Why is preheating essential during welding of high-carbon equivalent pipeline steels?

Preheating is essential during the welding of high-carbon equivalent pipeline steels primarily to prevent hydrogen induced cracking (HIC) and to ensure adequate weldability. The carbon equivalent (CE) is a calculation that represents the hardening tendency of steel; higher CE values indicate a greater susceptibility to hardening and cracking during welding. High-carbon equivalent steels are more prone to forming a hard and brittle microstructure in the heat-affected zone (HAZ), the area surrounding the weld that experiences high temperatures during the welding process. This hard microstructure is susceptible to hydrogen cracking, also known as cold cracking. Atomic hydrogen, generated from moisture in the welding environment or from the welding electrode itself, can diffuse into the HAZ. The hydrogen then accumulates in the hard microstructure, creating internal pressure and leading to cracking. Preheating reduces the cooling rate of the weld and the HAZ, allowing more time for hydrogen to diffuse out of the metal before it can cause cracking. It also reduces the thermal stresses in the weldment, further decreasing the risk of cracking. Preheating also improves weldability by reducing the temperature gradient between the weld metal and the base metal, which promotes a more uniform microstructure and reduces the risk of incomplete fusion or lack of penetration. The specific preheating temperature required depends on the steel's carbon equivalent, the thickness of the material being welded, and the welding process being used. Proper preheating is a critical step in ensuring the integrity and reliability of welded joints in high-carbon equivalent pipeline steels.