ASSESSMENT OF FATIGUE CHARACTERISTICS OF GERMAN STEEL AND STAINLESS STEEL WELDMENTS

Abstract

Structural components are frequently subjected to complex time histories of stress for which the prevalent mode of failure is fatigue. For reliable and cost-effective fatigue life calculations of metallic materials, the systematic characterization of their fatigue behaviour and the detailed understanding of basic fatigue mechanisms are of prime importance. Material of Reactor Pressure Vessel (RPV) should have good fatigue characteristics to ensure safe operation of nuclear reactor. In order to be on safe side the fatigue characteristics of RPV material have to be quantified. In this present work the fatigue characteristics of 20MnMoNi55 steel (the material used for Indian PHWR, German steel) and AISI 304 austenitic stainless steel weldments have been, found using S-N curve. Welds between different steels such as SA 302 Grade B, SA 533 Grade B Class 1, SA 508 Class 2 etc and austenitic stainless steel such as AISI type 304, 31.6 etc are used widely in steam generation of nuclear power plants. Austenitic stainless steel tubes are used in the high temperature sections, such as final stage super heater, reheater and nozzle. The other parts where temperature are lower, ferritic steels are used, which is more economical. 20MnMoNi55 steel and austenitic steel (AISI 304) are joined by. gas tungsten arc welding (GTAW) process with AISI 308 SS filler metal. After welding, ultrasonic non-destructive testing is done in all the weldment to ensure the defect free weld joint. The yield strength (YS) of as-welded and heat treated weldment are calculated to find out the maximum stress value. The fatigue test is carried out for as-welded and heat treatment specimens at different stress values. It is observed that the fatigue lives of heat treated weldment are more as compared to as-welded condition. Mechanical properties such as YS and microhardness are estimated for both as-welded and heat treated specimens. Scanning electron microscopy (SEM) is employed to study the failure mechanism of fatigue fractured surfaces. SEM images of fractured surface are analyzed to study the fatigue failure mechanism of as-welded and heat treated weldment