Detrimental effect of coarse titanium-niobium nitrides on the fracture toughness of the CGHAZ in a 500 MPa offshore steel for cold climate conditions

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Offshore steels for cold climate conditions require not only relatively high strength to improve the cost-efficiency of the structures, but also excellent toughness at low temperatures to guarantee the safety of the structures in harsh environments. The most challenging locations to fulfil both requirements are in the welded joints of these steels because the weld thermal cycles cause detrimental changes to the steel microstructure. Titanium and niobium are often used in these steels to form nanoscale precipitates and reduce austenite grain growth during the thermal cycle. However, there is a risk that in addition to the nanoscale precipitates formed from the solid state, microscale coarse inclusions are also formed from the melt. These inclusions are hard and brittle and thus likely to deteriorate mechanical properties such as fracture toughness and impact toughness. In this study, two experimental 500 MPa offshore steels with differing inclusion structures were studied to find out if titanium based nitrides significantly deteriorate the fracture toughness of the coarse grained heat-affected zone (CGHAZ) at -40 °C. A Gleeble 3800 was used to simulate the CGHAZ, and the fracture toughness was determined by three point bending testing. Inclusions were characterized using FESEM-EDS. It was found that fracture toughness of the steel with coarse nitrides was lower and the scatter higher than in the steel with a higher calcium based inclusion content and without the coarse nitrides. Fractographic examination showed that the failure in the samples with the lowest toughness was initiated by coarse titanium-niobium nitrides. To avoid toughness problems caused by the CGHAZ under cold conditions, it is necessary to prevent the formation of coarse nitrides during steel production.</p></div>