The Effect of Microalloy Additions on Ferritic Weld Metal Microstructures

Abstract

The objective of this research was to gain a further understanding of ferritic steel weld metal microstructures and the compositional factors affecting their formation. Continuous cooling transformation (CCT) diagrams have been produced isolating the effects of low and high concentrations of the microalloying elements vanadium, titanium, niobium and aluminium on the development of asdeposited and reheated weld metal microstructures, with low oxygen contents. The effects of carbon and oxygen content have also been considered. In order to achieve this, thermal analysis and dilation techniques were used for simulating as-deposited and reheated weld metal. The type and proportions of ferritic constituents were significantly affected by the addition of microalloying elements, even at low concentations. The transformation temperatures were depressed and the phase field on the CCT diagrams displaced to longer times. In the simulated as-deposited weld metal, low microalloy additions suppressed primary ferrite(PF) and ferrite side Plate(FSP) growth, initially increasing the proportion of acicular ferrite(AF) and at faster cooling rates, lath ferrite(LF). High AF contents could be achieved in low oxygen content weld metal, in the absence of a suitable oxide inclusion population, to act as nucleation sites. High microalloying elements significantly increased the proportions of LF, even at slow cooling rates. High titanium and niobium concentration refined the prior austenite grain size, promoting PF at slow cooling rates...