630 (17-4PH) Stainless Steel: The Precipitation-Hardening Workhorse

What Is 630 (17-4PH)

630, better known as 17-4PH, is a precipitation-hardening martensitic stainless steel with a typical composition of 15 to 17.5 percent chromium, 3 to 5 percent nickel, 3 to 5 percent copper, and 0.15 to 0.45 percent niobium plus tantalum. It is supplied in a solution-treated condition, Condition A, which is relatively soft and easily machined, then hardened by a low-temperature aging treatment that precipitates fine copper-rich particles throughout the martensitic matrix.

The Precipitation Hardening Process

After machining in the soft Condition A, components are heated to a specific aging temperature and held for several hours. Aging precipitates fine, uniform particles that dramatically increase strength and hardness with minimal distortion. The key manufacturing benefit is that complex parts are machined to tight tolerances in the soft state, then hardened with a simple low-temperature treatment, eliminating the warping and scaling of traditional quenching.

Aging Conditions and Properties

The H designation refers to the aging temperature in degrees Fahrenheit. H900, aging at 482 degrees C, produces the highest strength and hardness, up to about HRC 44, with lower toughness. H1150, aging at 621 degrees C, gives lower strength but significantly better toughness and stress corrosion cracking resistance. Intermediate conditions such as H1025 and H1075 provide a balance, allowing engineers to tailor properties to the application.

Corrosion Resistance

17-4PH offers good resistance to atmospheric corrosion, fresh water, seawater splash, and many mild chemical and food processing environments, similar to 304. Its corrosion resistance is not as high as 316L in acidic or high-chloride conditions, and higher-strength aged conditions can be susceptible to hydrogen embrittlement and stress corrosion cracking under certain conditions.

Machining and Welding

Machining is best performed in the solution-annealed Condition A. Welding is possible in the annealed condition using matching filler, followed by a full re-solution and aging treatment to restore optimum properties in the weld zone. When purchasing, specify the required condition, for example Condition A for fabrication or an aged condition such as H900, and request certified mill test reports.

Applications

17-4PH is used for aircraft fittings, turbine blades, pump shafts, gears, nuclear reactor components, high-strength fasteners, and molds and dies. Its combination of high strength, moderate corrosion resistance, and fabrication flexibility makes it the workhorse of the precipitation-hardening family.

Frequently Asked Questions

Q: What is the composition of 630?

A: 15 to 17.5 percent chromium, 3 to 5 percent nickel and copper, with niobium.

Q: What is Condition A?

A: The solution-treated, soft condition supplied for machining.

Q: What hardness does H900 give?

A: Up to about HRC 44.

Q: What is H1150 used for?

A: Higher toughness and stress corrosion cracking resistance.

Q: Can 17-4PH be welded?

A: Yes, in the annealed condition, followed by re-solution and aging.

Q: What applications use 17-4PH?

A: Aircraft fittings, turbine blades, shafts, fasteners, and molds.

Contact us for 630 17-4PH bars, forgings, and plates in your required condition with certified mill test reports.