Effect of cooling rate on intercritically reheated microstructure and toughness in high strength low alloy steel

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Abstract
High strength low alloy steels have been shown to be adversely affected by the existence of regions of poor impact toughness within the heat affected zone (HAZ) produced during multipass welding. One of these regions is the intercritically reheated coarse grained HAZ or intercritical zone. Since this region is generally narrow and discontinuous, of the order of 0·5 mm in width, weld simulators are often employed to produce a larger volume of uniform microstructure suitable for toughness assessment. The steel used for this study was a commercial quenched and tempered steel of 450 MN m·2 yield strength. Specimen blanks were subjected to a simulated welding cycle to produce a coarse grained structure of upper bainite during the first thermal cycle, followed by a second thermal cycle where the peak temperature Tp2 was controlled. Charpy tests carried out for Tp2 values in the range 650–850°C showed low toughness for Tp2 values between 760 and 790°C, in the intercritical regime. Microstructural investigation of the development of grain boundary martensite–retained austenite (MA) phase has been coupled with image analysis to measure the volume fraction of MA formed. Most of the MA constituent appears at the prior austenite grain boundaries during intercritical heating, resulting in a ‘necklace’ appearance. For values of Tp2 greater than 790°C the necklace appearance is lost and the second phase areas are observed throughout the structure. Concurrent with this is the development of the fine grained, predominantly ferritic structure that is associated with the improvement in toughness. At this stage the microstructure is transforming from the intercritical regime structure to the supercritically reheated coarse grained HAZ structure. The toughness improvement occurs even though the MA phase is still present, suggesting that the embrittlement is associated with the presence of a connected grain boundary network of the MA phase. The nature of the second phase particles can be controlled by the cooling rate during the second cycle and varies from MA phase at high cooling rates to a pearlitic structure at low cooling rates. The lowest toughness of the intercritical zone is observed only when MA phase is present. The reason suggested for this is that only the MA particles debond readily, a number of debonded particles in close proximity providing sufficient stress concentration to initiate local cleavage. MST/1642