630 (17-4 PH) vs 631 (17-7 PH): Precipitation-Hardening Stainless Steel Comparison

630 and 631 are the two most common precipitation-hardening (PH) stainless steels, yet they are frequently confused in supplier literature and design reviews. Both reach tensile strengths around 1300–1400 MPa through aging, both are magnetic after hardening, and both are used in aerospace and high-performance machinery. The differences are structural and practical: 630 hardens directly from martensite and is easy to machine; 631 is austenitic after annealing and must be transformed to martensite by a thermal or thermomechanical treatment before aging. This article compares the grades on composition, heat treatment, mechanical properties, corrosion resistance and cost so that the right one is specified for the job.

1. Designations and Standards

Property	630 (17-4 PH)	631 (17-7 PH)
UNS number	S17400	S17700
ASTM	A564 (bar), A693 (plate/sheet/strip)	A693 (plate/sheet/strip)
EN	1.4542 / X5CrNiCuNb16-4	1.4568 / X7CrNiAl17-7
JIS	SUS630	SUS631

2. Chemical Composition (ASTM A564 / A693)

Element (wt%)	630 (S17400)	631 (S17700)
Carbon, max	0.07	0.09
Manganese, max	1.00	1.00
Silicon, max	1.00	1.00
Phosphorus, max	0.040	0.040
Sulfur, max	0.030	0.030
Chromium	15.0–17.5	16.0–18.0
Nickel	3.0–5.0	6.5–7.75
Copper	3.0–5.0	-
Niobium + Tantalum	0.15–0.45	-
Aluminium	-	0.75–1.5

630 hardens by copper precipitation; 631 hardens by the precipitation of nickel-aluminium intermetallic compounds. 631 carries roughly twice the nickel of 630, which is the main reason for its higher cost and for its austenitic parent structure before transformation.

3. Heat Treatment Routes and Mechanical Properties

The two grades are not heat treated the same way:

630: solution anneal at 1040 ± 15 °C, cool to 32 °C or below (Condition A), then age directly - e.g. H900 at 480 ± 6 °C for 1 h, or H1150 at 620 ± 6 °C for 4 h. The martensite formed on cooling is aged as-is.

631: annealed it is austenitic. To harden, it must first be transformed: TH1050 (anneal, refrigerate or cold work to convert austenite to martensite, then age at 565 °C for 90 min) or RH950 (refrigerate at −75 °C, then age at 510 °C for 1 h).

Grade / Condition	Tensile Strength, MPa	Yield Strength 0.2%, MPa	Elongation, %	Hardness, HRC
630 H900 (ASTM A564 min)	1310	1170	10	40 (typical ≈ 44)
630 H1150 (ASTM A564 min)	930	725	16	28 (typical ≈ 31)
631 TH1050 (typical per A693 practice)	≈ 1240	≈ 1035	≈ 9	≈ 43
631 RH950 (typical per A693 practice)	≈ 1380	≈ 1210	≈ 9	≈ 44

For 631, RH950 reaches the highest strength; TH1050 is preferred where a balance of strength, toughness and dimensional control is needed. Both conditions of 631 offer noticeably better low-temperature toughness than 630, which is why 631 appears in cryogenic and aerospace applications down to about −200 °C. 630 retains useful strength to about 260–300 °C and is often the cheaper route to a high-strength part; 631 is generally used where sheet, strip or spring performance at temperature is required.

4. Corrosion Resistance Comparison

630: corrosion resistance comparable to 304 in atmospheric, freshwater and mild chemical service; performs acceptably in oilfield brines but is not recommended for unprotected marine immersion. Higher aging temperatures (H1150) improve resistance to stress-corrosion cracking and are required for sour-service hardness limits.

631: slightly better chloride resistance than 630 because of its higher chromium, tolerating short-term salt-spray exposure; still below 316L in marine service, and its performance depends on the transformation/aging condition.

Both: clearly outperform martensitic grades such as 410/420 in corrosive settings, but both fall below duplex grades (e.g. UNS S32205) in aggressive chloride environments.

5. Fabrication, Cost and Selection Guidance

Machining: 630 is machined in Condition A with behaviour similar to 304, then aged - the preferred workflow for complex parts. 631 is harder to machine in the annealed austenitic state and work-hardens rapidly; machining before transformation treatment is the usual route.

Forming: 631 in the annealed condition can be deep drawn or formed before hardening, which suits spring and diaphragm geometries. 630 forms acceptably in Condition A but is less ductile than austenitic grades.

Welding: neither grade is welded for critical load-bearing duty without re-solution annealing and re-aging; welding disrupts the hardened microstructure and the heat-affected zone loses a large part of its strength. Where welded joints are unavoidable, design for post-weld heat treatment.

Cost: as a market rule of thumb, 630 is roughly 15–20 % less expensive than 631 in equivalent product form, driven by its lower nickel content.

Select 630 for shafts, valve stems, pump parts, oil/gas downhole tools, fasteners and medical components requiring strength plus moderate corrosion resistance.

Select 631 for springs, bellows, diaphragms, aircraft structural sheet, missile and satellite brackets, and cryogenic or LNG-related components.

Frequently Asked Questions

What is the main difference between 630 and 631?

Metallurgy and cost. 630 (17-4 PH) is a martensitic PH grade hardened by copper precipitation - cheaper, easier to machine, and versatile. 631 (17-7 PH) is a semi-austenitic PH grade that needs a transformation treatment (TH1050/RH950) and offers higher sheet strength and better cryogenic toughness at higher cost.

Can 631 be used as a direct replacement for 630?

Only after re-evaluating the design. 631 gives higher strength in sheet form but costs more, is harder to machine, and has different heat-treatment requirements. Substitution without re-qualification is not recommended.

Why does 631 need a transformation treatment before aging?

Because it is austenitic after annealing - the martensite that must be aged does not exist yet. The austenite is converted to martensite by refrigeration (RH950) or by a combination of cold working and conditioning (TH1050), and only then does aging produce the strengthening precipitates.

Which grade is better for cryogenic service?

631. Its low-temperature toughness is superior, which is why it appears in aerospace and LNG applications down to about −200 °C. 630 is not specified for cryogenic service.

Are 630 and 631 magnetic?

Both are essentially non-magnetic in the annealed (austenitic/solution-treated) condition and become magnetic after hardening - 630 because martensite forms on cooling, 631 because the austenite is transformed to martensite during the hardening sequence.

Which heat treatment of 630 is best for maximum toughness?

H1150 (620 °C, 4 h) or H1150M gives the best toughness and stress-corrosion resistance with tensile strength around 930 MPa. Choose H900 (1310 MPa) only when maximum strength outweighs toughness and corrosion requirements.