Why is hardening of the weld and base metal HAZ considered important?

The hardening of the weld and base metal heat-affected zone (HAZ) is considered important primarily because of the susceptibility to hydrogen assisted cracking (also known as hydrogen-induced cracking) in carbon and low alloy steels.

When welding, hydrogen can be introduced into the weld metal and HAZ either from the atmosphere or from moisture present on the surface of the base materials. If the weld and HAZ regions become overly hardened, they can lose their ductility, making them more susceptible to cracking under stress, particularly when hydrogen is present. This is critical in ensuring the integrity and longevity of welded structures, especially in high-strength applications.

Understanding this aspect helps welding inspectors and metallurgists to assess the impact of welding on the mechanical properties of the weld and the surrounding material, thereby preventing potential failures that can arise from the combination of hardening and hydrogen exposure.