Why is pre-heating required in some welding processes according to the AWS D1.1 code?

Pre-heating is required in certain welding processes according to the AWS D1.1 code primarily to prevent cracking in certain materials. This requirement is particularly important when working with high-strength steels or materials that are prone to hardening or hydrogen-induced cracking.

When metal is heated during the welding process, the expansion and subsequent cooling can create stresses that lead to cracking. Pre-heating raises the overall temperature of the metal before welding begins, reducing the temperature differential between the weld and the base material. This approach helps to mitigate the risk of thermal shock and resulting stresses that could cause cracks. Furthermore, it can also assist in driving off moisture and contaminants that could lead to hydrogen absorption during the welding process, further reducing the likelihood of cracking.

Overall, the focus on preventing cracks ensures structural integrity and reliability in welded connections, adhering to safety and performance standards outlined in the AWS D1.1 code.