The influence of thermomechanical treatment on the creep behaviour of din 1.4970 austenitic stainless steel at 973 k

The creep-rupture behaviour of a Type DIN 1.4970 austenitic stainless steel has been investigated at 973 K (700° C) in a high vacuum for three conditions of thermomechanical treatment and a wide range of applied stresses. This type of steel is a candidate for use in the German SNR-300 Fast Breeder Reactor where it shall be used after a 13% cold-working treatment and subsequent aging at 1073 K (800° C) for 24 hours ("standard condition"). As an alternative, two other conditions were alsoinvestigated, namely aged at 1073 K ( 800° C) for 24 hours before the cold-working (condition 2) and cold worked only (condition 1 ). Because of various experimental efforts in this laboratory and elsewhere to study helium induced embrittlement effects in $\alpha$implanted $\underline{foil}$ specimens, all tests were performed using foil specimens of 105 $\mu$m thickness which were solution annealed at 1373 K (1100° C) before the above thermomechanical treatments were applied. The rupture lives and the minimum creep rates were found to be highly dependent on the applied stresses. The treatment of condition 1 material yielded a product as streng as the standard condition 3, while the condition 2 material was less creep resistant. Structural changes as well as f ractography were followed using metallographic, transmission and scanning electron microscope techniques. Transgranular ductile fracture was clearly observed in all three conditions. TEM investigations showed that dispersive TiC precipitates were present in the matrix of condition 3 material before creep testing contrary to condition 1 and 2 material. In condition 1 the TiC dispersion was already found after short creep times, while no dispersive TiC precipitates were found in condition 2 material in every test condition