Metallurgical analysis on a bending failed pump-shaft made of 17-7PH precipitation-hardening stainless steel

Abnormal jump and bending occurred on a shaft used in an erect multiple-grades pump when performing a trial run. The failed shaft is made of 17-7PH stainless steel. Microstructure and mechanical properties of the failed pump-shaft material were examined. Two different microstructural zones were exhibited. One is composed of the tempered martensite coexisting the retained austenite, which is the normal microstructure of this grade steel. However, a lot of patchy austenitic regions with needle-like bainite appear, whose amount is up to 17.0% by quantitative metallurgical analysis. The total austenite content was determined to be 40.6 vol.% by XRD. The tensile strength of the failed shaft material is lower and the hardness value is much lower than that of the specified. The simulated heat-treatment tests were conducted on the failed shaft material according to the specified technology. The hardness value of the shaft material treated by the standard process is within the specified range. And the tensile strength values of the material retreated are greater than those of the failed shaft material but the toughness indexes do not reduce. No patchy austenite structure was observed on the specimen retreated and the austenite amount decreased intensely. Presence of a lot of patchy austenite zones in the failed shaft material results in low hardness, tensile strength and poor rigidity of the shaft so that the bending failure occurred on the shaft when performing a trial run. It is suggested that the improper heat-treatment process is responsible for the formation of patchy austenite zone.