Abstract
Welding thermal cycles with heat inputs ranging from 25 to 75 kJ/cm were performed on a Gleeble 3500. The impact energy improved significantly (from 10 to 112 J), whereas the simulated coarse-grain heat-affected zone (CGHAZ) microstructure changed from lath bainite ferrite (LBF) and granular bainite ferrite (GBF) + martensite/austenite (M/A) to acicular ferrite (AF) + polygonal ferrite (PF) + M/A as the heat input increased. Simultaneously, the mean coarse precipitate sizes and the degree of V(C,N) enrichment on the precipitate surface increased, which provided favorable conditions for intragranular ferrite nucleation. The Ar3 of CGHAZ increased from 593 °C to 793 °C with increasing heat inputs; the longer high-temperature residence time inhibited the bainite transformation and promoted the ferrite transformation. As a result, acicular ferrite increased and bainite decreased in the CGHAZ. The CGHAZ microstructure was refined for the acicular ferrite segmentation of the prior austenite, and the microstructure mean equivalent diameter (MED) in the CGHAZ decreased from 7.6 µm to 4.2 µm; the densities of grain boundaries higher than 15° increased from 20.3% to 45.5% and significantly increased the impact toughness. The correlation of heat input, microstructure, and impact toughness was investigated in detail. These results may provide new ideas for the development of high welding heat input multiphase steels.
Highlights
Due to its good toughness and high strength, high-strength micro-alloy steel has been utilized broadly to build large ships, high-rise buildings, and heavy-duty steel bridges [1,2,3,4].high-strength steel comes with many problems and challenges during welding.For example, high-strength steel suffers from cold cracking when welded in a wet environment [5,6]
Coarse grains formed in the coarse-grain heat-affected zone (CGHAZ) at a peak temperature of 1320 ◦ C or higher [7,8,9,10,11]
Lan et al [18,19] reported the impact toughness of CGHAZ in low-carbon bainitic steel deteriorated with an increased heat input, which was primarily associated with the large M/A constituent and coarse bainite
Summary
Due to its good toughness and high strength, high-strength micro-alloy steel has been utilized broadly to build large ships, high-rise buildings, and heavy-duty steel bridges [1,2,3,4]. In CGHAZ, austenite grains coarsened and brittle structures, such as a M/A constituents, coarse-grain boundary ferrite, and granular bainite, formed in the grains and significantly deteriorated the toughness [11,12,13,14] This reduction is serious in high heat inputs. Lan et al [18,19] reported the impact toughness of CGHAZ in low-carbon bainitic steel deteriorated with an increased heat input, which was primarily associated with the large M/A constituent and coarse bainite. The evolution mechanism of the simulated CGHAZ microstructure and the impact toughness of low-carbon N-contained steel with different welding heat inputs both require further clarification. Low-carbon Mo-V-Ti-N-B steel was modeled for a welding simulation, and the relationships between heat input, microstructure, and impact toughness of the CGHAZ were investigated in detail. This research plays a significant role in advancing the steel of high-heat-input welding in engineering applications
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