What is the minimum preheat for A36 steel?
For A36 welded with non-low-hydrogen SMAW (Category A), minimum preheat is 32°F (0°C) up to 3/4", 150°F (65°C) for 3/4"–1-1/2", 225°F (110°C) for 1-1/2"–2-1/2", and 300°F (150°C) over 2-1/2". With low-hydrogen electrodes, SAW, GMAW, or FCAW (Category B), preheat is lower: 32°F up to 3/4", 50°F for 3/4"–1-1/2", 150°F for 1-1/2"–2-1/2", 225°F over 2-1/2".
Does GMAW require preheat for structural steel?
Yes. GMAW falls under Category B in D1.1 Table 5.11 (same as low-hydrogen SMAW, SAW, and FCAW). For common structural steels like A36 and A992, minimum preheat with GMAW is 32°F (0°C) up to 3/4" thickness and 50°F (10°C) for 3/4"–1-1/2". The 32°F minimum means no preheat is needed above freezing.
What is Category B in D1.1 preheat?
Category B covers steels welded with low-hydrogen electrodes (E7018, E7016, etc.), SAW, GMAW, or FCAW. It applies to common structural steels: A36, A572 Gr.50, A992, A588, A709 Gr.50. Category B has lower preheat than Category A because low-hydrogen processes produce less diffusible hydrogen, reducing the risk of hydrogen-induced cracking.
What is the difference between preheat and interpass temperature?
In D1.1, the minimum preheat temperature is the base metal temperature before welding begins. The minimum interpass temperature is the lowest temperature the weld area may cool to between passes. Table 5.11 sets both to the same value — the listed temperature applies to both preheat and between-pass cooling. You cannot let the weld cool below the listed temperature during multi-pass welding.
Do I need preheat for plate under 3/4 inch?
For most structural steels (Category A and B) up to 3/4" thick, the minimum preheat is 32°F (0°C) — ambient temperature unless conditions are below freezing. Per Table 5.11 footnote a, when base metal temperature is below 32°F, preheat to at least 70°F (20°C) and maintain during welding. Category C steels require 50°F (10°C) minimum even under 3/4".
What does D1.1 Table 5.11 cover?
Table 5.11 specifies minimum preheat and interpass temperatures for prequalified WPS. It organizes steels into categories (A–G) based on composition and strength, then assigns temperature requirements by welding process and material thickness. Table 5.11 applies only to prequalified WPS — procedures qualified by testing under Clause 6 may use different preheat values if supported by the qualification test.
What is the difference between H4 and H8 electrodes?
H4 and H8 are diffusible hydrogen designations per AWS A4.3. H8 means the electrode or flux deposits no more than 8 mL of diffusible hydrogen per 100g of deposited weld metal. H4 is stricter — no more than 4 mL/100g. Certain high-strength steels in Table 5.11 (A913, A1066) qualify for lower preheat categories with H8 consumables. A913 Gr.80 requires H4 — the most restrictive classification.