What Is ASTM A217 C5?
When to Specify A217 C5
ASTM A217 Grade C5 is a 5 chromium, 1/2 molybdenum cast alloy steel for valve bodies and pressure parts operating up to 650°C (1200°F) in high-temperature, sulfur-bearing environments. The 5% chromium content provides substantially better sulfidation resistance than WC6 (1.25Cr) and WC9 (2.25Cr), making C5 the standard cast material for refinery heater and high-sulfur crude processing piping.
Sulfidation Resistance by Chrome Content
High-temperature sulfidation (reaction of sulfur compounds with steel above 260°C) is a primary corrosion mechanism in refinery service. Chromium forms a protective chromium sulfide layer that reduces metal loss. The Modified McConomy curves quantify sulfidation rates by alloy:
| Grade | Cr Content | Sulfidation Rate (relative) | Max Service Temp |
|---|---|---|---|
| WCB (carbon) | 0.30% max | 1.0x (baseline) | 425°C |
| WC6 | 1.00-1.50% | 0.5x | 565°C |
| WC9 | 2.00-2.75% | 0.3x | 595°C |
| C5 | 4.00-6.50% | 0.1x | 650°C |
| C12 | 8.00-10.0% | 0.05x | 650°C |
C5 reduces sulfidation rates by approximately 90% compared to carbon steel, a critical advantage in heater transfer lines and sulfur recovery units.
Chemical Composition and Properties
| Property | Value |
|---|---|
| Specification | ASTM A217 / ASME SA-217 |
| Grade | C5 |
| Carbon | 0.20% max |
| Chromium | 4.00-6.50% |
| Molybdenum | 0.45-0.65% |
| Manganese | 0.40-0.70% |
| Silicon | 0.75% max |
| Tensile strength (min) | 485 MPa (70 ksi) |
| Yield strength (min) | 275 MPa (40 ksi) |
| Elongation (min) | 20% |
| Reduction of area (min) | 35% |
| Hardness | 241 HBW max |
| Heat treatment | Normalize + temper, or quench + temper |
| Forged equivalent | A182 F5 |
| Matching pipe | A335 P5 |
Applications
- Refinery heaters: crude heater, vacuum heater, and coker heater transfer line valves
- Sulfur recovery units: Claus reactor outlet valves and tail gas piping
- High-sulfur crude processing: atmospheric and vacuum column overhead valves
- Catalytic reformers: reactor circuit valves operating in H2/H2S mixtures
C5 cast valve bodies pair with A182 F5 forged flanges and fittings, A234 WP5 buttweld fittings, and A335 P5 seamless pipe. Mixing chrome grades within a single piping class requires dissimilar-metal weld procedures and is generally avoided.
Quality and Testing
All C5 castings require heat treatment (normalize + temper or quench + temper). Mill test certificates per EN 10204 3.1 document chemistry, mechanicals, and heat treatment condition. Radiographic examination is standard for valve castings per ASME B16.34. Hydrostatic testing follows the applicable valve standard. The casting process requires foundry expertise in managing the higher chromium content to minimize segregation and hot cracking.
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