The influence of preheat and microstructure on cold cracking for high strength and low hydrogen flux cored arc (FCA) weld metal was analyzed using Y-groove test. The weld microstructure showed a mixture of acicular ferrite, bainite, and predominantly martensite. As the preheat temperature increased, the volume fraction of acicular ferrite increased and that of martensite decreased. This result was consequently associated with a reduced cold cracking ratio. Cold cracking progressed mainly along the prior austenite grain boundary of martensite, and the fractograph of cold cracking consisted of intergranular fractures and quasi-cleavage fractures. With increasing the preheat temperature and decreasing the carbon equivalent, the fraction of intergranular fractures decreased and quasi-cleavage fractures increased. The microstructure related with intergranular fractures was martensite; quasi-cleavage fractures were associated with acicular ferrite.
Cold crack fractography identified intergranular fracture (IF) and quasi-cleavage fracture (QCF). Bainite and martensite predominated on the surface and subsurface of IF while the QCF showed inclusions that nucleated acicular ferrite. The influence of inclusions on cold cracking can be described as follows; i) The inclusions acted as nucleation sites of QCF, ii) The inclusions contributed to the nucleation and growth of micro-cracks, and iii) New cracks were produced from inclusions when a crack impinged on the inclusions. Inclusions smaller than 2 μm increased cold crack resistance by contributing to the nucleation of acicular ferrite. Inclusions larger than 2 μm increased the cold crack susceptibility by inducing crack nucleation in welds.
Mn and Ni segregation band exits in the intergranular region. And localized Mn and Ni segregation leads to the heterogeneity of microstructure in welds metal. Intergranular fracture is originated in the locally higher Mn and Ni segregation region because of higher volume expansion. The area of non segregation region generated the quasi-cleavage fracture because lower tensile stress is applied in the lower volume fraction of martensite. In higher Mn Segregation Region, Cold crack occurs in the higher volume fraction of martensite region because of high volume expansion. And high tensile stress leads to the intergranular fracture. Quasi-cleavage fracture is generated in the non-segregation region and resist the propagation of cold crack, when the intergranular crack propagates.