Fatigue crack path in Cu-Ni-Mo alloyed PM steel

A detailed qualitative fatigue crack path study is made on Ni Cu alloyed powder metallurgy (PM) steel with Mo prealloyed base powder. The microstructure is characterized by upper bainitic cores of the base powder particles surrounded by a Cu-rich martensite with islands of Ni-rich austenite. Earlier observations have shown that the visible surface cracks do not pass through the Ni-rich areas. The aim of the present study is to find the crack path in relation to the Cu-rich martensite and the Ni-rich austenite also below the surface. Successive grinding in steps of about 20 mum is made, and the surface crack in each level is recorded by light optical microscope (LOM). The main part of the crack path is found through high temperature bainite or along the interface between martensite and high temperature bainite. Ni-rich austenite is surrounded by martensite that hinders the crack from entering into austenite. The result indicates that austenite as such is not a strong crack stopper.