Influence of Ni on Austenite Transformation
The affect of a single element on the austenite transformation can be studied by looking at the influence of elements such as nickel on the mechanism of austenite formation. Nickel is one of the most commonly used alloying elements in industrial steel and stainless steel. By looking at the effect nickel can have on austenite transformation, one can gain knowledge of the thermodynamics of this phenomenon.
When nickel is present, it influences the transformation process by changing the kinetics of austenite transformation. Specifically, it modifies the Grain Coarsening (GC) and Grain Refinement (GR) rates. The result of this change in reaction kinetics is a decrease in the onset temperature of austenite transformation and a decrease in the transformation rate itself. The result is that the austenite transformation occurs at lower temperatures with lower rate coefficients.
At higher nickel concentrations (> 20wt%Ni) there is a phenomenon called Widmanstätten grain coarsening which reduces the grain refinement process. This phenomenon occurs when nickel is more soluble in austenite above the upper critical temperature than other elements (e.g. carbon). As this phenomenon occurs, it limits the normal grain refinement process and creates more Widmanstätten grain boundaries, making it easier for larger grains to form. The Widmanstätten grain structure has been observed in steels containing up to 21wt%Ni and temperatures as high as 900°C.
In addition to its influence on the grain structure, nickel can also influence the magnetic properties of austenitic steels. Nickel alters the magneto-crystalline anisotropy of the steel, and can cause changes in the coercivity and saturation magnetization. At higher nickel concentrations, greater anisotropy is observed, resulting in greater coercive force.
Finally, nickel decreases the ductility of austenitic steels and increases their strength. This is due to the cementite particles present in the grains, which increase the hardness of the steel. The higher the concentration of nickel, the greater the presence of cementite in the steel, and the higher the strength.
In conclusion, nickel has a significant effect on the austenite transformation process. It alters the kinetics of austenite formation, as well as the crystal structure, magnetic properties and strength of the steel. It is therefore important to consider the effect of Ni on the austenite transformation when selecting steels for manufacturing processes.
