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 (T
PH), prior to welding. It has been suggested that T
PH for any given steel should be about 50 F above the martensite start temperature (M
S) for the particular steel being welded. Most low alloy steels, however, have fairly high M
S 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 T
PH temperatures below M
S. A case in point is AISI 4130 with an M
S of 700 F; For this steel, federal, military, industry and company specifications typically list T
PH temperatures in the 200-600 F range, all below M
S.
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 T
PH. This is so whether preheating is above or below M
S. 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 M
S.
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.