Acicular ferrite development in autogenous laser welds using cerium sulphide particle dispersed steels

Abstract

Acicular ferrite development has been investigated in autogenous laser welds using cerium sulphide particle dispersed steels. Around 80% acicular ferrite has been obtained in weld metal irrespective of [Al]/[O] ratio. Acicular ferrite is high when combined sulphur and oxygen levels are between 200 and 550 ppm, but is markedly reduced below 200 ppm. Large acicular ferrite variations cannot be explained by changes in particle number density and austenite grain size, but may be attributed to changes in particle type. Phases promoting high acicular ferrite are (Ce,La)2O2S, Ce4O4S3, MnS and TiN, with some Al2O3, CeAlO3 and rare earth sulphides. Reduced acicular ferrite results from an absence of TiN or MnS. It is argued that phases may radically influence acicular ferrite content by altering the thermodynamic driving force for intragranular transformation and varying nucleation and growth kinetics through epitaxial relationships. Where aluminium is the primary deoxidising agent, loss of acicular ferrite at [Al]/[O] ratios greater than unity may potentially be explained by unfavourable Al2O3 clustering and formation of eutectic type II MnS colonies. When cerium is the primary deoxidising and desulphurising agent, this is not observed.