Metallurgical Factors Influencing the Susceptibility of Hydrogen Assisted Cracking in Dissimilar Metal Welds for Application Under Cathodic Protection

Abstract

Dissimilar metal weld (DMW) overlays of Nickel-based filler metals on low-alloy steel pipes are used in the oil and gas industry in order to eliminate the need for field post weld heat treatment (PWHT) of adjacent closure welds. Brittle failures have been reported along some of these DMW interfaces, especially in AISI 8630-IN625 weld combinations during subsea service under cathodic protection (CP). These failures have been attributed to hydrogen assisted cracking (HAC) due to local hydrogen embrittlement of susceptible microstructures that form at the fusion boundary during welding and PWHT. Testing at The Ohio State University using the delayed hydrogen cracking test (DHCT) has concluded that this type of HAC is strongly affected by the base metal/filler metal combination, and by the welding and PWHT procedures. These controlling factors determine the microstructures that form at the fusion boundary. Thermo-CalcTM and DictraTM software was used to simulate the above-mentioned controlling factors. Based on the simulations conducted, carbon concentration near the fusion boundary was monitored closely. The results of the simulations yielded results that correlate well with DHCT experiments. Also, the controlling factors of HAC can be optimized to reduce the accumulation of carbon at the fusion boundary.