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

Introduction: Strength Beyond Annealing

630 (17-4 PH, UNS S17400, EN 1.4542) and 631 (17-7 PH, UNS S17700, EN 1.4568) are martensitic precipitation-hardening stainless steels. Both are supplied in a soft annealed condition and then hardened by a low-temperature aging treatment that precipitates fine intermetallic particles. The result is stainless steel with yield strengths in the 1000 to 1500 MPa range. The two grades differ in alloy system, heat treatment response and intended markets: 630 is the general industrial workhorse, while 631 reaches higher strength for aerospace duty.

Chemical Composition Compared (ASTM A693)

Element (wt %)	630 (17-4 PH)	631 (17-7 PH)
Carbon (C)	0.07 max	0.09 max
Manganese (Mn)	1.00 max	1.00 max
Phosphorus (P)	0.040 max	0.040 max
Sulfur (S)	0.030 max	0.030 max
Silicon (Si)	1.00 max	1.00 max
Chromium (Cr)	15.00 - 17.50	16.00 - 18.00
Nickel (Ni)	3.00 - 5.00	6.50 - 7.75
Copper (Cu)	3.00 - 5.00	-
Niobium + Tantalum (Nb+Ta)	0.15 - 0.45	-
Aluminum (Al)	-	0.75 - 1.50
Iron (Fe)	Balance	Balance

Heat Treatment and Resulting Strength

Condition	Treatment	Tensile (min)	Yield (min)	Hardness
630 H900	Solution anneal ~1040 C, age 482 C / 1 h	1310 MPa	1170 MPa	~40 HRC
630 H1150	Age 621 C / 4 h	930 MPa	725 MPa	~33 HRC
631 TH1050	Solution anneal ~1065 C, age 510 C	1450 MPa	1310 MPa	~44 HRC
631 RH950	Refrigeration + age 510 C	1620 MPa	1520 MPa	~48 HRC

Values follow ASTM A564. For 630, aging at 482 C gives maximum strength and aging at 621 C gives better toughness. For 631, the TH1050 route combines a high-temperature transformation with aging at 510 C, and the RH950 route adds a sub-zero treatment for the highest strengths.

Applications

630 is used for oil and gas downhole tools, pump shafts, valve stems, fasteners, gears, turbine components and orthopedic surgical instruments, where high strength, moderate corrosion resistance and economy are balanced. 631 is used for aircraft structural parts, landing-gear components, springs, bellows, fasteners and missile hardware that demand aerospace-grade strength, fatigue resistance and toughness at cryogenic temperature.

Corrosion Resistance and Limitations

Both grades offer corrosion resistance between the 304 and 430 levels in many environments, but neither is intended for marine or strong-chloride service; pitting and stress corrosion can occur. For corrosion-dominated design, a molybdenum austenitic grade such as 316L is the standard answer.

Cost and Fabrication Trade-Offs

630 is roughly 15 to 20 percent cheaper than 631 on a material basis and is easier to machine and weld, making it the default precipitation-hardening grade for industrial components. 631 has lower formability and requires more specialized fabrication, which is accepted in aerospace where its strength-to-weight and cryogenic performance justify the cost.

Selection Guidelines

Prioritize 630 for industrial high-strength parts where economy and machinability matter. Select 631 where aerospace-grade strength, fatigue and cryogenic toughness are required. For chloride or seawater environments, use a corrosion-resistant grade instead of either PH steel.

FAQ

1. What does PH mean in 17-4 PH?

PH stands for precipitation hardening. The steel is hardened by aging, which precipitates fine particles that strengthen the martensitic matrix, rather than by quenching and tempering alone.

2. What is the EN equivalent of 17-4 PH and 17-7 PH?

17-4 PH is EN 1.4542 (X5CrNiCuNb16-4). 17-7 PH is EN 1.4568 (X7CrNiAl17-7). The UNS numbers are S17400 and S17700 respectively.

3. Can 630 and 631 be welded?

Yes, in the annealed or overaged condition. After welding, a full solution anneal and re-aging restores the required properties in the heat-affected zone.

4. Why is 631 used in aerospace?

Its aluminum-based precipitation system delivers yield strengths above 1300 MPa with good fatigue and cryogenic toughness, at lower density cost than many competing ultra-high-strength alloys.

5. Which grade is better for seawater?

Neither. Both are susceptible to pitting and stress corrosion cracking in chloride service. Specify 316L or a duplex grade for seawater.

6. How is the final condition identified on a certificate?

The condition is stated with the aging temperature, for example 630 H900 or 631 TH1050, together with the UNS designation S17400 or S17700, and the governing specification such as ASTM A564.