Fracture toughness improvement mechanism of calcium treated steel

The effect of Ca addition on fracture toughness has been examined by means of the analysis of fracture mechanics and tensile tests together with micro -structural observation. The Ca addition results in the spheroidization and reduction of the number of non-metallic inclusions, while the matrix struc ture does not show significant changes. Fracture mechanics test in terms of J-integral using a three point bending rest shows the increase in the slop e of the R-curve, indicating enhanced increase in the stable crack growth r esistance. The dimple morphology on the ductile fracture surface is classif ied into two types, i.e. primary and secondary dimples. By Ca addition, the depth of the primary dimple and local shear area on the fracture surface i ncrease. It implies that Ca addition causes increased local plastic deforma tion in the course of stable crack extension, resulting in the improvement of fracture toughness. On the other hand effect of Ca addition does not app ear in tensile test. It suggests that the effect of micro-structural factor s on mechanical properties depends on the testing methods.