SUS630 (JIS G4303) — known internationally as 17-4PH — is a precipitation-hardening martensitic stainless steel that achieves high-strength stainless performance impossible with conventional grades: 930–1310 MPa tensile strength with SUS316-class corrosion resistance, through a single low-temperature aging treatment rather than quenching. The 15–17.5% Cr / 3–5% Ni / 3–5% Cu composition forms a nearly full martensite on air cooling from solution treatment, then precipitates copper-rich particles at 480–620°C to raise strength without the distortion of conventional hardening. The result is a grade used in aerospace structural components, medical instruments, high-performance pump shafts, and precision machinery parts where strength, corrosion resistance, and dimensional stability must coexist.
- International Equivalent Grades
- Chemical Composition
- Precipitation Hardening: How It Works
- Mechanical Properties by Condition
- Corrosion Resistance
- Machinability
- Common Mistakes
- When to Choose SUS630
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4303 | SUS630 | Japan | Reference |
| ASTM A564 / A693 | AISI 630 / Type 630 / 17-4PH | USA | ✅ Nearest Exact |
| UNS | S17400 | USA (UNS) | ✅ Nearest Exact |
| EN 10088-1 | 1.4542 / X5CrNiCuNb16-4 | Europe | ✅ Nearest Exact |
| AMS | AMS 5604 (sheet/strip), AMS 5643 (bar) | Aerospace | ✅ Aerospace specification |
| GB/T 4237 | 05Cr17Ni4Cu4Nb | China | ✅ Nearest Exact |
2. Chemical Composition
| Element | SUS630 (JIS G4303) | AISI 630 (ASTM A564) | 1.4542 (EN 10088-1) |
|---|---|---|---|
| C | ≤ 0.07% | ≤ 0.07% | ≤ 0.07% |
| Si | ≤ 1.00% | ≤ 1.00% | ≤ 0.70% |
| Mn | ≤ 1.00% | ≤ 1.00% | ≤ 1.50% |
| Cr | 15.50–17.50% | 15.50–17.50% | 15.00–17.00% |
| Ni | 3.00–5.00% | 3.00–5.00% | 3.00–5.00% |
| Cu | 3.00–5.00% | 3.00–5.00% | 3.00–5.00% |
| Nb+Ta | 0.15–0.45% | 0.15–0.45% | 0.20–0.45% (Nb) |
Copper (3–5%) is the precipitation-hardening agent. Niobium stabilizes carbon and acts as a secondary precipitation strengthener. These two elements distinguish SUS630 from all other stainless grades.
3. Precipitation Hardening: How It Works
SUS630’s strengthening mechanism differs fundamentally from both the austenitic 300-series (not hardenable) and the martensitic 400-series (hardened by carbon martensite).
Step 1 — Solution Treatment (Condition A)
Heat to 1020–1060°C (1868–1940°F), hold, then air cool. The material forms martensite on cooling (unlike austenitic grades), reaching approximately 35HRC in Condition A. The copper and niobium are dissolved in supersaturated solid solution — the matrix is soft martensite, not the strong final structure.
Step 2 — Aging (Precipitation)
Heat Condition A material to the aging temperature and hold. Copper-rich precipitate particles nucleate and grow throughout the martensite matrix, creating obstacles to dislocation movement and raising strength dramatically — with minimal dimensional change (typically < 0.05% linear). This is the manufacturing advantage: parts can be finish-machined in Condition A, then aged to final strength with negligible distortion.
4. Mechanical Properties by Condition
| Condition | Aging Temp | Tensile Strength | 0.2% Proof Stress | Elongation | Hardness |
|---|---|---|---|---|---|
| Condition A (solution treated) | — | ~1000 MPa (145 ksi) | ~793 MPa (115 ksi) | ~10% | ~35HRC |
| Condition H900 | 480°C (896°F) | ≥ 1310 MPa (190 ksi) | ≥ 1170 MPa (170 ksi) | ≥ 10% | ~40–43HRC |
| Condition H925 | 495°C (923°F) | ≥ 1170 MPa (170 ksi) | ≥ 1000 MPa (145 ksi) | ≥ 10% | ~38–42HRC |
| Condition H1025 | 550°C (1022°F) | ≥ 1070 MPa (155 ksi) | ≥ 1000 MPa (145 ksi) | ≥ 12% | ~35–38HRC |
| Condition H1075 | 580°C (1076°F) | ≥ 1000 MPa (145 ksi) | ≥ 862 MPa (125 ksi) | ≥ 13% | ~31–35HRC |
| Condition H1150 | 620°C (1148°F) | ≥ 930 MPa (135 ksi) | ≥ 724 MPa (105 ksi) | ≥ 16% | ~28–32HRC |
H900 maximizes strength (1310 MPa) but has the lowest toughness and stress-corrosion cracking resistance. H1150 sacrifices ~30% strength for significantly better toughness, ductility, and corrosion resistance. For aerospace structural parts, H900–H925 is standard. For general engineering and marine environments, H1075–H1150 is often preferred.
5. Corrosion Resistance
SUS630 in the H1025–H1150 conditions offers corrosion resistance comparable to SUS304 — significantly better than martensitic grades (SUS410, SUS420J2) at the same hardness level. H900 condition has noticeably lower resistance due to incomplete aging and residual stress effects.
| Condition | PREN (approx.) | vs SUS304 | vs SUS316 |
|---|---|---|---|
| H1150 | ~18–20 | Comparable | Lower (no Mo) |
| H1025 | ~17–19 | Slightly lower | Lower |
| H900 | ~15–17 | Lower | Significantly lower |
SUS630 does not contain Mo, so it should not be specified for high-chloride environments where SUS316 is the minimum. It is well-suited to mildly corrosive environments: freshwater, dilute acids, food-processing atmospheres, and outdoor conditions without Cl⁻ accumulation.
6. Machinability
In Condition A (~35HRC), SUS630 machines at approximately 50–60% of AISI 1212 — comparable to SUS304 but with less work-hardening tendency due to the martensitic structure. Standard carbide tooling at moderate speeds works well. After aging to H900 (~43HRC), conventional machining becomes difficult; grinding is the primary final finishing operation. Recommended machining sequence: rough and semi-finish in Condition A → age to final condition → finish grind to tolerance.
7. Common Mistakes
8. When to Choose SUS630
Aerospace structural components, high-performance turbine shafts, precision mechanical parts requiring 1100+ MPa with stainless behavior. Not for sustained-load chloride environments.
Pump and valve shafts, medical instruments, mold components, precision machinery in mildly corrosive environments. Combines ~1000 MPa strength with good toughness and SUS304-class corrosion resistance.
Parts requiring impact resistance and corrosion resistance with moderate strength (~930 MPa). Lower stress-corrosion cracking susceptibility makes this condition appropriate for pressurized or sustained-load applications in mildly corrosive environments.
High-chloride environments (marine immersion, chloride process fluids) — specify SUS316 or duplex instead. When strength above ~1300 MPa is needed continuously — high-alloy tool or maraging steels achieve this more reliably. When the application requires full non-magnetic properties — SUS630 is martensitic and ferromagnetic.
9. FAQ
Q: Is SUS630 the same as 17-4PH?
Yes. 17-4PH is the trade name (17% Cr, 4% Ni, precipitation hardening) coined by Armco Steel in the 1950s; AISI 630 and JIS SUS630 are the standard designations for the same alloy system. EN 1.4542 is the European equivalent. Material certified to any of these designations is interchangeable for most engineering purposes.
Q: What is the best condition for precision machined parts?
Machine in Condition A (solution treated, ~35HRC), then age to the required condition. The aging treatment causes minimal dimensional change (<0.05% linear), allowing tight tolerances to be held through the final heat treatment step. For extremely tight tolerances (<±0.005 mm), consult with your heat treater regarding expected dimensional change for your specific geometry.
Q: Can SUS630 be welded?
Yes, but welding requires care. Weld with ER630 filler in Condition A, then re-solution-treat the entire assembly and re-age to restore full mechanical properties. Welding after aging and attempting to re-age only the weld zone will produce inconsistent properties. For components that cannot be re-heat-treated after welding, austenitic 300-series grades are more practical.
Summary
- SUS630 = AISI 630 = 17-4PH = EN 1.4542: 17% Cr, 4% Ni, 4% Cu precipitation-hardening martensitic stainless
- Achieves 930–1310 MPa tensile strength via copper precipitation aging — no distortion-inducing quench required
- Six standard conditions (H900–H1150): lower aging temperature = higher strength, lower toughness and SCC resistance
- Corrosion resistance ≈ SUS304 in H1025–H1150; lower in H900. No Mo — not for high-chloride environments
- Machine in Condition A, then age — dimensional change through aging is minimal (<0.05%)
- H900 in sustained-load chloride environments causes SCC — use H1075/H1150 or switch to SUS316/duplex
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