Effect of Composition, Microstructure and Hardness after Heat Treatment on Machinability of Steel Forgings

Machining is a major process performed on the forged steels. Any change in composition or heat treatment of these steels changes their microstructure as well as properties. Knowing the limited focus on understanding the machinability of forging steels, this study focuses on comparison of dry machinability of ferritic-pearlitic, quench-tempered and bainitic steels under different cutting conditions with a variation in its composition and heat treatment performed on them. The machinability in the dry condition is assessed in terms of cutting and thrust forces, machining temperature, tool abrasion wear on the flank face of tools and chip segmentation. In ferritic-pearlitic steels, prominent mode of chip segment formation is Mode-II fracture (shearing), whereas in the quench-tempered steels, it changes from Mode-I (tearing) to Mode-II (shearing) as the sulfur content increases. In the case of bainitic steels, Mode-I fracture is often prominent. The ferritic-pearlitic and quench-tempered steels depict a reduction in cutting forces in between 5 and 12% and machining temperature in between 12 and 20% due to addition of sulfur even though its strength is higher due to higher vanadium content.