Comparing high-speed steels for wood machining

High-speed steel is primarily manufactured for cutting tools. In the forest products and woodworking industries, HSS is a common tool material for saw blades, planer blades, moulder and shaper knives, router bits and other applications.

After annealed stock is manufactured into a relatively soft tool, HSS can be uniformly hardened and subsequently tempered. The most important property of all HSS is its retention of hardness at elevated temperatures. Because high temperatures exist at the tool edge and accelerate tool wear, HSS is a good selection for many wood machining processes. However, many different HSS's and their chemical compositions and properties are diverse.

Further, HSS properties particularly wear resistance, require proper heat treating. Consequently, the forest products and woodworking industries are confronted with a large number of combinations of HSS and heat treatments. This study compared seven HSS's to determine their wear characteristics. The results will help tool and machinery manufacturers select a HSS for wood machining.

HSS is heat treated to change its structure, which enhances its physical and mechanical properties for its intended final use. Heat treating is a sequence of heating and cooling steel in the solid state to develop the required properties. This process is as important as alloy composition for HSS properties in service.

Heat treating HSS includes four major steps:

1. Preheating - heating below the austenitizing temperature to reduce thermal shock, minimize distortion, and reduce the austenitizing temperature.
2. Austenitizing - heating the steel above a critical temperature so it approaches a uniform solid solution.
3. Quenching - cooling the steel in oil, water, molten salt, or air to develop hardness and obtain the desired structure (martensite).
4. Tempering - reheating below the critical temperature to obtain the desired combination of hardness, strength, and ductility.

Generally, harder HSS is more wear resistant than softer HSS, but exceptions exist, as this study shows.

The addition of alloying elements to HSS singly or in various combinations can affect one or more of the following five properties:

1. Strength in large sections;
2. distortion in the hardening process;
3. resistance to wear at the same hardness;
4. toughness at the same hardness in small sections; and
5. hardness and strength at elevated temperatures.

Testing methods  

Knives were milled from annealed bar stock of seven unhardened HSS grades. The 1/8 x  3/8 x 1-inch knives were then heat treated in salt baths to two hardness levels. The schedules included preheating and heating temperatures to minimize thermal gradients. The knives were then sharpened for turning tests on MDF in accordance with past methods. The turning test combinations were replicated nine times for each HSS with the depth of cut at 0.005 inch and 550 rpm. The tool force components parallel (Fp) and normal (Fn) to the direction of tool travel relative to the workpiece were recorded with an oscillographic recorder attached to a lathe dynamometer. The length of cut was approximately 7,600 inches for each replication.

Previous results have shown that tool forces are linearly related to edge recession. Thus, tool forces after a predetermined length of cut (7,600 in.) were selected as a tool wear index for these tests and applied as a method for ranking the HSS's for wear resistance in wood machining. The workpiece sample disks were MDF cut from a single source with a density of 46.5 pcf maintained at approximately 8 percent moisture content. The round ¾ -inch-thick MDF test specimens were randomly selected for the turning tests that were replicated nine times for each HSS and heat treatment combination. The terminal tool force components (Fp and Fn) were compared by an analysis of variance (least significant differences). For the purpose of this paper the results are summarized as 1/X where X is the normal force Fn ( see Figure 1 ).

Results and discussion  

Overall, HSS heat treated to a high hardness wore less than HSS heat treated to a low hardness (Figure 1). However, some HSS grades showed better wear resistance over other grades that had increased hardness. For example, M-2 (Rc 65.5) had a higher wear index (1/X) and a lower normal force than T-15 (Rc 66.3) as shown in Figure 1. Other examples in Figure 1 show that HSS's of the same hardness had different wear indexes. Consequently, although hardness is a general indicator of wear resistance, other factors influence wear.

Generally, harder tool materials are more resistant to chemical attack at elevated temperatures (refractory) and/or are better heat conductors. The data in this study suggest that this factor may explain the variation in wear resistance for different HSS grades with the same hardness. For example, the analysis of variance for the normal tool force at the high hardness heat treatment shows M-2 had a lower force than T-15, but T-15 was slightly harder than M-2. Further, M-2 is the same hardness as Vasco Wear at the low hardness heat treatment but their mean normal tool forces are not the same (although they are not statistically different). Hence, chemical attack or hot corrosion could be responsible for some of the wear resistance observed.

An HSS within each high- or low-hardness heat treatment may not necessarily wear less, as indicated by an inspection of the mean tool forces alone ( Figure 1 ). For example, Alloy Z has the lowest mean tool forces in the low hardness heat treatment, but Alloy Z is also slightly harder than the other HSS's. If the tool steels had the same hardness, 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 HSS's at the low hardness heat treatment. Consequently, the alloying elements, their percentage of the steel, and the heat treatment are all important 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 HSS's are plotted versus 1/X, where X is the mean normal force, along with the standard deviation. A higher 1/X indicates a 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.

Wear resistance  

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 HSS's 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. Thus, refractory properties as well as others are also important for selecting HSS for wood machining.

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 HHS microstructure and mechanical properties should be checked after heat treating knife stock or tools such as router bits to insure 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. These results should help provide a basis for selecting tool steel for woodworking.


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About the author

Harold A. Stewart wrote a tooling column for FDM magazine, looking at ways to improve the cutting process. He was previously an educator and consultant to wood products companies.