Effect of microstructural features on steel machinability

Non-metallic inclusions can have a tremendous impact on the machinability of steel. Some oxide inclusions such as alumina, spinel, and other refractory abrasive oxide inclusions can have a detrimental effect on machinability. Modification of oxide inclusions by calcium treatment can promote the formation of low melting point oxides such as anorthite and gehlenite inclusions that can assist in chip formation and form a lubricating layer on the rake surface of the machine tool. MnS is known to be beneficial to improving the machinability of steel. They assist in chip formation by being heavily deformed in the primary shear zone. Sulfide inclusions can also be extruded onto the rake surface of the machine tool to form a lubricating layer to resist crater wear. Machinability tests were conducted on three grades of industrially produced steels, and on a grade produced from a laboratory heat to investigate the effect of non-metallic inclusions on the machinability of different steels. From the results it is shown that encapsulating oxide inclusions by a sulfide shell can reduce the tool wear during machining and can assist in chip formation. The number of inclusions per unit area have a direct effect on the flank wear during machining of clean steel. Having an optimal amount of inclusions present in the steel can lead to lower flank wear. Calcium treatment can modify the oxide inclusions to be deformable during machining to assist chip formation, and stabilize the lubricating layer on the rake surface of the machine tool. The effect of grain refinement can lead to a slight improvement in the machinability of a super austenitic stainless steel with an increased amount of TiN and spinel inclusion