Effects of absorbed hydrogen and thermal exposure on mechanical properties, crack opening and fatigue crack propagation of welded steel

Abstract

Accurate quantification of the current accumulated material damage in the steel wall of a reactor vessel is essential in assessing the safety and integrity of the structure. In this study, a framework for mechanism-based structural life monitoring and assessment procedure is proposed and examined. The methodology is based on the competition between damage evolution and continual strength degradation of the material throughout the design life of the component. In this respect, damage evolution characteristics of the welded vessel steel are established through controlled laboratory experiments. Two types of steels, type-316 austenitic stainless steel and A516 Gr70 pressure vessel steel are used in this research. The samples (SS316 austenitic stainless steel) were heat treated (HT) at 500°C, 800°C and 1000°C followed by furnace cooling. This work examines the effect of different microstructures of austenitic stainless steel on both static and fatigue responses of the alloy. Type A516 steels are commonly used in welded construction of pressure vessels and boilers. Prolonged exposure to high operating temperature and fluctuating pressure could induce undesirable microstructure evolution, particularly in the vicinity of the welded connection. This in turn, modifies the long-term mechanics response of the structure and affects structural reliability prediction. This research attempts to quantify the effects of absorbed hydrogen and thermal aging on crack-tip plastic zone, impact toughness and fatigue crack growth response of the Type A516 Gr70 steel plate and the associated heat affected zone (HAZ). Ductileto- brittle transition temperature (TDBTT) values of Base Metal (BM) and Weld Metal (WM) are -26°C and -20°C respectively, while TDBTT value of HAZ is -32°C. Results show that a crack continuously grows in the base metal or HAZ with increasing applied crack tip driving force, Ka. The threshold stress intensity factor range, Kth for HAZ (13.2 MPam) is lower than that for the base metal (15.3 MPam). Prolonged thermal exposure further lowers Kth of HAZ to 11.4 MPam.