EFFECT OF MORPHOLOGY OF NONMETALLIC INCLUSIONS ON FRACTURE-TOUGHNESS OF QUENCHED AND LIGHT-TEMPERED 0.4-C-CR-MO-NI STEELS

Several 0.4 C-Cr-Mo-Ni steels with different sulfur and calcium contents have been studied to determine the effect of the morphology of nonmetallic inclusions on the fracture toughness (K-IC) of quenched and light-tempered low-alloy structural steels. Stringy MnS inclusions existing in the steel containing sulfur at the commercial level led to a remarkable anisotropy regarding K-IC independent of test temperatures of 193 and 293 K. The fine elliptical MnS inclusions that precipitated in the desulfurized steel having a low sulfur content of 0.002 mass% had a detrimental effect on K-IC independent of testing orientation and temperature; however, when the steel was desulfurized to a very low sulfur content (0.0008 mass%), a beneficial effect was found regarding that property. Fine particle inclusions appearing in the calcium-treated steel coupled with a low sulfur content (0.002 mass%) were beneficial for K-IC independent of testing orientation and temperature; whereas two cluster types of composite inclusions associated with the calcium-treated steel containing sulfur at the commercial level were detrimental to the isotropy of the property. The results are described and discussed.