Frequently Asked Questions
Preheating is the process applied to raise the temperature of the parent steel before welding. It is used for the following main reasons:
- The slower cooling rate encourages hydrogen diffusion from the weld area by extending the time period over which it is at elevated temperature (particularly the time at temperatures above approximately 100°C) at which temperatures hydrogen diffusion rates are significantly higher than at ambient temperature. The reduction in hydrogen reduces the risk of cracking.
- To slow the cooling rate of the weld and the base material, potentially resulting in softer weld metal and heat affected zone microstructures with a greater resistance to fabrication hydrogen cracking.
Preheat can be applied through various means. The choice of method of applying preheat will depend on the material thickness, weldment size and the heating equipment available at the time of welding. The methods can include furnace heating for small production assemblies or, for large structural components, arrays of torches, electrical strip heaters, induction heaters or radiation heaters.
It is important to apply preheat correctly, with appropriate monitors and controls, and also to monitor the interpass temperature (the temperature of the workpiece between welding the first and subsequent passes), to ensure that it does not fall below the preheat temperature. (See FAQ: Which is important - Preheat or interpass?).
Common techniques for monitoring preheat are temperature indicating crayons (see FAQ: What is a Tempil stick?) and thermocouples or contact thermometers. Preheat should be monitored at a distance of 4t (where t is the thickness of the material to be joined) away from the longitudinal edge of the groove for t<50mm  or at a minimum distance of 75mm from the joint preparation for t>50mm and on the reverse side of the plate to the heat source [1,2]. Further details of preheat application and reporting are given in reference 1.
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- BS EN ISO 13916:1997: 'Welding: Guidance on the measurement of preheating temperature, interpass [interrun] temperature and preheat maintenance temperature', British Standards Institution, 1997.
- BS EN 1011-2: 2001: 'Welding. Recommendations for welding of metallic materials. Arc welding of ferritic steels', British Standards Institution, 2001.