Heat-Affected Zone and Stainless Sensitization
The Heat-Affected Zone (HAZ) is the region of base metal adjacent to the weld that was not melted but was heated to temperatures high enough to alter its microstructure. Understanding the HAZ is critical because most weld failures initiate in the HAZ rather than the weld metal itself. In carbon steel, the HAZ extends approximately 1/4" to 3/4" from the fusion line (the boundary between melted weld metal and unmelted base metal). The portion of the HAZ closest to the fusion line (the coarse-grain HAZ) is heated above 1,800°F — at these temperatures, austenite grain growth occurs rapidly, producing large grains that have lower toughness than fine-grained base metal. The portion furthest from the fusion line (the fine-grain HAZ) is heated between approximately 1,350°F and 1,800°F — this temperature range actually refines the microstructure, improving toughness above the original base metal in some alloys. In high-strength low-alloy (HSLA) steels, the HAZ may also experience softening (a reduction in yield strength) through over-tempering of the martensite microstructure used to achieve their strength. Sensitization in stainless steel is a specific HAZ metallurgical problem critical to corrosion resistance. Standard austenitic stainless steels (304, 316) contain 18–20% chromium, which forms a passive chromium oxide film providing corrosion resistance. When heated to 800–1,500°F (the sensitization temperature range), carbon in the stainless combines with chromium to form chromium carbide precipitates at grain boundaries. This depletes the regions adjacent to grain boundaries of dissolved chromium, leaving them with less than the 10.5% chromium minimum for corrosion resistance — a sensitized joint rusts preferentially along grain boundaries in corrosive environments. Prevention: use L-grade (low carbon) stainless filler and base metal (304L, 316L — maximum 0.03% carbon vs. 0.08% for standard grades), use minimum heat input, limit interpass temperature, or specify stabilized grades (321 or 347, which add titanium or niobium to preferentially combine with carbon instead of chromium).