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Understanding tool-steel service life: why the ill-tempered and mistreated take early retirement.

When tools are made from tool steels, the steel's inherent ideal characteristics, plus those actually derived from its heat treatment and manufacture, will seriously affect its service life.

Obviously, chemical composition is critical. You woldn't pick a steel low in heat-resistant alloys for a high-heat application, or use a highly alloyed tool steel for a simple carpenter's wood chisel. But even with the right alloy choice, it pays to understand the signs that the cutting tool or die that your shop is counting on so heavily has not been properly finished.

Making the grade

What are the key mechanical properties of a cutting tool or die? The hardness, toughness, heat resistance, and wear resistance of the final heat-treated tool. Carbon is the most important tool-steel element--it gives steel its hardness and contributes greatly to its wear resistance. Chromium, vanadium, molybdenum, tungsten, and cobalt (with sufficient carbon) contribute to wear resistance and can develop heat resistance and red hardness. Silicon and nickel contribute toughness to tool steels, and silicon can add heat resistance as well. Sulfur, carefully added in percentages up to 0.1, can improve machinability with no effect on other mechanical properties.

There are three general categories of tool steels (covering 71 of the 85 AISI standard analyses for tool steels):

1) Hot-work tool steels are used for forging, die-casting, and light-metal extrusion dies where the work temperature exceeds 400 F.

These grades (AISI H12, H13, H21, and H42) are typically low in carbon with high percentages of alloying elements, particularly chronium, with varying amounts of molybdenum, tungsten, vanadium, and silicon. They exhibit excellent toughness, adequate hardness (Rc 55), and good working hardness to 1000 F, with the higher alloyed steels yielding greater heat resistance and elevated-temperature wear resistance at some expense in toughness.

A recent advance has been the higher vanadium steels with added carbon (Latrobe's Koncor, for example) that solve wear-resistance problems at high temperature, but are not advised for heat cracking or checking problems.

2) Cold-work tool steels are used for cold-heading, blanking, trimming, and other work where the temperature of die and work does not exceed 400 F.

These grades range from the simplest, W1, which contains only carbon as an alloying element, to D7 with 19.25-percent alloy content (2.25-percent C). Generally, wear resistance increases with carbon content, and toughness decreases. There have been many recent wear-resistance advances, culminating in D7, hardened to Rc 67, the best abrasion-resistant tool steel for cold-work applications.

3) High-speed steels are used in cutting tools of all kinds, and sometimes in cold-work applications for their wear resistance.

There are over 28 HS grades, but only seven comprise the majority of tonnage produced: T1, T15, M1, M2, M3, M10, M36, and M42 (the latest super-hard, super-high-speed steel). Although the T1 analysis is essentially unchanged from that developed in the 1900s at Bethlehem Steel, the vast increase in red hardness of this group gives these steels ability to cut under speeds and feeds that would cause immediate failure in lower-alloy grades.

Mistreated

What happens when recommended heat-treatment procedure are not followed exactly? With underhardening (heating to a lower temperature), the tool will show substantial dimensional movement after hardening (expansion), and will be deficient in red hardness and room-temperature hardness. The only advantage is the tool is considerably tougher than those hardened at normal temperatures and tempered back to the same hardness.

Overheating tool steels beyond normal hardening temperatures coarsens the grain and increases brittleness, developing a retained austenite microstructure. These steels are relatively soft, shrink drastically from expected dimensions, and have little value as a tool. They can be partially salvaged by refrigeration and tempering, but remain coarsely grained.

Since HSS hardening temperatures are so high, incipient melting will occur when these temperatures are exceeded. A spider-web of eutectic microstructure results from the partial melting and refreezing of the carbide-rich areas. The tool is quite unsuitable because of extreme brittleness, and unsalvageable short of reheating and reforging to a smaller dimension. Ill-tempered

Overtempering--heating a hardened tool steel to a higher temperature than required to reach the desired Rockwell-produces a softer tool. Such tools can be salvaged by reannealing and rehardening, but with some risk of surface-condition problems and size change.

An undertempered tool is usually discovered by hardness testing--it's too hard. This is simply corrected by heating to a higher temperature. Undertempered tools should not be used in production, not only because of their higher hardness, but also because of their brittleness caused by high levels of residual stress.

Undertempering sometimes cannot be discovered by hardness tests. For example, a HSS tool untempered at 900 F will have precisely the same Rockwell as one tempered at 1125 F, and thus falsely imply good service life. Fortunately, a check of its microstructure will show the undertempered tool much lighter than one properly tempered. Also, due to a dip in the tempering characteristic curve, a tool with a Rockwell on the low side--which would seem to indicate overtempering--may in fact be undertempered. This can be resolved by retempering to the right temperature. If undertempered, the hardness will increase to the full Rockwell value.

If tools are ground sufficiently after hardening, surface effects generated during hardening will probably be removed and of no great importance. But if tools are hardened in the finished condition, correct hardening-furnace atmospheres are very important.

Carburizaion is particularly dangerous in the hot-work steels that depend to a large extent on their relatively low carbon content for toughness. Carburizing these steels results in a brittle surface layer that can easily develop heat checking in service, or even chip out or cause catastrophic die failure. HSS carburization is less frequent, but similarly develops an extremely brittle surface layer that, if concentrated at the tool tip, can cause rapid failure.

Slight amounts of carburization on HSS tools can be an advantage with rugged tools requiring maximum wear resistance. If atmospheric control on furnaces is to vary, it is much better for tool performance to err on the carburizing side, rather than on the decarburizing side.

A tough grind

The ideal grinding of hardened steel is like a miniture milling operation--sharp points of abrasive grains remove tiny chips just like the cutting edge of a milling cutter. When this is achieved, the cutting-tool clearance angles can be achieved with little damage to the fragile working edges.

Unfortunately, when a hard cutting tool is ground with an improperly dressed wheel, a too-hard wheel, or incorrect feeds and speeds, rubbing instead of cutting occurs, generating damaging heat at the wheel/cutting-tool interface. The heat-affected zone is extremely shallow, but moves rapidly across the tool surface, causing three serious metallurgical effects.

First, the surface zones of the steel are rapidly expanded, then rapidly cooled by conduction to the cooler metal underneath. When the grinding effect is irregular, this sets up surface stresses (usually tension) that can distort fragile tools or reduce the strength of more solid tools to the extent that they break under loads that would normally cause no difficulty.

Second, under severe grinding conditions, the rapid expansion and contraction of the very thin surface layer (0.0001" to 0.0003") can cause formation of very shallow surface cracks that act as stress raisers. These cracks sometimes progress to catastrophic failure in the toolroom, even before the tool is used. Once in use, the tool will invariably give poor production life, or fail by edge chipping or the breakage of the entire tool.

Third, severe grinding heat can soften or retemper the tool. Rehardening the surface layer leads to retempering; the tool is softened in these areas, and its value as a cutting tool severely decreased. Softening of a few thousandths at critical relief angles can cause very rapid wear, destruction of clearances, consequent generation of even more heat during cutting, and rapid tool breakdown. When the surface layer of ground tools is actually rehardened, not only is the rehardening ineffective, but the affected layer is left highly stressed and untempered, and these high internal stresses lead to rapid tool failure.

As the alloy content of the cutting tool is increased--particularly in vanadium carbides--grinding difficulty is greatly increased. Just as carbon tool steel is much easier to grind than HSS, some HSS grades are easier to grind than the high-carbon/high-chrome D2 cold-work tool steels. For example, D7 is extremely difficult to grind in the hardened condition because of its high alloy content and high vanadium-carbide content. Yet, some of the newer super-HSS grades at Rc 69-70 are considerably easier to grind than high-vanadium HSS at Rc 65-66.

An important factor is proper heat treatment. Tools with the optimum balance between hardness and toughness will grind easier than those either overheated to a brittle structure or undertempered so austenite remains. Since an overheated tool has less ductility, the effects of grinding that cause incipient cracking will occur at an even lower stress level (sometimes even below that developed under proper grinding conditions). A steel with considerable retained austenite will be difficult to grind because the transformation of austenite by grinding heat adds to grinding stresses, often causing cracking under conditions that would not normally cause this to happen.

For more information on metallurgical factors affecting tool-steel service life, circle 526.
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Dec 1, 1985
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