Improvement of Submerged-Arc Weld Heat-Affected Zone Toughness by Titanium Additions to Linepipe Steels

Abstract

The effect of titanium additions on the base plate properties and coarsegrained heat-affected zone (CGHAZ) toughness of a partially acicular ferrite linepipe steel has been studied using laboratory melts. The heats were made with titanium levels between 0 and 0.039 weight percent, and were chill cast using cooling rates similar to those in commercial slab casting. The slabs were controlled rolled to 15-mm plate, and the CGHAZ grain size and toughness were evaluated using both double-pass submerged-arc welds (≈2 kJ/mm heat input) and simulated CGHAZ coupons. Titanium additions between 0.005 and 0.011 weight percent restricted austenite grain coarsening during both slab reheating and thermal cycles of the weld heat-affected zone (HAZ). This improved the plate Charpy fracture appearance transition temperature and the CGHAZ toughness of simulated weld HAZs, measured by using both crack opening displacement and instrumented impact tests. Higher titanium additions (0.015 to 0.039 weight percent) at high titanium/nitrogen ratios (3.00 to 4.6) exhibited improved grain size control in comparison with the base chemistry heats, but no significant improvement in CGHAZ toughness. The precipitate size distribution and chemistry in the as-cast, as-rolled, and welded conditions have been characterized to assess their effect on grain coarsening.