Variables affecting cutting performances are estimated by several investigators to run into millions. These figures are raised to astronomical proportions and are therefore of no practicable value if we take into account variations in the analysis of the steel being cut, and the effect of the various phases in steelmaking, i.e., rolling, heat-treatment, cold-working, etc. Naturally, metallurgical control is essential if satisfactory machinability is to be attained. Specific cases are given, illustrating how increases in hardening elements will affect machinability. Studies of chip sizes and characteristics, supplemented by color photographs, give interesting information on machining properties and tool wear. Microhardness tests are used to study metallurgical differences and the effect of elements such as nitrogen and boron on the various constituents of the steel.

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