Precipitation Hardening Stainless Steels

Important properties of the precipitation hardening (PH) stainless steels are ease of fabrication, high strength, good ductility, and excellent corrosion resistance. There are two main types of PH stainless steels: semiaustenitic and martensitic. The semiaustenitic grades are essentially austenitic in the solution annealed condition. After fabrication operations are completed, they can be transformed to martensite by an austenite conditioning heat treatment that converts the austenite to martensite followed by precipitation hardening. The martensitic types are already martensitic in the solution annealed condition and only require precipitation hardening after fabrication.

Stainless steels are more difficult to forge than alloy steels because they maintain greater strength at elevated temperatures, and they have to be forged at lower temperatures to avoid microstructural damage, and the PH grades are the most difficult of the stainless steels to forge.21 For example, the PH grades require about 30-50% greater pressures than 4340; therefore, heavier duty equipment is required. However, the PH stainless steels are much less prone to decarburization during forging than alloy steels. Power drop hammers are used for open die forgings and mechanical presses for small forgings. Hydraulic presses are often used for the final forging operations because there is less chance of overheating due to the slower forging action than with hammers. However, die life is shorter for hydraulic presses due to the longer contact times with the hot metal. Both gas-fired and electrically heated furnaces can be used for preheating. Oil-fired furnaces are avoided due to the potential for sulfur contamination. The maximum forging temperatures for the PH grades are in the range of 2150-2250° F. Due to their low thermal conductivities, it takes longer to heat the PH steels to the forging temperature; however, once the forging temperature is reached, they should not be soaked but immediately forged. The steel must be reheated if it falls below 1800° F. To prevent cracking due to thermal gradients during heating, the PH grades can be preheated in the 1200-1700° F range. The forging dies should be heated to at least 300° F and lubricated before each blow. The forging flash must be hot trimmed to prevent flash-line cracks, which can penetrate into the forging. Hot trimming is done immediately after forging before the forging falls below a red heat. The cooling rate after forging for the martensitic grades is important because they convert directly to martensite on cooling from the austenite range, and too fast a cooling rate can result in cracking as a result of residual stresses. Slow furnace cooling, or an equalization temperature hold (e.g., 1900° F for 30min), during cooling are used to prevent cracking. Although stainless steels do not scale as badly as carbon or alloy steels, they form a hard and abrasive scale that should be removed prior to machining or cutting tool life will be adversely affected.

The semiaustenitic alloys are generally supplied from the mill in the solution annealed condition (Condition A). In Condition A, these alloys can be formed almost as easily as if they were true austenitic stainless steels.22 17-7PH has approximately the same chromium and nickel contents as austenitic type 301 stainless but also contains 1.2% aluminum for precipitation hardening. After fabrication in the soft condition, the austenite is conditioned to allow transformation to martensite. Because of their relatively high hardness in the solution annealed condition, the martensitic types are used principally in the form of bar, rod, wire, and heavy forgings, and only to a minimum extent the form of sheet. The martensitic precipitation hardening steels, before aging, are similar to the chromium martensitic stainless steels (e.g., 410 or 431) in their general fabrication characteristics.

Stainless steels are difficult to machine because of their high strength, high ductility and toughness, high work hardening rates, and low thermal conductivities. They are often characterized as being "gummy" during machining, producing long stringy chips which seize or form built-up edges on the cutting tool, resulting in reduced tool life and degraded surface finishes. The chips removed during machining exert high pressures on the nose of the tool, and these pressures, when combined with the high temperature at the chip-tool interface, cause pressure welding of portions of the chip to the tool. In addition, their low thermal conductivities contribute to a continuing heat build-up. In general, more power is required to machine stainless steels than carbon steels; cutting speeds are usually lower; a positive feed must be maintained; tooling and fixtures must be rigid; and flood cooling should be used to provide lubrication and temperature control. The martensitic PH grades are somewhat easier to machine in the solution annealed condition than the semiaustenitic grades because they are harder and cut cleaner. The following guidelines should be used when machining stainless steels:23

• Because more power is generally required to machine stainless steels, equipment should be used only up to about 75% of the rating for carbon steels,

• To avoid chatter, tooling and fixtures must be as rigid as possible. Overhang of either the workpiece or the tool must be minimized,

• To avoid glazed work hardened surfaces, particularly with the semi-austenitic alloys, a positive feed must be maintained. In some cases, increasing the feed and reducing the speed may be necessary. Dwelling, interrupted cuts, or a succession of thin cuts should be avoided,

• Lower cutting speeds may be necessary. Excessive cutting speeds result in tool wear or tool failure and shutdown for frequent tool replacement. Slower speeds with longer tool life are often a better answer to higher output and lower costs,

• Both high speed steel and carbide cutting tools must be kept sharp with a fine surface finish to minimize friction with the chip. A sharp cutting edge produces the best surface finish and provides the longest tool life, and

• Cutting fluids must be used to provide proper lubrication and heat removal. Fluids must be carefully directed to the cutting area at a sufficient flow rate to prevent overheating.

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. Post-weld annealing is recommended for some grades.

The conditioning treatment for the semiaustenitic alloys consists of heating to a high enough temperature to remove carbon from solid solution and precipitate

Fig. 5.20. Comparison of Heat Treatments for PH Stainless Steels

it as chromium carbide (Cr23C6). Removing carbon and some chromium from the austenite matrix makes the austenite unstable and on cooling to the Ms temperature, the austenite transforms to martensite. As shown in Fig. 5.20, 17-7PH is conditioned at 1400° F and then cooled to 60° F to produce the T condition. If the conditioning is done at a higher temperature (1750° F), fewer carbides are precipitated and the steel must be cooled to a lower temperature (-110° F) to transform the austenite to martensite. The final step is PH, which is carried out in the 900-1200° F range. During PH, aluminum in the martensite combines with some of the nickel to produce precipitates of NiAl and Ni3Al.

Since the martensitic PH grades are martensitic after solution annealing, they do not require conditioning but only a precipitation hardening treatment. As shown in the right side of Fig. 5.20, 15-5PH is solution annealed at 1950° F

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