Heat Treatment of Tool Steels
Tool steels are usually supplied in the annealed condition, around 200/250 Brinell (about 20 HRC), to facilitate machining. In this condition, most of the alloy content exists as alloy carbides, dispersed throughout a soft matrix. These steels must be heat treated to develop their characteristic properties. The heat treating process alters the alloy distribution and transforms the soft matrix into a hard matrix capable of withstanding the pressure, abrasion and impacts inherent in metal forming. Each step of the heat treating cycle is designed to perform a specific function, and, like links in a chain, the final product is only as good as its weakest component. Although it may only represent 10% or less of the cost of the tool, the heat treat process is probably the single most important factor in determining the performance of a tool. There is no such thing as an acceptable shortcut in heat treating tool steels.
Preheating, or slow heating, of tool steels provides two important benefits. First, most tool steels are sensitive to thermal shock. A sudden increase in temperature of 1500/2000°F may cause tool steels to crack. Second, tool steels undergo a change in density or volume when they transform from the as-supplied annealed microstructure to the high temperature structure, austenite. If this volume change occurs nonuniformly, it can cause unnecessary distortion of tools, 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 this critical transformation temperature, and then held long enough to allow the full cross-section to reach a uniform temperature. Once the entire part is equalized, further heating to the austenitizing temperature will allow the material to transform more uniformly causing less distortion to occur.
The useful alloy content of most tool steels exists as carbide particles within the annealed steel. This alloy content is at least partially diffused into the matrix at the hardening or austenitizing temperature. The actual temperature used depends mostly on the chemical composition of the steel. The temperature may be varied somewhat to tailor the resulting properties to specific applications. High temperatures allow more alloy to diffuse, permitting slightly higher hardness or compressive strength. At lower temperatures, less alloy diffuses into the matrix, and the matrix is therefore tougher, or less brittle, although it may consequently not develop as high a hardness. The hold times used depend on the temperatures. Diffusion of alloy occurs faster at higher temperatures, and soak times are decreased accordingly.
For the best combination of properties, we generally recommend using the lowest hardening temperature which will produce adequate hardness for your application.
The times shown in the table are typical for relatively small sections (under 2″) and represent total soak time after material has reached the aim temperature. Larger sections need to be held longer to allow the center to reach temperature. The extended soak times depend on furnace equipment, load size and heat treat experience.
(1) Tools should be held in preheat range just long enough for temperature to equalize throughout material. A second preheat step at 1850/1900°F is recommended for vacuum or atmosphere furnaces when hardening temperature is over 2000°F.
(2) Higher austenitizing temperatures are used for slightly greater hardness; lower temperatures may provide slightly improve toughness.
(3) Hold times are typical soak times after material has reached the aim temperature. Longer times are for low austenitizing temperatures; high temperatures require shorter soaks. Variations in furnace type, load or part size, etc., may require varying allowances for parts to reach aim temperature.
(4) Interrupted oil quench may be required for very large sections.
(5) Although high speed steels may be air-hardened, a salt bath or other similar equipment is required to attain maximum hardness.
(6) Multiple tempers are mandatory for most grades. Consult individual data sheets for specific requirements.
Once the alloy content has been redistributed as desired during austenitizing, the steel must be cooled fast enough to fully harden to martensite, which will provide the material’s strength. How fast a steel must be cooled to fully harden depends on the chemical composition. In general, low alloy steels (O1) must be quenched in oil in order to cool fast enough. The drastic quench may cool some portions of a tool significantly faster than other portions, causing distortion or even cracking in severe cases. Higher alloy content allows steel to develop fully hardened properties with a slower quench rate. Air-hardening steels cool more uniformly, so distortion and risk of cracking are less than with oil-hardening steels.
For the higher alloyed tool steels which harden from over 2000°F, the quench rate from about 1800°F to below 1200°F is critical for optimum heat treat response and material toughness.
No matter how tool steels are quenched, the resulting structure, martensite, is extremely brittle, and under great stress. If put into service in this condition, most tool steels would shatter. Some tool steels will spontaneously crack in this condition even if left untouched at room temperature. For this reason, as soon as tool steels have been quenched by any method to hand-warm (about 125/150°F), they should be immediately tempered.
Tempering is performed to stress-relieve the brittle martensite which was formed during the quench. Most steels have a fairly wide range of acceptable tempering temperatures. In general, use the highest tempering temperature which will provide the necessary hardness for the tool. The rate of heating to, and cooling from the tempering temperature is not critical. Sudden drastic temperature swings should be avoided. The material should be allowed to cool completely to room temperature (50/75°F) or below between and after tempers. Most steels must be held at temperature for a minimum of two to four 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 two hours regardless of size.
The heat-treat process results in unavoidable size increases in tool steels because of the changes in their microstructure. Most tool steels grow between about 0.0005 and 0.002 inch per inch of original length during heat treatment. This varies somewhat based on a number of theoretical and practical factors. Most heat treaters 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, inadequate cooling between tempers, or other factors in the process, may cause some of the high-temperature structure, austenite, to be retained at room temperature. In other words, during the normal quench, the structure is not completely transformed to martensite.
This retained austenite condition usually is accompanied by an unexpected shrinkage in size and sometimes by less ability to hold a magnet. This condition often can be corrected simply by exposing tools to low temperatures, as in cryogenic or refrigeration treatments, to encourage completion of the transformation to martensite.
Most tool steels actually develop their hardened structure (martensite) during the quench, between about 600°F and 200°F. For various reasons, however, in some cases, transformation to martensite may not be complete even at 125/150°F. In such cases, some of the high temperature microstructure, austenite, may be retained after normal heat treating. A2 and D2 are two common grades which may contain significant (20% or more) retained austenite after normal heat treating. Retained austenite may be undesirable for a number of reasons. By cooling the steel to cryogenic (sub-zero) temperatures, this retained austenite may be transformed to martensite. The newly formed martensite is similar to the original as-quenched structure and must be tempered. Cryogenic treatments should include a temper after freezing. Often the freezing may be performed between normally scheduled multiple tempers. Technically, cryogenic treatments are most effective as an integral part of the original quench, but due to the high risk of cracking, as discussed in the “Quenching” section above, we recommend tempering material normally at least once before performing any cryogenic treatments.
Exposure to oxygen at the austenitizing temperatures causes scaling and decarburization of the tool surfaces. Decarburization causes a permanent loss in attainable hardness at the tool surface. For this reason, some type of surface protection during austenitizing is required. Vacuum, controlled-atmosphere, or neutral salt bath furnaces all offer surface protection. If neutral-atmosphere furnaces are not available, parts may be wrapped in stainless foil to minimize oxygen exposure.
Salt furnaces usually offer the quickest and most uniform heating but leave a residue which must be cleaned from the tool surface. Salt bath heat treating has traditionally been used for high-speed steel cutting tools and often cannot accommodate large tools or high-volume hardening.
Vacuum furnaces offer the best surface protection but usually require longer process cycles. The quench rate may be limited because of the inability to remove heat from a hot part fast enough to obtain maximum hardness. Vacuum heat treating may result in slightly lower hardness than achieved in a salt bath. Wrapping parts in foil may also slow the quench rate because of the slight insulating effect of the foil layer. In addition, the type of foil must be chosen to withstand the austenitizing temperature used.
When heat treating several parts, it is important to load furnaces so that there is clear circulation around each part. During austenitizing, each part should be allowed to heat up relatively uniformly so that excessive soak times are not encountered. Excessive soak times can lower the material’s toughness. Also, good circulation around tools promotes faster quenching, which is good for metallurgical properties, and also promotes more uniform cooling, which helps control distortion.
Recommended heat treatments for specific tool steels are described thoroughly in the individual data sheets. However, many practical concerns may affect the heat-treat process. Concerned tool builders should discuss heat treatment with their heat treaters to find the best process to suit their tools and applications.
Edited by Eric
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