Steel C12A-AC (Spheroidize annealed)
Heat-treated state
ASTM A217 Grade C12A
Reference only: the raw casting is hard, brittle and crack-prone and must never be shipped or welded in this state.
Values for the AC condition
Other C12A tempers and conditions
C12A is listed in 6 conditions; see the grade overview at C12A.
Compare all 6 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed | Intermediate softening condition only — castings must be re-normalized and tempered before service. | 360 | 560 | 26 | 9 HRC | 3 % IACS |
N+T high temper | Choose when weld-repair ductility, toughness or a ≤210 HB hardness cap is more important than peak strength. | 435 | 610 | 24 | 13 HRC | — |
N+T plus post-weld heat treatment 750–770 °C | Condition of production/repair-welded castings after mandatory PWHT; slightly softer HAZ than the base N+T state. | 450 | 630 | 21 | 17 HRC | 2.9 % IACS |
N+TDefault Normalized and tempered | Standard mandatory ASTM A217 delivery condition for all C12A castings in creep service. | 470 | 650 | 21 | 19 HRC | — |
Quenched and tempered | Used on very heavy valve bodies where air cooling would not fully transform the core to martensite. | 520 | 700 | 19 | 22 HRC | — |
ACThis page Spheroidize annealed | Reference only: the raw casting is hard, brittle and crack-prone and must never be shipped or welded in this state. | 800 | 1,150 | 5 | 43 HRC | 2.6 % IACS |
What is C12A-AC?
C12A-AC is C12A heat treated to AC — reference only: the raw casting is hard, brittle and crack-prone and must never be shipped or welded in this state. C12A-AC has a yield strength of 800 MPa (116 ksi) and a tensile strength of 1,150 MPa (167 ksi) — stronger than 86% of steel grades. Elongation at break is 5% and the elastic modulus is 216 GPa (31.3 Msi). C12A-AC has a density of 7.77 g/cm³ (0.281 lb/in³), lighter than 92% of steel grades. It melts at 1,420°C (2,588 °F). N+T is the default condition FabDigit quotes; AC is available on request or by drawing note.
Advantages
- Good corrosion resistance — 30/100, better than 95% of steel grades
- High strength — 800 MPa (116 ksi), better than 86% of steel grades
Limitations
- Limited formability — 2/100, worse than 100% of steel grades
- Limited ductility — 5%, worse than 98% of steel grades
- Difficult to machine — 15/100, worse than 98% of steel grades
- Low electrical conductivity — 2.6 % IACS, worse than 97% of steel grades
- Difficult to weld — 12/100, worse than 96% of steel grades
C12A-AC properties
Typical room-temperature values for C12A-AC; 22 are specific to this condition. Other listed values are C12A reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
C12A-AC has a density of 7.77 g/cm³ (0.281 lb/in³), lighter than 92% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
C12A-AC has a yield strength of 800 MPa (116 ksi) and a tensile strength of 1,150 MPa (167 ksi) — stronger than 86% of steel grades. Elongation at break is 5% and the elastic modulus is 216 GPa (31.3 Msi).
Thermal6
C12A-AC is rated for continuous service to 200°C (392 °F). It conducts heat at 24 W/m·K (13.9 BTU/hr·ft·°F), worse than 90% of steel grades. Thermal expansion is 10.8 µm/m·K (6 µin/in·°F).
Electrical3
C12A-AC conducts electricity at 2.6 % IACS, worse than 97% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (30/100), better than 95% of steel grades.
Sustainability4
Producing a kilogram of C12A-AC takes about 33 MJ of energy and emits 2.4 kg of CO₂ — more than 76% of steel grades. Typical recycled content is 80%.
Manufacturability7
C12A-AC's machinability is poor (15/100, worse than 98% of steel grades); weldability not recommended (12/100); formability not recommended (2/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with C12A-AC
Is C12A easy to machine?
Machine in the tempered condition with rigid setups, carbide inserts and moderate speed (≈60–100 m/min); the martensitic matrix work-hardens and scale from heat treatment is very abrasive, so take the first cut under the scale.
Can C12A be welded?
Weld with matching 9Cr-1Mo-V filler (AWS E9015-B9 / ER90S-B9), 200–250 °C preheat, ≤300 °C interpass, cool to 80–120 °C to complete martensite transformation, then PWHT 750–770 °C — never leave the joint untempered.
Can C12A be formed, bent or molded?
Cast to shape only; use generous risers and chills for the long freezing range, control nitrogen and avoid hot tearing in section changes, and always re-normalize + temper after any upgrading weld.
What surface finishes work on C12A?
Shot blast to remove heat-treat scale; parts are painted or aluminized for external protection, and no anodizing applies — internal steam surfaces rely on the Cr oxide formed in service.
C12A 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 | 0.08 – 0.12 | 0.1 |
| MnMn+Ni often limited to ≤1.0 % to keep Ac1 high | 0.3 – 0.6 | 0.45 |
| Si | 0.2 – 0.5 | 0.35 |
| P | ≤ 0.02 | 0.01 |
| S | ≤ 0.01 | 0.01 |
| Cr | 8 – 9.5 | 8.8 |
| Mo | 0.85 – 1.05 | 0.95 |
| VMX carbonitride former | 0.18 – 0.25 | 0.21 |
| Nblisted as Cb in ASTM A217 | 0.06 – 0.1 | 0.08 |
| Nessential for VN precipitation strengthening | 0.03 – 0.07 | 0.05 |
| Ni | ≤ 0.4 | 0.2 |
| Almax; low Al preserves free N | ≤ 0.02 | 0.01 |
| Ti | ≤ 0.01 | 0 |
| Zr | ≤ 0.01 | 0 |
| Fe | balance | — |
C12A-AC — frequently asked
What is C12A-AC used for?
C12A-AC is typically used for ultra-supercritical steam valve bodies, turbine casings and high-temperature power plant castings. In short: cast P91 equivalent; highest creep strength here; ultra-supercritical steam grade.
What is the yield strength of C12A-AC?
C12A-AC has a typical yield strength of 800 MPa (116 ksi) and a tensile strength of 1,150 MPa (167 ksi) — stronger than 86% of steel grades. Strength varies by condition: see the 6 listed tempers.
Is C12A-AC easy to machine?
Not especially — machinability is rated poor (15/100, worse than 98% of steel grades). Machine in the tempered condition with rigid setups, carbide inserts and moderate speed (≈60–100 m/min); the martensitic matrix work-hardens and scale from heat treatment is very abrasive, so take the first cut under the scale.
Can C12A-AC be welded?
Not readily — weldability is rated not recommended (12/100). Weld with matching 9Cr-1Mo-V filler (AWS E9015-B9 / ER90S-B9), 200–250 °C preheat, ≤300 °C interpass, cool to 80–120 °C to complete martensite transformation, then PWHT 750–770 °C — never leave the joint untempered.
What surface finishes work on C12A-AC?
Shot blast to remove heat-treat scale; parts are painted or aluminized for external protection, and no anodizing applies — internal steam surfaces rely on the Cr oxide formed in service.
Can C12A-AC be formed, bent or molded?
Cast to shape only; use generous risers and chills for the long freezing range, control nitrogen and avoid hot tearing in section changes, and always re-normalize + temper after any upgrading weld.
What is the maximum service temperature of C12A-AC?
C12A-AC is rated for continuous use to about 200°C (392 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between C12A N+T and C12A-AC?
N+T is the default condition — standard mandatory ASTM A217 delivery condition for all C12A castings in creep service. AC: reference only: the raw casting is hard, brittle and crack-prone and must never be shipped or welded in this state. Yield strength is 470 MPa in N+T versus 800 MPa in AC.
Sources
- ASTM A217/A217M — Steel Castings, Martensitic Stainless and Alloy, for Pressure-Containing Parts at High Temperature — ASTM International (2021)
- ASME BPVC Section II Part D — Properties (Materials) — ASME (2023)
- ASM Handbook Vol. 1: Properties and Selection — Irons, Steels, and High-Performance Alloys — ASM International (1990)
- The Grade 91 Steel Handbook / EPRI Guidelines for Grade 91 Fabrication and Service — EPRI (2015)
- GB/T 16253 / DL/T (ZG1Cr9Mo1VNbN) (2019)
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.