HARD TURNING OF PARTS MADE FROM SINTER HARDENABLE POWDERS

摘要

This study'deals with the quantitative evaluation of the machinability of fully martensitic P/M parts made using sinter hardenable powders pre-alloyed with manganese sulfide particles. The main goal of this work was to determine if P/M parts having a typical particle hardness of 60 HRC could be brought to final dimension using a turning operation and remain cost efficient. The results indicate that Polycrystalline Cubic Boron Nitride (PCBN) tools have a tool life that is at least 10 times longer than other types of cutting tools usually specified for machining P/M parts in turning (ex. cermets). Furthermore, experimental evidence shows that the extremely high temperature generated in the secondary shear zone (chip/tool interface) induces a chemical reaction between the manganese contained in the powder and the material that makes up the PCBN tool. This chemical reaction has been shown to greatly accelerate tool wear. Moreover, it seems that there is a threshold temperature below which the reaction between the PCBN tool and the chip does not occur. This threshold temperature is greatly influenced by the cutting speed used. For the case studied here, no significant chip/tool interaction could be observed when the cutting speed was lower than 152 m/min. (500 sfm). A new approach to improve machinability of very hard P/M parts was also investigated. This technique, based on localized surface decarburization, allows to machine a part to its final dimension by cutting only through the decarburized surface layer, leaving the rest of the part fully martensitic. Although still limited in its application, localized surface decarburization has been shown to be an interesting way of significantly increasing tool life.