Abstract
The susceptibility to delayed fracture of 18%Ni Maraging and SNCM 8 steels which were heat-treated variously, was studied in 3%NaCl solution by means of the following three methods; constant load, constant displacement and constant strain rate methods. Delayed fracture surfaces of these steels were observed by scanning electron microscopy.
The following results were obtained:
(1) Heat treatment and loading condition have an effect on the crack propagation rate of delayed fracture of the steels, but have only a little effect on the morphology of the fracture surface. There is a wide range of the stress intensity factor over which the crack propagation rate is essentially constant.
(2) The value of lower critical stress obtained from the constant load method is not directly related to the value of threshold stress intensity factor obtained from the constant displacement method.
(3) In the strain rate range of 2×10−6 to 8×10−4 sec−1 at the constant strain rate method, the fracture behavior of SNCM 8 steel tempered at 300°C depends on the strain rate, and the smaller the strain rate the lower the reduction of area of this steel.
