The additional steps of the overall heat treating process serve to eliminate this characteristic. The process of martensitic transformation was named after Adolf Martens, a prominent 19th century German metallurgist. There are four basic steps in the process of heat treating tool steel: Preheating, Heating also caused austenitizing , Quenching, and Tempering. Depending on the tool steel being treated and the ultimate applications for which it is intended, other steps can be added to the process as well.
The temperature of the treatment, the duration of the treatment, and the frequency of the treatment for example, if a certain step must be done multiple times are all dependent on the type of tool steel that is being treated, as well as the end product that the tool steel will be used for. Heating tool steel rapidly from room temperature to the point where the atomic structure changes to austenite can significantly degrade or completely destroy the product. Transforming tool steel from the annealed phase to the austenite phase alters the volume of the steel.
Rapidly heating tool steel to these temperatures can cause thermal shock, which in turn causes the tool steel to crack. Additionally, depending on the shape and configuration of the tool steel, rapid changes in volume can cause it to warp to a point where it is unusable. These problems can be avoided by a thorough pre-heating process that takes the tool steel from room temperature to a point just below the target austenitization point.
The duration of the preheating process must be sufficient to ensure that the tool is heated uniformly throughout. Once the preheating process is completed and the tool steel is stable, austenitization can commence. The transformation of ferrite to austenite occurs at various temperatures, depending on the component content of the alloy being treated. When an alloy reaches the critical austenitization temperature, the micro atomic structure opens so that it can absorb more carbon from the already present iron carbides.
It is extremely critical that this process be precisely controlled both in terms of process temperature and duration. Incomplete initial austenitization can leave undissolved carbides in the atomic matrix.
Metallurgical engineers determine the optimum time and temperature for heating based on many factors, such as the tools steel being treated and the desired end results. For example, generally speaking a lower austenitizing temperature increases the toughness of the end product, whereas higher temperatures will increase the hardness of it.
Quenching is the process of rapidly cooling the hot austenite into the much harder, desired endstate martensite micro atomic structure. As with the heating process, the duration and process methodology used for quenching are configured based on the desired final product.
For low alloy tool steel that must be quenched quickly in order to preserve the martensite structure, oil is typically the medium that provides the best results.
For higher alloy tool steel, air cooling is the most effective approach. Additionally, for certain types of steel, a water quenching process is recommended. As with all of the steps in the tool steel hardening process, quenching must be meticulously measured, managed, and controlled. This lack of uniformity can distort the finished shape or cause cracking. Tempering tool steel makes the newly formed martensite less brittle. Without proper tempering, martensite will crack—or even shatter—very easily.
Proper tempering is an essential step in the overall tool steel heat treating process. With that said, the precision required for proper austenitization is much less critical during the tempering step, although the rapid heating of the tool steel should be avoided. The heat intensity is typically determined by the hardness required for the finished material—a higher tempering temperature yields a harder product. Instead of a precise value, most alloys have a relatively wide range of acceptable tempering temperatures.
First, most tool steels are sensitive to thermal shock. 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. This problem is especially evident where differences in geometry or section size can cause some parts of the tool to transform before other parts have reached the aim temperature. In general, higher temperatures allow more alloy to diffuse, permitting slightly higher hardness and strength.
Soak times at austenitizing temperature are usually extremely short — in the neighborhood of one to five minutes once the tool has reached temperature. 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. High temperatures allow more alloy to diffuse, permitting slightly higher hardness or compressive strength.
The hold times used depend on the temperatures. Diffusion of alloy occurs faster at higher temperatures, and soak times are decreased accordingly. The austenitizing temperature that is selected depends strongly upon the alloy content of the steel. The aim properties including hardness, tensile strength, grain size, etc. Higher temperatures allow more alloy to diffuse, which usually permits a higher hardness.
This is true as long as the temperature does not exceed the incipient melting temperature of the steel. If lower austenitizing temperatures are used, then less diffusion of alloy into the matrix occurs. 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. No matter how tool steels are quenched, the resulting martensitic structure is extremely brittle and under great stress. Some tool steels will spontaneously crack in this condition even if left untouched at room temperature. How fast a steel must be cooled to fully harden depends on the chemical composition.
In general, low alloy steels must be quenched in oil in order to cool fast enough. 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.
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. This process is called quenching. Generally, lower alloy steels such as 01 must be quenched in oil in order to cool fast enough.
Higher alloy content steels can develop fully hardened properties by undergoing a slower quenching process. 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. Facts about the Elements: Erbium. Part One: Phase Transformations. Report Abusive Comment Thank you for helping us to improve our forums.
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