When engineers specify a precipitation-hardening stainless steel forging, they routinely face a choice between two ASTM A564 grades whose datasheets look nearly identical: Grade 634 (UNS S35500) and Grade 630 (UNS S17400). Both achieve tensile strengths above 1,310 MPa at peak-aged conditions. Both are produced as open-die forgings and seamless rolled rings under ASTM A705. But their metallurgy, corrosion envelope, and heat treatment complexity are fundamentally different — differences that are critical to get right at design stage and nearly impossible to correct after procurement.
Bottom Line: Which Grade Should You Choose?
Choose Grade 634 (AM-355 / UNS S35500) when your service environment contains chlorides, seawater, or reducing acids; when operating temperatures exceed 316°C; when cryogenic toughness is required; or when you need to cold-form the material before final hardening. Its 2.5–3.25% molybdenum content gives it a PREN of ~24–26 versus Grade 630's ~16–18 — a critical advantage in corrosive environments.
Choose Grade 630 (17-4PH / UNS S17400) when the service environment is mild to moderately aqueous, when NACE sour-service compliance via the H1150D condition is required, or when a simpler single-step aging heat treatment is preferred. Grade 630 has the broadest global supply chain depth and the widest industry acceptance for structural aerospace and oil-and-gas applications.
At equal peak UTS (≥1,310 MPa), the decision comes down to environment, temperature range, and process complexity — not raw strength.
1. Metallurgical Foundations
The most important distinction between Grade 634 and Grade 630 is not their peak strength — it is their microstructure class. This determines heat treatment complexity, corrosion behavior, and elevated-temperature capability.
Grade 630 (17-4PH): Martensitic Precipitation-Hardening
Grade 630 is a martensitic PH stainless steel. After solution annealing (Condition A) at ~1040°C, the chromium-nickel-copper balance produces a martensitic matrix that is already relatively strong — typically ~1,000 MPa UTS — without any intermediate transformation step. Subsequent aging at temperatures from 480°C (H900) to 621°C (H1150) precipitates copper-rich particles throughout the martensite, raising strength further in a single, predictable cycle. The resulting microstructure in all aged conditions is predominantly martensitic, giving Grade 630 isotropic properties in forgings and excellent process predictability.
Grade 634 (AM-355): Semi-Austenitic Precipitation-Hardening
Grade 634 belongs to a fundamentally different subclass: semi-austenitic PH stainless steel. In the solution-annealed condition (Condition A), the alloy is predominantly austenitic, not martensitic. The microstructure only transforms to martensite after a controlled conditioning (equalization) step at ~930–955°C, followed by cooling to below −73°C to complete martensite transformation. A subsequent aging step then precipitates strengthening phases into that freshly formed martensite.
This two-stage transformation pathway gives Grade 634 three capabilities Grade 630 lacks: (1) superior cold formability in austenitic Condition A; (2) excellent cryogenic impact toughness; and (3) access to sustained elevated-temperature strength above 316°C. Grade 634 also contains 2.5–3.25% molybdenum — completely absent in Grade 630 — which is the single most impactful compositional difference for corrosion performance.
The choice between Grade 634 and Grade 630 is not about which grade is stronger. At peak conditions, tensile strength is equivalent. The real question is which microstructure and alloying strategy fits your service environment, heat treatment capability, and design margins.
— Jiangsu Liangyi Technical Engineering Team, 25+ years PH stainless forging experience2. Chemical Composition Compared
The table below shows full composition ranges per ASTM A564 / ASTM A705 for both grades. Elements that drive material selection decisions are annotated in the right column.
| Element | Grade 634 (S35500) | Grade 630 (S17400) | Engineering Significance |
|---|---|---|---|
| Carbon (C) | 0.10 – 0.15% | ≤ 0.07% | 634's elevated C enables nitrogen strengthening and stabilizes martensite after conditioning |
| Chromium (Cr) | 15.0 – 16.0% | 15.5 – 17.5% | Both provide stainless behavior; 630's wider range supports more flexible melting practice |
| Nickel (Ni) | 4.0 – 5.0% | 3.0 – 5.0% | 634's tighter Ni control stabilizes Ms (martensite start) temperature for predictable conditioning |
| Molybdenum (Mo) | 2.5 – 3.25% | — (absent) | Decisive difference. Mo adds ~8 PREN points to 634 (PREN ~24–26 vs 630's ~16–18), enabling resistance to pitting in chloride environments, seawater, and reducing acids |
| Nitrogen (N) | 0.07 – 0.13% | ≤ 0.05% | 634 uses N as a solid-solution and precipitation strengthener. 630 minimizes N to ensure reliable martensite transformation |
| Copper (Cu) | — (absent) | 3.0 – 5.0% | Cu is the primary precipitation-hardening agent in 630; absent in 634, which uses a C-N-Mo mechanism. Note: Cu presence may be relevant for FDA/food-contact applications |
| Niobium (Nb) | — | 0.15 – 0.45% | Nb in 630 refines grain size and stabilizes carbides during aging |
| Manganese (Mn) | 0.50 – 1.25% | ≤ 1.00% | 634's Mn works with N to stabilize austenite and contributes to solid-solution strength |
The single most consequential compositional difference is the presence or absence of molybdenum. PREN = %Cr + 3.3×%Mo. Grade 630 achieves PREN ~16–18. Grade 634, with 2.5–3.25% Mo, achieves PREN ~23–26. This 8-point gap translates directly into resistance to pitting in seawater (critical above 20°C), chloride process streams (critical above 60°C), and reducing acid environments. Specifying Grade 630 where Grade 634 should be used in chloride service is one of the most common — and costly — material selection errors in PH stainless steel procurement.
3. Mechanical Properties: Full Condition Matrix
Both grades are available in multiple heat-treatment conditions. The table below covers all conditions commonly specified in forging applications per ASTM A564 / ASTM A705.
| Grade / Condition | UTS min (MPa) | 0.2% YS min (MPa) | Elongation min (%) | Hardness max (HRC) |
|---|---|---|---|---|
| 634 — Condition A (Annealed) | ≥ 1,000 | ≥ 690 | ≥ 8 | — |
| 634 — SCT 850 (Peak aged) | ≥ 1,310 | ≥ 1,000 | ≥ 12 | 45 |
| 634 — SCT 950 | ≥ 1,170 | ≥ 895 | ≥ 14 | 40 |
| 634 — SCT 1000 | ≥ 1,000 | ≥ 795 | ≥ 15 | 37 |
| 630 — H900 (Peak aged) | ≥ 1,310 | ≥ 1,170 | ≥ 10 | 44 |
| 630 — H925 | ≥ 1,170 | ≥ 1,000 | ≥ 10 | 42 |
| 630 — H1025 | ≥ 1,070 | ≥ 1,000 | ≥ 12 | 39 |
| 630 — H1075 | ≥ 1,000 | ≥ 862 | ≥ 13 | 37 |
| 630 — H1150 | ≥ 862 | ≥ 724 | ≥ 16 | 35 |
| 630 — H1150M (Double aged) | ≥ 793 | ≥ 586 | ≥ 18 | 28 |
At peak conditions, Grade 634 SCT 850 and Grade 630 H900 achieve identical minimum UTS of ≥1,310 MPa. The key difference: Grade 634 at SCT 850 delivers higher minimum elongation (12% vs 10%) at equivalent UTS, reflecting better fracture toughness from the semi-austenitic conditioning step. Grade 630 at H900, however, delivers a higher minimum yield strength (≥1,170 MPa vs ≥1,000 MPa), which is advantageous in designs where dimensional stability under sustained stress governs.
4. Heat Treatment Routes Compared
Heat treatment sequences for these two grades differ significantly in complexity — a practical consideration that affects supplier selection, process certification requirements, and cost.
Grade 630 (17-4PH) — Two-Stage (Solution Anneal + Single-Step Aging)
Solution Anneal — Condition A
Heat to ~1040°C, hold, cool rapidly below 32°C. Material transforms to martensite in a single pass. Standard supply condition from most forging manufacturers worldwide. Simple, well-documented furnace operation.
Age Hardening (H900 through H1150M)
Single isothermal hold at target temperature for 1–4 hours, then air cool. Copper-rich (Cu₃Nb) precipitates form throughout the martensitic matrix. Process complete. This is Grade 630's decisive manufacturing advantage — it requires only one furnace cycle after solution annealing, and is mastered by virtually every heat-treatment facility in the world.
Grade 634 (AM-355) — Three-Stage (Solution Anneal + Conditioning + Aging)
Solution Anneal — Condition A
Heat to 1025–1065°C, hold, cool. Material is predominantly austenitic in this condition — soft and highly formable. This enables cold-forming and deep-drawing operations that Grade 630 in Condition A (martensitic, HRC ~35) cannot tolerate.
Equalization / Conditioning at 930–955°C + Sub-Zero Cooling
Hold for 3 hours at 930–955°C, then cool to below −73°C (refrigeration treatment required). The austenite transforms to martensite. Inadequate sub-zero cooling leaves retained austenite that suppresses final mechanical properties — the most common failure mode at suppliers without validated PH stainless process capability. Hardness must be verified after this step.
Age Hardening — SCT 850, 950, or 1000
Heat to 480°C (SCT 850), 510°C (SCT 950), or 538°C (SCT 1000) for 3 hours, then air cool. Carbon-nitrogen-rich precipitates form in the conditioned martensite. Full time-temperature trace documentation is mandatory for aerospace and oil-and-gas code compliance. At Jiangsu Liangyi, all three furnace cycle logs are included as standard in the EN 10204 3.1 Mill Test Certificate. For full size range, heat treatment options, and MTC scope, see our ASTM A564 Grade 634 forgings product page.
Grade 634's three-stage heat treatment requires validated sub-zero cooling capability (to at least −73°C) and documented hardness verification after the conditioning step. When sourcing Grade 634 forgings, always request:
- Complete time-temperature furnace logs for all three stages
- Hardness test data confirming martensite transformation after sub-zero cooling
- Final mechanical test certificates: tensile, yield, elongation, and Charpy impact (if specified)
- EN 10204 3.1 (or 3.2 third-party witness) material test certificate
5. Corrosion Resistance
Both grades resist atmospheric and fresh-water corrosion well. Divergence becomes critical in chloride-bearing, reducing-acid, and high-temperature environments.
| Service Environment | Grade 634 (AM-355) | Grade 630 (17-4PH) |
|---|---|---|
| Atmospheric / fresh water | Excellent | Excellent |
| Seawater splash zone | Good – Excellent (Mo passive film) | Moderate — pitting risk in immersion |
| Chloride streams < 60°C | Good (PREN ~24–26) | Limited — SCC risk under sustained stress |
| Chloride streams > 60°C | Acceptable with surface finish control | Not recommended |
| Sour service (H₂S + CO₂) | NACE MR0175 (select conditions) | NACE MR0175 compliant (H1150 / H1150D) |
| Reducing acids (dilute HCl, H₂SO₄) | Good — Mo enhances passive film | Poor — no Mo protection |
| Oxidizing acids (HNO₃) | Good | Good |
| High-temp. service > 316°C | Good to ~400°C | Strength degrades; not recommended |
| Cryogenic (< −100°C) | Excellent impact toughness | Martensitic structure embrittles |
6. Forgeability and Manufacturability
Both grades are open-die forgeable and seamless ring-rollable on standard industrial equipment. Their pre-conditioning microstructure differences produce meaningful effects on forging process behavior.
- Hot forging temperature range (both grades): 1,010°C to 1,175°C. Work must be completed above ~980°C to avoid hot shortness and cracking.
- Grade 634 in Condition A is austenitic and tolerates larger reductions per pass, making it favorable for complex cross-sections and large seamless rings with significant diameter changes.
- Grade 630 in Condition A is already martensitic (~HRC 35) and requires more careful pass scheduling for heavy-section forgings.
- Post-forge treatment: Grade 630 requires one furnace cycle. Grade 634 requires three stages including validated sub-zero cooling to −73°C.
- Machining: Both grades are best machined in the annealed condition. Carbide tooling with flood coolant is standard. Grade 634 at SCT 850 is harder to machine than Grade 630 at H1075/H1150 due to its higher C+N content.
- NDT: Both grades support full UT (ASTM A388), MT, and PT inspection. For critical applications, ASTM A705 (forgings) applies in place of ASTM A564 (bars). Macrostructure per ASTM A604.
- Welding: Grade 630 is more widely welded in industry. Grade 634 welding requires careful post-weld heat treatment to restore the conditioned martensite in the heat-affected zone.
7. Applications by Industry
Grade 634 / AM-355
UNS S35500 · Semi-Austenitic- Subsea valve bodies and Christmas tree components in chloride + H₂S environments
- High-pressure pump impellers and casings on offshore oil and gas platforms
- Gas turbine compressor discs and rings requiring elevated-temperature strength above 316°C
- Cryogenic storage and LNG plant equipment requiring impact toughness at −196°C
- Aerospace structural forgings — ultra-high strength with controlled toughness
- Chemical plant shafts in reducing-acid service (dilute HCl, H₂SO₄)
- Marine propulsion shafts and rudder stocks in seawater immersion
Grade 630 / 17-4PH
UNS S17400 · Martensitic- Aerospace structural fasteners, brackets, and attach fittings (H900–H1025)
- Oil and gas downhole tool mandrels (NACE H1150D sour-service path)
- Pump shafts and valve stems in mild aqueous service
- Nuclear-related components per ASTM A705 nuclear grade
- Paper and pulp industry (excluding bleach-plant contact)
- Medical and surgical instruments where Cu content is acceptable
- General structural components where single-step heat treatment is required
8. Decision Framework: Grade 634 vs Grade 630
Use the table below to map your specific design constraints to the appropriate grade. This framework is based on 25+ years of material selection consultations with engineers across oil and gas, aerospace, power generation, and marine sectors.
| Design Constraint | Grade 634 (AM-355) | Grade 630 (17-4PH) |
|---|---|---|
| Chloride-bearing service environment | ✓ PREN ~24–26; Mo provides decisive advantage | Risk above 60°C; pitting and SCC failure mode |
| Seawater immersion service | ✓ Good with controlled surface finish | Not recommended for immersion service |
| NACE MR0175 / ISO 15156 sour service | ✓ Compliant in select conditions | ✓ H1150D — well-documented NACE path |
| Sustained temperature > 316°C | ✓ Usable to ~400°C without overaging | Not recommended; strength and microstructure degrade |
| Cryogenic toughness required | ✓ Excellent Charpy impact at −196°C | Martensitic structure embrittles below −40°C |
| Cold forming before final hardening | ✓ Austenitic Condition A is highly formable | HRC ~35 in Condition A; cold forming limited |
| Reducing acid process stream | ✓ Mo-enhanced passive film provides protection | No Mo; passive film inadequate in reducing acids |
| Single-step heat treatment preferred | 3 stages + sub-zero cooling required | ✓ Single aging cycle (1–4 hours) |
| Maximum yield strength at peak condition | SCT 850: ≥ 1,000 MPa YS; higher elongation | H900: ≥ 1,170 MPa YS; superior for stress-critical design |
| Broadest global supply chain | Specialist grade; fewer qualified suppliers | ✓ 17-4PH: widest global availability |
| Copper-free requirement (FDA / food contact) | ✓ No copper in composition | 3.0–5.0% Cu; may restrict FDA food contact use |
The Bottom Line
Grade 630 (17-4PH) is the right default for the majority of structural forging applications where the service environment is mild to moderately aqueous, NACE sour-service compliance via the H1150D condition is required, and heat treatment simplicity and supply chain depth are priorities. Its copper-precipitation mechanism is extremely well characterized globally, and its single-step aging cycle is mastered by every certified heat treatment facility in the world.
Grade 634 (AM-355) is the right choice when designing for chloride-bearing environments — especially above 60°C — when elevated-temperature strength above 316°C is required, when cryogenic impact toughness is a design criterion, or when the austenitic Condition A formability needs to be exploited before final hardening. In these scenarios, the three-stage heat treatment and sub-zero cooling requirement are fully justified by the performance advantage.
The most common and costly material selection error in PH stainless steel procurement is specifying Grade 630 where Grade 634 should be used — specifically in chloride-rich or chemically aggressive environments. When the service environment involves significant chloride exposure, Grade 634's 2.5–3.25% molybdenum content is the non-negotiable deciding factor. If Grade 634 is the right choice for your project, our ASTM A564 Grade 634 forgings page covers available forging sizes, heat treatment conditions, NDT options, and how to request a quote.
Choose Grade 634 (AM-355) When:
- Chloride or marine immersion environment
- Sustained temperatures above 316°C
- Cryogenic toughness required
- Cold forming needed in Condition A
- Reducing acid process streams
- Copper-free composition required
Choose Grade 630 (17-4PH) When:
- Mild to moderate service environment
- NACE MR0175 sour service (H1150D)
- Single-step heat treatment required
- Aerospace structural fasteners / fittings
- Maximum yield strength at H900
- Widest global supply chain required
10. Frequently Asked Questions
The following questions are answered based on ASTM A564, ASTM A705, and 25+ years of engineering experience with PH stainless steel forgings.
What is the main difference between ASTM A564 Grade 634 and Grade 630? +
Grade 634 (UNS S35500 / AM-355) is a semi-austenitic precipitation-hardening stainless steel containing 2.5–3.25% molybdenum, providing superior corrosion resistance (PREN ~24–26) and elevated-temperature capability to ~400°C. Grade 630 (UNS S17400 / 17-4PH) is a martensitic PH stainless steel with no molybdenum (PREN ~16–18) but simpler single-step aging and the broadest global supply chain. Both achieve ≥1,310 MPa UTS at peak-aged conditions. The decision is driven by service environment and heat treatment capability — not raw strength.
Which grade has better corrosion resistance in chloride environments? +
Grade 634 (AM-355) is significantly more resistant to pitting and crevice corrosion in chloride environments due to its 2.5–3.25% molybdenum content (PREN ~24–26 vs Grade 630's ~16–18). Grade 630 (17-4PH) carries a risk of stress corrosion cracking (SCC) in chloride environments above 60°C. For seawater immersion or chloride process streams, Grade 634 is the strongly recommended choice.
What are the heat treatment conditions for Grade 634 (SCT 850, SCT 950, SCT 1000)? +
Grade 634 (AM-355) requires a three-stage heat treatment: (1) Solution anneal at 1025–1065°C — Condition A (austenitic); (2) Equalization at 930–955°C for 3 hours, then sub-zero cooling to below −73°C to transform austenite to martensite; (3) Age hardening at 480°C for SCT 850, 510°C for SCT 950, or 538°C for SCT 1000, held 3 hours, then air cooled. SCT 850 achieves the highest strength (UTS ≥1,310 MPa); SCT 1000 the highest ductility (elongation ≥15%).
What are the heat treatment conditions for Grade 630 (H900 through H1150D)? +
Grade 630 (17-4PH) heat treatment involves solution annealing to Condition A at ~1040°C, then single-step age hardening: H900 (480°C), H925 (495°C), H1025 (552°C), H1075 (579°C), H1100 (593°C), H1150 (621°C), H1150M (double-aging cycle), or H1150D (double-aging cycle for NACE MR0175 sour service compliance). H900 delivers peak strength (UTS ≥1,310 MPa, YS ≥1,170 MPa); H1150 delivers peak ductility (elongation ≥16%).
Can Grade 634 forgings be used in NACE MR0175 sour service? +
Yes, Grade 634 (AM-355) can be qualified for NACE MR0175/ISO 15156 sour service in specific heat treatment conditions with hardness control. Grade 630 (17-4PH) in the H1150D double-aged condition is more widely documented in the industry for NACE sour service. Always confirm the specific condition, hardness limit, and applicable NACE MR0175 table with your forging supplier and the project engineer before final specification.
What is the maximum operating temperature for Grade 634 vs Grade 630? +
Grade 634 (AM-355) maintains useful mechanical properties up to approximately 400°C (750°F). Grade 630 (17-4PH) is not recommended for sustained service above 316°C (600°F), as overaging begins to degrade strength. For elevated-temperature applications above 316°C, Grade 634 is the preferred choice.
Does ASTM A564 or ASTM A705 apply to Grade 634 and Grade 630 forgings? +
ASTM A564 covers hot-rolled and cold-finished age-hardening stainless steel bars and shapes. For forgings specifically, ASTM A705 (Age-Hardening Stainless Steel Forgings) is the applicable standard. Both Grade 634 and Grade 630 are covered under ASTM A705. Most forging manufacturers, including Jiangsu Liangyi, produce forgings per ASTM A705 with material data cross-referenced to ASTM A564 for chemical composition limits.
What forging sizes does Jiangsu Liangyi supply in Grade 634 and Grade 630? +
Jiangsu Liangyi Co., Limited supplies Grade 634 and Grade 630 forgings with single-piece weights from 30 kg to 30,000 kg (30 metric tons). Maximum dimensions: seamless rolled rings up to 6,000 mm OD; forged shafts up to 15 meters long; forged discs and blocks up to 3,000 mm diameter. Standard lead time is 3–4 weeks; 4–8 weeks for custom-machined components. The factory is located in Jiangyin City, Jiangsu Province, approximately 150 km from Shanghai Port.
11. About Jiangsu Liangyi — Grade 634 and Grade 630 Forging Manufacturer
Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified open-die forging and seamless rolled ring manufacturer located in Chengchang Industry Park, Jiangyin City, Jiangsu Province, China. We supply ASTM A564 Grade 634 (UNS S35500) forgings and ASTM A564 Grade 630 (UNS S17400) forgings to customers in more than 50 countries, including the United States, Germany, Saudi Arabia, the UAE, Singapore, and Australia.
- Quality system certification: ISO 9001:2015
- Single-piece forging weight: 30 kg to 30,000 kg
- Seamless rolled rings: OD up to 6,000 mm
- Forged shafts: up to 15 meters in length
- Complete in-house heat treatment including sub-zero conditioning for Grade 634
- UT, MT, PT non-destructive testing per ASTM A388 and ASTM A604
- Mill Test Certificates issued per EN 10204 3.1 format as standard; EN 10204 3.2 third-party witness inspection available on request
- Lead time: 3–4 weeks standard; 4–8 weeks custom-machined
- Location: 150 km from Shanghai Port, 200 km from Ningbo Port
For technical questions about Grade 634 vs Grade 630 for your specific application, contact our engineering team. Material selection consultation is included with every quotation.