Abstract
Welded stainless steel components used in power plants and chemical industries are subjected to mechanical load cycles at elevated temperatures which result in early fatigue failures. The presence of weld makes the component to be liable to failure in view of residual stresses at the weld region or in the neighboring heat affected zone apart from weld defects. Austenitic stainless steels are often welded using Tungsten Inert Gas (TIG) process. In case of single pass welding, there is a reduced weld penetration which results in a low depth-to-width ratio of weld bead). If the number of passes is increased (Multi-Pass TIG welding), it results in weld distortion and subsequent residual stress generation. The activated flux TIG welding, a variant of TIG welding developed by E.O. Paton Institute, is found to reduce the limitation of conventional TIG welding, resulting in a higher depth of penetration using a single pass, reduced weld distortion and higher welding speeds. This paper presents the fatigue crack growth rate characteristics at 823 K temperature in type 316LN stainless steel plates joined by conventional multi-pass TIG (MP-TIG) and Activated TIG (A-TIG) welding process. Fatigue tests were conducted to characterize the crack growth rates of base metal, HAZ and Weld Metal for A-TIG and MP-TIG configurations. Micro structural evaluation of 316LN base metal suggests a primary austenite phase, whereas, A-TIG weld joints show an equiaxed grain distribution along the weld center and complete penetration during welding (Fig. 1). MP-TIG microstructure shows a highly inhomogeneous microstructure, with grain orientation changing along the interface of each pass. This results in tortuous crack growth in case of MP-TIG welded specimens. Scanning electron microscopy studies have helped to better understand the fatigue crack propagation modes during high temperature testing.
Highlights
The Austenitic Stainless steels are known to possess adequate high-temperature physical properties and excellent corrosion resistance
It may be noted that in case of Activated flux TIG (A-TIG) welding, during solidification, maximum thermal gradient is observed in the transverse direction and the same is indicated from the orientation of columnar grains
It was observed that at the specific high temperature, the crack growth rates exhibited by the weld prepared using MPTIG and A-TIG technique was lower than the parent base metal
Summary
The Austenitic Stainless steels are known to possess adequate high-temperature physical properties and excellent corrosion resistance. The available literature presents the details of the effect of different welding process on the fatigue behavior for variety of austenitic stainless-steel weldments [6]. These studies indicate a reduction in crack growth rates in the weldments compared to the base metal. The present work presents the high-temperature fatigue crack growth in type 316L(N) stainless steel welds, joined using two processes: a) conventional multi-pass TIG (MP-TIG) and b) Activated Flux TIG (A-TIG) welding process. The use of conventional TIG for the joining the stainless steel is limited to plates having a maximum thickness of 3 mm, which is a serious limitation [12] for many power plant component building. The load-COD response during fatigue cycling was recorded at periodic intervals of (~ 750 cycles) at a low frequency of 0.5 Hz, over 5 cycles in an otherwise constant amplitude load cycling of the specimen at a typical test frequency of 10 Hz.,
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