Fusion boundary microstructure evolution associated with embrittlement of Ni–base alloy overlays applied to carbon steel

Abstract

The embrittlement of the fusion boundary region of Ni–base Alloy 625 overlay welds on AISI 8630 steel was evaluated. Metallurgical characterization, thermodynamic and kinetic simulations, and hydrogen charging experiments were conducted in an effort to identify the embrittlement mechanism and simulate the type of failures that occur in service under cathodic protection. Optical microscopy, scanning electron microscopy, electron backscattered diffraction, electron probe microanalysis, X-ray diffraction, and hardness mapping were all used to characterize the microstructure and assist in identifying the microstructural features in the transition region of this dissimilar alloy combination that can promote susceptibility to hydrogen-assisted cracking (HAC). A special low-angle sectioning technique was developed that expanded the very narrow transition zone between the Alloy 625 overlay and the heat-affected zone of the 8630 steel. Using this technique, a “featureless zone” resulting from planar solidification was evaluated. This was the region in which HAC was observed during service under cathodic protection. Carbon migration studies conducted using Dictra™ showed that considerable carbon buildup (over 1 wt.%) occurs in the transition zone during postweld heat treatment. This level of carbon was confirmed using electron probe microanalysis. The high carbon content stabilizes the austenite phase in this region and results in local hardness levels exceeding 500 VHN. Metallographic sections of samples that failed under cathodic charging revealed that cracking occurs preferentially within this hardened region. The paper describes the metallurgical basis for the formation of this transition zone in weld overlays of Ni–base filler metals on carbon steels.