Effect of morphology of nonmetallic inclusions on tensile properties of quenched and tempered 0.4C-Cr-Mo-Ni steel

Several 0.4C-Cr-Mo-Ni steels with different sulfur and calcium levels have been studied to determine the effect of the morphology of nonmetallic inclusions on the mechanical properties of quenched and tempered low-alloy structural steels. Stringy MnS inclusions appearing in the steel containing sulfur at a commercial level produced a remarkable anisotropy in the fracture ductility (εf) for tempers of 473 and 923 K. The fine elliptical MnS inclusions that precipitated in the desulfurized steels at low sulfur levels of 0.002 mass% had a detrimental effect on the εf for the temper of 473 K but a beneficial effect on the εf for the temper of 923 K. Fine particle inclusions that appeared in the calcium-treated steel coupled with a low sulfur content of 0.002 mass% were quite effective in improving the isotropy regarding the fracture ductility for tempers of 473 and 923 K. However, two cluster types of composite inclusions associated with the calcium-treated steel containing sulfur at a commercial level produced a detrimental effect on the isotropy regarding the εf independent of the tempering conditions. The results are described and discussed.