A Chart Method to Determine Necessary Preheat Temperature in Steel Welding.

Abstract

Preheating is carried out to avoid cold cracking in steel welding. The occurrence of cold cracking is considered to be governed by accumulating diffusible hydrogen, welding residual stresses and hardness at the crack initiation site. The hydrogen accumulating at the crack site depends on the initial diffusible hydrogen content in weld metal, the weld heat input and the wall thickness. The local residual stress is governed by the weld metal yield strength, the joint restraint and the notch stress concentration factor. The HAZ hardness is influenced by the steel chemical composition, the weld heat input and the plate (wall) thickness.These influential factors affect cold cracking independenly or in an interacted manner. It must be, thus, difficult to predict the necessary preheat temperature by a theoretical method or simple formulae. The method presently proposed is completely based on the empirical results by y-groove weld cracking tests. The present method determines the necessary preheat temperature through the charts describing the following respective effects : 1) steel composition ; 2) diffusible hydrogen content of weld metal ; 3) weld heat input ; 4) wall thickness ; 5) weld metal yield strength ; 6) joint restraint. As to the steel composition, this method uses CEN carbon equivalent that preferably assesses weldability of a wide variety of steels. Also, this method considers a logarithmic dependence of the weld metal hydrogen and the analysis of hydrogen diffusion in a weld has proved that the hydrogen effect on cold cracking must be logarithmic.