Stirling engine heater tube failures

The cause of leaks which had developed in the austenitic stainless steel heater tubes during operation was determined. It is concluded that: (1) the tube leaks were due to intergranular cracking of the tube wall, apparently resulting from a combination of high thermal stresses and grain boundary oxidation; (2) the severe grain boundary attack was associated with carbide precipitation and attendant depletion of chromium in the grain boundaries, producing localized regions of reduced oxidation resistance; and (3) appreciable intergranular oxidation was noted on the I.D., as well as the O.D., of the tubes, indicative of contamination of the hydrogen working gas. Use of a stabilized grade of stainless steel to reduce the incidence of intergranular oxidation is recommended. However, in view of the apparent high thermal stresses imposed on the tubes during operation, a material of greater high temperature strength than 310 stainless steel, such as N-155, is recommended for this application.