How to set welding preheat and interpass temperature
AWS D1.1 sets minimum preheat and interpass temperatures by steel category and material thickness. For a low-hydrogen process on common structural steels, the minimums step from 32 °F up to 3/4 in, to 50 °F over 3/4 through 1-1/2 in, 150 °F over 1-1/2 through 2-1/2 in, and 225 °F over 2-1/2 in. Without a low-hydrogen process those same thickness bands jump to 32, 150, 225 and 300 °F.
Preheat slows the cooling rate so the heat-affected zone does not harden abruptly, and it gives diffusible hydrogen time to escape before it can cause delayed cold cracking. The carbon equivalent shown alongside the tabulated value is a sensitivity indicator, not a substitute: a high CE means the joint is more hardenable and often justifies preheating above the table minimum.
Interpass temperature has both a floor and a ceiling. The floor is the preheat temperature, held between passes. The ceiling protects toughness; AWS D1.1 caps interpass temperature at 550 °F for quenched and tempered steels unless the producer permits more, and project specifications often set tighter limits. Confirm the category for your exact steel specification and process against the published AWS D1.1 table, and have a welding engineer or Certified Welding Inspector approve the values written into the WPS.
Frequently Asked Questions
They follow the minimum preheat and interpass temperature table in AWS D1.1 (Table 5.8, formerly Table 3.2) for the common steel categories welded with and without low-hydrogen processes. Always confirm the category for your specific steel specification against the published table.
Carbon equivalent by the IIW formula is carbon plus manganese over 6, plus chromium, molybdenum and vanadium over 5, plus nickel and copper over 15. A higher value means a more hardenable heat-affected zone and greater sensitivity to hydrogen cracking, so it supports the tabulated preheat rather than replacing it.
Yes. AWS D1.1 limits the maximum interpass temperature for quenched and tempered steels to 550 degrees Fahrenheit unless the steel producer allows otherwise, and many project specifications set lower limits to protect toughness.