STUDIES ON FATIGUE CRACK GROWTH BEHAVIOUR IN NARROW GAP GMA WELD OF SUBMARINE BUILDING STEEL

Abstract

In present study, the critical welded joints with ultra-narrow, extra narrow and conventional grooves of submarine pressure hull have been prepared by P-GMAW welding and variation in fatigue crack growth rates of critical welded joint with ultra-narrow, extra narrow and conventional grooves has been calculated on the basis of Paris equation using ASTM 647. It is well known that the submarine structure encounters cyclic stresses produced by variation in hydrostatic pressure due to changes in depth during sea maneuvers. The cyclic stresses can cause fatigue failure without any prior warning. By using Paris law based fatigue testing, the characteristics of fatigue failure, life prediction of submarine can be carried out and repairs timeline can be formulated. The welded joints of high strength low alloy steels have different behavior to fatigue based on the microstructure and residual stress developed due to their exposure to weld thermal cycle. Hence, study of effect of microstructure and residual stress on mechanical properties and fatigue crack growth rate of ultra-narrow, extra narrow and conventional groove weld joints has been carried out. In the study, it was observed that the weld zone and HAZ of ultra-narrow groove weld joint possess finer microstructure as compared to conventional groove weld joint along with relatively reduced residual stresses which resulted in better mechanical properties and better resistance to fatigue failure in case of ultra-narrow groove weld joint as compared to conventional groove weld joint.