PSI - Issue 60

Behrooz Tafazzolimoghaddam et al. / Procedia Structural Integrity 60 (2024) 575–581 Author name / StructuralIntegrity Procedia 00 (2019) 000–000

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1. Introduction Hardfacing is a common practice in the nuclear industry to protect critical components against degradation mechanisms such as galling, wear, and self-welding. This is normally achieved by depositing a layer of alloys with superior wear resistance and hardness onto the surface of the component to protect it from wear related degradation. Historically, Cobalt-base alloys such as Co-Cr Stellite 6 and Co-Cr-W Stellite 20 were the obvious choice for hardfacing due to their excellent resistance to wear at high temperatures; however, the nuclear industry is moving away from these alloys because of the induced radioactivity considerations [1]. When Cobalt based alloy is used in the nuclear reactor environment, Co 60 isotope is formed. Co 60 emits high-energy gamma rays causing difficulties controlling the personnel radiation exposure during maintenance, handling and decommissioning of the parts. Nickel-based hardfacing alloys such as 75Ni13.5Cr2.7B-3.5Si alloys (commercially known as Colmonoys or Deloro) were developed to replace the Co-base alloys in fast reactor applications. Ni-based alloys offer good wear resistance at high temperatures [2]. Bhaduri et al. [3] showed that the dose rate and induced activity decreased by two to three orders of magnitudes when 75Ni13.5Cr2.7B-3.5Si alloy is used instead of Stellite. This reduction of radiation allows lower shielding thickness and consequently lower maintenance and handling work. Plasma Transfer Arc (PTA), Gas Tungsten Arc (GTA) and Laser surfacing are the major processes used for hardfacing over large areas. Application of intense localized high temperature in these processes creates large residual stresses in the deposit. Furthermore, the presence of dissimilar material regions during thermal loading and their different thermal properties adds another source of stress. Residual stresses reduce the potential safe service load of a component and ultimately can lead to crack initiation which is not acceptable for safety-critical nuclear components. There are several factors involved in the formation of the residual stresses that can be grouped into design parameters (i.e., part dimensions), material properties (i.e., material microstructure), and fabrication parameters (i.e., heat input, preheating, sequence) [4]. Balaguru et al. [5] simulated the PTA hardfacing of SS 304 with 75Ni13.5Cr2.7B-3.5Si using FE analysis and demonstrated the effect of heat input, the deposition process and the component geometry on the magnitude and distribution of the residual stresses. Kumar and Pai [6] reported on the issues of 75Ni13.5Cr2.7B-3.5Si application for hardfacing of seal assembly components for the Intermediate Heat Exchanger (IHX) of Prototype Fast Breeder Reactor (PFBR). PTA and GTA surfacing processes were used for the deposition, however difficulties arose due to the significant difference between the coefficient of thermal expansion between 75Ni13.5Cr2.7B-3.5Si coating and the austenitic stainless steel substrate. Residual stress was generated due to differential shrinkage of the overlay and the substrate material due to the process-induced thermal gradient, difference in coefficient of thermal expansion and their melting points. Here again, the magnitude of the stresses is controlled by the heat input, overlay thickness, preheat temperature, the component geometry and deposition direction [6] [7] [8]. It has been seen that preheat temperature of 450 ° C and above reduces cracking propensity of the 75Ni13.5Cr2.7B-3.5Si deposit. Hardfacing can influence the fatigue life of components. Hutasoit et al. [9] reported that their hardfaced specimens had lower fatigue life compared to unclad specimens due to the residual stress caused by the laser cladding process. Furthermore, it was observed that thinner coatings have higher fatigue life since the value of residual stress is lower than in the thick deposits [3]. To verify the structural integrity of the hard-faced parts, it is important to have an accurate knowledge of the residual stress and the mechanisms by which the residual stresses are generated. In this study, the contour method is used for the residual stress measurement in PTA hard-faced AISI 316 stainless steel (SS) plate with 75Ni13.5Cr2.7B-3.5Si alloy. The aim is to identify the effect of PTA surfacing process parameters / patterns on the residual stress evolution, microstructure, and the hardness of the overlay alloy as well as the substrate. 2. Materials and Methods 2.1. Contour Method The contour method is a destructive measurement technique that provides a two-dimensional map of the residual stresses perpendicular to the desired cross-section formed by a planar cut. When a planar cut is performed on a component having residual stresses, the cut surfaces adjust themselves to the now stress-free cut plane by deforming in the opposite direction of the released stresses. If the deformations are elastic, the initial residual stress can be back-