The purpose of preheating prior and during the welding process is to assist with hydrogen diffusion from the weld and heat-affected zone (HAZ). It also slows down the cooling rate from the welding, this is necessary to avoid cracking. The need for preheat is determined by the standards and the requirements increase with thicker wall thickness, and carbon / alloy content of the steel.
RCR Heat Treatment has various methods for carrying out pre-heat process. The most common is by resistance heating using low voltage ceramic heating elements.
To avoid overheating, thermocouple and heater placement are vital during the set-up, especially when working with P91 and other CSEF steel.