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What this calculator does
This calculator computes minimum preheat temperature using the International Institute of Welding (IIW) carbon equivalent formula, cross-referenced against AWS D1.1 Structural Welding Code — Steel preheat and interpass temperature requirements. It accounts for base metal hardenability, section thickness, welding process, and ambient conditions.
Preheat is required when steel has sufficient hardenability to form martensite in the heat-affected zone (HAZ) during cooling. Martensite is hard and brittle — it cracks under restraint stress. Preheating slows the cooling rate, allows hydrogen to escape, and reduces HAZ hardness.
When preheat is mandatoryAWS D1.1 Table 4.5 requires preheat whenever CE exceeds approximately 0.40 on material over 1" thick, or when ambient temperature is below 32°F regardless of CE. ASME B31.3 and B31.1 pipe codes have separate preheat tables for pressure systems.
The carbon equivalent formula
The IIW carbon equivalent (CE) combines alloying elements that affect hardenability into a single number:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
- CE below 0.35 — generally weldable without preheat on thin to moderate sections at ambient temperature
- CE 0.35 to 0.45 — preheat required on heavier sections or low ambient temperatures
- CE 0.45 to 0.60 — preheat required in most cases; control heat input and interpass temperature
- CE above 0.60 — high preheat required; post-weld heat treatment (PWHT) may also be necessary
Common steel grades — CE reference
| Steel grade | Typical CE | Weldability | Notes |
|---|---|---|---|
| ASTM A36 | 0.25–0.40 | Excellent | Most common structural steel |
| ASTM A572 Gr.50 | 0.35–0.45 | Good | Preheat often needed over 3/4" |
| ASTM A514 / T-1 | 0.45–0.65 | Fair | Always preheat; low heat input required |
| 4130 Chromoly | 0.55–0.75 | Fair | Always preheat; PWHT often required |
| 4140 Chromoly | 0.65–0.90 | Difficult | High preheat (400–600°F); PWHT for pressure service |
| 1018 Mild steel | 0.20–0.30 | Excellent | Rarely needs preheat except in freezing conditions |
How to apply preheat on the job
- Heat the zone — at least 3 inches on each side of the joint, not just the weld face.
- Verify temperature — contact pyrometer or Tempilstik; infrared guns are less accurate on shiny surfaces.
- Soak time — allow preheat to soak through full thickness (roughly 2–3 minutes per inch after surface temp is reached).
- Interpass — maintain preheat between passes; do not let the joint cool below minimum preheat.
Cold weather ruleAWS D1.1 requires minimum 70°F (21°C) preheat when base metal is below 32°F (0°C), regardless of CE or thickness.
Frequently asked questions
Do I need preheat for MIG welding mild steel?
On A36 or 1018 under 3/4" thick above 32°F — usually not. Over 1" thickness, cold weather, or higher-carbon steels often require preheat. Use this calculator to confirm.
What's the difference between preheat and PWHT?
Preheat is applied before and during welding to control HAZ cooling rate. PWHT is applied after welding for stress relief — typically 1100–1200°F. This calculator covers preheat only.
Where do I find CE values for my steel?
The mill test report (MTR) lists actual chemistry for your heat of steel. Without an MTR, use maximum values from the ASTM specification — conservative but safe.