Effects of ER308L buttering and post-buttering heat treatment on the microstructure and mechanical properties of API 5L X65/AISI304 dissimilar joint

Abstract

This study investigates the effects of ER308L butter layer on the microstructure and mechanical properties of API 5L X65/dissimilar joint. High strength low alloy API 5L X65 and AISI 304 steel grades are widely used in a wide range of industrial applications, and more specifically in the oil and gas sector. Dissimilar welds between API 5L X65 pipeline and austenitic stainless steel joints have long been used in the oil/gas and petrochemical sectors. Using buttering layer to improve properties of these types of dissimilar joints has lately received significant attentions. In the present work, the ER308L butter layer was applied on the API 5L X65 side through gas tungsten arc welding, followed by a stress-relief heat treatment at 600 °C. Buttered API 5L X65 samples were welded to AISI 304 samples through gas tungsten arc welding in 5 passes. Microstructures of joints were evaluated by scanning electron microscope (SEM) and optical microscope (OM). Mechanical properties were also assessed by tensile, impact, and Vickers micro-hardness tests. Fractography and phase identifications were carried out using SEM and energy dispersive spectroscopy (EDS). Results showed that a martensite layer on the fusion zone with a thickness of about 30 μm was observed at the interface of API 5L X65/butter layer. It appears that compositional changes played a significant role in the formation of martensitic structures and type-II grain boundaries. Results also showed that the fracture positions in both heat treated and as-welded samples were in the heat affected zone (HAZ) of API 5L X65. The implications of buttering and post-buttering stress-relief heat treatment for the microstructure and mechanical properties are comprehensively discussed.