High strength and abrasion resistant steel can sometimes be welded with the steel at room temperature without the risk of hydrogen cracking, particularly for thin plates. However, when single plate thickness is 0.75" or above, some preheating above room temperature is generally required. Most mills provide detailed preheating recommendations for their products. When comparing recommendations, be sure to check the heat input. If the heat input is 25-30 KJ/in (0.9-1.1 KJ/mm), for example, the preheat temperature would typically be higher than if the heat input was 50-70 KJ/in. (1.9-2.7 KJ/mm), sometimes by as much as 100C (210F). When quenched and tempered steels are welded, a soft zone forms near the fusion limit. The width of the soft zone varies with heat input. If the heat input remains low, the soft zone should not affect the strength of the welded joint. For thicknesses below 0.375" (9.5mm), heat input should generally not exceed 28 KJ/in. (1.1 KJ/mm).
Preheat and interpass temperatures should be maintained during welding by using a gas burner, heating blankets or, when the temperature is reached, by using the welding heat itself. Temperatures should be measured on both sides of the plate (for example, by using temperature crayons or heat guns). Under conditions of severe restraint, for tack welding, or in damp weather conditions, preheat temperatures would typically be increased.