Microstructural characterization of the transition zone between a C–Mn steel pipe and a 70%Ni30%Cu alloy cladding welded by HW-GTAW

Abstract

Offshore seawater systems are commonly manufactured using a monolithic copper–nickel alloy (90%Cu10%Ni). However, because of the high cost of this material, they have been replaced in some Brazilian oil and gas platforms by C–Mn steel pipes clad with a Ni–Cu alloy (70%Ni30%Cu, Monel 400®). Therefore, considering the complex transition zone (TZ) in dissimilar weldments and its influence on the performance of the clad component, the present work aims to characterize the TZ of an API 5L X65 steel pipe clad with a 70%Ni30%Cu alloy by hot-wire gas tungsten arc welding (HW-GTAW). The TZ was evaluated using optical microscopy, scanning electron microscopy, electron backscatter diffraction, Vickers microhardness measurements, and bending tests. It exhibited beach, peninsula, and island macrosegregation morphologies with martensite (high hardness) and type II grain boundaries as well as an abrupt chemical composition gradient and Zone M. Moreover, the cladding solidified epitaxially, even though the cladding and the substrate had different crystal lattices. A solidification grain boundary crack, observed in the pipe end, nucleated in the substrate and propagated through the beach macrosegregation and posteriorly along the cladding grain boundaries. Despite this crack, the API 5L X65 steel pipe clad with 70%Ni30%Cu was approved according to ISO 15614 Part 7 and is a potential candidate for seawater systems.