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Turn up the heat on today's tooling.

Special care needed to heat treat today's new-generation materials.

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 Advanced Materials is a leading peer-reviewed materials science journal published every two weeks. Advanced Materials includes Communications, Reviews, and Feature Articles from the cutting edge of materials science, including topics in chemistry, physics,  after a brief updating on some metallurgical met·al·lur·gy  
1. The science that deals with procedures used in extracting metals from their ores, purifying and alloying metals, and creating useful objects from metals.


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 TO DEGRADE, DEGRADING. To, sink or lower a person in the estimation of the public.
     2. As a man's character is of great importance to him, and it is his interest to retain the good opinion of all mankind, when he is a witness, he cannot be compelled to disclose
 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 annealing (ənēl`ĭng), process in which glass, metals, and other materials are treated to render them less brittle and more workable.  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 Fine Tuning is the name of XM Satellite Radio's eclectic music channel. The program director for Fine Tuning is Ben Smith.

The channel is described as "A musical oasis for the sophisticated listener culled from every imaginable genre and country.
" 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 car·bu·rize  
tr.v. car·bu·rized, car·bu·riz·ing, car·bu·riz·es
1. To treat, combine, or impregnate with carbon, as when casehardening steel.

2. To carburet.
 or decarburization de·car·bu·rize  
tr.v. de·car·bu·rized, de·car·bu·riz·ing, de·car·bu·riz·es
To decarbonize.

 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 carbide, any one of a group of compounds that contain carbon and one other element that is either a metal, boron, or silicon. Generally, a carbide is prepared by heating a metal, metal oxide, or metal hydride with carbon or a carbon compound.  and ferrite fer·rite  
1. Any of a group of nonmetallic, ceramiclike, usually ferromagnetic compounds of ferric oxide with other oxides, especially such a compound characterized by extremely high electrical resistivity and used in computer memory
. 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 aus·ten·ite  
A nonmagnetic solid solution of ferric carbide or carbon in iron, used in making corrosion-resistant steel.

[After Sir William Chandler Roberts-Austen (1843-1902), British metallurgist.
. 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 quenching

Rapid cooling, as by immersion in oil or water, of a metal object from the high temperature at which it is shaped. Quenching is usually done to maintain mechanical properties that would be lost with slow cooling.
 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 o·ver·heat  
v. o·ver·heat·ed, o·ver·heat·ing, o·ver·heats
1. To heat too much.

2. To cause to become excited, agitated, or overstimulated.

 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 Chromium carbide (Cr3C2) is an extremely hard refractory ceramic material. It is usually processed by sintering. It has the appearance of a gray powder with orthorhombic crystal structure. Its CAS number is 12012-61-9.  are the first elements to dissolve; they do so at 1900F. Molybdenum molybdenum (məlĭb`dənəm) [Gr.,=leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.94; m.p. about 2,617°C;; b.p. about 4,612°C;; sp. gr. 10.22 at 20°C;; valence +2, +3, +4, +5, or +6.  and tungsten carbides tungsten carbide
An extremely hard, fine gray powder whose composition is WC, used in tools, dies, wear-resistant machine parts, and abrasives.
 start to dissolve at 1450F.

Vanadium carbides Vanadium carbide, VC, is an extremely hard refractory ceramic material. It is commercially used in tool bits cutting tools. It has the appearance of gray metallic powder with cubic crystal structure.  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

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 deleterious adj. harmful.  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 pre·de·ter·mine  
v. pre·de·ter·mined, pre·de·ter·min·ing, pre·de·ter·mines
1. To determine, decide, or establish in advance:
 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 AISI American Iron and Steel Institute
AISI African Information Society Initiative
AISI Alberta Initiative for School Improvement (Canada)
AISI As I See It
AISI American International Supply, Inc (Oakland, CA) 
 Type M4) and MicroMelt T-15 alloy (AISI Type T15) are P/M P/M Powder Metallurgy
P/M Pipe Major (director of bagpipe music in a Scottish pipeband)
P/M Projectile/Mortar
 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 vanadium (vənā`dēəm), metallic chemical element; symbol V; at. no. 23; at. wt. 50.9415; m.p. about 1,890°C;; b.p. 3,380°C;; sp. gr. about 6 at 20°C;; valence +2, +3, +4, or +5. Vanadium is a soft, ductile, silver-grey metal.  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 quench  
tr.v. quenched, quench·ing, quench·es
1. To put out (a fire, for example); extinguish.

2. To suppress; squelch:
, 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
Geographic Code:1USA
Date:Nov 1, 1998
Previous Article:Feeling the need for speed.
Next Article:PCD-tipped inserts.

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