Volume 04 - Heat Treating Part 15 pot

Data on the deep-drawing characteristics of commercially pure fine silver as a function of annealing temperature are given in Table 2.. Table 3 Effect of cold work on the softening tempe

• Billet is preheated in molten salt bath at 630 °C (1165 °F) and extruded

• Extruded billet is machined into blank that is solution heat treated at 800 °C (1475 °F) for 8 h in vacuum and water quenched

• Blank is aged at 385 °C (725 °F) for 4.5 h in argon and water quenched

Example 2: U-0.75Ti

Bars of DU-0.75Ti alloy, 36 mm (1.4 in.) in diameter are to be heat treated to the following specifications: hardness, 38 to

44 HRC; minimum 0.2% yield strength, 725 MPa (105 ksi); minimum elongation, 12%; and maximum hydrogen content,

1 ppm The procedure is to:

• Cut extruded bar stock to length

• Pickle in 1:1 nitric acid to remove copper sheath

• Rinse and air dry

• Place rods vertically in a basket

• Solution treat 2 1

2 h at 850 °C (1560 °F) in a vacuum of 7 × 10-3 Pa (5 × 10-5 torr), or better

• Quench into circulating water at 455 mm/ min (18 in./min)

• Air dry

• Age 16 h at 350 °C (660 °F) in an inert gas recirculating furnace

Example 3: U-6Nb Alloy

A U-6 Nb alloy is to be formed into a hemisphere with the following mechanical properties: ultimate tensile strength, 770 MPa (112 ksi) min; yield strength at 0.2% offset, 360 to 485 MPa (52 to 70 ksi); and total elongation, 25% min

• Billet is preheated to 800 to 850 °C (1475 to 1560 °F) and forged

• Forged billet is homogenized at 1000 °C (1830 °F) for 4 h in vacuum

• Forged billet is preheated to 850 °C (1560 °F) in molten salt bath and cross rolled into plate

• Plate is preheated in argon furnace to 850 °C (1560 °F) and formed into a hemisphere

• Hemisphere is solution heat treated to 800 °C (1475 °F) for 1 to 2 h in vacuum and water quenched

• Hemisphere is aged in argon at 200 °C (400 °F) for 2 h

References cited in this section

5 K.H Eckelmeyer, A.D Romig Jr., and L.J Weirick, The Effect of Quench Rate on the Microstructure,

Mechanical Properties, and Corrosion Behavior of U-6 Wt Pct Nb, Met Trans A, Vol 15A, 1984, p 1319

6 K.H Eckelmeyer and F.J Zanner, The Effect of Aging on the Mechanical Behavior of U-0.75 wt.% Ti and

U-2.0 wt.% Mo, J Nucl Mater., Vol 62, 1976, p 37

7 G.H Llewellyn, G.A Aramayo, M Siman-Tov, K.W Childs, G.M Ludtka, "Computer Simulation of Immersion of Uranium-0.75 wt.% Titanium Alloy Cylinders," Y-2355, Martin Marietta Energy Systems, Inc., June 1986

8 G.H Llewellyn, G.A Aramayo, G.M Ludtka, J.E Park, M Siman-Tov, "Experimental and Analytical Studies in Quenching Uranium-0.75% Titanium Alloy Cylinders," Y-2397, Martin Marietta Energy Systems, Inc., Feb 1989

Licensing and Health and Safety Requirements

Possession of more than 15 lb (6.8 kg) of depleted uranium in any form requires a license from the U.S Nuclear Regulatory Commission Title 10, Part 40, of Federal Regulations describes the steps necessary and the requirements to obtain such a license In addition, all other local, state, and federal regulations are effective as applicable

The greatest potential source of contamination in the heat-treating area is uranium oxide The area should be isolated from the remainder of the plant, and everyone entering should be required to wear disposable protective footwear Smoking and eating should be restricted

Caution: The toxicity of depleted uranium if it enters the blood stream may result in poisoning similar to that caused by lead, arsenic, mercury, or any other heavy metal

A more detailed discussion about health and safety requirements is provided in Volume 2 of ASM Handbook, formerly 10th Edition Metals Handbook The important fact to remember is that each new operation or procedure involving

uranium alloys should be individually evaluated to determine the correct protective clothing and equipment, dosimetry, and handling requirements for that particular job The prior processing history of the heat treatment samples is likewise important in this consideration since operations which change the state of the uranium, like casting, can make concerns about daughter-product beta radiation more important than normal low-level alpha radiation associated with depleted uranium

Annealing of Precious Metals

Gaylord Smith, Inco Alloys International, Inc.; Al Robertson, Englehard Corporation

Introduction

THE PRECIOUS METAL GROUP consists of silver, gold, platinum, palladium, rhodium, iridium, ruthenium, and osmium Significant production of wrought product forms is limited to the first four elements and their alloys The last four elements become increasingly intractable or less ductile and consequently cannot be practically fabricated into engineering products Because of the dissimilarities of the physical metallurgy of elements within the precious metal group, the annealing practice for each member of the group will be considered separately For each element and its alloys,

a brief description of the compositions, uses, annealing practices, and nominal annealing effects on key properties is given

Silver and Silver Alloys

Consumption of silver and silver alloys in wrought product form is large and exceeds that of any other members of the precious metal group Because of the extensive use of silver, data on annealing practice and the effects of annealing on mechanical properties are more available for silver than for the remainder of the precious metal group

Commercially Pure Fine Silver

Commercially pure fine silver contains, by definition, at least 99.9% Ag It is widely used in the electrical and electronics industries as contacts and conductors and in the chemical industry as linings for reactors and process/storage vessels, particularly caustic evaporators and crystallizers

Annealing Practice. Commercially pure fine silver is typically annealed between 300 and 350 °C (570 and 660 °F) following at least 50% cold work However, data exists in the literature for annealing times up to 1.5 h at temperatures as high as 565 °C (1050 °F) Most annealing of silver, however, is done at approximately 500 °C (930 °F) Under extreme conditions of cold work, ultra-pure (99.99% pure) silver can recrystallize at temperatures as low as room temperature Silver is typically annealed in air at temperatures below 350 °C (660 °F) without adverse effects However, higher annealing temperatures (550 to 650 °C, or 1020 to 1200 °F) can result in oxygen adsorption due to the high solubility and diffusion rate (under 0.025 mm at >800 °C/h, or 1440 °F/h) of oxygen in silver Very pure silver has a hardness of 25 HV after a hydrogen anneal at 650 °C (1200 °F) and 27 HV after annealing in air at the same temperature Oxygen present in silver tends to react with impurities and has the beneficial effect of inhibiting grain growth Silver containing oxygen will become embrittled when annealed in hydrogen Hydrogen annealing of thin material sections of silver can cause the formation of blisters This effect is similar to that known to occur when tough pitch copper, that is, copper refined in a reverberatory furnace to adjust the oxygen content to 0.2 to 0.5%, is annealed in hydrogen Thus, deoxidized silver is essential where hydrogen annealing is practiced

Effect of Annealing Temperature on Mechanical Properties. The effect of annealing temperature on the tensile strength and elongation of wire cold drawn 49% prior to annealing is presented in Fig 1 Ductility is maximized at

approximately 370 °C (700 °F) Higher temperatures reduce ductility and ultimately increase tensile strength as grain growth and perhaps oxygen adsorption begin to adversely influence tensile properties Comparable data are presented in Table 1 for commercially pure fine silver sheet, 0.81 mm (0.032 in.) thick Data on the deep-drawing characteristics of commercially pure fine silver as a function of annealing temperature are given in Table 2 Annealing lowers Poisson's ratio to 0.37 from 0.39 for hard-drawn material The most frequently reported room-temperature value for the elastic modulus of commercially pure fine silver is 71 GPa (10.3 × 106 psi) This value was determined on material strained 5% and then annealed 0.5 h at 350 °C (660 °F) Cold work and annealing temperature, as well as compositional variations, apparently can affect the elastic modulus The shear modulus is reduced by annealing from the cold worked state from 26.9 GPa (3.90 × 106 psi) for hard-drawn material to 26.6 GPa (3.86 × 106 psi) for annealed commercially pure fine silver

Fig 1 Tensile properties of commercially pure fine silver 2.3 mm (0.091 in.) diam wire

Effect of Cold Work on Recovery and Recrystallization. Data for the onset of softening (recovery) for commercially pure fine silver as a function of the degree of cold rolling at 20 °C (68 °F) are shown in Table 3 Commercially pure fine silver that has been cold worked extensively, that is, greater than about 95%, can recrystallize at relatively low temperatures The effect of small amounts of a second element has been found to measurably influence the recrystallization temperature under these conditions A brief summary of these observations is given in Table 4 Because

of the uncertainty of overall purity and whether the temperature given is the start or finish of recrystallization, these data are only indicative of a general trend

Table 3 Effect of cold work on the softening temperature of commercially pure fine silver

Softening temperature Reduction by rolling at

Element Nominal composition,

of jewelry Coin silver is used for coins and in certain electrical contacts where pure silver is deemed too soft and prone to pitting Spring-type electrical contacts are made from the eutectic alloy

Effect of Annealing Temperature on Mechanical Properties. Figure 2 shows the effect of annealing temperature on the strength and elongation of cold drawn 2.3 mm (0.091 in.) diam sterling silver and eutectic alloy wire Commercial silver-copper alloys are typically annealed between 480 and 535 °C (900 and 1000 °F) followed by furnace cooling under protective atmosphere, which can result in some age hardening Process annealing can be conducted at temperatures as high as 675 °C (1250 °F) usually in a steam atmosphere or salt bath However, at the higher temperature, quenching is required for producing full softness Alternating between oxidizing and reducing atmospheres during annealing is damaging to this alloy compositional range Where light oxidation has occurred, pickling in a hot (approximately 50 °C, or 120 °F) sulfuric acid solution (5 to 10%) is suitable

Fig 2 Tensile properties and electrical conductivity of silver-copper alloys 2.3 mm (0.091 in.) diam wire, cold

drawn (CD) 49% before annealing (a) Sterling silver (92.5Ag-7.5Cu) (b) Eutectic alloy (72Ag-28Cu)

Figure 3 records the nominal electrical resistivity of annealed as well as annealed/aged silver-copper wire as a function of copper content The silver-copper alloys can be age hardened as depicted in Fig 4 The solubility of copper in silver at

650 °C (1200 °F) is about 4% and at 730 °C (1350 °F) about 6%, thus sterling silver annealed at these temperatures is duplex with small amounts of the copper-rich phase scattered through the silver-rich matrix Aging treatments cause precipitation of the copper-rich phase and, if prolonged, increase the electrical conductivity considerably Coin silver will remain duplex after any annealing treatment, and ages in much the same manner as sterling silver Both alloys respond to aging at 280 °C (535 °F) (Fig 4) The mechanical properties of sterling silver and coin silver are virtually the same after the usual annealing treatment at about 650 °C (1200 °F) because the composition of the silver-rich phase is essentially the same Alloys containing 20 to 30% Cu have much more of the copper-rich phase and show less age hardening

Fig 3 Effect of copper content on the electrical resistivity of annealed silver-copper alloys and annealed/aged

silver-copper alloys

Fig 4 Effect of copper content on properties of silver-copper alloys

References cited in this section

1 A Butts and C.D Coxe, Ed., Silver: Economics, Metallurgy, and Use, R.E Kriegier Publishing, 1967, p 141

2 A Butts and C.D Coxe, Ed., Silver: Economics, Metallurgy, and Use, R.E Kriegier Publishing, 1967, p 144

3 A Butts and C.D Coxe, Ed., Silver: Economics, Metallurgy, and Use, R.E Kriegier Publishing, 1967, p 149

4 A Butts and C.D Coxe, Ed., Silver: Economics, Metallurgy, and Use, R.E Kriegier Publishing, 1967, p 148

Gold and Gold Alloys

There are a number of types of pure and alloy gold systems of commercial significance each with different annealing practices Included here will be annealing information on pure gold, color and white gold, gold-platinum, gold-palladium-iron, and cast and wrought gold alloys for dentistry

Pure Gold

The usual grade of refined gold is 99.99% pure and is suitable for jewelry and dental applications Coin gold contains 89.9 to 91.7% Au, with the balance being copper

Annealing Practice. Pure gold can be annealed in air at 305 °C (580 °F) to control grain size but it is usually not required because pure gold easily recrystallizes at room temperature Wrought annealed pure gold has a room temperature tensile strength of 131 MPa (19 ksi), 45% elongation, and a hardness of 25 HB The elastic modulus is 80 GPa (11.6 × 106psi)

Color Gold Alloys

Most of the commercially important color gold alloys are based on the gold-silver-copper system although zinc and nickel are frequent modifiers The composition of typical color gold alloys is given in Table 5 These alloys are used principally

in jewelry; slip rings and bushings in electrical devices; and in dental applications

Table 5 Chemical composition of typical color gold alloys

Annealing Practice. Typical annealing temperatures for the color gold alloys are in the range of 500 to 700 °C (930 to

1290 °F) depending on the exact composition It is recommended that color gold alloys be quenched in water after annealing to avoid age hardening This has the secondary effect of removing any oxide scale that may have formed during

annealing in air Commercially, most annealing of color gold alloys is done in a 7% H2-N2 atmosphere with slow cooling instead of quenching

However, nickel-containing white gold alloys should be air cooled as quenching introduces high residual stress levels in these alloys Aging of the two-phase alloys is customarily done at 260 to 315 °C (500 to 600 °F)

Effect of Annealing and Aging on Hardness. Figure 5 depicts the effect of annealing, followed by quenching, on the hardness of the color gold alloys Also shown is the effect of aging at 260 to 315 °C (500 to 600 °F) as a function of silver content Age hardening temperatures can vary from between 100 °C (210 °F) and 425 °C (800 °F) depending upon the alloy that is being used Aging times can vary from 5 min to 2 h, with increasing time associated with increasing strength and lower ductility Longer aging times may result in overaging and subsequent decreasing hardness In Fig 5, note the lack of an aging response for the extremes of the silver content for the 10 and 14 k alloys, where k is karats

Fig 5 Variation of hardness with silver content for gold-silver-copper alloys

Gold Alloys in Dentistry

A large number of alloys are used in dentistry in the form of wrought plate and wire, casting, and solder These alloys require high strength and corrosion resistance Both requirements are met by complex alloys of gold, platinum, palladium, silver, copper, and zinc These alloys age harden readily Typical compositional limits of wrought alloys are given in Table 6 and for cast alloys in Table 7

Table 6 Compositions and colors of wrought precious-metal alloys used in high-strength dental wires

4 62-64 7-13 6 max 9-16 7-14 2 max 1 max Light gold

(a) Numbers are for identification in this article only

(b) Fractional percentages of iridium, indium, and rhodium are omitted here

Table 7 Composition limits of alloys used in dental castings

(a) Numbers are for identification in this article only

(b) American Dental Association Specification No 5

Annealing Practice. The wrought alloys are typically annealed and quenched from 705 to 870 °C (1300 to 1600 °F) Aging of these alloys can be accomplished by cooling slowly and uniformly from 450 to 250 °C (840 to 480 °F) Manufacturer recommendations for each alloy must be carefully followed for specific mechanical properties The cast gold alloys used in dentistry are annealed by heating to 705 °C (1300 °F) for 10 min and quenching in water Reheating to

450 °C (840 °F) in about 30 min will produce maximum strength and hardness with loss of considerable ductility Surface discoloration can be removed by pickling in 15% HC1 or hot 50% H2SO4

Effect of Annealing and Aging. The annealed and aged physical and mechanical properties of wrought gold alloys for dental applications are presented in Table 8 The properties for the cast gold alloys are given in Table 9

Table 8 Physical and mechanical properties of wrought high-strength precious-metal dental wires

See Table 6 for chemical compositions

Tensile strength (b) Proportional limit

Soft (c) Hard (d) Soft Hard

5 14-20 1-3 135-200 230-290 900-930 1650-1710 14.1-15.2 0.509-0.549

(a) Numbers are for identification in this article only

(b) Tension tests on wires 1.0 mm (0.040 in.) in diam Most elongation data on 8-in gage lengths

(c) Quenched from 705 to 870 °C (1300 to 1600 °F) depending on type of alloy

(d) Cooled slowly and uniformly from 450 to 250 °C (840 to 480 °F) in 30 min This is a severe hardening treatment used in testing to determine the behavior of wire under adverse conditions Manufacturers recommend hardening treatments for specific uses

(e) Brinell hardness numbers obtained with the Baby Brinell testing machine

(f) Not appreciably affected by heat treatment

Table 9 Physical and mechanical properties of gold-alloy castings used in dentistry

Modulus of elasticity of these alloys ranges from 76 to 125 GPa (11 to 18 × 106 psi) See Table 7 for compositions of alloys

Tensile strength Proportional limit Liquidus temperature Alloy (a) Treatment (b) Hardness,

Fig 6 Curves of equal resistivity for gold-platinum-iron alloys annealed 1 h at 500 °C (930 °F) Numbers from

1 to 15 refer to alloys for which resistivity is shown in Table 10 Numbers 500 to 1000 refer to resistivity values given in ohm · circular mils/ft

Annealing Practice. Peak resistivity is generally achieved by annealing cold worked material at 500 °C (930 °F) for 1

h Higher temperatures result in lower resistivity values and lower temperatures require longer times to reach peak resistivity

Effect of Annealing on Resistivity. Table 10 presents the resistivity of the alloys of Fig 6 as cold worked (about

50%) and as annealed for 1 h at 500 °C (930 °F) The tensile strength of the peak resistivity alloy is 1380 MPa (200 ksi)

as heat treated and 690 MPa (100 ksi) as cold worked

Table 10 Electrical resistivity of gold-platinum-iron alloys shown in Fig 6

Cold worked Annealed 1 h

Reference cited in this section

5 E.M Wise, Gold: Recovery, Properties, and Applications, Van Nostrand, 1964, p 264

Platinum Group Elements and Alloys

The platinum group metals are used in a wide range of applications and product forms Applications for wrought products include electrical contacts, brushes, precision potentiometer wire, laboratory ware, thermocouple alloys, catalysts, anodes, spinnerets, crucibles, sputter targets, spark plugs, igniters, fiberglass bushings, and magnets

Platinum and Platinum Alloys

Platinum is the best known and least rare of the platinum group metals and has the widest range of uses because of its general corrosion resistance, high melting point, appearance, and ductility

Annealing Practice. The annealing temperature range for platinum is 600 to 1000 °C (1110 to 1830 °F) The most common commercial annealing practice is torch annealing The maintenance of oxidizing conditions throughout the anneal is essential to avoid contamination Hot reducing atmospheres can contaminate pure platinum and contact with asbestos pads or charcoal is not recommended Alumina supports work well For high-purity wire and sheet, it is essential

to remove iron contamination after processing and before annealing by pickling in hot hydrochloric acid The entire range

of platinum-palladium alloys can be annealed at 900 °C (1650 °F) However, the palladium-rich alloys should be annealed in inert or nitrogen atmospheres Platinum-rhodium alloys containing 3.5 to 40% Rh are typically annealed between 900 and 1000 °C (1650 and 1830 °F) Platinum-iridium alloys containing 0.4 to 10% Ir are commonly annealed between 1000 and 1200 °C (1830 and 2190 °F) For alloys of higher iridium content, temperatures nearer 1400 °C (2550

°F) are recommended Platinum alloys containing more than 10% Ir can be age hardened by quenching from 1700 °C (3090 °F) followed by heating to 805 °C (1480 °F) Pt-5% Ru can be annealed at 1000 °C (1830 °F) and Pt-10% Ru at

1100 °C (2010 °F)

Pt-23% Co and similar alloys close to the 50/50 atomic composition can be annealed by quenching from above the transformation temperature of 830 °C (1525 °F) Optimum magnetic properties can be developed in these materials by following quenching with a heat treatment designed to develop partial ordering in the system

Effects of Annealing on Mechanical Properties. The hardness of commercially pure platinum (99.95% min) and chemically pure platinum (99.99% min) as a function of annealing temperature for various percent reductions is plotted in Fig 7 The tensile strength and hardness of annealed and cold worked platinum-palladium compositions as a function of palladium content are given in Fig 8 and 9 Table 11 presents the annealed and cold worked properties of the common platinum-rhodium alloys and Table 12 presents these properties for the common platinum-iridium alloys Figure 10 shows the tensile strength of platinum-ruthenium alloys in the annealed and cold worked condition as a function of ruthenium content

Table 11 Typical properties of platinum-rhodium alloys

Tensile strength

Density Electrical resistivity (b)

(d) Hard, as cold worked, 75% reduction

Table 12 Typical properties of platinum-iridium alloys

Tensile strength

Density Electrical resistivity Iridium, % Temper

Fig 7 The hardness of platinum as a function of annealing temperature and percent reduction (a)

Commercially pure (99.85% min) (b) Chemically pure (99.99% min)

Fig 8 Tensile strength of annealed platinum-palladium alloys as a function of palladium content

Fig 9 Hardness of platinum-palladium alloys as a function of palladium content

Fig 10 Tensile strength of platinum-ruthenium alloys as a function of ruthenium content Alloys initially

reduced by 75%, then annealed 15 rain at 1000 to 1100 °C (1830 to 2010 °F)

Palladium and Palladium Alloys

While palladium resembles platinum in appearance and tensile properties, its corrosion resistance and melting point are lower The major use of palladium and its alloys has been for contacts in light-duty relays, where its freedom from tarnish and cold fusion provides extreme reliability and noise-free transmission Palladium hardened with ruthenium provides an all precious metal white jewelry alloy that sets off diamonds to advantage

Palladium dental casting alloys have, to a great extent, replaced the high gold dental casting alloys as the material of choice for porcelain bonding and other dental restoration work Palladium-silver alloys are used for the production of high-purity hydrogen by taking advantage of the selective high diffusion rate of this gas through the alloy

Annealing Practice. Commercially pure palladium (99.85% min) can be annealed at about 800 °C (1470 °F) Nitrogen-hydrogen mixtures, nitrogen, argon, or steam provide a suitable annealing atmosphere Slow cooling in air from

815 °C (1500 °F) to 425 °C (800 °F) will cause a blue oxide film to form To avoid this, the metal should be quenched in water or cooled in a nitrogen atmosphere Cooling in hydrogen will cause a phase change to occur with accompanying distortions Alloys in the palladium-silver-copper system can be annealed at about 800 °C (1470 °F) in nitrogen The age hardenable alloys in this system (see Fig 11) should be cooled fairly rapidly for maximum softness If much zinc is present, oil quenching may be required Treatment for 1

4 to 5 h between 400 and 500 °C (750 and 930 °F) hardens most

of these alloys effectively Alloys in the palladium-silver-gold system can be annealed at 850 °C (1560 °F) in nitrogen Pd-4.5% Ru is effectively annealed in a N2-36.7% H2 atmosphere at 900 °C (1650 °F)

Fig 11 Maximum hardness of aged palladium-silver-copper alloys

Effect of Annealing on Mechanical Properties. The tensile properties of cold drawn deoxidized palladium as a function of annealing temperature are presented in Fig 12 The hardness of commercially pure (99.85% min) and chemically pure (99.99% min) palladium versus annealing temperature are given in Fig 13 for various degrees of reduction The tensile strength and hardness of annealed and cold worked palladium-silver alloys as a function of silver content are shown in Fig 14 The maximum hardness of aged palladium-silver-copper alloys is shown in Fig 15 The effect of annealing temperature on the tensile properties of Pd-4.5% Ru are shown in Fig 16

Fig 12 Tensile properties of cold drawn deoxidized palladium as a function of annealing temperature (held for

5 min)

Fig 13 The hardness of palladium as a function of annealing temperature and percent reduction (a)

Commercially pure (99.85% min) (b) Chemically pure (99.99% min)

Fig 14 Plot of mechanical properties of palladium-silver alloys as a function of silver content (a) Tensile

strength (b) Hardness

Fig 15 Maximum hardness of annealed palladium-silver-gold alloys

Fig 16 Effect of annealing temperature on the tensile properties of Pd-4.5Ru

Rhodium, Iridium, and Ruthenium

Wrought products of these elements are produced although not to the extent that platinum and palladium are produced Iridium and rhodium can be produced using arc melting and electron-beam (EB) furnaces These elements can be produced using powder metallurgy, although generally only ruthenium is fabricated via powder metallurgy

Annealing Practice. These elements must be annealed in an inert atmosphere or vacuum to prevent oxidation Commercial iridium and ruthenium can be annealed at 1315 °C (2400 °F)

Effect of Annealing on Hardness. The hardness of commercial rhodium, iridium, and ruthenium as a function of annealing temperature for varying degrees of cold work is given in Fig 17

Fig 17 Plot of hardness versus annealing temperature (held for 1 h) for varying degrees of cold work of

selected noble metals (a) Commercial rhodium (b) Commercial iridium (c) Electron-beam-melted ruthenium

AR, as-rolled Source: Ref 6

Reference cited in this section

6 R.W Douglass, C.A Krier, and R.I Jaffee, Report from Battelle Memorial Institute to Office of Naval Research, 31 August 1961

Temper Colors for Steels

Introduction

It is common and long-time knowledge that steel heated in contact with air at temperatures in the tempering range takes

on various temper colors due to the formation of a thin oxide film There is, however, little detailed information on this

phenomenon in the readily available literature As an example, the 1948 ASM Metals Handbook gives the following

8in.) diam hot-rolled bars of a standard SAE 1035 steel Samples that were 50 mm (2 in.) long were cut from the bars and carefully machined and cleaned to give smooth, bright surfaces They were then heated for various times at several temperatures in air-circulating furnaces controlled to within ±3 °C (±5 °F) of the desired temperatures The results of this study are shown in Fig 1

Fig 1 Temper colors after heating 1035 steel in circulating air (atmospheric pressure)

Austenitizing Temperatures for Steels

Introduction

Temperatures recommended for austenitizing carbon and low-alloy steels prior to hardening are given in Table 1 (for direct-hardening grades) and Table 2 (for carburized steels) Table 2 is applicable to carburized steels that have been cooled slowly from the carburizing temperature and are to be furnace hardened in a subsequent operation

Table 1 Austenitizing temperatures for direct-hardening carbon and alloy steels (SAE)

(a) Commonly used on parts where induction hardening is employed All steels from SAE 1030 up may have induction hardening applications

(b) This temperature range may be employed for 1095 steel that is to be quenched in water, brine, or oil For oil quenching, 1095 steel may alternatively be austenitized in the range 815 to 870 °C (1500 to 1600 °F)

(c) This range is recommended for steel that is to be water quenched For oil quenching, steel should be austenitized in the range 815 to 870 °C (1500 to 1600 °F)

Table 2 Reheating (austenitizing) temperatures for hardening of carburized carbon and alloy steels (SAE)

Carburizing is commonly carried out at 900 to 925 °C (1650 to 1700 °F); slow cooled and reheated to given austenizing temperature

Temperature Rise

The difference in temperature rise within thick and thin sections of articles of varying cross section is a major problem in practical heat-treating operations When temperature uniformity is the ultimate objective of the heating cycle, this is more safely attained by slowly heating than by rapidly heating Furthermore, the maximum temperature in the austenite range should not exceed that required to achieve the necessary extent of solution of carbide The temperatures listed in Tables 1 and 2 conform with this requirement When heating with significant cross-section variations, provisions should be made for slower heating to minimize thermal stresses and distortions

• Aecm, Ae1, Ae3, Ae4

• Defined under transformation temperature

• aerated bath nitriding

• A type of liquid nitriding in which air is pumped through the molten bath creating agitation and increased chemical activity

• aging

• A change in the properties of certain metals and alloys that occurs at ambient or moderately elevated temperatures after hot working or a heat treatment (quench aging in ferrous alloys, natural or artificial aging in ferrous and nonferrous alloys) or after a cold working operation (strain aging) The change in properties is often, but not always, due to a phase change (precipitation), but never involves a change in chemical composition of the metal or alloy See also age hardening , artificial aging , interrupted aging , natural aging , overaging , precipitation hardening , precipitation heat treatment , progressive aging , quench aging , step aging

• air-hardening steel

• A steel containing sufficient carbon and other alloying elements to harden fully during cooling in air or other gaseous mediums from a temperature above its transformation range The terms should be restricted to steels that are capable of being hardened by cooling in air in fairly large sections, about 2 in (50 mm) or more in diameter Same as self-hardening steel

• allotropy

• A near synonym for polymorphism Allotropy is generally restricted to describing polymorphic behavior in elements, terminal phases, and alloys whose behavior closely parallels that of the predominant constituent element

In ferrous alloys, annealing usually is done above the upper critical temperature, but the temperature cycles vary widely in both maximum temperature attained and in cooling rate employed, depending on composition, material condition, and results desired When applicable, the following commercial process names should be used: black annealing , blue annealing , box annealing , bright annealing , cycle annealing , flame annealing , full annealing , graphitizing , intercritical annealing , isothermal annealing , malleablizing , order hardening , process annealing , quench annealing , spheroidizing , subcritical annealing

time-In nonferrous alloys, annealing cycles are designed to: (a) remove part or all of the effects of cold working (recrystallization may or may not be involved); (b) cause substantially complete coalescence of precipitates from solid solution in relatively coarse form; or (c) both, depending

on composition and material condition Specific process names in commercial use are final annealing , full annealing , intermediate annealing , partial annealing , recrystallization annealing , stress relieving ,anneal to temper

• athermal transformation

• A reaction that proceeds without benefit of thermal fluctuations; that is, thermal activation is not required In contrast, a reaction that occurs at constant temperature is an isothermal transformation ; thermal activation is necessary in this case and the reaction proceeds as a function of time

• ausforming

• Thermomechanical treatment of steel in the metastable austenitic condition below the recrystallization temperature followed by quenching to obtain martensite and/or bainite

• austempering

• A heat treatment for ferrous alloys in which a part is quenched from the austenitizing temperature

at a rate fast enough to avoid formation of ferrite or pearlite and then held at a temperature just above Ms until transformation to bainite is complete Although designated as bainite in both austempered steel and austempered ductile iron (ADI), austempered steel consists of two phase mixtures containing ferrite and carbide, while austempered ductile iron consists of two phase mixtures containing ferrite and austenite

• austenite

• A solid solution of one or more elements in face-centered cubic iron Unless otherwise designated (such as nickel austenite), the solute is generally assumed to be carbon

• austenitic grain size

• The size attained by the grains of steel when heated to the austenitic region; may be revealed by appropriate etching of cross sections after cooling to room temperature

• austenitizing

• Forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing)

or above the transformation range (complete austenitizing) When used without qualification, the term implies complete austenitizing

• B

• bainite

• A metastable aggregate consisting of dispersed carbide in ferrite resulting from the transformation

of austenite at temperatures below the pearlite range but above Ms Its appearance is in the form

of relatively coarse ferrite laths between which carbides are precipitated as platelets if formed in the upper part of the bainite transformation range; acicular, resembling tempered martensite, if formed in the lower part