Abstract
To evaluate the influence of internal strain, separating it from the influence of microstructure, upon the change of delayed fracture strength of steel due to tempering after quench-hardening, the change of delayed fracture strength with cold-drawing and annealing after drawing was investigated for 0.38%C-0.95%Cr-0.18%Mo steel whose microstructure had been made to contain stable spheroidized carbides by annealing. The delayed fracture test was carried out in 0.1N-HCl solution by a cantilever type bending machine. The results are summarized as follows:
(1) The hardness and static bending strength of the steel increased and delayed fracture ratio (the ratio of delayed fracture limit to static bending strength) decreased gradually with increasing cold reduction in drawing.
(2) The delayed fracture limit reached a maximum value with the cold reduction of 16%. It is thought that the result is attributed to the cause that at that cold reduction the load carrying capacity of the steel balanced with the susceptibility to delayed fracture, both of which increased with increasing cold reduction of steel.
(3) The delayed fracture strength of the steel in short life region increased by cold working. It is thought that the result comes from the decrease of diffusion rate of hydrogen due to the trapping effect of lattice defects produced by cold working.
(4) The delayed fracture limit and delayed fracture ratio of cold-drawn steel increased with increasing annealing temperature gradually up to 300°C and rapidly over 300°C.
(5) The delayed fracture ratio varied in good correlation with the change of internal strain induced by cold working or annealing after that. It was suggested that microscopic internal strains play an important role in susceptibility to delayed fracture of steel.
