Metallurgy & Corrosion Design Guide
Killed Steel & PWHT
Chapter from the Metallurgy & Corrosion Design Guide.
KILLED CARBON STEEL & PWHT REQUIREMENTS
Killed vs. Semi-Killed Steel
Killed Steel: fully deoxidized with silicon and/or aluminum during steelmaking
• Si killed: Si content 0.15-0.30% — deoxidized with silicon
• Al killed: Al content 0.015-0.060% — deoxidized with aluminum (finer grain)
• Si-Al killed: both deoxidizers — preferred for critical service
Why It Matters:
• Killed steel has fewer internal voids, porosity, and non-metallic inclusions
• Better resistance to HIC (fewer initiation sites for hydrogen blistering)
• More uniform mechanical properties
• Required by most refinery specs for sour service, H2 service, and pressure vessels
Semi-Killed or Rimmed Steel:
• NOT acceptable for sour service, H2 service, or pressure vessels per ASME VIII
• Contains porosity and segregation — susceptible to HIC
• May be found in older piping or structural steel — verify during turnaround inspections
PWHT Requirements
| Service | PWHT Required? | Temperature | Hold Time | Basis | Cooling Rate | Notes |
|---|---|---|---|---|---|---|
| Wet H2S (sour) | Yes — mandatory | 1,100-1,150°F | 1 hr/inch, 1 hr min | NACE SP0472, API 945 | ≤400°F/hr (max) | Reduces residual stress to prevent SSC |
| Amine service | Yes — mandatory | 1,100-1,150°F | 1 hr/inch, 1 hr min | API 945, NACE RP0170 | ≤400°F/hr | All welds including socket welds |
| Caustic (>50% or >140°F) | Yes — mandatory | 1,100-1,150°F | 1 hr/inch, 1 hr min | NACE RP0403 | ≤400°F/hr | Caustic SCC prevention |
| HF acid service | Yes — mandatory | 1,100-1,150°F | 1 hr/inch, 1 hr min | NACE 5A171 | ≤400°F/hr | Strict CS composition also required |
| H2 service (>400°F) | Yes — recommended | 1,100-1,200°F | 1 hr/inch | API RP 934-A | Controlled | For Cr-Mo steels: mandatory |
| Carbon steel >1.25" thick | Yes — ASME code | Per ASME VIII Table UCS-56 | Per code | ASME VIII Div.1 | Per code | Code-mandated for thick sections |
| General CS piping | Per P-number | Per ASME B31.3 | Per code | ASME B31.3 Table 331.1.1 | Per code | Check code exemptions |
Source: Metallurgy_Corrosion_Design_Guide_v1.xlsx · sheet “Killed Steel & PWHT”
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