The microstructure of carbon-free Bainitic steel, C45 steel, is composed of primary prior austenite grain and secondary ferrite laths. Ferrite laths grow in the matrix of prior austenite grain and various carbides are also present in the ferrite lath. Due to the high carbon content of C45 steel, secondary carbide precipitates form which take the form of globular and platelet shaped particles.The average grain size of C45 steel is usually between 10-20 micrometers in size, although some instances of larger grains may exist.
The matrix of the microstructure is composed primarily of ferrite, which is further broken down into two main types: Widmanstätten ferrite and lower bainite. Widmanstätten ferrite consists of ferrite grown along the austenitic grain boundaries, shaped much like the interlocking teeth of a scissor blade. This growth of ferrite along austenitic boundaries is also referred to as grain boundary ferrite. Lower bainite is a form of ferrite which is produced during isothermal transformation, that is, during cooling at constant temperatures. This ferrite forms after the Widmanstätten ferrite and occupies the centres of the prior austenite grains.
The microstructure of C45 steel also contains scattering of carbides of various sizes. These carbides are usually seen in the form of platelet and globular particles, most of which contain some form of TiC. A higher temperature of nitriding produces moreTiC, while a lower temperature produces more M7C3, as observed with hard-nitrided layers. Due to the high carbon content of C45 steel, the carbon tends to precipitate out of the matrix and form carbides or graphite.
In conclusion, C45 steel typically presents with a microstructure composed of primary austenite grains, secondary ferrite laths, Widmanstätten ferrite and lower bainite, and various carbide particles. The average grain size is 10-20 micrometers in size, with some examples displaying larger grains. Carbide particles take the form of platelet and globular particles and mainly consist of TiC.
