Relationship between creep rupture properties and microstructures of Type 316 stainless steels with different carbon and nitrogen contents

It is quite important to understand the relationship between creep rupture properties and microstructures for estimating of long term creep behaviour of fast reactor structural materials. For this purpose, the microstructural changes of a newly developed low carbon medium nitrogen Type 316(316FR) stainless steel has been investigated comparing with medium carbon low nitrogen Type 316(SUS316) steel. Creep rupture tests were conducted at 550degC and 600degC up to 40000 h. The microstructures of specimens ruptured and also those aged were observed with electron microscope, and solute concentrations were assessed by analyzing extract residues of aged specimens. 316FR has higher rupture strength and ductility than those of SUS316 for long time creep. Laves phase precipitates mainly on the grain boundaries at 550 and 600degC and also in matrix after 10000 h creep at 600degC for 316FR. The Laves phase on the grain boundaries and in matrix does not decrease rupture ductility but that in matrix may result in loss of rupture strength due to solute molybdenum depletion. Solid solution hardening by nitrogen is effective for a long period of time, because of extremely small amount of nitrides. On the other hand, large amount of carbides precipitate on the grain boundaries and in matrix in SUS316. They cause loss of the rupture strength due to the decreasing in solute carbon content. Precipitation of carbides on the grain boundaries results in the decrease in ductility because of the grain boundary embrittlement. (author)