Experimental evaluation of temper aging embrittlement of cast austenitic stainless steel from PWR

Instrumented impact and nano-indentation tests were carried out to investigate the temper aging effect on the strength and toughness of a cast austenitic stainless steel from PWR primary pipe. SEM and TEM were used to investigate the fracture photography and the micro-structure evolution during temper. From the instrumented Charpy impact tests, the dynamic ultimate strengths of the temper aged specimens increase while the impact energy reduces with the increment of the temper aging time. The fracture surface patterns are changed from the ductile fracture with shallow dimples to the cleavage fracture caused by the dislocations piling up in the ferrite matrix. TEM investigation reveals that mechanical behavior of aging effect is caused by dislocations and precipitate interactions in ferrite phase. The Cr-enriched δ′ phase hindering the dislocations movement results in the increase of the internal stress and stress triaxiality, then causes an increment in strength/hardness and a sharp decline in the impact energy. The nano-indentation hardness increases while the nano-indentation plastic energy decreases and results of the ferrite phase changes much more than those of the austenite phase for temper aging.