Effect of Microstructure on mechanical and Charpy impact properties offerrite-martensite dual phase API X52 pipeline Steel

The microstructural changes that occur during intercritical annealing treatment at 780°C of API X52 steel have been investigated. Three treatments were developed to produce dual phase (DP) microstructure: Intermediate Quenching Treatment/Air (IQ/A), Intermediate Quenching Treatment/Water (IQ/W) and Step Quenching Treatment (SQ).The IQ/W resulted in the formation of fine and fibrous martensite morphology uniformly distributed in the ferrite matrix. However, the IQ/A treatment showed a spherical network of martensite along the ferrite/ferrite grain boundaries. The SQ yielded blockly and banded martensite and ferrite morphology. The experimental results show that API X52 (DP) steel with finely dispersed microstructures (IQ/W) have higher Charpy impact properties and lower ductile–brittle transition temperature (DBTT)than (DP) steel with banded microstructures.A carefully conducted comparison of fracture surfaces of the representative specimens obtained after IQ and SQ treatments has been studied.

Keywords:

Intercritical annealing Dual-phase steel,

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