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
