Failure analysis of holding yokes made of investment cast 17-4 PH stainless steel

Holding yokes made of investment cast 17-4PH stainless steel were too soft (below 20 HRC). Optical microscopy of the parts showed that there is a high fraction of retained austenite after oil quench due to high amount of molybdenum found in parts as an impurity. Subzero treatment in liquid nitrogen after solution heat treating was used to convert retained austenite to martensite but parts became too brittle and fractured during post-heat treating machining. Double and triple aging at 538 °C (1000 °F) after subzero cooling was done on parts. Detailed fractography and tensile testing along with optical microscopy (OM) investigation proved that repeated aging at 538 °C (1000 °F) led to lower strain microstructure and improved elongation without reduction in tensile strength. Tensile and non-destructive testing after machining proved that problem has been solved.