Abstract
Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.
Highlights
The combination of an austenite grain coarsening and rapid cooling promotes the formation of brittle microstructural constituents, containing high proportions of upper bainite (UB), grain boundary ferrite (GBF) and Widmanstatten ferrite (WF), and martensite-austenite (M-A) modules, which may be detrimental to toughness of the coarse-grained heat-affected zone (CGHAZ)
The objective of the present study is to investigate the effect of initial base metal microstructure on prior austenite grain size, and to correlate the heat input, prior austenite grain size, room temperature microstructure with impact toughness of the CGHAZ
The microstructure for the CGHAZ with heat input of 100 kJ/cm mainly consists of grain boundary ferrite and acicular ferrite in both the air-cooled and water-quenched initial microstructure
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The balance between strength and toughness in high-strength low-alloy steels can be disrupted by welding, which is characterized by rapid heating and cooling, producing the heat-affected zone (HAZ), especially the coarse-grained heat-affected zone (CGHAZ). The combination of an austenite grain coarsening and rapid cooling promotes the formation of brittle microstructural constituents, containing high proportions of upper bainite (UB), grain boundary ferrite (GBF) and Widmanstatten ferrite (WF), and martensite-austenite (M-A) modules, which may be detrimental to toughness of the CGHAZ. It is generally believed that a microstructure consisting of acicular ferrite and lower inclusion content provides the optimum strength and toughness for both weld metal and CGHAZ [4,5,6,7]
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