The effect of the carbon and nitrogen contents on the fracture toughness of Type 347 austenitic stainless steels

Ductile J-R fracture resistance of commercial and modified Type 347 stainless steels for nuclear primary piping is evaluated at 316 deg. C, and the effects of the carbon and nitrogen contents are investigated in the light of the microstructure and microscopic fracture behaviors. The test alloys are selected for controlling the distribution of the carbide precipitates by changing the carbon and niobium contents with an additional variation in the nitrogen contents. The commercial alloys exhibit a relatively low fracture resistance due to the coarse Nb(China) particles in the matrix which act as void formation sites. The reduction of the carbon and niobium contents in the alloys has successfully improved the fracture resistance by eliminating the aforementioned coarse brittle particles. The improvement is greater in the alloys with a low nitrogen content. This arises from the enhanced formation of the coarse Nb(China) particles by the nitrogen in the higher nitrogen alloys especially with a carbon content below 0.04 wt.%. From the simple calculations based on the fracture micromechanism, the critical particle size for crack initiation is found to be larger than that causing crack growth for the Type 347 alloys in this investigation. This critical particle size difference is explained through the triaxiality difference between the stationary crack and the growing crack