Mechanisms of plastic deformation and fracture of austenitic chromium-nickel steel irradiated during 45 years in WWER-440

Abstract

The test results of tensile round bars from austenitic stainless steel of 18Cr-10Ni-Ti grade (Russian analog of AISI 321 steel) irradiated during 45 years in Pressure Water Reactor of WWER-440 type with various damage doses are represented over wide temperature range. The standard mechanical properties including the fracture stress and the fracture strain are determined, and the fracture mechanisms are studied. A phenomenon of decrease in the fracture strain with decrease of temperature is revealed that is accompanied by transition to intergranular brittle fracture. This phenomenon is termed as low temperature helium embrittlement (LTHE). On the basis of the analysis of other published works it has been concluded that LTHE is sufficiently common phenomenon that is revealed for various austenitic steels irradiated with mixed neutron spectrum.It is shown that the strain-induced martensite and low cohesive strength of grain boundary (GB) due to the helium accumulation on GBs are two necessary factors which cause LTHE of the irradiated austenitic steels. Mechanism of microcrack nucleation on weakened GB is proposed that takes into account the morphology of the strain-induced martensite formed near GB.Some features of plastic deformation over the temperature range of LTHE are revealed and explained as follows: (i) a running neck and large yield drop; and (ii) a difference in the true stress-strain curves for the cross-section with the originally arisen neck and for other cross-sections of tensile round bar over initial plastic strain range.