Pre / Post Heating Service Provider In Ahmedabad, Gujarat, India.

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Pre / Post Heating

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.

Post-heating refers to the maintenance of preheat after the weld has been completed, to allow increased rates of hydrogen evolution from the weld to occur. The post-heat temperature may be the same as, or greater than, the original preheat temperature specified. Post-heating is different to post weld heat treatment.

Post-heat is not usually mandatory in standards, but can be specified by clients. There is some guidance (approximate temperature and length of time required) in standards but this is non-specific and further advice may need to be taken. A post-heat requirement in a weld procedure usually implies the need for preheat.

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