Abstract
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.
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