Preheating and post-cooling procedures are important to minimize cracks, warpage, porosity, shrinkage stresses and hard zones (ref. table 1). A good preheat is very important with complex shapes and base metals of high carbon or alloy steels. Always avoid the quench effect from cold floors or drafts. Some post-cooling requires the use of a furnace.
Cracking: When the molten weld cools, it contracts, causing stress between the hot weld and the cooler base metal. This can cause cracking during or after welding. Preheating the base metal reduces the temperature differential between it and the weld metal. This reduces the susceptibility to cracking.
Porosity: If moisture is present on the base metal during hardfacing, hydrogen can be trapped in the weld metal and cause porosity as it solidifies. Preheating eliminates moisture on the base metal.
Warpage: The weld metal contracts as it cools, causing stresses with the cooler base metal; it may become warped or distorted. Reducing the temperature differential between the weld metal and the base metal by preheating will minimize warpage.
Hard Zone: A fast cooling rate during hardfacing can cause some alloy steels to harden and crack in the heat zone. Preheating slows the cooling rate and provides a more ductile microstructure.