Preheat temperature charts by material and thickness. Interpass temperature control, post-weld stress relief procedures, PWHT requirements, and methods for verifying temperature.
Preheat and post-weld heat treatment (PWHT) control the thermal cycle that determines whether a weld cracks or survives. Preheat slows the cooling rate going into the weld. PWHT removes residual stress and tempers hard microstructures after welding. Skip either one when it’s needed and you’ll find cracks, sometimes immediately, sometimes months later under load.
Preheat works by raising the base metal temperature before welding so the weld zone cools more slowly after the arc passes. Slower cooling prevents martensite formation in the HAZ, gives hydrogen time to diffuse out of the joint, and reduces thermal shock. The required temperature depends on three factors: carbon equivalent of the base metal, material thickness, and hydrogen level of the consumable.
For mild steel (A36, A572), preheat is usually needed above 1 inch thick, starting at 150F and increasing with thickness. Medium-carbon steels (1045, 4140) need 400-600F. Chromoly (4130) needs 300-400F on thick sections. High-carbon and tool steels need 800-1200F. AWS D1.1 Table 3.2 lists specific minimums by steel group and thickness.
Interpass temperature is the maximum temperature the joint reaches before you start the next pass. Too hot between passes causes grain growth, sensitization in stainless, and loss of toughness in quenched-and-tempered steels. For most structural carbon steel, keep interpass under 600F. For stainless, under 350F. For quenched-and-tempered plate (T-1, AR400), under 400F.
PWHT (stress relief) heats the completed weldment to a temperature below the lower critical point, holds it there, and slow-cools. For carbon steel, the standard cycle is 1100-1200F, held for 1 hour per inch of thickness, with controlled heating (400F/hour) and cooling (500F/hour) rates. This reduces residual stress from about 90% of yield down to about 10-20%.
Temperature verification uses contact pyrometers, temperature-indicating crayons (Tempilstiq), or thermocouples for furnace PWHT. The guides below cover preheat charts by material, PWHT procedures, and practical heating methods for shop and field work.
PWHT guide for welders. When post-weld heat treatment is required, temperature and hold time by material, heating and cooling rates, local vs furnace PWHT, and thermocouple placement.
Preheat temperature chart for welding. Minimum preheat by material and thickness, AWS D1.1 Table 3.3, ASME B31.3, measurement methods, and preheat maintenance.
Tack weld placement, size guidelines, spacing by material thickness, removal vs incorporation, AWS D1.1 tack requirements, and sequence for distortion control.