Investigations on High Cycle Fatigue and Tensile behaviour of 316L N SS hardfaced with Ni Cr B Si alloy by GTA PTA and Laser cladding processes

Abstract

The grid plate is a key structural element situated at the bottom of the reactor core, designed to support and accurately align all the components of reactor core. The grid plate consists of top and bottom plates. Bottom plate of grid plate is placed on core support structure (CSS) in reactor core of Sodium-cooled Fast Reactor (SFR). This grid plate is used to support the CSS and maintain the verticality of the reactor core components. Grid plate and CSS are made of stainless-steel grade 316L(N) material. Components in the reactor core are surrounded by liquid sodium coolant present inside the reactor core. This liquid sodium coolant serves as a metallic gum and can cause self-welding of bottom plate of grid plate and CSS. To avoid this phenomenon, hardfacing of bottom surface of the grid plate is done using Ni-Cr-B-Si alloy cladding material. This hardfacing can be done using three different methods like plasma transferred arc hardfacing (PTA), gas tungsten arc hardfacing (GTA) and laser cladding (LC) methods. Due to variation in co-efficient of linear thermal expansion between cladding and base metal (also called as substrate) and also due to mixing of sodium at varying temperatures, Ni-Cr-B-Si cladding is subjected to tensile and cyclic loads. This causes tensile and fatigue fractures with cracking/debonding of the cladding. Hardfacing increases the hardness of the specimen. Though the increase in hardness is preferred for the specimens, it can also lead to cracking/debonding if there is a sudden rise in the hardness immediately after the interface in the cladding region. To asses this structural integrity, high cycle fatigue (HCF) tests are required to be carried out on the hardfaced specimens to recommend the most suitable hardfacing process from above said three processes. newlineFrom the literature, it is known that several studies have already been carried out on dilution and shear strength of hardfaced specimens. However, no research has been carried out using tensile and HCF tests for the above mentioned three processes to recommend the best hardfacing process out of those three. The current research work is thus carried out to recommend an effective hardfacing process from GTA, PTA and LC processes which can give greater tensile strength and fatigue strength without debonding and cracking of the cladding. Microstructural and hardness studies are performed to understand the effect of different processes on tensile and fatigue behaviour. To analyse the tensile behaviour and fatigue fracture initiation and propagation characteristics, tensile tests and axial high cycle fatigue (HCF) tests are conducted, followed by macroscopic and fractography evaluation. newlineInitially, hardfacing of Ni-Cr-B-Si alloy on 316L(N) stainless steel plate is done by PTA, GTA newlineand LC processes. Tensile and fatigue specimens are prepared from these hardfaced plates according to ASTM A-370 and ASTM E-466 standards respectively. All the tests are carried out on samples prepared from all the three processes. Before going for the mechanical testing, microstructural studies are carried out using metallurgical microscope. The grain sizes were measured as per the ASTM E112-13. Microstructure is examined at the coating, interface and substrate region. Micro hardness is observed on the hardfaced specimens at these three locations using Vickers hardness testing machine. Tensile tests are conducted to evaluate the yield strength, ultimate tensile strength and percentage elongation of all the specimens. Later, HCF tests and fractographic studies are carried out to measure the fatigue limit of the specimens and understand the fracture nucleation and propagation characteristics. newlineFrom the experimental results, it is observed that LC specimens have least grain size ~11and#956;m followed by GTA specimens with grain size of ~29 and#956;m and then PTA showed highest grain size of ~42 and#956;m near the interface (~200and#956;m) in the cladding region. Consequently, the hardness near the interface (~200and#956;m) in the cladding region is highest for LC specimens followed by GTA and then least for PTA. Tensile tests concludes that GTA process gives ~42 % and ~46 % higher tensile strength than LC and PTA processes respectively. GTA specimens show ductile behaviour under tensile tests with no cracking in the cladding while PTA specimens show brittle behaviour due to increase in hardness at the interface with no cracks in the cladding. LC specimens also show brittle behaviour but with cracks in the cladding. HCF tests shows that GTA process has ~17 % higher fatigue strength than LC process and ~42% higher fatigue strength than PTA process. GTA specimens do not exhibit any fatigue cracks at the cladding region for any cyclic load whereas LC and PTA specimens show fatigue cracking and debonding at the cladding. newlineThough the highest hardness near the interface is obtained by LC process followed by GTA and PTA process, tensile strength and fatigue strength of LC process is less than that of GTA but higher than that of PTA. Therefore, for longer life of cladding without debonding at the interface, GTA process is the most suitable process among the three processes. Hence GTA process is recommended in the current research work for uninterrupted services of SFR. newlineKey words: Ni-Cr-B-Si alloy, 316L(N) stainless steel, Hardfacing, Grid plate, Fatigue, carbides newline newline