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Weld Preheating Low Alloy Steels

Weld Preheating
Low alloy steels are defined as consisting of less than 10.5% Ni, Cr, Mo, and other alloy elements. In general, low alloy steels are required to be preheated to some temperature (TPH), prior to welding. It has been suggested that TPH for any given steel should be about 50 F above the martensite start temperature (MS) for the particular steel being welded. Most low alloy steels, however, have fairly high MS temperatures, making welding at or above them somewhat uncomfortable to the welder, thereby potentially compromising weld quality. For such steels, therefore, manufacturers often opt for TPH temperatures below MS. A case in point is AISI 4130 with an MS of 700 F;  For this steel, federal, military, industry and company specifications typically list TPH temperatures in the 200-600 F range, all below MS.

Why Preheat?

Preheating drives moisture and other contaminants off the joint; moisture, lubricants and other contaminants are sources of hydrogen. More importantly, preheating serves to reduce the rate at which the metal cools down from the welding temperature to TPH. This is so whether preheating is above or below MS. Cooling rate reductions will lead to a general reduction in residual stress magnitudes, and also allow more time for hydrogen removal.

Most low alloy steels that may be susceptible to hydrogen-induced cracking transform from austenite during cooling through the 800-500 C (1470-930 F) temperature range. The length of time a steel spends in this range during cooling, will establish its microstructure and, hence, its susceptibility to cold cracking. To maximize cracking resistance, a microstructure that is free of untempered martensite is desired; that is, the austenite would have transformed to ferrite + carbide and no austenite will be available to transform to martensite upon reaching MS.

For more information about preheating low alloy steels, contact Hotfoil-EHS at 609-588-0900 or by visiting their web site at https://hotfoil-ehs.com.
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