Preheating is the process involves heating the surrounding base metal near the joint to be welded, to a specific desired temperature prior to the welding or thermal cutting operation.
Purpose of Preheat:
1) It raises the overall temperature of the material, which results in a slower cooling rate of the base materials, HAZ and the weld to prevent steel from cracking (by reducing the hardness) those are susceptible to formation of brittle microstructure (martensite) if cool fast.
2) The slower cooling rate provides sufficient time to diffuse out the hydrogen from the weld metal without causing cracking.
3) It helps to reduce the shrinkage stresses that can lead to cracking and distortion especially in case of highly restrained joints.
4) Preheating introduces the necessary heat into the weld area to ensure proper penetration. This benefits thick materials and those that conduct heat quickly. By preheating, you can use less heat in the welding arc and still achieve optimal penetration, because the base material starts out at an elevated temperature. And preheating also helps in thermal cutting of high thickness material.
When Preheat is required?
Preheating is especially important when welding:
- Highly restrained weld joints.
- Base materials having high carbon equivalent
- High hydrogen content of weld metal.
- Thick materials (preheat varies by material type).
- Base materials that tend to be more brittle, such as cast iron.
How to measure Preheat:
Preheat measuring often is done with Temperature indicating stick (tempilstik) and infrared digital thermometers. Generally, the preheat temperature should be measured for a distance at least equal to the thickness of the thickest welded part (but not less than 3 in [75 mm]) in all directions from the point of welding and be preferably measured on the opposite face to the one being welded.. The preheat temperature should be verified directly before welding begins.
Preheat Requirements as per various codes and standards: