Steel 31CrMo12 +A (Annealed)
Heat-treated state
EN 31CrMo12
Choose the soft annealed bar (≤ 248 HB) when rough machining or cold working comes first, then quench, temper and nitride afterwards; yield is only 470 MPa in this state.
Values for the +A condition
Other 31CrMo12 tempers and conditions
31CrMo12 is listed in 8 conditions; see the grade overview at 31CrMo12.
Compare all 8 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
+AThis page Annealed | Choose the soft annealed bar (≤ 248 HB) when rough machining or cold working comes first, then quench, temper and nitride afterwards; yield is only 470 MPa in this state. | 470 | 800 | 18 | 230 HB | — |
Hot rolled | Order untreated hot-rolled or forged stock only when you will anneal or quench and temper in house; hardness near 270 HB is not controlled and machining response varies. | 550 | 900 | 14 | 27 HRC | 6.9 % IACS |
Normalized | Pick normalized forgings or billets for a refined, uniform microstructure at 600 MPa yield and 280 HB — between annealed and quenched-and-tempered — as an intermediate stage before final Q&T. | 600 | 950 | 15 | 29 HRC | 6.9 % IACS |
+QT | Specify for 250-500 mm forgings and bar, where deep hardenability still holds 740 MPa yield and Rm 880-1080 MPa at the core; strength drops further as section size grows. | 740 | 980 | 15 | 31 HRC | 6.6 % IACS |
+QT | Use for 100-250 mm sections quenched and tempered to Rm 980-1180 MPa; the slower cooling of medium bar leaves core strength at 850 MPa yield, slightly below thin sections. | 850 | 1,060 | 13 | 34 HRC | 6.6 % IACS |
+QTDefault Quenched and tempered | Choose oil quenched and high-temperature tempered bar up to 100 mm diameter (Rm 1030-1230 MPa) as the usual stock condition for finish machining before gas nitriding. | 900 | 1,100 | 13 | 35 HRC | — |
+QT + gas nitrided | Select the finished state — quenched and tempered, then gas nitrided to 800-950 HV with a 0.3-0.6 mm case — for maximum wear and fatigue resistance; no machining is possible after nitriding. | 900 | 1,100 | 12 | 66 HRC | — |
Peeled/turned and cold-drawn or ground bar | Pick peeled, cold-drawn or ground bright bar when tight diameter tolerance and surface quality let you go straight to finish machining and nitriding; core is annealed or quenched and tempered. | 900 | 1,100 | 11 | 35 HRC | 6.6 % IACS |
What is 31CrMo12 +A?
31CrMo12 +A is 31CrMo12 heat treated to +A — choose the soft annealed bar (≤ 248 HB) when rough machining or cold working comes first, then quench, temper and nitride afterwards; yield is only 470 MPa in this state. 31CrMo12 +A has a yield strength of 470 MPa (68.2 ksi) and a tensile strength of 800 MPa (116 ksi) — stronger than 57% of steel grades. Elongation at break is 18% and the elastic modulus is 210 GPa (30.5 Msi). 31CrMo12 +A 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). +QT is the default condition FabDigit quotes; +A is available on request or by drawing note.
Advantages
- High service temperature — 480°C (896 °F), better than 88% of steel grades
Limitations
- Poor heat conductor — 34 W/m·K (19.6 BTU/hr·ft·°F), worse than 77% of steel grades
- Low electrical conductivity — 6.6 % IACS, worse than 76% of steel grades
31CrMo12 +A properties
Typical room-temperature values for 31CrMo12 +A; 12 are specific to this condition. Other listed values are 31CrMo12 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
31CrMo12 +A 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).
Mechanical16
31CrMo12 +A has a yield strength of 470 MPa (68.2 ksi) and a tensile strength of 800 MPa (116 ksi) — stronger than 57% of steel grades. Elongation at break is 18% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
31CrMo12 +A is rated for continuous service to 480°C (896 °F). It conducts heat at 34 W/m·K (19.6 BTU/hr·ft·°F), worse than 77% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
31CrMo12 +A conducts electricity at 6.6 % IACS, worse than 76% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (15/100), better than 75% of steel grades.
Sustainability4
Producing a kilogram of 31CrMo12 +A takes about 34 MJ of energy and emits 2.3 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 30%.
Manufacturability7
31CrMo12 +A's machinability is good (55/100, better than 62% of steel grades); weldability fair (35/100); formability fair (40/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with 31CrMo12 +A
Is 31CrMo12 easy to machine?
In the quenched and tempered condition (~340 HB) it turns and mills normally with coated carbide at roughly 0.7–0.8 times the cutting speed used for 42CrMo4. After nitriding only grinding or honing is possible, and stock removal must stay below 0.05 mm per face to preserve the case.
Can 31CrMo12 be welded?
Weldability is poor because of the high carbon equivalent: preheat to 250–350 °C, use low-hydrogen or matching Cr-Mo consumables and temper immediately after welding below the original tempering temperature. Nitrided parts should not be welded.
Can 31CrMo12 be formed, bent or molded?
Warm or hot forge annealed material at 1100–850 °C followed by slow cooling or annealing. In the Q&T condition cold forming is not practical, and bending or stamping do not apply.
What surface finishes work on 31CrMo12?
Stress relieve at approximately 550–600 °C after rough machining to limit nitriding distortion; anodizing does not apply. Final surfaces can be black-oxided, phosphated, hard chrome plated, or left as the nitrided layer with optional post-oxidation for wear and corrosion resistance.
31CrMo12 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 | 0.28 – 0.35 | 0.31 |
| Si | ≤ 0.4 | 0.25 |
| Mn | 0.4 – 0.7 | 0.55 |
| Pimpurity limit | ≤ 0.03 | — |
| Simpurity limit; free-machining variant may be 0.020–0.040 | ≤ 0.04 | — |
| Cr | 2.8 – 3.3 | 3.05 |
| Mo | 0.3 – 0.5 | 0.4 |
| Fe | balance | — |
31CrMo12 +A — frequently asked
What is 31CrMo12 +A used for?
31CrMo12 +A is typically used for heavy machinery shafts, gears and high-wear tooling components. In short: aluminium-free nitriding steel; highest core strength here; deep hardenability, large sections.
What is the yield strength of 31CrMo12 +A?
31CrMo12 +A has a typical yield strength of 470 MPa (68.2 ksi) and a tensile strength of 800 MPa (116 ksi) — stronger than 57% of steel grades. Strength varies by condition: see the 8 listed tempers.
Is 31CrMo12 +A easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). In the quenched and tempered condition (~340 HB) it turns and mills normally with coated carbide at roughly 0.7–0.8 times the cutting speed used for 42CrMo4. After nitriding only grinding or honing is possible, and stock removal must stay below 0.05 mm per face to preserve the case.
Can 31CrMo12 +A be welded?
With care — weldability is rated fair (35/100). Weldability is poor because of the high carbon equivalent: preheat to 250–350 °C, use low-hydrogen or matching Cr-Mo consumables and temper immediately after welding below the original tempering temperature. Nitrided parts should not be welded.
What surface finishes work on 31CrMo12 +A?
Stress relieve at approximately 550–600 °C after rough machining to limit nitriding distortion; anodizing does not apply. Final surfaces can be black-oxided, phosphated, hard chrome plated, or left as the nitrided layer with optional post-oxidation for wear and corrosion resistance.
Can 31CrMo12 +A be formed, bent or molded?
Warm or hot forge annealed material at 1100–850 °C followed by slow cooling or annealing. In the Q&T condition cold forming is not practical, and bending or stamping do not apply.
What is the maximum service temperature of 31CrMo12 +A?
31CrMo12 +A is rated for continuous use to about 480°C (896 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
Sources
- EN 10085 — Nitriding steels — Technical delivery conditions — CEN (2001)
- DIN 17211 (withdrawn, historical nitriding steel data) — DIN (1987)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.4A — Steel Heat Treating Fundamentals (nitriding) — ASM International (2013)
- Nitriding steel 1.8515 / 31CrMo12 bar datasheet — Böhler / Ovako / Lucchini (typical mill data) (2022)
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.