Microstructure and Charpy impact properties of ferrite-martensite dual phase API X70 linepipe steel

The microstructural changes which occur during intercritical annealing treatment at 780 °C of API X70 High Strength Low Alloy (HSLA) Steel have been investigated. Three treatments were developed to produce dual phase microstructure: Intermediate Quenching Treatment/Air (IQT/A), Intermediate Quenching Treatment/Water (IQT/W) and Step Quenching Treatment (SQT). The IQT/Water treatment resulted in the formation of fine and fibrous martensite morphology uniformly distributed in the ferrite matrix. However, the IQT/Air treatment showed a spherical network of martensite along the ferrite/ferrite grain boundaries. The SQT treatment yielded blocky and banded martensite and ferrite morphology. The experimental results show that the API X70 HSLA steel with finely dispersed microstructures (IQT/Water treatment) has higher Charpy impact properties and lower ductile-brittle transition temperature DBTT than HSLA steel with banded microstructures. A carefully conducted comparison of fracture surfaces of the representative specimens obtained after IQT and SQT treatments has been studied.