Steel C12-AC (Spheroidize annealed)
Heat-treated state
EN C12 · ASTM A217 Grade C12
Intermediate foundry state only — hard, brittle untempered martensite; must be heat treated before machining and pressure use.
Values for the AC condition
Other C12 tempers and conditions
C12 is listed in 4 conditions; see the grade overview at C12.
Compare all 4 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Full annealed and tempered | Choose when maximum machinability and lowest residual stress are needed on complex castings; slightly lower strength than N+T. | 440 | 630 | 24 | 11 HRC | 2.9 % IACS |
N+TDefault Normalized and tempered | Standard supply condition for A217 C12 pressure castings; best balance of creep strength, toughness and hardness for valve bodies to ~600 °C. | 470 | 660 | 22 | 200 HB | — |
Quenched and tempered | Used on heavy-section castings where accelerated cooling is needed to obtain uniform tempered martensite and the highest yield strength. | 520 | 710 | 20 | 220 HB | — |
ACThis page Spheroidize annealed | Intermediate foundry state only — hard, brittle untempered martensite; must be heat treated before machining and pressure use. | 760 | 1,050 | 6 | 38 HRC | 2.7 % IACS |
What is C12-AC?
C12-AC is C12 heat treated to AC — intermediate foundry state only — hard, brittle untempered martensite; must be heat treated before machining and pressure use. C12-AC has a yield strength of 760 MPa (110 ksi) and a tensile strength of 1,050 MPa (152 ksi) — stronger than 85% of steel grades. Elongation at break is 6% and the elastic modulus is 207 GPa (30 Msi). C12-AC has a density of 7.79 g/cm³ (0.281 lb/in³), lighter than 90% of steel grades. It melts at 1,425°C (2,597 °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 — 760 MPa (110 ksi), better than 85% of steel grades
Limitations
- Difficult to weld — 10/100, worse than 99% of steel grades
- Limited formability — 3/100, worse than 99% of steel grades
- Difficult to machine — 15/100, worse than 98% of steel grades
- Limited ductility — 6%, worse than 97% of steel grades
- Low electrical conductivity — 2.7 % IACS, worse than 97% of steel grades
C12-AC properties
Typical room-temperature values for C12-AC; 22 are specific to this condition. Other listed values are C12 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
C12-AC has a density of 7.79 g/cm³ (0.281 lb/in³), lighter than 90% of steel grades. It melts at 1,425°C (2,597 °F).
Mechanical17
C12-AC has a yield strength of 760 MPa (110 ksi) and a tensile strength of 1,050 MPa (152 ksi) — stronger than 85% of steel grades. Elongation at break is 6% and the elastic modulus is 207 GPa (30 Msi).
Thermal6
C12-AC is rated for continuous service to 350°C (662 °F). It conducts heat at 25 W/m·K (14.4 BTU/hr·ft·°F), worse than 88% of steel grades. Thermal expansion is 11.2 µm/m·K (6.22 µin/in·°F).
Electrical3
C12-AC conducts electricity at 2.7 % 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 C12-AC takes about 32 MJ of energy and emits 2.5 kg of CO₂ — more than 70% of steel grades. Typical recycled content is 75%.
Manufacturability8
C12-AC's machinability is poor (15/100, worse than 98% of steel grades); weldability not recommended (10/100); formability not recommended (3/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with C12-AC
Is C12 easy to machine?
Machine in the N+T or A+T condition at ~200 HB with rigid setups, carbide tooling and moderate speeds; work-hardening and gummy chips make sharp geometry and generous coolant important.
Can C12 be welded?
Weldable with matching E505/ER80S-B8 (9Cr-1Mo) consumables but requires 200–300 °C preheat/interpass, low-hydrogen practice and mandatory PWHT at 730–760 °C to avoid hard, crack-prone HAZ.
Can C12 be formed, bent or molded?
Supplied as sand castings — control feeding and slow post-pour cooling to prevent hot tears and quench cracks; no cold forming, and repair welding must be followed by full re-tempering.
What surface finishes work on C12?
Not anodizable; protect with phosphate + high-temperature aluminum/silicone paint, hard-facing on seats, or aluminizing for long-term oxidation service.
C12 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 |
|---|---|---|
| C0.20 max per ASTM A217 | ≤ 0.2 | 0.12 |
| Si | ≤ 1 | 0.5 |
| Mn | 0.35 – 0.65 | 0.5 |
| P | ≤ 0.04 | 0.02 |
| S | ≤ 0.05 | 0.02 |
| Cr | 8 – 10 | 9 |
| Mo | 0.9 – 1.2 | 1 |
| Fe | balance | — |
C12-AC — frequently asked
What is C12-AC used for?
C12-AC is typically used for high-temperature steam valve bodies, refinery process piping fittings, hot oil pump casings, cast elbows and tees for main steam lines, valve bonnets and yokes for high-pressure service and header connection castings. In short: 9Cr cast valve steel; cast counterpart to P9; better oxidation resistance than C5.
What is the yield strength of C12-AC?
C12-AC has a typical yield strength of 760 MPa (110 ksi) and a tensile strength of 1,050 MPa (152 ksi) — stronger than 85% of steel grades. Strength varies by condition: see the 4 listed tempers.
Is C12-AC easy to machine?
Not especially — machinability is rated poor (15/100, worse than 98% of steel grades). Machine in the N+T or A+T condition at ~200 HB with rigid setups, carbide tooling and moderate speeds; work-hardening and gummy chips make sharp geometry and generous coolant important.
Can C12-AC be welded?
Not readily — weldability is rated not recommended (10/100). Weldable with matching E505/ER80S-B8 (9Cr-1Mo) consumables but requires 200–300 °C preheat/interpass, low-hydrogen practice and mandatory PWHT at 730–760 °C to avoid hard, crack-prone HAZ.
What surface finishes work on C12-AC?
Not anodizable; protect with phosphate + high-temperature aluminum/silicone paint, hard-facing on seats, or aluminizing for long-term oxidation service.
Can C12-AC be formed, bent or molded?
Supplied as sand castings — control feeding and slow post-pour cooling to prevent hot tears and quench cracks; no cold forming, and repair welding must be followed by full re-tempering.
What is the maximum service temperature of C12-AC?
C12-AC is rated for continuous use to about 350°C (662 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between C12 N+T and C12-AC?
N+T is the default condition — standard supply condition for A217 C12 pressure castings; best balance of creep strength, toughness and hardness for valve bodies to ~600 °C. AC: intermediate foundry state only — hard, brittle untempered martensite; must be heat treated before machining and pressure use. Yield strength is 470 MPa in N+T versus 760 MPa in AC.
Sources
- ASTM A217/A217M — Steel Castings, Martensitic Stainless and Alloy, for Pressure-Containing Parts, Suitable for High-Temperature Service — ASTM International (2021)
- ASME BPVC Section II Part D — Properties (Metric), 9Cr-1Mo materials — ASME (2023)
- ASME B16.34 — Valves, Flanged, Threaded and Welding End — ASME (2020)
- ASM Handbook Vol. 1 — Properties and Selection: Irons, Steels, and High-Performance Alloys — ASM International (1990)
- Steel Castings Handbook, 6th ed. — Steel Founders' Society of America / ASM (1995)
- GB/T 16253 — (pressure-containing steel castings) — SAC (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.