Preheating involves heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature, called the preheat temperature, prior to welding. Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source.

Why Preheat?

There are four primary reasons to utilize preheat: it slows the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistance to cracking; the slower cooling rate provides an opportunity for hydrogen that may be present to diffuse out harmlessly, reducing the potential for cracking; it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints; and it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as weld metal notch toughness.

 
When no welding code is specified, and the need for preheat has been established, how does one determine an appropriate preheat temperature?

 
The two methods outlined in Annex XI of AWS D1.1 are: heat affected zone (HAZ) hardness control and hydrogen control.

 
The HAZ hardness control method, which is restricted to fillet welds, is based on the assumption that cracking will not occur if the hardness of the HAZ is kept below some critical value. This is achieved by controlling the cooling rate. The critical cooling rate for a given hardness can be related to the carbon equivalent of the steel. From the critical cooling rate, a minimum preheat temperature can then be calculated.

 
The hydrogen control method is based on the assumption that cracking will not occur if the amount of hydrogen remaining in the joint after it has cooled down to about 120°F (50°C) does not exceed a critical value dependent on the composition of the steel and the restraint. This procedure is extremely useful for high strength, low-alloy steels that have high hardenability. However, the calculated preheat may be somewhat conservative for carbon steels. The three basic steps of the hydrogen control method are: (1) calculate the composition parameter; (2) calculate a susceptibility index as a function of the composition parameter and the filler metal diffusible hydrogen content; and (3) determine the minimum preheat temperature from the restraint level, material thickness, and susceptibility index.

 
Reference: Taking your weld’s temperature
Keep reading, happy welding
Thank you,
KP Bhatt