630 (17-4 PH) vs 631 (17-7 PH) Precipitation-Hardening Stainless Steels: Versatile Strength vs Cryogenic Toughness

 

What are their core compositions and key mechanical property differences?

630 (17-4 PH) contains 15–17.5% Cr, 3–5% Ni, 3–5% Cu, 0.15–0.45% Nb, and ≤0.07% C. In the H900 heat-treated condition, it achieves a yield strength of ~1100 MPa and a hardness of 38 HRC, balancing strength, ductility, and corrosion resistance for general industrial use.631 (17-7 PH) features 16–18% Cr, 6.5–7.75% Ni, 0.75–1.5% Al, and ≤0.09% C, with no copper or niobium. Its TH1050 heat treatment yields a peak yield strength of ~1380 MPa and a hardness of 42 HRC, with exceptional toughness at subzero temperatures down to -200°C.Both grades are non-magnetic in the solution-annealed state and become magnetic after precipitation hardening.

How do their heat treatment processes differ for optimal performance?

For 630, the standard H900 cycle is: solution anneal at 1040°C, quench to room temperature, then age at 480°C for 1 hour to maximize strength. Aging at higher temperatures (e.g., 620°C for H1150 condition) reduces strength but improves corrosion resistance and toughness.631 requires a two-step process for peak performance: solution anneal at 1065°C, quench to room temperature, then age at 510°C for 1 hour (TH1050 condition). It can also be cold-worked before aging to further boost strength by 10–15%.Both grades must be machined in the solution-annealed state-their hardened condition is extremely difficult to machine without specialized carbide tools.

In which applications is one grade irreplaceable over the other?

Choose 630 for general high-strength, moderate-corrosion applications: oil and gas downhole tools, valve stems, hydraulic cylinder rods, and medical implants like orthopedic screws where biocompatibility and strength are critical.Opt for 631 for aerospace, defense, and cryogenic uses: aircraft landing gear components, missile guidance system parts, satellite brackets, and liquid natural gas (LNG) storage equipment that faces extreme temperature fluctuations.631 is also preferred for high-temperature service up to 315°C, while 630's strength declines rapidly above 260°C.

How do their corrosion resistances compare to other high-strength grades?

630 offers corrosion resistance comparable to 304 stainless steel, resisting atmospheric, freshwater, and mild chemical corrosion. It performs well in oilfield brines but is not recommended for marine environments without protective coatings.631's higher chromium content gives it slightly better chloride resistance than 630, tolerating short-term salt spray exposure in coastal aerospace facilities. It still falls short of 316L's marine corrosion resistance.Both grades outperform martensitic grades like 410 and 420 in corrosive settings, but are less resistant than duplex stainless steels (e.g., 2205) for harsh industrial environments.

What are the cost and fabrication trade-offs between the two grades?

630 is 15–20% cheaper than 631, making it the cost-effective choice for non-aerospace high-strength applications. Its copper content also improves machinability compared to 631.631 has lower formability than 630 due to its higher nickel and aluminum content, requiring more force for bending and deep drawing to avoid cracking.Welding is not recommended for critical load-bearing parts of either grade-welding disrupts the precipitation-hardened microstructure, reducing strength in the heat-affected zone by 30–40%.