Steel A231 OT-SR
Heat-treated state
ASTM A231
OT-SR = oT + stress relieved after coiling
Apply the 315–425 °C stress relief after coiling, a mandatory step that restores the elastic limit, stabilises free length and improves fatigue life while holding about 45 HRC.
Values for the OT-SR condition
Other A231 tempers and conditions
A231 is listed in 10 conditions; see the grade overview at A231.
Compare all 10 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed | Choose fully annealed bar or rod at 197 HB when parts must be machined, cold headed or re-heat-treated afterwards; strength is only 670 MPa tensile with 23% elongation. | 415 | 670 | 23 | 197 HB | — |
Normalized | Use air cooling from about 870 °C to homogenise the structure and remove forging banding as a pre-treatment before hardening; 940 MPa tensile is not a final service condition. | 615 | 940 | 22 | 27 HRC | 8 % IACS |
Cold drawn | Pick annealed then cold-drawn bar for dimensional accuracy and straightness on turned parts that will be quenched and tempered afterwards; drawing raises yield to 640 MPa at 14% elongation. | 640 | 790 | 14 | 21 HRC | 7.8 % IACS |
Q&T | Choose the 650 °C temper at about 30 HRC for large-section structural parts needing maximum toughness and machinability, accepting 1000 MPa tensile and 17% elongation. | 940 | 1,000 | 17 | 31 HRC | 7.5 % IACS |
Q&T | Use the 540 °C temper at about 38 HRC when impact toughness leads, as on shafts and pins; tensile drops to 1290 MPa but elongation reaches 13% and service temperature 400 °C. | 1,210 | 1,290 | 13 | 38 HRC | — |
OTDefault Oil tempered spring wire | Pick the as-supplied oil-quenched and tempered chromium-vanadium spring wire for cold-coiled springs; strength is graded by wire diameter and formability is limited at a score of 30. | 1,400 | 1,550 | 9 | 46 HRC | — |
OT-SRThis page OT + stress relieved after coiling | Apply the 315–425 °C stress relief after coiling, a mandatory step that restores the elastic limit, stabilises free length and improves fatigue life while holding about 45 HRC. | 1,430 | 1,520 | 9 | 45 HRC | 7.5 % IACS |
Q&T | Pick the 425 °C temper at about 43 HRC as the usual strength-toughness balance for suspension springs and high-stress fasteners, giving 1550 MPa tensile and 10% elongation. | 1,430 | 1,550 | 10 | 43 HRC | — |
Q&T | Choose the 315 °C temper at about 47 HRC for leaf springs and torsion bars needing 1860 MPa tensile with slightly more ductility than the 205 °C temper. | 1,620 | 1,860 | 9 | 47 HRC | — |
Q&T | Use the 845 °C oil quench with 205 °C temper for maximum strength, 1930 MPa tensile at 50 HRC, in high-stress spring plates and tooling, accepting the lowest toughness of the range. | 1,690 | 1,930 | 8 | 50 HRC | — |
What is A231 OT-SR?
A231 OT-SR is A231 heat treated to OT-SR — apply the 315–425 °C stress relief after coiling, a mandatory step that restores the elastic limit, stabilises free length and improves fatigue life while holding about 45 HRC. A231 OT-SR has a yield strength of 1,430 MPa (207 ksi) and a tensile strength of 1,520 MPa (220 ksi) — stronger than 96% of steel grades. Elongation at break is 9% and the elastic modulus is 207 GPa (30 Msi). A231 OT-SR has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,420°C (2,588 °F). OT is the default condition FabDigit quotes; OT-SR is available on request or by drawing note.
Advantages
- High strength — 1,430 MPa (207 ksi), better than 96% of steel grades
Limitations
- Difficult to machine — 20/100, worse than 97% of steel grades
- Limited ductility — 9%, worse than 93% of steel grades
- Low fracture toughness — 46 MPa·√m (41.9 ksi·√in), worse than 88% of steel grades
- Limited service temperature — 230°C (446 °F), worse than 88% of steel grades
- Difficult to weld — 18/100, worse than 87% of steel grades
A231 OT-SR properties
Typical room-temperature values for A231 OT-SR; 22 are specific to this condition. Other listed values are A231 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
A231 OT-SR has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical15
A231 OT-SR has a yield strength of 1,430 MPa (207 ksi) and a tensile strength of 1,520 MPa (220 ksi) — stronger than 96% of steel grades. Elongation at break is 9% and the elastic modulus is 207 GPa (30 Msi).
Thermal6
A231 OT-SR is rated for continuous service to 230°C (446 °F). It conducts heat at 44 W/m·K (25.4 BTU/hr·ft·°F), better than 57% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
A231 OT-SR conducts electricity at 7.5 % IACS, worse than 67% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (14/100), better than 70% of steel grades.
Sustainability4
Producing a kilogram of A231 OT-SR takes about 34 MJ of energy and emits 2.3 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 25%.
Manufacturability6
A231 OT-SR's machinability is poor (20/100, worse than 97% of steel grades); weldability poor (18/100); formability poor (25/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with A231 OT-SR
Is A231 easy to machine?
Turns and mills normally in the annealed condition at roughly 55–60 % of AISI 1212. In the oil-quenched-and-tempered state at 46 HRC, plan on grinding or hard turning with CBN or ceramic inserts.
Can A231 be welded?
A medium-carbon alloy steel with poor weldability: 200–300 °C preheat, low-hydrogen consumables and post-weld tempering are required. Finished springs should not be welded as a matter of practice.
Can A231 be formed, bent or molded?
Oil-tempered wire can be cold coiled with a spring index C of 4 or greater, and must be stress relieved at 315–425 °C after forming. Large sections are hot coiled and then through-quenched and tempered.
What surface finishes work on A231?
Corrosion resistance is poor, so use black oxide, phosphate plus oil, Dacromet or epoxy powder. After zinc or chrome electroplating, bake at 190–220 °C for at least 4 h to remove hydrogen; shot peening markedly increases fatigue life.
A231 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| C | 0.48 – 0.53 | 0.5 |
| Mn | 0.7 – 0.9 | 0.8 |
| SiSAE 6150 permits up to 0.35 | 0.15 – 0.3 | 0.22 |
| Cr | 0.8 – 1.1 | 0.95 |
| V0.15% minimum | ≥ 0.15 | 0.18 |
| P | ≤ 0.04 | — |
| S | ≤ 0.04 | — |
| Fe | balance | — |
A231 OT-SR — frequently asked
What is A231 OT-SR used for?
A231 OT-SR is typically used for coil springs, torsion bars and valve springs. In short: cr-V fatigue spring bar; 1550 MPa oil-tempered; not for welded assemblies.
What is the yield strength of A231 OT-SR?
A231 OT-SR has a typical yield strength of 1,430 MPa (207 ksi) and a tensile strength of 1,520 MPa (220 ksi) — stronger than 96% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is A231 OT-SR easy to machine?
Not especially — machinability is rated poor (20/100, worse than 97% of steel grades). Turns and mills normally in the annealed condition at roughly 55–60 % of AISI 1212. In the oil-quenched-and-tempered state at 46 HRC, plan on grinding or hard turning with CBN or ceramic inserts.
Can A231 OT-SR be welded?
Not readily — weldability is rated poor (18/100). A medium-carbon alloy steel with poor weldability: 200–300 °C preheat, low-hydrogen consumables and post-weld tempering are required. Finished springs should not be welded as a matter of practice.
What surface finishes work on A231 OT-SR?
Corrosion resistance is poor, so use black oxide, phosphate plus oil, Dacromet or epoxy powder. After zinc or chrome electroplating, bake at 190–220 °C for at least 4 h to remove hydrogen; shot peening markedly increases fatigue life.
Can A231 OT-SR be formed, bent or molded?
Oil-tempered wire can be cold coiled with a spring index C of 4 or greater, and must be stress relieved at 315–425 °C after forming. Large sections are hot coiled and then through-quenched and tempered.
What is the maximum service temperature of A231 OT-SR?
A231 OT-SR is rated for continuous use to about 230°C (446 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between A231-OT and A231-ANN?
OT is the default condition — pick the as-supplied oil-quenched and tempered chromium-vanadium spring wire for cold-coiled springs; strength is graded by wire diameter and formability is limited at a score of 30. ANN: choose fully annealed bar or rod at 197 HB when parts must be machined, cold headed or re-heat-treated afterwards; strength is only 670 MPa tensile with 23% elongation. Yield strength is 1,400 MPa in OT versus 415 MPa in ANN.
Sources
- ASTM A231/A231M — Chromium-Vanadium Alloy Steel Spring Wire — ASTM International (2022)
- SAE J404 / J1249 — Chemical Compositions of SAE Alloy Steels (6150) — SAE International (2020)
- GB/T 1222 (50CrVA) (2016)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- Handbook of Spring Design — Spring Manufacturers Institute (SMI) (2013)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.