Hot ductility behaviour of 18%Ni maraging steel

Abstract

The hot ductility behaviour of a material exhibits a good correlation with its heat affected zone (HAZ) hot cracking sensitivity. This study was aimed at evaluating the HAZ hot cracking susceptibility of 18%Ni maraging steel and exploring the relationship between hot ductility and microstructure of the HAZ in an 18%Ni maraging steel. Optical metallography and scanning electron microscopy (SEM) investigations revealed that the ductility loss was accompanied by fracture transition from ductile transgranular mode to brittle intergranular mode. Fracture specimens were examined using SEM with an attached energy dispersive X-ray spectrometer. The SEM fractographs indicate that the ductile transgranular dimple fracture (i.e. coarse) surface was the titanium rich inclusion precipitated at the grain boundary, which promotes microvoid nucleation, growth, and coalescence and leads to localisation deformation until final fracture. Results of hot ductility tests also indicate that the titanium rich inclusion can reduce the ultimate tensile strength without reducing the tensile ductility. The appearance of brittle intergranular fracture is caused by the constitutional liquation of titanium sulphide inclusions that induces formation of a liquid film penetrating the grain boundaries. The results show that titanium sulphide inclusion not only reduces the HAZ tensile ductility, but also reduces the ultimate tensile strength.