Kinetics and microstructural analysis of fatigue fracture progress in weld joints of duplex stainless steel grade 2205

Yurtışık, Koray
Despite its high efficiency, autogenous keyhole welding technique leads to an excessive δ-ferrite formation in as-welded duplex stainless steels that leads to deterioration of the corrosion properties of the material, as well as its toughness. Developing the hybrid plasma-gas metal arc welding procedure for duplex stainless steels, a narrow gap and single-pass weldment on 11 mm-thick duplex stainless steel (grade 2205) plates has been achieved. Contrary to other keyhole techniques, such as Laser beam welding, electron beam welding and conventional plasma arc welding, the developed procedure apparently enabled us to obtain a proper cooling rate and weld metal chemistry, which were the key factors determining the microstructure and consequently corrosion and mechanical properties of the material. Fatigue crack propagation behaviour of the fusion zone in both air and sour environments has been interested and investigated within the damage tolerant design framework. A comparative analysis basis methodology was employed during investigations; beside hybrid plasma-gas metal arc welding, gas metal arc welding and plasma arc welding were also operated to obtain two reference duplex stainless steel weldments. Simultaneous utilization of keyhole and filler metal deposition in this hybrid welding process enabled us to achieve less total heat input and arc time compared to conventional multi-pass gas metal arc welding. Therefore precipitation secondary phases were considerably supressed. Moreover, microstructural examinations and analyses provided data indicating a sufficient reconstructive transformation of primary austenite in the fusion zone of the weldment obtained using hybrid plasma gas-metal arc welding, and that was not possible using autogenous plasma arc welding. Fatigue crack propagation tests were conducted in laboratory air and 3.5% NaCl solution on compact tension specimens, which were notched at the centre of fusion zone of the weldments. Controversial effect of the secondary austenite on crack propagation behaviour was the primary interest among the others, such as residual stress, phase balance and precipitates of chromium-nitrides and intermetallic phases. On the one hand, retarding the crack propagation in near threshold stress intensity factor range, precipitation of the secondary austenite in multi-pass weldment was observed to be beneficial for as-welded duplex stainless steel as long as the material would give service in air. On the other hand, the secondary austenite led to chromium and molybdenum depleted zone, where pitting and crevice resistance of the material was relatively low. Crack propagation rate of secondary austenite free single-pass weldment that was obtained employing the hybrid welding was lower than the one of multi-pass weldment in the sour environment. Crack propagation rate data was supported with comprehensive fractography and micro analysis works.
Citation Formats
K. Yurtışık, “Kinetics and microstructural analysis of fatigue fracture progress in weld joints of duplex stainless steel grade 2205,” Ph.D. - Doctoral Program, Middle East Technical University, 2013.