Cracking due to Cu and Ni segregation in a 17-4 PH stainless steel piston rod

• The white belts with Cu and Ni segregated in a 17-4 PH stainless steel have been clearly characterized.
• The details of the Cu and Ni segregation belts causing cracking were presented.
• The white belts with Cu and Ni segregated were revealed to be composed of martensite and austenite.
• The cracks could propagate along the interface between martensite and δ ferrite in the matrix.

The 17-4 PH stainless steel is employed to produce piston rods in industry due to the high strength and toughness, good workability and nice corrosion resistance. In the present failure analysis, obvious long cracks were observed along the longitudinal direction on the surface of the commercial 17-4 PH stainless steel piston rod after heat treatment. The cracks were carefully looked into by observing the crack tip and characterizing the microstructure along the cracks. The results showed that the cracks were mostly initiated from the surface of the rod and propagated along the phase boundary between martensite and δ ferrite. The EDXA showed that the segregation of Cu and Ni should be responsible for the cracking after heat treatment. In order to define when the crack was coming into being, oxidation film along the crack was considered as a clue. The scrutiny of the oxidation film on the crack edge illustrated that the crack should be formed right in the heat treatment of aging.