2.6precipitation and transformation-hardening steels (stainless)

2.6.0.1

Metallurgical Considerations — The transformation and precipitation-hardening stainless steels are martensitic or semiaustenitic stainless steels that are hardenable by heat treatment.* The martensitic alloys require only a single step heat treatment to develop maximum strength. The others are austenitic in the fully annealed condition but become martensitic during subsequent heat treatment or as a result of extensive cold working. During a final heat treatment designed to temper the martensite, several of these steels are hardened further by the precipitation of copper, aluminum, or titanium.

Some dimensional change may be experienced during the heat treatment of the semiaustenitic steels. A dimensional expansion of approximately 0.0045-in./in. occurs during the transformation from the austenitic to the martensitic condition; during aging, a contraction of about 0.0005-in./in. takes place.

2.6.0.2

Manufacturing Considerations — The martensitic precipitation-hardening steels, before age hardening, are similar to the straight-chromium martensitic stainless steels (Type 410 or 431) in their general fabricating characteristics. The semiaustenitic grades, in the annealed condition, are similar to the austenitic stainless steels (Types 301, etc.) in this respect, and are readily cold formed. Forming of hardened steels after final heat treatment should be avoided.

These alloys can be welded by the conventional methods used for the austenitic stainless steels. Inert-gas-shielded welding is recommended to prevent the loss of titanium or aluminum in certain of these alloys. Postweld annealing is recommended for some grades.

The heat treatments for these steels are compatible with the cycles used for honeycomb panel brazing. Vapor blasting of scaled parts, after final heat treatment, is recommended because of the hazards of intergranular corrosion in inadequately controlled acids pickling operations.

2.6.0.3

Environmental Considerations — The precipitation-hardening stainless steels have good strength and oxidation and corrosion resistance in their service range. Prolonged exposures above 600°F and below the tempering range may cause further hardening, with possible decrease in ductility. Prolonged exposures in or above the temperature range result in loss of strength due to overtempering, overaging, or reaustenizing.

2.6.1.0

Comments and Properties — AM-350 has high strength up to 800 °F and good oxidation resistance up to about 1000 °F. The alloy can be hardened by subzero cooling and tempering (Condition SCT).

Manufacturing Considerations — AM-350 is readily formed, welded, and brazed. Its forming characteristics are similar to the AISI 300 series stainless steels; however, it does have a higher rate of strain hardening. When fabricating AM-350 in the annealed condition, proper design allowance must be made for growth which occurs upon hardening. To obtain proper response to the SCT treatment after welding, the alloy must be reannealed.

Environmental Considerations — AM-350 shows good corrosion-resisting properties in ordinary atmospheres and also in a number of chemical environments. Exposure in the 600 to 800°F range for 1,000

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2.6.1.1

SCT 850 Condition — Effect of temperature on various mechanical properties of AM-350 is presented in Figures 2.6.1.1.1 through 2.6.1.1.4. Typical stress-strain and tangent-modulus curves at several temperatures are shown in Figures 2.6.1.1.6(a) and (b).

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2.6.2.0

Comments and Properties — AM-355, like AM-350, has high strength up to 800°F and good oxidation resistance up to 1000°F. The AM-355 alloy is generally hardened by subzero cooling and tempering (Condition SCT).

AM-355 is available in all mill products. The manufacturing considerations for AM-355 are similar to those for AM-350. Machining of AM-355 bars and forgings is best accomplished after overtempering at 1000°F to 1100°F.

The differences between AM-350 and AM-355 are a result of higher carbon, lower chromium, and reduced delta ferrite in AM-355. This difference in composition makes AM-355 slightly stronger but slightly less corrosion resistant than AM-350.

Environmental Considerations — Exposure in the 600 °F to 800 °F range for 100 hours at stress levels below the short time yield strength tends to increase room-temperature yield strength and room-temperature tensile strength slightly, with little change in elongation. Typical data are shown in Table 2.6.2.0(a).

Specifications and Properties — Material specifications for AM-355 are presented in Table 2.6.2.0(b). The room temperature properties of AM-355 SCT are shown in Table 2.6.2.0(c) through (e). The physical properties of this alloy are presented in Figure 2.6.2.0.

Table 2.6.2.0(b). Material Specifications for

AM-355 Stainless Steel

a Noncurrent specification.

2.6.2.1

SCT Condition — Elevated-temperature properties for AM-355 in the SCT (subzero cooled and tempered) condition are presented in Figures 2.6.2.1.1 through 2.6.2.1.4.

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2.6.3.0

Comments and Properties — Custom 450 is a martensitic, precipitation-hardening stainless steel used for parts requiring corrosion resistance and high strength at temperatures up to 800°F for aged conditions. It is available in the form of forgings, billet, bar, wire, strip, and welded tubing.

Manufacturing Considerations — Custom 450 is normally supplied and fabricated in the solution-treated condition except wire for cold heading is supplied in the H1150M condition. Forming, machining, and joining operations are similar to those employed for other precipitation hardening stainless steels.

Heat Treatment — Among the alloys of its type, Custom 450 is the only one recommended for use in the solution-treated condition at temperatures up to 500°F. The alloy can also be heat treated to various strength levels having a wide range of properties. Consult the applicable material specification or MIL-H-6875 for specific heat treatment procedures.

In all heat treat conditions, Custom 450 has excellent ductility and toughness. Cryogenic properties are optimum in the H1150 condition. Maximum strength is achieved with the 900°F aging treatment while optimum fatigue life is exhibited with a 1050°F age.

When the as-supplied solution-treated condition is altered during processing by hot working, severe cold working, or welding, parts should be resolution annealed prior to aging. A dimensional contraction of about 0.0002 in./in. with the 900 °F age and about 0.001 in./in. for the 1050°F aging treatment can be expected.

Environmental Considerations — The general corrosion resistance of Custom 450 is similar to AISI Type 304 stainless steel. Custom 450 shows excellent resistance to atmosphere corrosion and mild chemical environments. It has good resistance to stress corrosion cracking in the solution-treated condition. Like all martensitic precipitation hardening alloys, if stress corrosion is of concern, it should be aged at the highest temperature compatible with strength requirements. It offers the best resistance to stress corrosion cracking and hydrogen embrittlement when aged at 1150°F. The general corrosion resistance is very slightly decreased by the higher aging temperatures.

Material specifications for Custom 450 are shown in Table 2.6.3.0(a). The room-temperature mechanical properties are presented in Tables 2.6.3.0(b) and (c). The effect of temperature on thermal expansion is shown in Figure 2.6.3.0.

2.6.3.1

H900 Condition — Elevated temperature curves are presented in Figures 2.6.3.1.1, 2.6.3.1.2, and 2.6.3.1.5. A tensile stress-strain curve at room temperature is shown in Figure 2.6.3.1.6. Fatigue data at room temperature are presented in Figure 2.6.3.1.8.

2.6.3.1.8

Technical Memorandum (Progress Report), “Evaluation of Custom 455 and Custom 450 for MIL-HDBK-5,” Carpenter Technology (November 14, 1974) (MIL-HDBK-5 Source M-350).

2.6.3.2

H1050 Condition — Elevated temperature curves are presented in Figures 2.6.3.2.1, 2.6.3.2.2, and 2.6.3.2.5. A tensile stress-strain curve at room temperature is shown in Figure 2.6.3.2.6. Fatigue data at room temperature are presented in Figure 2.6.3.2.8.

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2.6.4.0

Comments and Properties — Custom 455 is a precipitation hardenable stainless steel with a martensitic structure in both the solution annealed and hardened conditions. It is used for parts requiring corrosion resistance and high strength at temperatures up to 800°F. It is produced by consumable electrode remelting and is available in the form of forgings, billet, bar, wire, strip, and welded tubing.

Manufacturing Considerations — Custom 455 is normally supplied and fabricated in the solution annealed condition. Forming, machining, and joining operations are similar to those employed for other precipitation hardening stainless steels. Optimum weld ductility is obtained by postweld solution annealing prior to aging.

Heat Treatment — The alloy can be heat treated to several strength levels. Consult the applicable materials specification or MIL-H-6875 for specific procedures. The minimum recommended hardening temperature to produce the optimum combination of strength, fracture toughness, and stress corrosion cracking resistance is 950°F. Higher strength is attainable with the 900 °F aging treatment but at a sacrifice of fracture toughness and stress corrosion cracking resistance. Like other precipitation hardening stainless steels, the fracture toughness and stress intensity below which stress corrosion cracking does not occur improve with increasing aging temperature within the range of 900°F to 1000°F.

Usually parts are aged directly from the as-supplied solution annealed condition. When this condition has been altered during processing by hot working, severe cold working, or welding, the parts should be resolution annealed prior to aging. A dimensional contraction of about 0.0009 in./in. should be expected with the 950°F aging treatment.

Environmental Considerations — The general corrosion resistance of Custom 455 is about equivalent to that of AISI Type 430 stainless steel.

Hydrogen embrittlement tests in 5 percent by weight acid saturated with H2S at room temperature show the same degree of susceptibility as other high-strength martensitic stainless steels.

When stress-corrosion cracking is of concern, one should use the highest aging temperature consistent with the strength properties required. The 900°F aging treatment should not be employed when stress corrosion cracking is a consideration. Consult the material producers literature for available stress corrosion data.

Like other precipitation hardening stainless steels, Custom 455 increases slightly in tensile strength and loses some toughness when exposed for long periods of time at temperatures around 700°F. For most applications, the loss in toughness which occurs is not detrimental to performance.

Specifications and Properties — Material specifications for Custom 455 are presented in Table 2.6.4.0(a). The room-temperature mechanical properties of Custom 455 are presented in Table 2.6.4.0(b). Physical properties at elevated temperatures are presented in Figure 2.6.4.0.

Table 2.6.4.0(a). Material Specifications for Custom

455 Stainless Steel

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2.6.4.1

H950 Condition — Elevated temperature curves are presented in Figure 2.6.4.1.1, 2.6.4.1.2, and 2.6.4.1.5. A tensile stress-strain curve at room temperature is shown in Figure 2.6.4.1.6. Fatigue data at room temperature are presented in Figure 2.6.4.1.8(a) and (b).

2.6.4.2

H1000 Condition — Elevated temperature curves are shown in Figures 2.6.4.2.1, 2.6.4.2.2, and 2.6.4.2.5. A tensile stress-strain curve at room temperature is presented in Figure 2.6.4.2.6. Fatigue data at room temperature are shown in Figure 2.6.4.2.8.

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2.6.5.0

Comments and Properties — Custom 465® stainless is a double-vacuum melted, martensitic, age-hardenable alloy. This alloy was designed to have excellent notch tensile strength and fracture toughness over a wide range of section sizes. In the H950 condition, the alloy achieves a minimum ultimate tensile strength of 240 ksi while retaining good toughness and resistance to stress-corrosion cracking. Overaging to the H1000 condition provides a greater level of toughness at a minimum ultimate tensile strength of 220 ksi. Custom 465 stainless provides a superior combination of strength, toughness and stress corrosion cracking resistance compared with other high-strength PH stainless alloys such as Custom 455® stainless or PH13-8Mo® stainless. Other combinations of strength and toughness are possible employing age-hardening temperatures between 900 °F and 1150 °F. Custom 465 stainless is available in the form of forgings, billet, bar, wire and strip.

Manufacturing Considerations —Custom 465 stainless normally is supplied and fabricated in the solution-annealed condition. Billet products will be provided in the hot finished condition. Forming, machining, and joining operations are similar to those employed for other precipitation-hardening stainless steels. Optimum weld strength and ductility are obtained by postweld solution annealing and subzero cooling prior to aging. Pyromet®X23 stainless filler metal should be considered under multi-bead GMA welding conditions.

Heat Treatment —Among the corrosion-resistant alloys of its type, Custom 465 stainless provides the highest minimum combinations of strength and toughness in the H950 and H1000 conditions. Usually, parts are aged directly from the mill-supplied, solution-annealed condition. However, if material has been hot worked or welded, components should be reannealed (1800°F/982°C) and subzero cooled (-100°F/- 73 °C, 8-hour hold) prior to age hardening. Components should be cooled rapidly from the annealing temperature. Section sizes up to 12" (305 mm) can be cooled in a suitable liquid quench medium. The subsequent subzero treatment should be applied within 24-hours of solution annealing. The refrigeration treatment after annealing is important for achieving optimum aging response by eliminating small amounts of retained austenite from the microstructure. The mill-supplied solution anneal includes the subzero treatment.

Aging treatments are performed by heating components to the specified temperature, holding for four hours, followed by cooling in air, oil or other suitable liquid quench medium. The 4-hour aging cycle is important developing optimum toughness and ductility at the specified strength levels. Increased cooling rates from the aging temperature tend to improve toughness and ductility and may be beneficial for 3" (76mm) section sizes and greater.

Environmental Considerations—The general corrosion resistance of Custom 465 stainless approaches that of Type 304 stainless. Exposure to 5% neutral salt spray at 95°F (35°C) (per ASTM B117) caused little or no corrosion after 200 hours regardless of condition (i.e., annealed or H900-H1100 conditions).

Double cantilever beam tests conducted in 3.5% NaCl (pH 6) show Custom 465 stainless to possess inherently good resistance to stress corrosion cracking which improves with increasing aging temperature. Typical results for 1/2" thick double cantilever beam specimens (T-L orientation) from 4-1/2" x 2-3/4" forged bar exposed to 3.5 wt. % NaCl (pH 6) for 1270 hours by constant immersion per NACE Standard TM0177-96 (Reference 2.6.5.0), are shown in Table 2.6.5.0(a).

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2.6.5.0

NACE Standard TM0177-96. TM0177-96, Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments.

• 2.6.6.1.8(a) Deel, O. L., and Mindlin, H., "Engineering Data on New Aerospace Structural Materials," AFML-TR-72-196, Vol. II (September, 1972) (MCIC 85292) (MIL-HDBK-5 Source M-466).
• 2.6.6.1.8(b) Deel, O. L., and Mindlin, H., "Fatigue Evaluation of PH13-8Mo Stainless Steel," Battelle Memorial Institute (July 31, 1970) (MCIC 79332) (MIL-HDBK-5 Source M-34).
• 2.6.6.1.8(c) Unpublished data, Lockheed-Georgia Company, Report No. ER 9347 (October 2, 1968) (MIL-HDBK-5 Source M-44).
• 2.6.6.1.8(d) Unpublished data, Letter report to Paul Ruff from ARMCO (March 29, 1972) (MIL-HDBK-5 Source M-141).
• 2.6.7.2.8(a) Unpublished data, Armco Research Lab, Armco Steel Corp., Baltimore, Maryland (April 11, 1977) (MIL-HDBK-5 Source M-364).
• 2.6.7.2.8(b) Doepher, P. E., “Effect of Manufacturing Process on Structural Allowables,” AFWAL-TR-85-4049 (May 1985) (MIAC 126632).
• 2.6.8.1.8(a) Illg, W., and Castle, C. B., "Fatigue of Four Stainless Steels and Three Titanium Alloys Before and After Exposure to 550 °F—Up to 8800 Hours," Langley Research Center, NASA TN D-2899 (July 1965) (MCIC 61319) (MIL-HDBK-5 Source M-579).
• 2.6.8.1.8(b) Illg, W., and Castle, C. B., "Axial-Load Fatigue Properties of PH15-7Mo Stainless Steel in Condition TH1050 at Ambient Temperature and 500°F," Langley Research Center, NASA TN D-2358 (July 1964) (MCIC 56366).
• 2.6.8.1.8(c) Roach, T. A., "Development of Fatigue Data for Several Alloys for Use in Aerospace Design," Air Force Flight Dynamics Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, Technical Report AFML-TR-69-175 (June 1969) (MCIC 76622) (MIL-HDBK-5 Source M-316).
• 2.6.9.1.8(a) Wolanski, Z. R., "Material Evaluation—17-4PH Cres, H-900 Condition Fatigue Characteristics," General Dynamics—Fort Worth (June 12, 1964) (MCIC 66105).
• 2.6.9.1.8(b) Larsson, N., "Fatigue Testing of Precipitating Steel 17-4PH With Aging as the Final Process," Aeronautical Research Institute of Sweden, Technical Note HU-1964 (August 1978) (MCIC 106285).

2.6.5.1

H950 and H1000 Condition — Figure 2.6.5.1(a) presents the typical tensile stress-strain curves at room temperature. Figures 2.6.5.1(b) and (c) present the full-range tensile stress-strain curves at room temperature for the H950 and H1000 conditions.

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2.6.6.0

Comments and Properties — PH13-8Mo is a martensitic precipitation-hardening stainless steel used for parts requiring corrosion resistance, high strength, high fracture toughness, and oxidation resistance up to 800°F. When used at temperatures between 600°F and 800°F, some loss in notch toughness will occur. The loss is time-temperature dependent and will occur gradually over thousands of hours at 600 °F and hundreds of hours at 800 °F. Depending upon the application, this loss in notch toughness may not be important and useful engineering properties may still be available. Good transverse mechanical properties are one of the major advantages of PH13-8Mo. PH13-8Mo is produced by double vacuum melting and is available in the form of forgings, plate, bar, and wire, normally furnished in the solution-treated (A) condition.

Manufacturing Considerations — Forming, joining, and machining operations are usually performed on material in Condition A, using similar procedures and equipment to those employed for other precipitation-hardening stainless steels. Best machinability is exhibited by Conditions H1150 and H1150M. A dimensional contraction of 0.0004 to 0.0006 and 0.0008 to 0.0012 in./in. occurs upon hardening to the H1000 and H1100 conditions, respectively.

Heat Treatment — PH13-8Mo must be used in the heat-treated condition and should not be placed in service in Condition A. The alloy can be heat treated to various strength levels having a wide range of properties. Consult the applicable material specification or MIL-H-6875 for specific heat treatment procedures.

Environmental Considerations — PH13-8Mo is nearly equal to 17-4PH in general corrosion resistance and surpasses the other hardenable stainless steels in stress-corrosion resistance. However, for tensile application where stress corrosion is a possibility, PH13-8Mo should be aged at the highest temperature compatible with strength requirements and at a temperature not lower than 1000 °F for 4 hours minimum aging time.

Specification and Properties — A material specification for PH13-8Mo is presented in Table 2.6.6.0(a). The room-temperature mechanical and physical properties for PH13-8Mo are presented in Table 2.6.6.0(b) and (c). The physical properties of this alloy at elevated temperatures are presented in Figure 2.6.6.0.

2.6.6.1

H950 and H1000 Conditions — Elevated temperature curves for tensile yield and ultimate strengths are presented in Figure 2.6.6.1.1. Typical tensile and compressive stress-strain and tangent-modulus curves for the H1000 condition at room temperature are depicted in Figures 2.6.6.1.6(a) and (b). Figure 2.6.6.1.6(c) contains typical full-range stress-strain curves at room temperature for various heat-treated conditions. Unnotched and notched fatigue information for H1000 condition at room temperature is presented in Figures 2.6.6.1.8(a) through (c).

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2.6.8.0

Comments and Properties — PH15-7Mo is a semiaustenitic stainless steel used where high strength and good corrosion and oxidation resistance are needed up to 600 °F. This steel is supplied in Condition A for ease of forming or in Condition C when highest strength is required.

Manufacturing Considerations — PH15-7Mo in Condition A is readily cold formed. Conventional inert-gas shielded arc and resistance techniques are generally used for welding. The heat treatments for this steel are compatible with the cycles used for honeycomb panel brazing. Vapor blasting of scaled Condition TH1050 parts is recommended because of the hazards of intergranular corrosion in adequately controlled pickling operations.

In hardening this steel from Condition A to Condition TH1050 a net dimensional growth of 0.0004 in./in. should be anticipated. Use of this steel in Conditions T and T-100 is not recommended.

Environmental Considerations—The resistance of PH15-7Mo to stress-corrosion cracking in chloride environments has been evaluated and found to be superior to that of the alloy steels and the hardenable chromium steels. Conditions C and CH 900 provide maximum resistance to stress corrosion.

Specification and Properties — A material specification for PH15-7Mo stainless steel is presented in Table 2.6.8.0(a). The room-temperature properties of PH15-7Mo are shown in Tables 2.6.8.0(b) and (c). The physical properties of this alloy at room and elevated temperatures are presented in Figure 2.6.8.0.

Table 2.6.8.0(a). Material Specification for PH15-

7Mo Stainless Steel

2.6.8.1

TH1050 Condition — Effect of temperature on various mechanical properties for this condition is presented in Figures 2.6.8.1.1 and 2.6.8.1.4. Typical stress-strain and tangent-modulus curves at room temperature and elevated temperature are presented in Figures 2.6.8.1.6(a) through (c). Unnotched and notched fatigue information at room and elevated temperatures are illustrated in Figures 2.6.8.1.8(a) through (f).

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