2.3 Preheat and Interpass Temperature
Key Takeaways
- Preheat slows cooling rate, reduces martensite, allows hydrogen diffusion, reduces residual stress, and dries surfaces
- Carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) predicts hardenability and cracking risk
- CE < 0.35 = excellent weldability; CE > 0.55 = poor weldability requiring extensive precautions
- AWS D1.1 preheat requirements are based on steel category, thickness, process, and hydrogen level
- Preheat measurement per D1.1: opposite face from heat source, 3 inches from weld edge
- Maximum interpass temperature (typically 400–600°F) prevents grain growth and loss of toughness
Preheat is the application of heat to the base metal before welding begins. Interpass temperature is the temperature of the weld area between successive passes in a multi-pass weld. Both are critical controls for preventing hydrogen-induced cracking and managing HAZ microstructure.
Why Preheat Is Applied
Preheat serves four primary purposes:
- Slows the cooling rate → reduces martensite formation → reduces cracking risk
- Allows hydrogen to diffuse out of the weld and HAZ → reduces HIC risk
- Reduces residual stresses → less thermal gradient between weld and surrounding metal
- Drives off surface moisture → reduces hydrogen introduction from the base metal surface
Carbon Equivalent (CE) Formula
The carbon equivalent is a calculated value that predicts the hardenability and cracking susceptibility of a steel. The most common formula (IIW formula) is:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
| CE Value | Weldability | Typical Action |
|---|---|---|
| < 0.35 | Excellent | Preheat usually not required |
| 0.35–0.45 | Good | Preheat may be required for thick sections |
| 0.45–0.55 | Fair | Preheat required; low-hydrogen practices essential |
| > 0.55 | Poor | High preheat + low-hydrogen + controlled procedures essential |
AWS D1.1 Preheat Requirements
AWS D1.1 Table 5.8 specifies minimum preheat and interpass temperatures based on:
- Steel category (grouped by ASTM specification)
- Thickness of the thickest part at the point of welding
- Welding process (SMAW with low-hydrogen vs. other processes)
- Hydrogen level of the consumable
Typical minimum preheat values per AWS D1.1:
| Thickness | Category A (A36, A572 Gr 50) | Category B (A588) |
|---|---|---|
| Up to 3/4" | 32°F (0°C) | 50°F (10°C) |
| 3/4" to 1-1/2" | 150°F (66°C) | 150°F (66°C) |
| 1-1/2" to 2-1/2" | 225°F (107°C) | 225°F (107°C) |
| Over 2-1/2" | 300°F (150°C) | 300°F (150°C) |
Measuring Preheat
- Temperature-indicating crayons (Tempilstik): Melt at a specific temperature; place on base metal to verify minimum preheat
- Contact pyrometer/thermometer: Direct temperature reading
- Infrared thermometer: Non-contact reading (less accurate on shiny surfaces)
Measurement location: Per AWS D1.1, preheat shall be measured on the face of the base metal opposite the side being heated, at a distance of 3 inches (75 mm) from the edge of the weld joint (for material over 3" thick, measure at 3 inches from the joint on the heated side).
Interpass Temperature
- Minimum interpass temperature = same as minimum preheat temperature (maintain throughout welding)
- Maximum interpass temperature is specified to prevent:
- Excessive grain growth in the HAZ
- Loss of mechanical properties (especially impact toughness)
- Typically 400–600°F (200–315°C) depending on the steel and application
For the Exam: Preheat measurement location is frequently tested. For AWS D1.1, measure on the opposite face from the heat source, 3 inches from the weld edge. Also know the CE formula and its relationship to cracking susceptibility.
According to AWS D1.1, where should preheat temperature be measured?
A steel with a carbon equivalent (CE) of 0.50 would be classified as having what level of weldability?
What is the PRIMARY purpose of applying preheat before welding?