ICMCTF2007 Session G6: Surface Pre-Treatment, Coating Post-Treatment and Duplex Technology

The physical properties in the surface layer of carbide tools have a significant influence on the tool wear behaviour. In gear hobbing with carbide tools, the phenomenon that the tool life decreases with increasing number of reconditioning cycles has been identified. It is shown that there is a significant correlation between the amount of residual stress in the subsurface and the tool life, and that there is a steady decrease of residual compressive stress from cycle to cycle. This decrease can be linked to the thermal load in the PVD process.

Through micro blasting, the initial stress can be readjusted, and good tool lives or tool lives that are superior to the life of unblasted new tools could be obtained in laboratory studies and trials in an industrial environment. Besides these successes, disappointments have also been encountered. Since many parameters can and must be adjusted in the micro blasting process, blasting is likely to have no or even a negative effect if the set of parameters is not set right. In order to get a better understanding of the aerosol micro blasting technology, fundamental studies on the influence of blasting parameters on the energy content of the aerosol have been conducted. In these studies, several phenomena have been identified. Finally, blasting of coated tools has also been applied successfully with regard to tool life increase. In ongoing research, the effects of blasting of coated tools with regard to residual stress in the coating and the coating´s cycle fatigue are studied. These findings will also be presented.

A commercially available process which thermally cycles products to cryogenic temperatures has been assessed with regard to performance benefits claimed for products treated by the process. This was measured by accelerated drill testing in P20 steel to determine any change in lifetime. Drills were HSS and the treatment applied at selected points during the heat treatment of the drill blanks prior to grinding and also as finished. When tested all applications failed to show any significant improvement in drill life when compared to an untreated group of drills from the same batch of drill blanks. The trials were repeated and analysed using design of experiment techniques, on a larger sample of finished jobber drills, manufactured for internal testing purposes, which included the effect of a surface coating.

TiN was applied to a selected mix of treated and untreated drills and the design analysed for the effect of both coating and cryogenic treatment. A further group of TiN coated untreated drills was cryogenically treated and tested. Although the results from this trial show large differences between the samples, unless early life failures are censored, these are not statistically significant given the variance in the raw data. Microstructure and fracture cross sections are also presented as well as a critique of the statistical techniques employed to reach this conclusion.