Grygorenko S. Structural non-homogeneity of low-alloy low-carbon pipe steel joints made by the pressure welding using heating

The work is devoted to the establishment of laws of formation of structural non-homogeneities in low-carbon low-alloy pipe steel (X60-X70 class) joints made by the pressure welding using heating. The effect of environment, thermodeformational parameters of welding, preparation of contact surfaces and oxide films on the formation of structure in the zone of joints produced by the pressure welding in vacuum and in the air was studied. The ferrite band (FB) is formed in the joint line as a result of decarburization of metal layers adjacent to the surface being welded. This decarburization is due to the presence of surface oxide films and thermodynamic instability of a pearlitic cementite in the fields of high inner stresses. The FB, forming after FBW, is located in the center of a light band observed on macrosections as a result of an optical effect. In the zone, which has a light etchability on the sections, the carbon content is reduced by 30 - 40 %. The mechanism of FB formation was established as a combination of action of diffusion and recrystallizing processes. The relation between the FB sizes and intensity of carbon mass transfer in the contact zone is shown. When the carbon-containing elements are introduced into the butt before welding , the FB is not formed. The examination of samples, subjected to impact tests, showed that the fracture is always occurred beyond the FB.