PreheatingSlow heating rates and appropriate preheat steps for tool steels provide multiple benefits. First, most tool steels are sensitive to thermal shock, and reducing thermal gradients produced by rapid heat rates minimizes the tendency of tool steels to crack. Also, tool steels undergo a volume change when they transform from their annealed microstructure to austenite while heating to elevated temperature. If this volume change occurs non-uniformly, it can cause unexpected distortion, especially in cases where differences in section size exist.
For most tool steels, select a preheat temperature just below the material’s critical transformation temperature (Ac1) and hold long enough to allow the full cross section to reach a uniform temperature. Another preheat and hold just above the criti-cal temperature allows the material volume change to occur uniformly, creating less distortion. Preheating should be followed by rapid heating to austenitizing temperature.
AustenitizingThe purpose of austenitizing is to allow carbide particles to partially or fully dissolve and diffuse into the matrix. Different types of carbides dissolve at different rates as a function of temperature, thus the appropriate austenitizing temperature depends primarily on the chemical composition of the steel. Also, the austenitizing temperature may be varied slightly to tailor the result-ing properties to specific applications.
In general, higher temperatures allow more alloy to diffuse, permitting slightly higher hardness and strength. At lower tempera-tures less alloy diffuses, and the resulting matrix is tougher and less brittle, although it may not develop as high a hardness.
Soak times at austenitizing temperature are usually extremely short – in the neighborhood of one to five minutes once the tool has reached temperature. Often load thermocouples are placed inside parts or in representative cross sections – the soak time being initiated once the center of the part has reached temperature. The optimum combination of properties is often obtained at the lowest hardening temperature that will produce adequate hardness for the intended application.
QuenchingAfter the alloy content has been redistributed during austenitizing, the steel must be cooled fast enough to transform to marten-site. Most tool steels actually develop a martensitic structure in the temperature range of 600°F (315°C) to 200°F (95°C). How fast a tool steel must be cooled, and in what type of quench medium to fully harden, depends on the chemical composition. Higher-alloy tool steels develop fully hardened properties with a slower quench rate. As a rule, use the slowest quench rate appropriate to develop an optimized part microstructure and hardness while minimizing distortion and the risk of cracking.
For the higher alloyed tool steels processed over 2000°F (1095°C), the quench rate from about 1800°F (980°C) to below 1200°F (650°C) is critical for optimum heat-treat response and material toughness.
No matter how tool steels are quenched, the resulting martensitic structure is extremely brittle and under great stress. If put into service in this condition, there is a significant risk that the tool will fail. 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 handling temperature, around 150°F (65°C), they should be tempered immediately, usually interpreted as within 15–30 minutes.
Deep-FreezingFor most tool steels, retained austenite is highly undesirable since its subsequent conversion to martensite causes a size (vol-ume) increase creating internal stress and leads to premature failure in service. By deep-freezing to -120°F (-85°C) or in some instances cryogenic cooling to -320°F (-195°C), retained austenite is transformed. The newly formed martensite is similar to the original as-quenched structure and must be tempered. Often deep-freezing is performed before tempering due to concerns over cracking, but it is sometimes done between multiple tempers.
TemperingTempering is performed both to stress-relieve the brittle martensite that was formed during the quench and to reduce the amount of retained austenite present. Most steels have a fairly wide range of acceptable tempering temperatures. In general, use the highest tempering temperature that will provide the necessary hardness for the tool. The rate of heating to and cooling from the tempering temperature is usually not critical. The material should be allowed to cool below 150°F (65°C) and often completely to room temperature between and after tempers. A good rule of thumb is to soak for one hour per inch of thickest section after the entire tool has reached temperature, but in no case less than two hours regardless of size.
Multiple tempers are typical, especially for many of the more complex tool steels (e.g. M-series and H-series) requiring dou-ble or even triple tempering to completely transform retained austenite to martensite. These steels reach maximum hardness after first temper and are designated as secondary hardening steels. The purpose of the second or third temper is to reduce the hardness to the desired working level and to ensure that any new martensite formed as a result of austenite transformation in tempering is effectively tempered.
AnnealingTool steels are usually supplied to customers in the annealed condition with typical hardness values around 200-250 Brinell (» 20 HRC) to facilitate machining and other operations. This is especially important for forged tools and die blocks where partial or full air hardening takes place, resulting in a buildup of internal stresses. Dies and tools that may need to be rehardened must be annealed.
Full annealing involves heating the steel slowly and uniformly to a temperature above the upper critical temperature (Ac3) and into the austenite range then holding until complete homogenization occurs. Cooling after heating is carefully controlled at a specific rate as recommended by the steel manufacturer for the grade of tool steel involved. Cooling is normally continued down to around 1000°F (540°C) when the steel may be removed from the furnace and air cooled to room temperature.
NormalizingThe purpose of normalizing is to refine the grains and to ensure that the microstructural constituents are evenly dispersed throughout the matrix. Excessive segregation can lead to poor fracture toughness or distortion in tools due, in part, to segrega-tion and differential transformation rates.
Normalizing involves heating slowly to the normalizing temperature (i.e. in the austenite range), holding at temperature suf-ficient to allow homogenization to occur and then air cooling to room temperature. Caution must be observed since many tool-steel grades air harden when cooled from austenitizing temperatures.
Stress ReliefIn instances where tools have been subject to aggressive machining, the build up of internal residual stresses must be re-moved. Stress relief is carried out at 925°F-1025°F (500°C-550°C) allowing the tools to cool to room temperature prior to heat treatment. Stress relief incorporated as a preheat step is often used.
Common Heat-Treatment IssuesDecarburization
This may occur during all heat-treatment processes (even in vacuum furnaces if leaks exist) and is to be avoided due to subse-quent detrimental effect on the hardness of the finished tool (unless removed by machining). The use of vacuum or protective at-mospheres will minimize or eliminate decarburization. Other techniques such as the use of a borax or glass coating have also been used.
The heat-treat process results in unavoidable size change – either an increase or decrease in dimensions due to changes in the tool microstructure. A combination of variables often contributes, including high alloy content, improper preheats, long soak times, higher-than-necessary austenitizing temperatures, variations in quenching, inadequate cooling between tempers or other factors in the process.IH