WCB vs WC6 vs WC9 Valves for Elevated Temperature Steam
Technical guide · Supreme Valves India · Updated 2026-10-25
For steam and other high-temperature service, the valve body material changes as the design temperature rises: carbon steel WCB first, then chrome-moly grades WC6 and WC9, and C12A for the hottest modern power plant steam. Getting the grade right matters because the failure modes at temperature, graphitisation and creep, develop slowly and aren't caught by a hydrotest.
The grades
| Cast grade | Nominal composition | Forged equivalent | Typical use |
|---|---|---|---|
| ASTM A216 WCB | Carbon steel | A105 | General service up to about 425 °C |
| ASTM A217 WC6 | 1¼Cr-½Mo | A182 F11 | Steam in roughly the 400 to 550 °C range |
| ASTM A217 WC9 | 2¼Cr-1Mo | A182 F22 | Higher temperature main steam, better creep strength |
| ASTM A217 C12A | 9Cr-1Mo-V | A182 F91 | Supercritical and ultra-supercritical steam |
Temperature ranges are typical practice, not code limits. The governing values are the ASME B16.34 pressure-temperature tables for the valve and the allowable stresses in the piping code.
Why carbon steel stops at about 425 °C
ASME B16.34 tabulates WCB ratings up to 538 °C, but prolonged exposure above about 425 °C causes graphitisation: carbides break down into graphite nodules, which can line up in weld heat-affected zones and cause sudden failure. Piping codes and owner specifications therefore limit carbon steel for long-term high-temperature service. Its allowable stress also falls quickly with temperature, so ratings drop sharply.
What chromium and molybdenum add
- Molybdenum improves creep strength, the resistance to slow deformation under sustained stress at high temperature.
- Chromium improves oxidation resistance and stabilises carbides against graphitisation.
WC9, with more of both, holds its strength better than WC6 at the top of the range, which is why it appears on main steam lines while WC6 is common for medium-high temperature steam, reheat and auxiliary lines.
Heat treatment and welding
- WC6 and WC9 castings are normalised and tempered; the MTC should state the heat treatment.
- Welding into the line needs preheat, controlled interpass temperature, matching filler metal and post-weld heat treatment as the code requires.
- Hardness after PWHT is often checked to confirm the treatment was effective.
- Consider trim and packing: high-temperature valves use stellite-faced seats, 13Cr or stainless stems and graphite packing.
Pressure ratings at temperature
At ambient temperature WCB, WC6 and WC9 have similar ratings in the same class. The difference shows at temperature: at 450 °C and above, the chrome-moly grades retain much more of their rating than WCB. Always check the B16.34 table for the actual design temperature before selecting the class.
Quick selection guide
- Up to about 425 °C: WCB (or A105 forged).
- About 425 to 550 °C steam: WC6 (F11).
- Higher temperature main steam or where creep margin matters: WC9 (F22).
- Supercritical steam: C12A (F91).
Send design pressure, temperature and the applicable code, and we will confirm material and class for each valve.
Frequently asked questions
What is the maximum temperature for WCB valves?
ASME B16.34 lists ratings for WCB up to 538 degrees C, but carbon steel is not recommended for prolonged service above about 425 degrees C because of graphitisation, which weakens the steel over time. Above that, chrome-moly grades such as WC6 or WC9 are specified.
What is the difference between WC6 and WC9?
WC6 is 1.25 percent chromium, 0.5 percent molybdenum steel; WC9 is 2.25 percent chromium, 1 percent molybdenum. WC9 has better creep strength and oxidation resistance at the top of the range, so it is used for higher temperature main steam lines, while WC6 is common for medium-high temperature steam.
Do WC6 and WC9 valves need special welding?
Yes. Chrome-moly steels need preheat, controlled interpass temperature and post-weld heat treatment, with filler metal matching the base material, as required by the piping code such as ASME B31.1 or B31.3. This applies when butt weld or socket weld valves are welded into the line.
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