Direct Answer
Preheat is the application of heat to the base metal before welding to slow the cooling rate of the weld joint, which reduces the risk of hydrogen-assisted cracking, lamellar tearing, and brittle fracture in the heat-affected zone. AWS D1.1 Table 4.5 specifies minimum preheat temperatures based on steel grade, base metal thickness, and diffusible hydrogen content of the filler metal.
Full Explanation
Preheat temperatures increase with: higher-strength steel (A572 vs. A36), greater base metal thickness (thicker steel cools faster), and higher carbon equivalent of the steel chemistry. Typical requirements: 1/8" to 3/4" A36 plate — no preheat required for most electrodes. 3/4" to 1.5" A572 Gr 50 — 50°F minimum. Over 2.5" A572 — 150–200°F minimum. High-restraint conditions (thick base plates, heavy column connections) always require preheat even at lower thicknesses. Interpass temperature (maximum temperature between weld passes) is also specified — typically 550°F maximum.
What This Means for Your Shop
Preheat compliance is verified during special inspection. A special inspector who measures base metal temperature below the specified minimum has the authority to stop welding — a halt that disrupts your production schedule and requires time to re-heat and document.
Common Mistakes
Preheating only the immediate weld area rather than the full volume specified by AWS D1.1 (typically 3" in all directions from the weld in thick material). A narrow preheat area loses heat quickly and may not maintain minimum temperature during the weld.
Recommended Process
Document preheat requirements for each weld condition in your WPS. Train welders to measure base metal temperature with a calibrated temperature stick before striking an arc. For thick-material connections, use oven preheat (propane torch array) rather than point heating.