Metalforming Magazine
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Matrix
High-Speed Steels
Economical
Alternative to Powders
If
fracturing, chipping and microchipping plague your tooling,
consider using a
new breed of tool steel.
BY
TOM SCHADE
Tom
Schade is vice president of International Mold Steel,
Florence, KY; tel. 800/625-6653;
www.imsteel.com.
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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
 In
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.
 Characteristics Defined
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.
 |
 Heat
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 high-speed
steel.
Reprinted with permission from
Metal Forming Magazine.
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