General Properties

17-4PH (Type 630), is a chromium-nickel-copper precipitation hardening stainless steel used for applications requiring high strength and a moderate level of corrosion resistance. High strength is maintained to approximately 600°F (316°C).

The S17400 alloy is martensitic in structure in the annealed condition and is further strengthened by a low temperature treatment which precipitates a copper containing phase in the alloy. In comparison to many alloys in the precipitation hardening family, the S17400 alloy requires a simple heat treatment; a one step process conducted at a temperature in the range 900°F (482°C) to 1150°F (621°C) depending on the combination of strength and toughness desired. A wide range of properties can be produced by this one step heat treatment. Heat treatment in the 900°F (482°C) range produces highest strength, although slightly less than that of alloys like S17700 or S15700. The latter precipitation hardening alloys generally require more steps to complete heat treatment.

Chemical Composition

CORROSION AND OXIDATION RESISTANCE

Tests have shown that the corrosion resistance of AL 17-4 Precipitation Hardening Alloy is comparable to that of Type 304 stainless steel in most media. In general, the corrosion resistance of AL 17-4 alloy is superior to that of the hardenable 400 series stainless steels.

As with other precipitation hardening alloys, AL 17-4 Precipitation Hardening Alloy is more susceptible to stress corrosion cracking at peak strength. Consequently, in applications in which chloride stress corrosion cracking is a possibility, the material should be precipitation hardened to produce the lowest hardness compatible with the intended end use. This is done by heat treating at the highest temperature which will produce suitable minimum properties.

Material in the annealed condition should not generally be put into service. In this condition, the material has an untempered martensite structure and is less ductile than aged material. The untempered martensite may be subject to unpredictable brittle fractures. In corrosive environments, the untempered martensite is more sensitive to embrittling phenomena such as hydrogen embrittlement than material which has had one of the precipitation hardening heat treatments. Similarly, untempered martensite is more sensitive to chloride stress corrosion cracking than material in which the martensite has been tempered.

The oxidation resistance of the AL 17-4 alloy is superior to that of 12 percent chromium alloys like Type 410, but slightly inferior to that of Type 430. Precipitation hardening will produce surface oxidation.