Thermal embrittlement of carbon steels in canistered waste disposal containers

摘要

It is known that fracture toughness in steels, especially in low steels, in severely reduced by exposure at the temperature range of 350-575(degrees)C through isothermal aging or slow cooling. It is also well recognized that segregation of impurities, such as Sb, P, Sn, As, along prior austenite grain boundaries is the main cause of thermal (temper) embrittlement. The most potent embrittling elements, in order of decreasing potency, are Sb, Sn, P, and As. However, Sb, Sn, and As are not generally present in steels. Therefore P is usually the most concerned element in steels. McMahon has concluded in his review that plain C steels containing less than 0.5 wt% Mn are not susceptible to temper embrittlement. However, he also noted the strong effect of Mn as an enhancement of the P aging of plain C-Mn steels, particularly high Mn contents, to disregard the possibility of thermal embrittlement. A516 plain carbon steels has been been proposed as a corrosion allowable material (CRM) for the outer barrier of canistered waste disposal containers. The expected peak temperature for a waste package container if as high as 200(degrees)C for many years. Therefore concern has been raised whether thermal embrittlement would occur in this steel after the long term temperature exposure expected in the current nuclear waste packages. The current report documents our recent analysis for this problem.