Study of cold cracking susceptibility of CO₂ gas shielded arc welded SA106C cast steel using implant tests

Implant tests were performed on the SA106C cast steel welded using CO₂ gas shielded arc welding,which intended to investigate the susceptibility of cold cracking (CR) and the fracture morphology.The results show that in the case of heat input being 16 kJ/cm and no preheating,the critical fracture stress of implant is 560 MPa,near to the value of tensile strength,consequently,its resistance to CR is so high that it can be welded without preheating.If the restraint stress is higer than tensile strength,the fracture will occur.The features of the fracture is typical hydrogen-induced crack (HIC).The rupture mode of the fracture is found to consist of sugar like intergrannular (IG) and quasi-cleavage (QC_HE),some sharp,perfectly straight,short sub-crack can be also observed additionally.With the crack propagating,the content of hydrogen is cut down,as a result,the proportion of IG decrease,wheras those of QC_HE and sub-crack increase signifcantly,and more plastic deformation appears clue in QC_HE.The reason for high resistance to CR of SA106C cast steel is mainly due to the lower content of hydrogen of CO₂ arc welding process,and also related to the microstructures and as-quenched hadness of over heated zone near the weld interface,which is largely bainite with hardness of 321 HV,less than critical hardness of 350 HV.