Metal Forming Magazine
Matrix High-Speed Steels
Alternative to Powders
fracturing, chipping and microchipping plague your tooling,
a new breed of tool
Schade is vice president of International Mold Steel,
Florence, KY; tel.
Powder metallurgy offers significant
advantages over traditionally melted
cold-work and high-speed steels. The
ability to produce highly alloyed steels
free of segregation, with uniform grain
structure and carbide distribution,
allows powdered-steel producers to make
claims about their steels’ high
performance. If your problems revolve
around pure abrasive wear, many of these
claims prove true.
Looking Beyond Abrasive Wear
the Japanese tooling industry, it has
long been accepted that abrasive wear
is not always the cause of premature
tool failure.Often, fracturing, chipping
and microchipping—popping of
individual or groups of carbides from a
cutting surface—are the culprits. Japanese
end-users sought a lower-cost material
that could address all toughness and fatigue
problems, so the Japanese
specialty-steel industry searched for a
class of wrought alloys to do just that.
The research efforts of Nachi-Fujikoshi
Corp, Daido Steel Ltd and others resulted in a new series of
matrix high-speed steels. As is common in Japan, these steels are
not classified by AISI or JIS standards. Each producer markets its
variation under a trade name.Nachi has the MDS series, MDS1, MDS3,
MDS7 (matrix high-speed) and MDS9 (high-toughness cold-work).
Daido has MH85, MH88 (matrix high speed) and DC53 (high-toughness
coldwork die steel).
Carbide Size and Quantity Limit Effectiveness
Consider D2, a low-cost high-hardenability cold-work die steel.
Large carbide particles and lack of toughness can limit the life
of tools made from D2 in certain applications, and the large
carbide particles also compromise machinability and grindability.
In the 1980s, Japanese researchers realized that by lowering
alloying elements and increased hot-working grain refinement, they
could produce super-tough fine-grained cold-work die steels with
advantages over D2. Thirty-percent improvement in machinability
and grindability, improvements in impact and fatigue strength, and
higher hardenability at high draw temperatures caused the Japanese
tool industry to gravitate to these steels throughout the 1990s.
The Japanese specialty-steel
industry then shifted its focus to overcoming the inherent
low-fracture resistance of high-speed and powdered steels. The
premise behind development of matrix high-speed steels: Carbides
were the problem, not the solution. Developers already had shown
with the high-toughness cold-work materials that reducing the size
and quantity of carbides had benefits. With matrix high-speed
steels, alloy elements are resubjected to a solid-solution
treatment with a base (matrix), reducing the quantity of carbides.
The end result offers high toughness and fracture resistance.
Additional benefits include improved machinability and
grindability. The trade off is reduced abrasive wear when compared
to M2 or powdered steels.
Toughness—Matrix high-speed tool steel exhibits high toughness and
fracture resistance due to the special characteristics of its
microstructure. Fig. 1 shows the Charpy impact value of the MDS
series and Fig. 2 (right) shows its deflective strength.When the
Charpy impact value is compared at the same hardness, the value is
higher for matrix high-speed steel than for M2 or D2. The same can
be said for deflective strength. Figs. 3 and 4 show similar
results when the matrix formula is compared to high-vanadium (10)
powdered high-speed steel. This underscores the retention of the
matrix steel’s wear resistance and improved fracture resistance.
Machinability—Benefits due to fine grain structure and
reduced carbides in matrix high-speed steels include improved
machinability and grinding after heattreatment. Fig. 5 compares
the grinding ratio (test-piece weight reduction divided by
grinding-wheel weight reduction) as the indicator of grindability—a
higher ratio indicates improved grindability. Reduced machining
and grinding costs significantly reduce tool costs.
treatment—Matrix high-speed steels are less sensitive than other
high-speed steels to the slower cooling rate of vacuum heattreat
furnaces. For example, the center hardness of an M2 100-mm bar loses
three points of HRC as a result of slower vacuum-furnace cooling.
Matrix high-speed steels do not lose HRC points. Table 1 shows
austenitizing and temper temperature ranges as well as expected
hardness ranges for MDS steels
Wear Resistance—Cold-work die
steels offer a wide hardness range, extending from HRC 58 to HRC 65.
Matrix high-speed formulations contain steels with differing
wear-resistance levels. For example, the hardness for one is HRC 58,
and HRC 62-64 for another. These levels are controlled by the quantity
of the matrix. The results of this control can be seen in Fig. 6,
where a lower wear ratio indicates superior wear resistance. Fig. 7
compares abrasive wear resistance to high-vanadium (10) powderedmetal
Reprinted with permission from Metal