Abstract
The effect of prestrain on the transition behaviour of fracture in a lamellar pearlite steel containing free ferrites has been investigated by means of low temperature tensile tests. The results obtained are summarized as follows:
(1) The types of initiation of micro-cracks can be divided into the following classes according to the temperature range. Below TC, almost all of micro-cracks occur at the boundary of pearlite colonies (type A) and micro-cracks in the free ferrite (type B) are observed barely. Between TC and TB, only those of type A are observed. Both type A and type C, which represents micro-cracks grown into the pearlite colony from a boundary of pearlite colony and a free ferrite, are observed near TB in the range between TB and TD·In the vicinity of TD, transgranular micro-cracks in the pearlite colony (type D) are observed in addition to those of types A and C.
(2) The non-ductile transition temperature rises with increasing prestrain.
(3) When prestrained, the peak of the fracture stress-temperature curve near the ductile transition temperature becomes very weak. The fracture stress of the prestrained specimens is lower than that of the non-strained.
