Influence of microstructure on susceptibility to weld defects in two high strength low alloy steels

Abstract

Hot cracking root causes in two HSLA welded steels were investigated and remedies were proposed to avoid hot cracking at centerline of the weld. The specimens were welded in Tjoint configuration keeping the constant welding parameters for both investigated alloys: Gr.50 (with lower content of alloying elements and yield strength of 345 MPa) and Gr.80 (with higher content of alloying elements and yield strength of 550 MPa). Metallographic studies were performed using optical and electron microscopy. Base metal, weld and HAZ were all characterized and discussed. Local chemical changes in centerline of the weld were also investigated with Energy Dispersive X-ray spectroscopy (EDS). All welds of Gr.80 showed hot cracking at the centerline while none of the Gr.50 welds were cracked. This can be explained by interlocking effect of secondary dendrite arms (SDA) in Gr.50 welds which enhances the resistance of weld centerline to hot cracking. Longer SDA is attributed to larger size of grains in HAZ of Gr.50 compared to grain size in Gr.80. The larger is the grain size of HAZ, the larger will be the width of primary dendrite arm (PDA) which in turn induces longer SDA. High residual stresses in Gr.80 base metal are also detected as a contributing factor. Existence of nonmetallic inclusions engulfed by gas porosities can also ease the crack propagation. Finally remedies and preventive action have been suggested to avoid formation of hot cracks.

Date	14 Jul 2017
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor)