What is the minimum preheat for A36 steel?
A36 is a Group I steel in D1.1:2025 Table 5.6 and falls into preheat Category A or B depending on your welding process per Table 5.11. With non-low-hydrogen SMAW electrodes like E6010 or E6011 (Category A), minimum preheat is 32°F (0°C) for material up to 3/4 in, 150°F (65°C) for 3/4 to 1-1/2 in, 225°F (110°C) for 1-1/2 to 2-1/2 in, and 300°F (150°C) over 2-1/2 in. With low-hydrogen processes — E7018 SMAW, SAW, GMAW, or FCAW (Category B) — preheat requirements are lower: 32°F up to 3/4 in, 50°F (10°C) for 3/4 to 1-1/2 in, 150°F for 1-1/2 to 2-1/2 in, and 225°F over 2-1/2 in. Preheat temperature shall be checked just prior to initiating the arc for each pass, per Clause 7.6.4.
Does GMAW require preheat for structural steel?
Yes. GMAW falls under Category B in D1.1 Table 5.11, grouped with low-hydrogen SMAW, SAW, and FCAW. For common structural steels like A36, A572 Gr.50, and A992, minimum preheat with GMAW is 32°F (0°C) up to 3/4" thickness, 50°F (10°C) for 3/4"–1-1/2", 150°F (65°C) for 1-1/2"–2-1/2", and 225°F (110°C) over 2-1/2". The 32°F minimum for thin plate means no active preheat is needed above freezing — ambient shop temperature satisfies the requirement. GMAW qualifies for Category B because solid wire electrodes like ER70S-6 inherently produce low diffusible hydrogen levels, typically well below the H8 threshold of 8 mL per 100g of deposited weld metal. This low hydrogen characteristic is why GMAW receives the same favorable preheat treatment as low-hydrogen SMAW electrodes like E7018. However, GMAW is never listed under Category A because non-low-hydrogen classification does not apply to the gas metal arc welding process.
What is Category B in D1.1 preheat?
Category B in D1.1 Table 5.11 covers steels welded with low-hydrogen electrodes (E7018, E7016, etc.), submerged arc welding (SAW), gas metal arc welding (GMAW), or flux-cored arc welding (FCAW). It applies to the most commonly used structural steels in building and bridge construction: A36, A572 Gr.50, A992, A588, A709 Gr.36, A709 Gr.50, A500 Gr.B/C, A53 Gr.B, and A106 Gr.B. Category B has lower preheat requirements than Category A because these processes and electrodes produce significantly less diffusible hydrogen in the weld deposit, which reduces the risk of hydrogen-induced cracking (also called cold cracking or delayed cracking). The preheat values for Category B are: 32°F (0°C) up to 3/4", 50°F (10°C) for 3/4"–1-1/2", 150°F (65°C) for 1-1/2"–2-1/2", and 225°F (110°C) over 2-1/2". In practice, Category B is the most frequently used preheat category because most structural steel fabrication uses low-hydrogen processes.
What is the difference between preheat and interpass temperature?
In D1.1, the minimum preheat temperature is the base metal temperature required before the first arc is struck. The minimum interpass temperature is the lowest temperature the weld zone may cool to between successive passes in a multi-pass weld. Table 5.11 sets both to the same value — the listed temperature applies to both initial preheat and between-pass cooling. You cannot let the weld area cool below the listed temperature during multi-pass welding. For example, if Table 5.11 specifies 150°F for your steel and thickness, you must preheat to at least 150°F before welding and maintain at least 150°F between every pass. Preheat temperature shall be checked just prior to initiating the arc for each pass per Clause 7.6.4. Some higher-strength steels also have a maximum interpass temperature limit to prevent grain coarsening — check the WPS for any upper bound.
Do I need preheat for plate under 3/4 inch?
For most structural steels in Categories A and B (including A36, A572 Gr.50, A992, and A500 Gr.B/C), minimum preheat for material up to 3/4 in thick is 32°F (0°C) per Table 5.11 — which means ambient shop temperature satisfies the requirement as long as conditions are above freezing. Per Table 5.11 footnote (a), when base metal temperature is below 32°F, you must preheat to at least 70°F (20°C) and maintain that temperature throughout welding. This footnote applies to all categories and is the primary cold-weather preheat rule. Category C steels (such as A588 and A709 Gr.50W with certain processes) require 50°F (10°C) minimum even under 3/4 in. For Categories D through G (higher-strength steels like A913 Gr.65 or A514), the under-3/4 in preheat may be 50°F to 150°F depending on hydrogen designation. Always check your specific steel in Table 5.6 to confirm the correct category.
What does D1.1 Table 5.11 cover?
D1.1 Table 5.11 specifies minimum preheat and interpass temperatures for prequalified welding procedure specifications (WPS). It organizes approved base metals into seven categories (A through G) based on chemical composition, yield strength, and sensitivity to hydrogen-induced cracking. Each category defines preheat requirements across four thickness ranges (up to 3/4", 3/4"–1-1/2", 1-1/2"–2-1/2", and over 2-1/2") and by welding process type. Categories A and B cover common structural steels with non-low-hydrogen and low-hydrogen processes respectively. Categories C through G address higher-strength and specialty steels that require specific hydrogen-controlled consumables (H8 or H4 designations). Table 5.11 applies only to prequalified WPS under Clause 5 — procedures qualified by testing under Clause 6 may use different preheat values if supported by the qualification test results and approved by the Engineer. For steels not listed in Table 5.6 or when Table 5.11 appears overly conservative, Annex B provides an analytical alternative using actual steel chemistry from mill test reports.
What is the difference between H4 and H8 electrodes?
H4 and H8 are diffusible hydrogen designations defined by AWS A4.3 testing methods. H8 means the electrode, wire, or flux combination deposits no more than 8 mL of diffusible hydrogen per 100 g of deposited weld metal. H4 is stricter — no more than 4 mL per 100 g. These designations matter for Table 5.11 because higher-strength steels are more susceptible to hydrogen-induced cracking and require tighter hydrogen control. For example, A913 Gr.50, 60, and 65 steels in Table 5.11 qualify for Category D preheat when welded with H8 consumables. A913 Gr.80 requires H4 consumables — the most restrictive classification — reflecting its higher yield strength and greater cracking sensitivity. Common H8 consumables include E7018-H8 SMAW electrodes and most GMAW solid wires like ER70S-6. H4 consumables typically require special handling and storage procedures to maintain their low-hydrogen rating.
What is the Annex B alternative to Table 5.11?
D1.1 Annex B provides two analytical methods for determining preheat using actual steel chemistry instead of specification-based categories. The HAZ hardness control method uses CE = C + (Mn+Si)/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 for fillet welds. The hydrogen control method uses Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B for all joint types. Annex B considers the actual chemistry from your mill test report, the hydrogen level of your consumables, and the restraint of your joint — three variables that Table 5.11 does not account for. Table 5.11 is the default for prequalified WPS. Annex B is the alternative when Table 5.11 appears overly conservative or not sufficiently demanding for your specific steel chemistry. The Engineer must approve the use of Annex B per Clause B1. Calculate your CE and Pcm with our
carbon equivalent calculator.