Comparing high-speed
steels for wood machining
Proper heat treatment is as important as the selection of HSS.
By Harold A. Stewart

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 HSSes 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 HSSes to determine their wear characteristics. The results will help tool and machinery manufacturers select an 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.

.035

.03

1/X

.025

60.7 61.8

65.3 65.5

65.5 66.3

67.3

Hardness vs. 1/X (X defined as: mean normal force)

0.7 61.8 5. 3 5. 5. 5 . 7. 3

59.5 9. 5

58.5

58.5
8. 5

60 60.1

60.5 60.1 0.5

59.5 9. 5

.02

.015

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 ap-proaches 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. 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.

T15

M2

VWEAR

M42

M4

MAT II

ALLOY Z

VWEAR

ALLOY Z

M4

M2

T15

M42

MAT II

Grade & Rc value

Figure 1 — Comparison of the mean normal forces (Fn) as 1/X where X is the mean normal force and standard deviation versus hardness (Rc) for the high-speed steel grades tested in turning tests. The results are for both the low and high hardness test treatments.

Testing methods

Knives were milled from annealed bar stock of seven unhardened HSS grades. The 1/8x3/8x1-inch knives were then heat treated in salt baths to two hardness levels. The schedules included

References:

http://fdmonline.com

http://www.fdmonline.com

http://fdm.hotims.com

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