D1.1 Preheat Requirements

Per D1.1 Table 5.8, A36 is Category B. For thicknesses over 3/4" through 1-1/2" using SMAW with low-hydrogen electrodes (or FCAW/GMAW/SAW), the minimum preheat and interpass temperature is 150°F (66°C). Without low-hydrogen electrodes, Category B requires 225°F at that thickness.

Category C includes higher-strength steels such as ASTM A514, A517, and A709 Grade HPS 100W. These require the highest preheat levels — 300°F minimum for thicknesses over 3/4" through 1-1/2", and up to 400°F for thicknesses over 2-1/2", per D1.1 Table 5.8.

Yes. D1.1 Table 5.8 includes separate columns for SMAW with low-hydrogen electrodes and SMAW with non-low-hydrogen electrodes. Using low-hydrogen electrodes (e.g., E7018) can reduce the required preheat by 50–100°F depending on the category and thickness range.

D1.1 does not specify a maximum interpass temperature for most carbon steels — it only sets minimums. However, for quenched and tempered steels (Category C), the maximum interpass temperature is 400°F (204°C) per Table 5.8, Note b, to avoid degrading the heat treatment.

For Category A steels (such as A36 and A53) at thicknesses of 3/4" or less, D1.1 Table 5.8 lists 32°F (0°C) as the minimum preheat — effectively ambient temperature in most conditions. However, Clause 5.6.1 still requires that the base metal be above 32°F before welding.