Effects of inclusion size and acicular ferrite on cold cracking for high-strength steel welds of YS 600 MPa grade

The influences of microstructure and inclusion on cold cracking were studied in high-strength steel welds of YS 600 MPa grade using the Y-groove test. The weld microstructure showed a mixture of acicular ferrite, bainite and predominantly martensite. 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 mu m increased the cold crack resistance by contributing to the nucleation of acicular ferrite. Inclusions larger than 2 mu m increased the cold crack susceptibility by inducing crack nucleation in welds.