431 Stainless Steel (UNS S43100)
The strongest and most corrosion resistant of the common martensitic grades: 16 percent chromium with nickel, heat treatable to 220 ksi bolts, for marine shafts, pump parts and high-strength stainless fasteners.
What 431 Stainless is.
Type 431 raises the chromium of the martensitic family to 15 to 17 percent and adds 1.25 to 2.50 percent nickel. The nickel keeps the structure hardenable despite the higher chromium, so 431 combines the best corrosion resistance of the hardenable stainless steels with strength to 220 ksi. It is the traditional grade for marine propeller shafts, pump shafts, aircraft parts and the strongest stainless bolting short of a precipitation-hardening alloy.
We machine 431 fasteners and components from certified A276 bar and heat treat to ASTM F593 Alloy Group 6 conditions H or HT. Because of its nickel it machines less freely than 410 and is usually finished after hardening and tempering.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | — | 0.20 |
| Manganese | Mn | — | 1.00 |
| Phosphorus | P | — | 0.040 |
| Sulfur | S | — | 0.030 |
| Silicon | Si | — | 1.00 |
| Chromium | Cr | 15.0 | 17.0 |
| Nickel | Ni | 1.25 | 2.50 |
Values per ASTM A276/A276M-17 Tables 1 and 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team.
Mechanical properties.
| Condition and nominal diameter (in.) | Tensile strength, min (ksi) | Yield strength 0.2% offset, min (ksi) | Elongation, min (%) | Reduction of area, min (%) | Hardness |
|---|---|---|---|---|---|
| Condition A (annealed) — all | — | — | — | — | 285 HBW max |
Values per ASTM A276/A276M-17 Tables 1 and 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. A276 controls 431 bar by annealed hardness; hardened strengths are per F593 below.
| F593 condition · marking | Nominal diameter (in.) | Tensile strength (ksi) | Yield strength, min (ksi) | Rockwell hardness |
|---|---|---|---|---|
| H (hardened, tempered ≥1050 °F) · F593S | ¼ to 1½ | 125 to 150 | 100 | C25 to C32 |
| HT (hardened, tempered ≥525 °F) · F593T | ¼ to 1½ | 180 to 220 | 140 | C40 to C48 |
Values per ASTM F593-24 Table 2, full-size tests (verified against the specification). Yield is 0.2 % offset; hardness is Rockwell. Machined-specimen limits differ slightly.
Specifications that apply.
How 431 Stainless ships from Houston.
- Hex and heavy hex bolts and cap screws
- Full-thread studs and threaded rod
- Socket head cap and set screws
- Hex, jam and lock nuts
- Washers and spacers
- Machined components from certified bar
431 Stainless fasteners we make.
Every item is manufactured to your drawing or standard in 431 Stainless, imperial or metric, with an EN 10204 3.1 certificate.
What is 431 stainless steel?
431 is the high-chromium, nickel-bearing member of the martensitic family. The extra chromium gives it corrosion resistance approaching 304 in many atmospheres, and the nickel keeps it hardenable, so a 431 bolt can be quenched and tempered to 180 ksi and still resist rusting on a boat deck. It is magnetic, tougher than 410 at the same strength, and harder to machine, which is why it is usually finished after heat treatment.
431 or 17-4 PH?
Both give high-strength stainless fasteners. 17-4 PH is hardened by a simple low-temperature age with almost no distortion, has better corrosion resistance and is the aerospace and oilfield standard. 431 is less costly, quenched and tempered like an alloy steel, and remains the choice for large marine shafts and where a drawing already calls for it.
Testing, certification and marking
- Tensile and hardness tested per lot on the finished product
- Grade or marking symbol F593S or F593T and manufacturer's identification on every piece
- EN 10204 3.1 material test report with heat number and chemistry; 3.2 witness available
- PMI verification on request
410 Stainless Steel (UNS S41000)
The basic hardenable stainless: 12 percent chromium, heat treatable to 120 ksi and beyond, magnetic, and corrosion resistant in mild atmospheres — for valve trim, pump parts and high-strength stainless bolts.
ASTM A1082 bolting · ASTM A564 bar17-4 PH Stainless (Type 630, UNS S17400)
The high-strength stainless — up to 190 ksi in H900, or 115 to 135 ksi with real toughness in the H1150 conditions — for pump shafts, valve stems, subsea and aerospace fasteners.
ASTM A582 bar · ASTM F593 bolts416 Free-Machining Stainless Steel (UNS S41600)
410 with sulfur added — the most machinable of all stainless steels, heat treatable to 190 ksi bolts, for turned pins, shafts, fittings and set screws.