Welding of the martensitic stainless steel CA6NM for the process FCAW: physical simulation of ZTA, treatment for stress relief, microestructural and mechanical characterization

Abstract

The CA6NM (ASTM 743, 13% Cr, 4% Ni e 0,4% Mo) martensitic stainless steel grade is used in manufacturing hydraulic turbines. In welding of the material for repair of cavitation damage, it is necessary to pre-heat and subsequently heat-treat the component in order to control the residual stresses after welding, due to the formation of hard and brittle martensite, which can lead to the development of cracks. From the literature, the process by vibration, as an alternative technique for relieving stress in welded components, may offer lower energy cost and production time, much less environmental impact and improved mechanical properties of the components. However, there are not yet sufficient information yet to propose a modification of the stress relieving procedure by the heat treatment currently used in industry. Therefore, the aim of this investigation are the evaluation of the different microstructures formed in the welded joints of CA6NM steel components, as well as their mechanical properties. The work seeks to provide detailed information about the weld, the heat affected zone (HAZ) and the base metal, with and without heat treatment or post weld heat treatment (PWHT) that can be compared to microstructural and mechanical properties of welded steel using vibration as a method of residual stress relief. The material will be welded using the FCAW process (Flux Cored Arc Welding) and AWS E410NiMoT1-4 /1 consumable. Three different residual stress relief treatments will be performed: Post Weld Heat Treatment (PWHT), Vibratory Stress Relief (VSR) and Vibratory Weld Conditioning (VWC). The latter is a new welding technology, which vibrates the workpiece during welding. The local heating of the HAZ region causes very complex microstructural changes, considered to be the most critical section of the weld. As these regions are usually quite small, it is difficult to characterize them, because of the insufficient volume of each sub-region of the HAZ. Work around this limitation, the base metal will be submitted to physical simulation in order to obtain the different regions of the HAZ. The welding simulation will be carried out on a Gleeble ® 3800, considering different thermal cycles and heat inputs, aiming at reproducing the same microstructure found in HAZ, but in higher volumes, allowing a better microstructural and mechanical characterization of the different regions of the HAZ. Real and physically simulated welding samples will be characterized via OM and SEM for microstructural analysis, as well as impact Charpy tests and microhardness tests to determine their mechanical properties. The knowledge of process parameters and the way the weldability can affect the microstructure and mechanical properties of steel are essential to technological development and to help the national industry in the search for more effective methods for stress relief, ensuring quality improvement and cost reduction of welded turbine components. (AU)