Comparison of simulated heat affected zone microstructures of niobium microalloyed steels subjected to multi-pass weld thermal cycles

A weldable material’s system design includes optimization of base metal (BM), heat-affected-zone (HAZ) and weld metal (WM) microstructure, for a given welding process condition, to achieve target mechanical properties. Traditional research focused on single-pass weld thermal cycles with different HAZ peak temperatures and mostly ignored the effect of initial microstructure. In this research, weldable materials system design methodology is applied to thick plate steels (up to 25 mm) made with niobium microalloying additions. The overarching goal of this research is to optimize the HAZ microstructure evolution to minimize the degradation of properties achieved by original thermomechanical processing, irrespective of the number of weld passes. Two niobium microalloyed steels with different initial microstructures were studied. The samples were subjected to simulated multipass weld HAZ thermal cycles typically utilised in linepipe production using a Gleeble thermomechanical simulator. Concurrent dilatometric measurements provided insight into phase transformation kinetics. Two sets of experiments were performed and the samples were characterized with optical and scanning electron microscopy, electron back scattered diffraction and microhardness testing. In the first experiment, for a given composition, samples with different initial microstructures (ferrite + pearlite or bainite + martensite) but the same initial composition were subjected to two pass thermal cycles. In both cases, the final microstructures in these two samples were found to be a mixture of ferrite and small fractions of martensite-austenite (MA) constituents. However, differences in MA content were observed. In the second experiment, steels with different initial microstructure and different compositions were subjected to multipass weld thermal cycles with peak temperatures above and below Ac1 and Ac3 temperatures. These thermal cycles are similar to those expected during multi-pass welding of a thick plate. Interestingly, the final microstructure was found to be invariant (polygonal ferrite) in both the samples. The differences in hardness between these samples were found to be small. This demonstrates that the effect of initial microstructure attained during original thermo-mechanical processing is minimal on the intercritical HAZ in multipass welds. (AU) Copyright © 2013 Companhia Brasileira de Metalurgia e Mineração (CBMM) All Rights Reserved