Effect of Normalizing and Solution Heat Treatments in the Microstructure of a Dissimilar Joint Welded Between an API 5L X-52 Steel Pipe and Inconel 625

Abstract

The purpose of this work is to compare changes in the microstructure and hardness of an API 5L X-52 steel pipe, internally cladded by welding with Inconel 625, after the execution of two heat treatments: normalization at 1100 °C for 60 min. and air cooled, and solubilization at 1030 °C with heating by electromagnetic induction for 30 s. and cooling in water. There were used techniques of optical microscopy (OM) and scanning electron microscopy (SEM), analysis by energy dispersive spectroscopy (EDS) and Vickers hardness. In the as-welded condition, the base metal (BM) had in its heat affected zone (HAZ) the formation of the coarse grain zone (CGZ) with average hardness of 234 HV. The partially diluted zone (PDZ) was presented clear, without grain boundaries, with high iron content and average hardness of 370 HV, indicating to be constituted by martensite. In the weld overlay, the microstructure is austenitic with dendritic cell shape with carbides of Nb and indications of precipitation of secondary Laves phases, with average hardness of 287 HV on its layers. In the condition after normalization, HAZ was suppressed and there was observed refinement in the BM microstructure; PDZ was maintened with its original morphology, but hardness was reduced about 15%; in the weld overlay, the microstructure became granular with indication of dissolution of the Laves phase and reduction of its hardness of 26% on average. In the condition after solubilization, HAZ of the BM was eliminated, PDZ maintened its morphology and previous microstructure, but its hardness reduced approximately 30%; in the weld overlay, microstructure and precipitates were maintened, but there was a reduction of hardness by about 20%. Finally, this work concludes that both heat treatments were considered beneficial to the properties of steel pipe API 5L X-52 internally cladded by Inconel 625. They promoted homogeneization of microstructure and hardness reduction of both BM and HAZ, significative reduction of PDZ hardness and, in the cladding layers, a reduction and adequation of hardness to the pertinent standards, allowing its use in aggressive enviromnents, mainly in the offshore and gas industry.

Key-words:
Cladding; Inconel 625; Normalizing; Solution heat treatment