Microstructure and mechanical properties correlation of weld joints of a high strength naval grade steel

Abstract

Low alloy steels of higher strength grades are conventionally used for pressure hull application. Strength required range from 560 MPa to 800 MPa and plate thickness requirement is often higher (40 mm or more). Welding of high strength steels with higher thicknesses is often a challenge. From the view point of structural integrity challenge is not only to obtain defect free weld joint but also to meet the strength and low temperature toughness requirement in both weld and HAZ. In hull construction, different types of welding processes are used depending upon the requirement, accessibility and criticality of the weld joints to be produced. Conventionally, welding processes include Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Submerged Arc Welding (SAW) & Gas Tungsten Arc Welding (GTAW). Number of weld consumables indigenously developed for a higher strength steel (YS~800 MPa), were used to develop the procedure, for producing weld joints, meeting the critical structural property requirements.Present study was focused on structural property correlation of weld joints made using these indigenous consumables. Weld joints were characterized in terms of impact toughness, Ductile Brittle Transition Temperature (DBTT), tensile properties and quantitative metallography. DBTT of weld joint is important from the aspect of structural integrity at sub-zero temperature. Impact toughness at -40° C varied from 39 J for SMAW process to 191 J for GTAW process. Yield strength of weld joints varied from 534 MPa for SMAW process to 907 MPa for GTAW process. Quantitative metallography was carried out to determine volume fraction of acicular ferrite (AF) in different welds. Heat input (HI) to produce a weld joint varied with welding processes used. HI for SMAW was 1.3 kJ/mm while it was 2.2 kJ/mm for SAW process. At HI of 1.85 kJ/mm (GTAW), a higher % fraction of AF (~77%) having Impact toughness of 191 kJ/mm at -40° C.