Correlation of microstructure with tensile and crack tip opening displacement properties at low temperatures in API linepipe steels

Abstract

The correlations of the microstructural factors with the tensile and crack tip opening displacement (CTOD) properties at −20 and −60 °C for three kinds of API linepipe steels were investigated. The C steel composed mainly of small-sized acicular ferrite exhibited excellent tensile and CTOD properties. On the other hand, the A and B steels with large-sized polygonal ferrite or granular bainite exhibited low CTOD properties at −60 °C. The effective grain size was inversely proportional to the CTOD value at low temperatures. In the A and B steels, the values of the plastic deformation area and the CTOD were low because the crack tips of the steels opened under a low maximum force due to the fracture mode of the unstable brittle crack extension behavior. In the C steel, however, the values of the plastic deformation area and the CTOD were high because the crack tip of the steel opened under a high maximum force due to the fracture mode of almost fully plastic behavior. The C steel showed the widest stretch zone and the highest CTOD value. The CTOD values and the portions of post elongation in the A and B steels decreased with decreasing test temperature.