Turn up the heat on today's tooling.
Experience and instinct may not be enough for those who heat-treat tools made of today's high-tech materials. Toolmakers can benefit from a better understanding of these advanced materials after a brief updating on some metallurgical concepts.
First, heat treating is the key to having good, reliable tools. One can start with the best alloy for the job and employ the finest machines and operators, but without sound heat-treating procedures and equipment it's all a waste of time and money.
Next, it's the toolmakers responsibility to capitalize fully on the special properties of materials. That can happen only by heat treating with special care, conscientiously following the recommendations of the manufacturers.
Why do it?
However basic it might sound, one needs to remember that all heating and cooling operations constitute heat treatment - regardless of purpose. Heat treatment takes place when a torch is applied to straighten a tool, or when a tool is placed in a furnace and left there a couple of days to promote stress-relief.
When a tool is heated, its metallurgical structure is changed. Thus it behooves the toolmaker to have a specific purpose in mind when heat treating. The toolmaker can improve the tool with good practice, or degrade it by making a mistake or using bad judgment.
Tool steel is supplied in the annealed condition so that it is easier to machine. After machining is completed, further annealing removes the stresses set up by the machining or grinding operations. It is advisable to follow the steel manufacturer's guidelines for stress relieving because procedures may vary greatly. Generally, the best sequence is to rough machine, stress-relieve, then finish-machine or grind.
Tools are hardened and austenitized to develop strength and wear-resistance. Again, optimum results are generally obtained by following the steel producer's recommendations.
Sometimes the toolmaker may be able to enhance a specific property, such as toughness or wear-resistance, by experimenting or "fine tuning" the heat-treating schedule provided. Occasionally, the toolmaker can pull it off without assistance, but more often the final result is achieved with technical assistance from the steel manufacturer. When heat-treating adjustments fail to produce the refinement desired, another steel might be considered.
When austenitizing, care must be taken to avoid carburization or decarburization by carefully monitoring the heat-treating atmosphere. Changing the surface composition by decarburization or carburization can cause stresses throughout the tool.
The process itself
It is helpful to understand what happens during the steel-hardening process. At room temperature, steel is carbide and ferrite. Upon heating, the steel expands. No metallurgical change takes place until the critical temperature is reached. Several important things happen at that point.
First, the matrix recrystallizes and becomes nonmagnetic. The name of this sort of iron is austenite. Next, the carbide begins to go into solution; then the steel, which was expanding, suddenly contracts. This metallurgical phenomenon during hardening makes it essential that the tools are heated uniformly, by using controlled heating rates. Otherwise, a tool placed in a furnace, without the benefit of uniform heating, will crack and become useless.
In the case of plain carbon steel, heating 100F above the critical temperature and quenching would harden the steel and give the properties desired. With highly alloyed steel, such as high speed steels, more than hardness is desired. Additional properties, such as red-hardness, are needed. To obtain these properties, it is important to dissolve and keep in solution as much carbide as possible. It may be necessary to heat almost 1000F above the critical temperature. However, heating too high will cause grain growth in the overheated austenite, making the finished tool brittle.
The chart (below) shows how much of each alloying element is dissolved during the hardening of Micro-Melt M4 alloy, and at what temperatures. The chromium carbides are the first elements to dissolve; they do so at 1900F. Molybdenum and tungsten carbides start to dissolve at 1450F.
Vanadium carbides do not start to dissolve until the temperature of the steel reaches 1900F. That is why heat treatment is recommended at 2100F instead of say, 1850F. The toolmaker who heats at that lower temperature is wasting money and time because he is not taking advantage of the alloy's carefully balanced analysis. That also means he will not get the properties desired.
Solution Treatment Variation Result Underheating Lower Strength No increase in toughness Overheating No increase in strength Decreased toughness Too rapid quench Possible quench cracking Lower strength Too slow quench Lower toughness AerMet-for-Tooling alloy must be solution-treated, refrigerated, and aged. In solution treatment, the carbides and key elements are dissolved and the alloy is in the solution-treated condition. Shown here are the effects of procedural variations on critical properties.
The ideal hardening temperature is that which takes into solution the maximum amount of carbide with the minimum amount of grain growth. Grain that is too coarse tends to lower toughness. Grain that is too fine does not allow the full development of the alloy's essential properties.
Quenching may be defined as a controlled cooling operation. Cooling may be conducted in various media - most commonly air, oil, or water. The medium used depends upon the chemical analysis and hardenability of the steel. In the heat-treating process, quenching is one of the most important and least understood operations.
At this stage the steel hardens or solutions, depending on the alloy. Consideration at this point must be given to the deleterious effects of warpage, size change, cracking, internal strains, and soft spots.
All advanced tooling materials must be cooled at a specific rate that is generally prescribed by the steel producer. The rate must be sufficient, regardless of the cooling medium, to ensure the properties desired. The best methods depend on the type of alloy, tool, and equipment used. One necessary precaution with all tool steels is to make sure the tools reach about 150F before the tempering operation.
Tempering is a heating operation applied to a tool after hardening to relieve strains and increase toughness. Bear in mind that the process usually causes the tool to lose some hardness. Tempering is achieved by heating to a specific temperature for a prescribed length of time. The total time required is the heating time plus the soaking time. Frequently misunderstood is the length of time required to heat a piece of steel to the low temperatures used in tempering. It actually takes three to four times longer to heat a steel section to 400F than it does to heat that same section to 1500F.
In no case should a tool be tempered for less than an hour. That is the minimum time needed to produce a reliable, good-performing tool. Additionally, careful control must be established to ensure that the tool is soaked for a precise, predetermined length of time. This must be a repeatable, disciplined process to ensure that key properties are obtained.
From a practical standpoint, it is well to consider that the useful strength of any tool is equal to the total strength of the steel, minus the amount of the internal stress introduced by quenching. That is why a major goal of tempering is to reduce that internal stress. Too much internal strain puts the tool at risk of breaking.
RELATED ARTICLE: Alloys emphasize strength
Advanced tooling materials are designed for the most sophisticated yet robust procedures, such as aerospace manufacturing. Accordingly, alloys are developed to emphasize strength in their applications.
For instance, Micro-Melt M-4 alloy (AISI Type M4) and MicroMelt T-15 alloy (AISI Type T15) are P/M high speed steels used mostly for cutting tools and more recently for punches and dies. The M-4 alloy is designed to ensure wear-resistance on the production floor.
Micro-Melt 10 alloy (AISI A11) and Micro-Melt 9 alloy are high vanadium P/M tool steels. These alloys have been used for a variety of punches, forming and compaction dies, and blades and extrusion tools.
Several nontool steels are used for difficult tooling applications because of their toughness. They include Carpenter AerMet-for-Tooling, NiMark 250 and NiMark 350, as well as high temperature Pyrotool 7, Pyromet A-286, Pyrotool A, Pyrotool V, Pyromet V-57, and Carpenter Waspaloy.
Not surprisingly, these materials require different heat treatment. The high vanadium P/M steels can be heat-treated from 1850F to 2150E The P/M high speed M-4 alloy and P/M T-15 alloy can be austenitized, quenched, and tempered at 2200F to 2250F. AerMet-for-Tooling and the managing alloys should be solution-treated and aged following the manufacturer's recommendations.
Overall, the heat treater should use a furnace that provides accurate control of temperature and the protective atmosphere that such alloys require.
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|Title Annotation:||heat treating|
|Author:||Garner, Harrison A.|
|Publication:||Tooling & Production|
|Date:||Nov 1, 1998|
|Previous Article:||Feeling the need for speed.|
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