Fatigue and Fracture of a 17-4 PH Shaft

A drive shaft from a fire suppression system pump experienced repeated failures. The shaft was originally constructed of case hardened 4340 steel, but after the second failure the shaft was replaced with 17-4 PH for increased strength and fracture toughness. The failure of the third shaft was investigated to determine the cause of failure. The crack appeared to have progressed through the shaft due to fatigue loading, and initiated at the key on the shaft which forms a natural stress riser. However, scanning electron microscopy imaging of the fracture surface revealed intergranular fracture along prior austenite grain boundaries and quasi cleavage throughout the fatigue crack region and in part of the final overload region. This fractography is consistent with hydrogen embrittlement of the material, and the material is susceptible due to the high strength of the shaft. The origin of the hydrogen is most likely microbiologically induced corrosion (MIC). The system is only operated very intermittently and communications with the plant have revealed that MIC has been a known issue in this system previously. The microbes likely poplulated the fracture surface as the fracture progressed, and in doing so provided a continuing source for hydrogen attack. Once the final overload fracture initiated, the depth of the hydrogen intrusion led to a brittle fracture mode on the first section of the overload that then transitioned to a ductile fracture as the depth of the crack exceeded the depth of hydrogen penetration into the material.