardenability required for hardening through the entire cross section

When service requirements mandate that hardening must produce more than 80% martensite, the section size that can be hardened to a prescribed depth decreases rapidly as the percentage of martensite required increases For example, let us assume that 95% martensite (51 HRC minimum hardness) is required in 8640H steel Then the largest section size that can be hardened to the center in oil would be 16 mm (5

8 in.); a 25 mm (1 in.) section could be hardened to only quarter-radius Again, on the basis of 95% martensite, the deepest hardening of standard steels, 4340H, will harden to the center of a 51 mm (2 in.) section; on the basis of 80% martensite (45 HRC), a 92 mm (35

three-8 in.) round will harden to the center in oil

The above examples emphasize the need for engineering judgment in requiring very deep hardening or unusually high percentages of martensite When these requirements are not wholly justified, the results are overspecification of steel at higher cost and greater likelihood of distortion and quench cracking

Quenching Media The cooling potential of quenching media is a critical factor in heat-treating processes because of

its contribution to attaining the minimum hardenability requirement of the part or section being heat treated The cooling potential, a measure of quenching severity, can be varied over a rather wide range by:

• Selection of a particular quenching medium

• Control of agitation

• Additives that improve the cooling capability of the quenchant

Any or all of these variables can be employed to increase quenching severity and provide the following advantages:

• Permit the use of less expensive (lower-alloy) steels of lower hardenability

• Optimize the properties of the steel selected

• Permit the use of less expensive quenching media

• Improve productivity and achieve cost reductions as a result of shorter cycle times and higher production rates

In practice, however, two other considerations modify the selection of quenching medium and quenching severity: the amount of distortion that can be tolerated and the susceptibility to quench cracking

In general, the more severe the quenchant and the less symmetrical the part being quenched, the greater the size and shape changes that result from quenching and the greater the risk of quench cracking Consequently, although water quenching

is less costly than oil quenching and water-quenched steels are less expensive than those requiring oil quenching, it is important that the parts to be hardened be carefully reviewed to determine whether the amount of distortion and the possibility of cracking as a result of water quenching will permit taking advantage of the lower cost of water quenching Oil, salt, and synthetic water-polymer quenchants are alternatives, but their use often requires steels of higher alloy content to satisfy hardenability requirements

A rule regarding selection of a steel and quenching medium for a given part is that the steel should have a minimum hardenability not exceeding that required by the quenching severity of the medium selected The steel should also contain the lowest carbon content compatible with the required hardness and strength properties This rule is based on the fact that the quench cracking susceptibility of steels increases with a decrease in Ms temperature and/or an increase in carbon content

Table 1 lists typical quenching severity, or H, values for the common quenching media and conditions These data are for media containing no additives Figure 16 shows the effects of additives and of other quenching media According to these data, considerable improvement in the cooling capability of quenchants can be obtained by such additions as water to hot

salt, proprietary additives to oil, and polyalkylene glycol (polymer) to water The polymer-water mixtures polyacrylamide gel (PAG), polyvinyl pyrrolidone (PVP), and polyvinyl alcohol (PVA) are gaining favor because they can be made to span the quenching severity range from oil to water by simple variation of the glycol (polymer) concentration in water Also, because they are free of fire hazards and obnoxious environmental pollution agents, they have no adverse effect on working conditions The quenching severity of these media should be tested at frequent intervals because dragout and thermal breakdown may affect their quenching efficiency

Table 1 Quenching severities, H, for various media and quenching conditions

Typical flow rates Typical H values Quenchant agitation

m/min sfm Air Mineral oil Water Brine

None 0 0 0.02 0.20-0.30 0.9-1.0 2.0

Mild 15 50 0.20-0.35 1.0-1.1 2.1

Moderate 30 100 0.35-0.40 1.2-1.3

Good 61 200 0.05 0.40-0.60 1.4-2.0

Fig 16 Approximate quench severities for quenching media containing additives to improve cooling capacity

Hardenability Versus Size and Shape When end-quench data such as those shown in Fig 2 are available, either of

two methods can be used to estimate the hardenability a steel part of given size and configuration must have to achieve the desired hardness, strength, and microstructure at critical locations when quenched in various production media These methods are:

• Method 1: The correlation of end-quench hardness data (Jeh) with equivalent hardness locations in variously quenched shapes

• Method 2: The correlation of end-quench cooling rate data (Jec) with equivalent cooling rate locations in variously quenched production shapes

Method 1 (Fig 17) is the more accurate and preferred method, because in practice it has been found that, when cooling at the same rates, large sections produce somewhat lower hardnesses than smaller sections, including end-quench and air hardenability bars This difference has been attributed to two factors (Ref 6):

• Higher contraction stresses in large parts accentuate the transformation of austenite

• Quenching severity, H, decreases with an increase in section size

Also, in using the cooling rate method (method 2), it is difficult to determine cooling rates with a high degree of accuracy Nevertheless, correlations that equate cooling conditions along the end-quench bar (Jec) with those in production shapes quenched in various liquid media are also extremely useful when attempting to establish the required hardenability and/or quenching conditions for a production part

Jominy equivalent hardness (J eh ) rates are determined by comparing the hardnesses of cross sections of parts receiving the established production heat treatment to hardnesses obtained on end-quenched bars of the same steel A typical procedure is as follows:

1 Select hardening and quenching conditions that the production hardening equipment can easily fulfill

2 Select a low-hardenability steel, such as 8620, 4023, or 1040, and manufacture a quantity of finished components: gears, bearings, shafts

3 Quench a number of these components (in the uncarburized condition) in the production facility

4 Measure the hardnesses obtained at all critical locations from the surface to the core

5 Compare the measured hardness values at these locations with equivalent hardness values produced at some end-quench (J eh ) location on a Jominy bar made from the same heat and end quenched from the same thermal conditions

6 The J values obtained in this fashion define the equal hardness cooling conditions for each location in the

production-quenched component

7 Finally, select from available end-quench data a steel that will produce the hardnesses required at each critical J eh location in the finished production part If end-quench data are not available, calculate a suitable composition by one of the standard methods

Fig 17 Determination of Jominy equivalent hardness (Jeh) rates

Reference cited in this section

6 D.J Carney, Another Look at Quenchants, Cooling Rates and Hardenability, Trans ASM, Vol 46, 1954, p

882

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

General Hardenability Selection Charts

Figures 18 and 19 show correlations of Jec equivalent cooling rates in end-quench hardenability specimens and round bars

of up to 102 mm (4 in.) in diameter when quenched in oil, water, brine, and hot salt at various controlled agitation rates They correlate bar diameter with equivalent positions on the end-quench hardenability specimen for ten modes of quenching, for both scaled and scale-free bars, and with data grouped according to bar location instead of by quenching mode

Oil (4),

230 m/min (750 sfm)

Oil (5),

60 m/min (200 sfm)

Oil (6),

15 m/min (50 sfm)

Fig 19 Correlation of Jecequivalent cooling rates in the end-quenched hardenability specimen and round bars quenched in salt at 205 °C (400 °F)

Table 2 has been devised to work with the charts in Fig 18 and 19 and includes most of the steels for which H-bands have been established, showing the location on the end-quenched specimen of the low limit of the H-band for six different hardness levels that might be specified for as-quenched hardness; 55, 50, 45, 40, 35, and 30 HRC The last two levels apply primarily to the core hardness of carburized parts

Table 2 Classification of H steels according to minimum hardness at various distances from quenched end

Typical values(a) obtained by the use of Fig 18 bar diameter (in.) for

equivalent cooling rate at:

radius

(a) If based on equivalent hardness, actual bar diameter will be less

(b) High residual alloy

The use of Fig 18 and Table 2 is described in the following example This method substantially reduces the amount of chart hopping that has in the past been needed to examine all the available steels for the purpose of selecting one

Example 1: Selection of a Steel with 38 mm (1 1

2 in.) Diam Section Equivalent Having 45 HRC at Half-Radius

This example traces the steps needed to select a steel that will harden to 45 HRC at half-radius in a part having a significant section equivalent to a 38 mm (11

2 in.) diam bar First, it is assumed that, to prevent distortion, the quench will be in oil at 60 m/min (200 sfm) (H = 0.5) and that a nonscaling atmosphere will be used for heating to the austenitizing temperature Therefore, the chart for half-radius in Fig 18(c) is applicable

The following steps will then lead to the selection of a steel First, trace horizontally at the level of 11

2 in diameter to the curve for oil quench at 60 m/min (200 sfm) (curve 5) From the point of intersection with this curve, trace vertically to the

x-axis to determine the location on the end-quenched bar that has the same cooling rate as the point at half-radius in the

16 from the end of the bar: 8640, 8740, 5150, and 94B30 If some additional hardenability is not undesirable, steels that will produce 45 HRC at 7

16can be included 4137, 8642, 6145, and 50B40 Steel 9261 is also in the same category, but it would not be applicable, because it is a spring steel used only when the asquenched hardness must be as high as 50 to 55 HRC Therefore, eight steels are available that will meet the hardenability requirements of the stipulated specification From knowledge of other characteristics of these steels, including machinability, forgeability, crackability, distortion, availability, and cost, the selector can decide which of these eight will be the most desirable for the part in question

Scaled Rounds When values for scaled round bars are desired, Fig 18(b), 18(d), and 18(f) can be used However,

prediction of results for sizes less than 25 mm (1 in.) in diameter should not be the basis for important decisions involving costly purchases without further checking, because values for these sizes were obtained by extrapolation

Figure 20 shows another correlation for rounds based on the equivalent hardness criterion In Fig 20, cooling conditions from the surface to the center of rounds of various sizes quenched in media ranging in quenching severity from 0.20 to infinity are correlated with Jeh; they are given in 1

16 in units producing the same hardness on the end-quench bar Figure

20 is especially useful for estimating through-section strength, because the entire hardness profile of the prospective steel (and, to a degree, microstructure as well) can be predicted for rounds with different diameters from one set of end-quench data Instructions for the procedure are given in the caption

Fig 20 Correlation of Jeh equivalent hardness positions in end-quenched hardenability specimen and various locations in round bars quenched in oil, water, and brine The dashed line shows the various positions in 1

2to 4

in diam rounds that are equivalent to the 8

16in distance on the end-quench bar To determine cross-sectional hardnesses from results of end-quench tests, pick out the end-quench hardness at an appropriate point on the bottom line and extend an imaginary line upward to the curved line that corresponds to the quenching severity needed to obtain that hardness for the given diameter of round

Rectangular or Hexagonal Bars and Plate Except in critical or borderline applications, size relationships for

rounds can be applied without correction to square or hexagonal sections Figures 18, 19, and 20 can also be used for

rectangular bars in which the ratio of width to thickness (W/T) is less than 4, but the value 1.4 times the thickness should

be used as the equivalent round Large plates cool considerably more slowly than bars The cooling rate relationships shown in Fig 21 and 22 apply to these shapes

Fig 21 Correlation of equivalent cooling rates in the end-quench specimen and quenched plates

Fig 22 Correlation between J ec and center cooling rates in plates quenched at various severities

Tubular Parts The application of end-quenched hardenability data to the selection of steel for hollow cylindrical

sections is based largely on production experience with similar parts There has been some progress in equating tubular sections to round bars and in developing dimensionless temperature-time charts for long hollow cylinders Hollomon and Zener (Ref 7) determined by calculation the diameter of solid steel cylinders that, when quenched in a given medium, could be expected to have the same hardness at the center as the minimum hardness in the wall of hollow cylinders when quenched in the same medium The rule of thumb of doubling the tube wall thickness to obtain the diameter of an equivalent solid bar is a useful first approximation

Estimating Hardenability When actual end-quenched hardenability data are unavailable, the hardening performance

of a steel of given chemical composition can be estimated from calculated hardenability data The various methods proposed for calculating hardenability are given in the section "Calculation of Hardenability" in this article Details can be found in Ref 3 and 8, 9, 10, 11, 12, 13

References cited in this section

3 M.A Grossmann, Hardenability Calculated from Chemical Composition, Trans AIME, Vol 150, 1942, p

227

7 J.H Hollomon and C Zener, Quenching and Hardenability of Hollow Cylinders, Trans ASM, Vol 33,

1944, p 1

8 J.M Hodge and M.A Orehoski, Relationship Between Hardenability and Percentage of Martensite in Some

Low-Alloy Steels, Trans AIME, Vol 167, 1946, p 627

9 I.R Kramer, S Siegel, and J.G Brooks, Factors for the Calculation of Hardenability, Trans AIME, Vol

167, 1946, p 670

10 A.F de Retana and D.V Doane, Hardenability of Carburizing Steels, Met Prog., Vol 100 (No 3), Sept

1971, p 65

11 C.F Jatczak, Trans AIME, Vol 4, 1973

12 E Just, New Formulas for Calculating Hardenability Curves, Met Prog., Vol 96 (No 5), Nov 1969, p 87

13 J.S Kirkaldy, Metall Trans., Vol 4, Oct 1973

Hardenability of Carbon and Low-Alloy Steels

Revised by Harold Burrier, Jr., The Timken Company

Use of the Charts

The true measure of applicability of any steel to a part requiring heat treatment is the relation of its hardenability to the critical cross section of the part at the time it is heat treated The term critical cross section refers to that section of the part where service stresses are highest and therefore where the highest mechanical properties are required For example, if the part is a rough forging 64 mm (21

2 in.) in diameter at the critical cross section, which is later machined to 50 mm (2 in.)

in diameter, and the finished part must be hardened to three-quarter-radius (that is, 6.4 mm, or 1

4in., deep), then the hardenability of the steel must be such that the rough forging will harden 13 mm (1

2 in.) deep

Figure 23 shows the correlation between cooling rates along the end-quench hardenability specimen and at four locations

in round bars up to 102 mm (4 in.) in diameter for both oil and water quenching at 60 m/min (200 sfm) The curves in Fig

23 provide data that can be used directly in steel selection Following is an example of their practical application to a specific problem of steel selection

Fig 23 Equivalent cooling rates for round bars quenched in water (a) and oil (b) Correlation of equivalent

cooling rates in the end-quenched hardenability specimen and quenched round bars free from scale Data for surface hardness are for mild agitation; other data are for 60 m/min (200 sfm)

Example 2: Use of Hardenability Charts to Verify that 4140H Steel Will Fulfill Hardness Specifications for a 44.45 mm (1.75 in.) Diam Shaft

A shaft 44.45 mm (1.75 in.) in diameter and 1.1 m (31

2 ft) long is required in a machine The engineering analysis indicates that the torsion requirements will approach a maximum of 170 MPa (25 ksi) and that the bending stresses will reach a maximum of 550 MPa (80 ksi) Because several other parts in production in the same plant are being made from 4140H steel, it is desired to know whether 4140H has enough hardenability for this shaft

Because the shear stress in torsion is about one-half that in bending, the latter will be the primary consideration In bending, stresses approach zero in the neutral axis; therefore, the steel need not be hardened completely to the center This

is helpful because the distribution of stress in quenching will decrease the danger of quench cracking and, after tempering, should leave the exterior portion of the shaft in compression

In order to withstand a fatigue load of 550 MPa (80 ksi) in bending, a minimum hardness of 35 HRC is required For this example, it will be assumed that 35 HRC should be obtained by tempering a structure that, as-quenched, contains at least 80% martensite From experience with similar parts, it is known that the 80% martensite structure should be present down

to the three-quarter-radius position in the shaft

Because 4140H has a minimum carbon content of 0.37%, the first operation on the charts (Fig 24) is to find the quenched hardness that corresponds to 0.37% C in an 80% martensite structure As shown in the top chart of Fig 24 the same data as in Fig 1(d) this as-quenched hardness is 45 HRC