TABLE OF CONTENTS Nickel is a very ductile metal, which does not undergo transformations in its basic fcc crystal structure up to its melting point and in general is more difficult to machine compared to other metals. This holds also for many nickel alloyed steels. Because most high strength PM steels are based on alloying with nickel, and since the machinability of newly developed nickel-less steels was not investigated so far, for nickel containing steels methods are needed to diminish the deteriorating effect of nickel on machinability.
In terms of the adverse effect on machinability, it is important to consider the higher concentration of nickel in interparticle necks areas between the starting iron powder particles. Ni alloyed sintered steels are commonly prepared from mixed or diffusion bonded, i.e. chemically heterogeneous powders, and during sintering the active diffusion of nickel occurs mainly in the small contact areas (due to higher dislocation density) than in the bulk material, esp. in the grain cores, the diffusivity of Ni in austenite being very slow. This also results in the formation of a nickel martensitic microstructure in localised areas. This type of martensite, which appears white after etching, is only slightly harder than ferrite. On the other hand, hard martensite shells form around the starting iron particles in combination with carbon, whereas the particle cores remain softer. This is a great problem for the machinability of nickel alloy steels. It must be noted that also wrought steels containing nickel exhibit poor machinability.
The...
