Welding Preheat Calculator

Estimate a first-pass preheat planning range from material composition, thickness, and ambient conditions. Uses the IIW carbon-equivalent formula as a simplified screening input.

Input Parameters

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Higher CE = higher crack risk.

Thicker sections require higher preheat due to faster cooling rates.

Cold ambient temperatures increase preheat requirements.

Enter parameters and click Calculate to see results

Welding Preheat Calculator Quick Answer

Preheat is used to slow cooling, reduce hard HAZ formation risk, and manage hydrogen or restraint-related cracking risk in susceptible steels and cast irons. Start with carbon equivalent, thickness, ambient temperature, and restraint, then confirm the value against the qualified welding procedure or responsible engineering requirement.

Understanding Preheat Requirements

Preheat Temperature Effect on Thermal Gradients

Illustrative thermal comparison

Thermal Gradient ComparisonAmbient Start (20°C)1500°C900°C400°CHigher gradientGradient: ~1500°C/cmCrack-risk indicationPreheat (250°C)1500°C900°C500°CLower gradientGradient: ~600°C/cmLower gradient examplePreheat by Carbon Equivalent:CE<0.3: low0.3-0.4: 100-150°C0.4-0.6: 200-300°C0.6-0.8: 300-400°CCE>0.8: 400-500°CMild Steel (0.2-0.3)Low routine preheatMed. Carbon (0.4-0.5)150-250°CHigh Carbon (0.6-0.8)300-400°CCast Iron (0.8+)400-500°C + PWHTPreheat can slow cooling, reduce hardness risk, and lower residual-stress gradients

Engineering note: CE > 0.6 usually warrants preheat and PWHT consideration because rapid cooling can increase hard HAZ and cracking risk.

Why Preheat?

Preheating can reduce the cooling rate of the weld, lowering the likelihood of brittle martensite in the heat-affected zone (HAZ). It can also reduce thermal gradients and residual stresses that can lead to cold cracking.

Carbon Equivalent (CE)

CE is a formula to assess hardenability and crack susceptibility of steel. Common formula:

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
  • CE < 0.35: low screening band, often minimal routine preheat
  • CE 0.35-0.50: moderate screening band, preheat often falls around 50-150°C
  • CE 0.50-0.70: elevated screening band, preheat often falls around 150-300°C
  • CE > 0.70: very elevated screening band, preheat and PWHT often needed

Temperature Verification

  • Temperature indicating crayons/sticks (Tempilstiks)
  • Contact pyrometers (thermocouple)
  • Infrared thermometers (check emissivity setting)

Preheat Zone

Many procedures heat a band around the joint, often on the order of 75 mm (3 inches) on each side. For thick sections, check whether broader part heating is needed to manage thermal gradients.

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