1995 Volume 81 Issue 7 Pages 721-726
For the purpose of high strength austempered ductile iron (ADI) has the potential to reduce the size and weight of automotive parts, however poor machinability has made it difficult to achieve broad commercial application.
To clarify the mechanism of poor machinability of ADI, (i) the chips was analysed by X-ray diffraction, (ii) the cutting front was observed after instantaneously stopping using a quick stop device, (iii) for the turning tool wear was measured and (iv) sawability of material was compared using a saw test.
The results were as follows :
In the low cutting-speed range, it was found that strain induced transformation from retained austenite (γR) to martensite (α') occurred, and this corresponded to the poorest machinability in the saw test.
In the high cutting-speed range.γR →α' transformation was limited to the damaged layer, because the temperature of chips exceeded Md point.
The tool wear during turning of ADI was generated closer to the cutting edge, as compared with turning of steels. Observation of chip forming state showed that this phenomenon resulted from the following two factors : a) the formation of saw-tooth like chips and the decrease of the tool-chip contact length due to the spheroidal graphite, and b) the increase of cutting force due to the high hardness of the bainitic phase and the damaged layer hardened by the γR →α' transformation.