A weld can look clean when you finish and still develop cracks later. On susceptible carbon and low-alloy steels, hydrogen, a hard microstructure and tensile stress can combine to cause hydrogen cracking. That’s one reason preheat matters before the first arc is struck.
Getting it right starts with knowing the material and following the welding procedure specification, or WPS. A quick pass with a torch and “that feels warm enough” won’t tell you whether the joint meets the required temperature.
What preheating actually does
Preheating raises the temperature of the base metal around the joint before welding. This slows the cooling rate and can limit the formation of hard, crack-sensitive structures in susceptible steels. It also gives diffusible hydrogen more time to escape during cooling.
Moisture and contamination are part of the problem, too. Clean joint surfaces and properly stored consumables remain essential. Preheat doesn’t make wet electrodes or a contaminated joint acceptable.
Does every piece of metal below 70°F need preheat?
No. There is no universal 70°F rule for every metal and every welding job.
The required temperature depends on the material, thickness, process and applicable procedure. Some joints require temperatures well above 70°F. Other applications have different requirements. Air temperature alone cannot settle the question: check the actual base-metal temperature against the job’s instructions.
A handy number on a shop reference sheet should never replace the approved WPS.
What determines the requirement?
Several factors affect the preheat decision:
- Steel chemistry and its tendency to harden
- Material thickness and heat flow away from the joint
- Joint configuration and restraint
- Hydrogen introduced by consumables or contamination
- Welding process and heat input
- Applicable code, project specifications and WPS
That’s why a temperature used successfully on one job shouldn’t automatically carry over to the next. A different grade, thickness or joint can change the requirements.
Keep preheat, interpass temperature and PWHT straight
Preheat is established before welding begins. Interpass temperature is checked before subsequent passes. The procedure may specify a minimum, a maximum, or both. More heat isn’t automatically better.
Post-weld heat treatment, or PWHT, is a separate controlled operation performed after welding when required. Preheating a joint doesn’t replace specified stress-relief treatment, and it doesn’t guarantee that distortion, residual stress or cracking will disappear.
Measure it instead of guessing
Use temperature-indicating crayons, thermocouples or another suitable instrument accepted by the procedure. Apply heat evenly and check the specified measurement locations. A hot surface directly under a torch doesn’t prove that the required section has reached a uniform temperature.
Keep checking between passes and after interruptions. Reaching the minimum once at the start doesn’t establish that the joint remains within its required temperature range throughout the weld.
Before striking the arc
Confirm the material identification and correct WPS. Check the minimum preheat, interpass limits and any required post-weld controls. Verify that the joint is clean and dry, the consumables are handled correctly, and the temperature measurement method is suitable.
If the material is unknown or the procedure is unclear, get qualified guidance before proceeding, especially on pressure equipment, structural work or critical repairs.
The practical takeaway is simple: identify the steel, follow the procedure and measure the temperature. Preheat works best as a controlled part of the welding process, rather than a number somebody remembers from another job.
Continue learning
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