Steel A231-ANN (Annealed)
Heat-treated state
ASTM A231
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.
Values for the ANN 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 |
|---|---|---|---|---|---|---|
ANNThis page 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 + 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-ANN?
A231-ANN is A231 heat treated to 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. A231-ANN has a yield strength of 415 MPa (60.2 ksi) and a tensile strength of 670 MPa (97.2 ksi) — weaker than 56% of steel grades. Elongation at break is 23% and the elastic modulus is 207 GPa (30 Msi). A231-ANN 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; ANN is available on request or by drawing note.
Limitations
- Low fracture toughness — 45 MPa·√m (41 ksi·√in), worse than 90% of steel grades
- Limited service temperature — 220°C (428 °F), worse than 89% of steel grades
A231-ANN properties
Typical room-temperature values for A231-ANN; 12 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-ANN 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-ANN has a yield strength of 415 MPa (60.2 ksi) and a tensile strength of 670 MPa (97.2 ksi) — weaker than 56% of steel grades. Elongation at break is 23% and the elastic modulus is 207 GPa (30 Msi).
Thermal6
A231-ANN is rated for continuous service to 220°C (428 °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-ANN 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-ANN 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-ANN's machinability is good (55/100, better than 62% of steel grades); weldability fair (35/100); formability good (60/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with A231-ANN
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-ANN — frequently asked
What is A231-ANN used for?
A231-ANN 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-ANN?
A231-ANN has a typical yield strength of 415 MPa (60.2 ksi) and a tensile strength of 670 MPa (97.2 ksi) — weaker than 56% of steel grades. Strength varies by condition: see the 10 listed tempers.
Is A231-ANN easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% 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-ANN be welded?
With care — weldability is rated fair (35/100). 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-ANN?
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-ANN 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-ANN?
A231-ANN is rated for continuous use to about 220°C (428 °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.