Valve Seat Fractures
Valve seats fractured during testing and during service. The seats were machined from grade 11L17 steel and were surface hardened by carburization. Investigation (visual inspection, hardness testing, 59x SEM images, and 2% nital etched 15x cross sections) supported the conclusion that the fracture occurred via brittle overload, which was predominantly intergranular. The amount of bending evidence and the directionality of the core overload fracture features suggest that the applied stresses were not purely axial, as would be anticipated in this application. The level of retained austenite in the hardened case layer likely contributed to the failure.