Ageing and cracking of simulated heat affected zones in 321 stainless steel

The heat affected zone (HAZ) of stabilised austenitic stainless steel welds may exhibit a serious form of intercrystalline cracking during service at high temperature. This type of embrittlement, well known as stress relief cracking, is related to thermal ageing: a fine and abundant intragranular Ti(C,N) precipitation appears near the fusion line during service at high temperature and modifies the mechanical behaviour of the HAZ. To analyse this embrittlement micro mechanism and to assess the lifetime of real components, different HAZ were simulated by various solution heat treatments, cold rolling and ageing conditions. The mechanical behaviour of these resulting materials was investigated using creep and tensile tests on smooth bars. Then, creep tests were carried out and simulated on notched bars by finite element calculations. A damage model was identified from intergranular damage measurements made on notched specimens and compared with calculated mechanical fields. Further tests on fatigue precracked specimens showed that crack propagation occurred under stress relaxation conditions in simulated HAZ material.