Search for: All records

The local yielding behavior in base metal, heat-affected zone, fusion boundary region, and weld metal of low-alloy steel/Alloy 625 filler metal welds was quantified using digital image correlation instrumented cross-weld tensile test. The tested welds exhibited undermatching, matching, or overmatching weld metal yield strength with significant gradients in the local yielding behavior. An undermatching weld yielded at 69 MPa below the base metal yield stress, accumulating to 0.72% total strain. The base metal in an overmatching weld had 110 MPa lower yield strength than the weld metal. The strong strain hardening response in the Alloy 625 weld metal, within the uniform elongation range, and its constraining effect on the fusion boundary region and heat affected zone, led to extensive strain accumulation, necking, and final failure in the base metal of all tested welds. The yielding behavior of the tested welds was compared to stress-based criteria, utilizing minimum specified and as-delivered yield and ultimate tensile strength, and to strain-based criteria. The capability of digital image correlation instrumented cross-weld tensile testing to quantify local yielding and strain accumulation demonstrates potential application in proving conformity to stress-based and strain-based design criteria of dissimilar and matching filler metal welds. 

Dissimilar metal welds (DMWs) are routinely used in the oil and gas industries for structural joining of high-strength steels to eliminate the need for post-weld heat treatment (PWHT) in field welding. Hydrogen-assisted cracking (HAC) can occur in DMWs during subsea service under cathodic protection. DMWs of two material combinations, 8630 steel/FM 625 and F22 steel/FM 625, produced with two welding procedures, non-temper bead (BS1) and temper bead (BS3), in the as-welded and PWHT conditions were investigated in this study. These DMWs were subjected to metallurgical characterization and testing with the delayed hydrogen cracking test (DHCT) to identify the effects of base metal composition, welding and PWHT procedures on their HAC susceptibility. The HAC susceptibility was ranked using the time to failure in the DHCT at loads equivalent to 90% of the base metal yield strength (YS) and the apparent stress threshold for HAC. A criterion for resistance to HAC in the testing conditions of DHCT was also established. The results of this study showed that 8630/FM 625 DMWs were more susceptible to HAC than the F22/FM 625 DMWs. PWHT did not sufficiently reduce the HAC susceptibility of the 8630/FM 625 and F22/FM 625 BS1 welds. DMWs produced using BS3 performed better than BS1 DMWs. The post-weld heat-treated F22/FM 625 BS3 DMW passed the HAC resistance criterion. 

Dissimilar metal welds (DMWs) are commonly used when a high strength steel is overlaid with a corrosion resistant alloy (CRA) for petrochemical applications. There have been reported failures of these DMWs during subsea service while under cathodic protection (CP). These failures are caused by local hydrogen embrittlement of susceptible microstructures that form at the weld fusion boundary. Hydrogen-assisted cracking (HAC) occurs as a result of the local embrittlement and is influenced by base/filler metal combinations, and welding and post-weld heat treatment (PWHT) procedures. A delayed hydrogen cracking test was used to simulate tensile load and hydrogen charging on 8630-FM 625 weld. The failure of this sample was recorded using a high-speed camera to capture the crack initiation and propagation during failure. Fractography was performed using a scanning electron microscope (SEM) along with energy dispersive spectroscopy (EDS). The fracture surfaces, EDS measurement and video timestamps revealed brittle fracture nucleation in the planar growth and CGHAZ regions of the weld. The cracking continued to propagate through the same regions of the weld leading to final ductile failure (microvoid coalescence) in the cellular dendritic region of the weld.