Annealing of nanostructured bainitic steel for improved machinability

Machinability can be thought of as the ability of a given machining operation to achieve the pre-set quality requirements for a given work material while keeping a pre-defined level of process efficiency. As such it can be regarded as a system property, depending on many variables ranging from the tool material, edge geometry, cutting fluid and the characteristics of the work piece. It is sometimes argued that the tool material is the dominating factor determining machinability. However once a certain industrial practice becomes established it is of interest to optimize the properties of the work pieces.

The introduction of embittering particles is commonly accomplished via alloying additions of S, P, Se and similar elements which form inclusions and thereby promote the formation and breakage of chips. The presence of such inclusions however adversely affects the mechanical properties of heat treated high strength steels, therefore this approach is limited to less demanding applications. There is a continuous effort in the development of annealed steel microstructures that could ease machining operations preferably without the need for detrimental additions of inclusion forming elements. Such potential can be found for instance in graphite forming steels.

The implication and usage of graphitized steels is challenging as the size and distribution of graphite particles are difficult to control in most steel. In the current work we present a different approach whereby graphite is precipitated from initial very fine carbide free bainitic microstructure. The high resolution transmission electron microscopy HRTEM method is applied for the characterization of the annealed microstructure, and the mechanical properties of the annealed steel determined and evaluated, with the prospect in mind of obtaining fully dense hardened microstructures with a high hardness of up to 62HRC and adequate toughness.