Abstract
A delayed fracture test has been made on SNCM8 steel tempered at 490°C (σY=127kg/mm2) in distilled water at 80°C using self-stressed WOL specimens. The results obtained are as follows :
1) Crack initiation time is almost identical with the total delayed fracture time in the case of conventional notched specimen.
2) Prevailing dimple pattern is observed in delayed fracture surface under higher applied stress, consisting with the fracture surface in air, while intergranular cracking is observed under lower stress. This trend agrees with the cases of delayed fracture in hydrogen gas and in distilled water with zinc anode.
3) Stretched zone is observed both in the tensile fracture and the delayed fracture. The width of stretched zone is proportional to (KIC/σY) 1.91 for the tensile fracture, while it is proportional to (KIC/σY) 1.13 for the delayed fracture if σY does not vary in the case and 127 kg/mm2 is applied. This indicates that the apparent decrease of flow stress σY should take place during the delayed fracture test. But the effect of hydrogen cannot be estimated because this material shows large stress relaxation.
4) On the basis of Hahn and Rosenfield's relationship between KIC, and tensile properties, it is estimated that the apparent considerable decrease of the true fracture strain of material near notch root combined with the apparent decrease of flow stress σY, occurs in the delayed fracture in the water.
