SEDL / STP / STP612-EB / STP27888S

Effect of Microstructure on the Thermal Fatigue Resistance of a Cast Cobalt-Base Alloy, Mar-M509

Beck, CG
Engineer and senior engineer, Westinghouse Research Laboratories, Pittsburgh, Pa.

Santhanam, AT
Engineer and senior engineer, Westinghouse Research Laboratories, Pittsburgh, Pa.

Pages: 18    Published: Jan 1976

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Abstract

A study was undertaken to determine whether the thermal fatigue response of the cast cobalt-base alloy, Mar-M509, could be modified by changing the casting parameters. Single-edge wedge-type specimens were tested in a fluidized bed using a thermal cycle of 1915/525°F with a cycle time of 6 min divided equally between heating and cooling. The results revealed that the thermal fatigue life of Mar-M509, from the point of view of both crack initiation and propagation, could be significantly improved by changing the casting variables. Cracking was found to be predominantly transgranular and often initiated at interdendritic carbides. A good correlation was obtained between the secondary dendrite arm spacing and the number of cycles for crack initiation, as well as the crack propagation rates. Small dendrite spacings reduced the thermal fatigue life by accelerating both the crack initiation and propagation. Precipitation of MC and M²³C⁶ carbide occurred during the thermal cycling and affected the crack propagation rate. Based on these results, a model for thermal fatigue cracking in Mar-M509 is presented.