Understanding the Role of Initial Microstructure on Intercritically Reheated Heat Affected Zone Microstructure and Properties of Multi-Pass Welds

Multi-pass welding of low alloy steels has been shown to cause regions of low toughness in the reheated heat affected zone (HAZ) due to the formation of islands of untempered martensite and austenite called 'M-A constituents'. Traditional research in this area has focused on studying the formation of these constituents and its effect on properties of one particular region of the HAZ zone called the 'inter-critically reheated coarse-grained heat affected zone'. Although, modern engineering steels are an eclectic mixture of microstructures, the effect of initial microstructure on the formation of these M-A constituents and on the properties of weld HAZ has mostly been ignored and has gone uninvestigated.;In this document, some of the previous work related to multi-pass weld HAZ properties is revisited and fundamental issues in need of further research related to initial microstructure and microstructural factors governing HAZ properties are highlighted. A study has been conducted to analyze the role of initial microstructure on HAZ properties of multi-pass welds on four pipeline grade microalloyed steels. These steels had minor differences in compositions with respect to certain alloying elements. For each of the four steels, two different microstructures were developed that acted as initial microstructures for HAZ simulation. One set of initial microstructure was developed using thermo-mechanical controlled processing (TMCP) and the second kind of microstructure was obtained using heat treatments. The steels with differing initial microstructures but same compositions were subjected to intercritically reheated HAZ thermal cycles using Gleeble thermo-mechanical simulator. The microstructures thus obtained were studied using advanced characterization techniques and their properties were evaluated and compared using mechanical testing.;Dilatometric analysis of phase transformations occurring during the HAZ simulation indicated difference in the transformation temperatures of steels with differing initial microstructures. The on-heating phase transformation start temperature was always found to be lower for the TMCP microstructure when compared to that initial microstructure formed from heat treatment. Furthermore, the amount of austenite formed during the intercritical thermal cycle by the steels was found to be different when the initial microstructures were different, with the TMCP microstructure always forming a lower amount of austenite. Microstructure characterization of HAZ regions obtained from different initial microstructures also showed differences in the amount of M-A constituents and the heat treated initial microstructure was always found to have higher amount of M-A constituents in the HAZ.;Room temperature mechanical testing on the HAZ microstructures formed from different initial microstructures did not show significant differences in the intercritically reheated HAZ tensile properties. However, profound differences were observed in the impact toughness properties of these HAZ regions. HAZ obtained from the heat treated microstructure always exhibited poor impact toughness at low testing temperatures. These results indicate that initial microstructure might have a significant effect on HAZ properties of multi-pass welds. The results of microstructure characterization and mechanical testing of HAZ regions formed from the two initial microstructures of all the four steels are discussed. Based on results, a possible mechanism of microstructure evolution is put forth to explain the reason behind the differences in the properties of HAZ regions formed from different initial microstructures.