The machinability of gray cast iron is well-knwn to depend on its graphite morphology and matrix structure. However, machinablility occasionally changes even if graphite morphology or matrix structure has not varied. We have therefore focused attention on seasoning and stress relief annealing known to improve machinability without changing the graphite morphology or matrix structure, and carried out machining tests and a series of ultrasonic transmission speed measurements, tensile test, hardness test and structure evaluation while varying the conditions for seasoning and stress relief annealing. As a result, it was found that seasoning applied specimens subject to seasoning greatly vary in machinability despite small changes in the ultrasonic transmission speed, tensile strength, and hardness. Furthermore correlation between ultrasonic transmission speed and amount of machining tool wear was seen in some parts of the stress-relief-annealed specimens with little changes in the optical microstructure and mechanical properties.
