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"Heat Treatment of Tool Steels"

 Most punches, dies, stamps, and other metalforming tools require high hardness, usually above about 55 Rockwell C, to perform successfully. However, they are supplied to tool builders in the annealed condition, around 200/250 Brinell (about 20 Rockwell C), to facilitate machining. They must be heat treated to develop their characteristic properties, making the tool capable of withstanding the pressure, abrasion, and impacts inherent in metalforming. Each step of the heat treating cycle performs a specific function. Like the links in a chain, the final product is only as good as its weakest component. Heat treating typically accounts for less than 10% of the cost of a tool, but it may be the single most important factor in determining the performance of the tool. There is no such thing as an acceptable shortcut in heat treating tool steels. 

The high hardness structure of tool steels, called martensite, cannot be produced directly from the soft annealed structure. Tool steels must undergo 2 intermediate structure changes during the heat treating process. First, the annealed structure, called ferrite, must be heated to become a high-temperature structure called austenite. Then, the austenite must be cooled relatively quickly to become martensite. In order to accomplish these structural changes in tool steels, 4 primary heat treating steps are used: preheating, austenitizing, quenching, and tempering. The table shows variations among those steps for 10 typical grades of tool steel. Each step is discused in detail in the following sections.

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Preheating, or slow heating, of tool steels provides 3 practical benefits, although it does not directly affect the final properties of the steel. First, most tool steels are sensitive to thermal shock. A sudden increase in temperature up to 1500/2000øF may cause tool steels to crack. Preheating to one or more intermediate temperatures allows more gradual heating.
Second, tool steels undergo a change in density when they transform from ferrite to austenite. If this volume change occurs non-uniformly, it can cause unnecessary distortion of tool, especially where differences in section cause some parts of a tool to transform before other parts have reached the required temperature. Tool steels should be preheated to just below the temperature at which austenite forms (called the critical temperature), and then held at that temperature long enough to allow the full cross-section to reach a uniform temperature. Then on further heating, the tool will transform more uniformly, causing less distortion to occur.

Third, steels conduct heat faster at higher temperatures. Thus, a preheated tool may require slightly less furnace time at the hardening temperature to reach uniformity. Minimizing time at temperature is a good practice for maximizing the toughness (impact resistance) of the tool. In some cases, particularly involving hardening temperatures over 2000F, multiple preheating steps may be used prior to reaching the final austenitizing temperature.

Once tool steels have been satisfactorily preheated, they are raised to their austenitizing temperature. The desired hardened structure, called martensite, cannot form directly from the annealed structure, called ferrite. The ferrite must first be transformed to an intermediate structure called austenite. Tool steel becomes austenite when it is heated above its critical temperature. The critical temperature for most tool steels is about 1500F.

After the structure changes to austenite, further heating is required to properly distribute the alloy content of the steel. Most of the useful alloy content of tool steels exists as microscopic carbide particles in the soft matrix of the annealed steel. These carbide particles must be at least partially dissolved into the matrix of the steel during the hold at the austenitizing or hardening temperature. The temperature used varies greatly from grade to grade, depending primarily on the composition of the steel. Typical temperature ranges are shown in the table.

In some grades, the austenitizing temperature may be varied to tailor the properties of the steel, in order to provide a little extra wear resistance or toughness for specific applications. Higher temperatures allow more alloy content to dissolve into the matrix, allowing slightly higher hardness and better wear resistance. Lower temperatures dissolve less alloy content into the matrix, and favor increased impact resistance, although the attainable hardness is slightly lower. The use of lower than maximum hardening temperatures, called underhardening, is often the most effective way to achieve the maximum toughness in the high alloy grades, where a range of hardening temperatures are possible.

The hold times used depend primarily on the austenitizing temperature. Diffusion of alloy content occurs faster at higher temperatures, and hold times are decreased accordingly. The hold times shown in the table are typical for small sections (under about 2"), and represent the total hold time after the tool has reached the aim temperature, regardless of part size or other factors. Larger sections require longer hold times to allow them to be heated through to the aim temperature. Extended hold times depend on the type of furnace equipment, load size, and heat treat experience.

 Once the alloy content has been dissolved as desired into the steel matrix during austenitizing, the steel must be cooled fast enough to keep the alloy content in place and transform the austenite to the high hardness martensite. How fast a steel must be cooled to fully harden depends on the chemical composition. Low alloy tool steels (O1, S5, L6 etc.) ol order to cool fast enough. This drastic quench can cool some portions of the tool much faster than other portions, increasing risk of distortion or even cracking in severe cases. Higher alloy tool steels (A2, D2, M4, 10V, etc.) may be cooled in air or inert gas. Air-hardening steels cool more uniformly, so distortion and risk of cracking are lower.

For tool steels which are quenched from over about 2000øF, the quench rate from about 1800øF down to about 1300øF must be rapid enough to avoid undesirable reactions which can impair toughness and hardness response. The actual transformation of austenite to martensite does not begin until the steel cools below about 700øF. The specific temperature at which martenite starts to form is called the "martenite start" or "MS" temperature. In tool most steels, the martensite forms between about 600øF and about 200øF. The amount of martensite depends principly on how close to the lower temperature, (martesnite finish or "MF" temperature), the steel gets. 100% martensite is not formed until the steel cools below the martesnite finishing temperature.

No matter how tool steels are quenched, the resulting martensite is extremely brittle, and under great stress. If put into service in this condition, most tool steels would shatter. Some tool steels may even crack spontaneously if allowed to remain for any time in this condition. For this reason, as soon as tool steels have been quenched to about 125/150øF, they should be immediately tempered.

  Tempering is performed to stress relieve the brittle martensite formed during the quench. Most steels have a fairly wide range of acceptable tempering temperatures (see table). For best relief of quenching stresses, use the highest tempering temperature which will give the desired hardness.

Most tool steels must be tempered at least 2 times, with triple tempering recommended for high alloy grades or high hardening temperatures. The rate of heating to or cooling from the tempering temperature is not critical, except sudden drastic temperature changes should be avoided. Tool steel should be allowed to cool completely to room temperature (50/75øF) between tempers.

Tool steels should be held at temperature a minimum of 2 hours for each temper. A rule of thumb is to allow one hour per inch of thickest section for tempering, but in no case less than 2 hours.
 When surface treatments are used, (nitriding, TiN coating, etc.), the heat treating process should be discussed with the surface treater. For best results, it is important that heat treating temperatures and processes are compatible with subsequent surface treatment temperatures.

 The heat treat process results in an unavoidable size increase in tool steels due to the changes in microstructure. Most tool steels will grow between about 0.0005" and 0.002" per inch of original length during heat treatment. This will vary somewhat based on a number of theoretical and practical factors. Many heat treaters will have a "feel" for what to expect from typical processes.

In certain cases, a combination of variables including high alloy content, long austenitizing time or high temperature, discontinuing the quench process too soon, or other factors in the process may cause the MS temperature to become depressed to below room temperature. In this case some of the high temperature structure, austenite, will be retained at room temperature (martensite is not completely formed). This retained austenite condition is usually accompanied by an unexpected shrinkage in size, and sometimes by less ability to hold a magnet. This condition can often be corrected simply by exposing tools to a low temperature (cryogenic or refrigeration treatments), to encourage continuation of the transformation to martensite by cooling the steel to below its MS.

Tool steels transform to martensite during quenching from about 600øF down to about 200øF. In some cases, as described above, the transformation to martensite may not be complete at the end of the quench (125øF). In such cases, some austenite may still be retained after the normal heat treatment. This retained austenite can sometimes lead to unexpected growth in service, causing loss of accuracy. A2 and D2 are two grades commonly prone to retained austenite after heat treating. By cooling the steel to sub-zero temperatures, this retained austenite may be transformed to martensite. The newly formed martensite is similar to the as-quenched martensite, and must be tempered. Cryogenic or refrigeration treatment should include a temper after freezing. The cold treatment is often performed between normally scheduled tempers. By minimizing retained austenite, certain kinds of dimensional stability problems can be avoided.
 

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