ness, the results could be different. The various alloying elements may affect the wear resistance because they affect the hardness, carbide composition, volume fraction and chemical reactivity.
The combination of alloying elements influences wear resistance. Alloy Z also includes aluminum, which in iron alloys has been shown to reduce high-temperature corrosion and oxidation. Because high-temperature mechanisms have been shown to cause wear in wood machining, the aluminum content in Alloy Z may be beneficial for machining wood. On the other hand, Matrix II and Vasco Wear are low in tungsten and have high mean tool forces at the high hardness heat treatment. Improved M-42 is also low in tungsten but the high cobalt content improved the wear resistance, most likely because cobalt increases hot hardness. Further, M-42 exhibits better wear resistance than some of the HSSes at the low hardness heat treatment. Consequently, the alloying elements, their percentage of the steel, and the heat treatment are all impor-
tant for extended tool life.
The general relation between hardness and tool wear is shown graphically in Figure 1. Low and high hardness heat treatments of HSSes are plotted versus 1/X, where X is the mean normal force, along with the standard deviation. A higher 1/X indicates longer tool life. The plots indicate exceptions to the direct relation between hardness and wear resistance. This reinforces the conclusion that the grade of HSS and the heat treatment are both important in wood machining.
The nominal mechanical properties indicate that M-42, M- 4 and especially T- 15 all have much better abrasive wear resistance than M- 2. However, wear tests in this study show that M- 2 resists wear during wood machining equally or slightly better
than any of the HSSes tested. Consequently, mechanisms other than abrasion cause tool wear when machining wood or even reconstituted wood products, such as MDF. As shown in previous research, mechanisms such as high-temperature corrosion/oxidation affect tool wear.
The proper heat treatment accurately applied to the HSS is as important as the selection of the HSS. For example, two pieces of M- 2 could be heat treated to the same hardness but have different toughness values because of different temperatures in heat treating. The HSS microstructure and mechanical properties should be checked after heat treating knife stock or tools such as router bits to ensure proper heat treating.
Because M- 2 and M-42 performed about the same and slightly better than T- 15 and other alloys after the high hardness heat treatment, M- 2 should be, and commonly is, an economical selection for tool steel, when properly heat-treated for wood machining. If tool steels perform equally in a wear test, the least expensive properly heat-treated steel should be selected.●
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