Workpiece steels protecting cutting tools from wear A study of the effects of alloying elements on material transfer and coating damage mechanisms

Sammanfattning: The vision of this thesis is to improve the machinability of workpiece steels. Workpiece material frequently transfers to the cutting tools during machining, and the transfer layers then forming on the tools may give both good and bad effects on machining performance and tool life. The objective of this work is to understand the effects of alloying element additions to workpiece steels on material transfer and the roles of the formed transfer layers on friction characteristics and wear of tools.To isolate and study the influence of the individual alloying elements, model steels are specifically designed. These steels include one reference with C as the only alloying element and others alloyed also with single additions or combined additions of 1 mass% Si, Mn, Cr and Al. The experiments are performed using both a sliding test, simulating the material transfer in milling, and a turning test.In a sliding contact, the mode of transfer is strongly dependent on the normal load and sliding speed. Material transfer initiates extremely fast, in less than 0.025 s, and characteristic transfer layers develop during the first few seconds. The different steel compositions result in the formation of different types of oxides in the transfer layers. At the workpiece/tool interface where the conditions involve high temperature, high pressure and low oxygen supply, easily oxidized alloying elements in the steel are preferentially transferred, enriched and form a stable oxide on the tool surface. The degree of enrichment of the alloying elements in the oxides is strongly related to their tendencies to become oxidized.The difference in melting temperature of the oxides, and thus the tendency to soften during sliding, explains the difference in the resulting friction coefficient. The widest differences in friction coefficients are found between the Si and Al additions. A Si containing oxide shows the lowest friction and an Al containing oxide the highest.The damage mechanism of coated tools is chiefly influenced by the form and shear strength of the transferred material. Absence of transfer layer or non-continuous transferred material leads to continuous wear of the coating. Contrastingly, continuous transfer layers protect it from wear. However, transfer layers with very high shear strength result in high friction heat and a large amount of steel transfer. This leads to rapid coating cracking or adhesive wear.