Assessment of nickel alloy 625 weld overlays deposited by the electroslag process

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Abstract
Nickel-base alloy weld overlays are commonly used in the oil and gas industry to extend the life of equipment under aggressive corrosion environments, since the overlays improve the corrosion resistance without a significant increase in the cost of manufacture when compared to massive equipment. Usually, the joints are welded by SMAW, GMAW or GTAW processes. In this respect, the electro slag welding process (ESW), which promotes high heat inputs and low dilution welds, can be an interesting option for this application as it provides high productivity as only one layer is necessary. The present work evaluates mechanical, microstructural and corrosion properties of an Alloy 625 weld overlay deposited on ASTM A516 Grade 70 carbon steel by the ESW process. The deposition was done with one and two layers in plates of dimension 50 mm × 400 mm × 400 mm, in the flat position and an average energy welding 11.7 kJ/mm. After welding, a post-weld heat treatment at 620 °C for 10 h was performed, this condition being compared with the as-welded condition. Bending tests showed no evidences of cracks. Microstructural evaluation performed using both optical (OM), scanning (SEM) and transmission electron microscopy techniques showed an austenitic microstructure of the weld deposit with a low proportion of secondary phases for all conditions, and the post-welding heat treatment did not promote significant changes in the mechanical properties. At the coarse grain heat-affected zone (CGHAZ), the occurrence of proeutectoid ferrite, pearlite and bainite were found for one layer deposit and refined pearlite and ferrite for two layer deposits where the microstructural constituents were observed due to the low cooling rate. In addition, no evidence of partially diluted zones (PDZ) were verified. Corrosion tests conducted on samples removed from the top layer of the weld overlay cladding deposit according to ASTM G 48 Method A Standard were considered satisfactory once no evidence of pittings was verified and the loss of mass was very much reduced.

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