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Heat treatment of tool steels

Heat treatment is a very important technology, it changes the microstructure of steel, and thus affects the mechanical properties of tool steel. Heat treatment is generally composed of heating, insulation and cooling three processes, specific to each stage there are some technical details, such as heating, how to heat up? How to cool down has become the key to heat treatment and the result of heat treatment.
Some fans see that "heat treatment of tool steel (1)" is very concerned about when to come out (2). This issue continues to introduce the heat treatment of tool steel, I hope that fans can learn from the key technology of heat treatment of tool steel.


Heating to hardening temperature

As is said, the stress contained in the material will deform during heat treatment. For this reason, thermal stress should be avoided during heating. Therefore, the basic rule of heating to the hardening temperature is: the heating process must be slow, only a few degrees per minute. In heat treatment, the warming up process is called slope (ramping).
residual stresses contained in the material  

During the hardening process, the slope should be carried out in different steps, stopping the process at intermediate temperatures, commonly known as the preheating step. The reason is that the temperature between the surface and the center of the component is equal. The usual preheating temperature is 600-650 degrees (1100-1200 degrees F) and 800-850 degrees (1450-1560 degrees F). Heating, heat preservation and cooling are the most important links in the process of steel heat treatment.

 Steel heats up during heat treatment
 In the case of large tools with complex geometries, a third step of preheating is recommended near the full austenite region.

Hardening temperature and holding time


 It is difficult to give accurate suggestions for all situations. In each case, factors such as the type of furnace, the hardening temperature, the burden weight related to the size of the furnace, and the geometry of different parts of the burden must be taken into account. The use of thermocouples can almost cover the temperature of different tools in different areas of the furnace.

 Hardening temperature and holding time
 When the core part of the furnace reaches a predetermined temperature, the ramp step ends. Then keep the temperature constant for a certain amount of time, this is called holding time.

the quenching process as expressed in a cct graph
 It is generally recommended that the holding time is 30 minutes. In the case of high speed steel, when the hardening temperature is higher than 1100 F (2000 degrees), the holding time will be shorter. If the holding time is lengthened, microstructure problems such as grain growth may occur.

Quench

The trade-off between quick and slow quenching rates should normally be chosen. In order to obtain the best microstructure and tool performance, the quenching rate should be fast. In order to minimize distortion, a slower quenching rate is recommended.
 Slow quenching results in a smaller temperature difference between the surface and the core of the component, and parts with different thicknesses will have a more uniform cooling rate. This is very important when quenching in the martensite range below Ms.


cooling rates form various media
 The formation of martensite leads to an increase in volume and stress in the material. This is why the quenching should be interrupted before reaching room temperature, usually at 50-70 C. However, if the quenching rate is too slow, especially for the heavier cross-section, the unexpected deformation in the microstructure may occur, thus posing a risk of poor tool performance.
 Nowadays, quenching medium used for alloy steel is hardening oil, polymer solution, air and inert gas. Air hardening is retained in steels with high hardenability, which in most cases is mainly attributed to the presence of a combination of manganese, chromium and molybdenum in the steel. The risk of deformation and hardening cracks can be reduced by step hardening or graded hardening. In this method, the material is quenched in two steps. First, it is cooled from the hardening temperature until the surface temperature is just higher than the Ms temperature. Then it must be kept there until the temperature is balanced between the surface and the core. Thereafter, the cooling process continues. This method allows the core and surface to transform almost simultaneously into martensite and reduce thermal stress. Quenching is also possible when quenching in a vacuum furnace.



tempering or step-quenching


 The maximum cooling rate available in a component depends on the thermal conductivity of the steel, the cooling capacity of the quenchant and the cross-sectional area of the component.
 Poor quenching rate leads to precipitation of carbide at the grain boundary of the component core, which is detrimental to the mechanical properties of the steel. In addition, for tools with larger cross-sections, the hardness at the surface of the larger parts can be lower than that of the smaller parts, because the large amount of heat transferred from the core to the surface produces a self-tempering effect.

Some practical issues

At high temperatures, steel is likely to undergo changes in oxidation and carbon content (carburization or decarburization). Protective atmosphere and vacuum technology are the answers to these questions. Decarburization causes low surface hardness and increases the risk of cracking. Carburizing, on the other hand, can lead to two different problems:

• the first and most recognizable is the formation of harder surface layers, which can have negative effects.

• the second possible problem is surface residual austenite. In many cases, residual austenite may be confused with ferrite when observed under an optical microscope.
 The two phases also have similar hardness. Thus, decarbonization at first can be identified as a problem that may be the exact opposite in some cases. For these reasons, it is critical that the heat treatment atmosphere does not affect the carbon content of the components.
 When heated, the sealed stainless steel foil wrapped in muffle furnace can provide some protection. Steel foil should be taken out before quenching.

top cooling lllustration from vacuum furnaces


High alloy steel hardening vacuum technology

Vacuum technology is the most commonly used technique in the hardening of high alloy steel. Vacuum heat treatment is a clean process, so parts do not need to be cleaned after heat treatment. It also provides reliable process control with high automation, low maintenance and environmental friendliness. All of these factors make vacuum technology particularly attractive for high-quality components.

The functions of different steps of the vacuum furnace are listed as follows:

• air is pumped from the heating chamber when the furnace is closed after charging operation to avoid oxidation.

• inject inert gas (most commonly nitrogen) into the heating chamber until the pressure reaches about 1-1.5pa.

• start the heating system. The presence of inert gases makes it possible to transfer heat through a convection mechanism. This is the furnace heated to about 850 ℃ the most effective way.

nitrogen gas stram passes through the heating chamber in different directions


• when the stove to about 850 ℃ temperature. The effects of radiation heating mechanisms will outweigh the effects of convection during heat transfer. Thus, the nitrogen pressure decreases in order to optimize the effects of radiation, and the convection heating mechanism is negligible under these new physical conditions. The new value of nitrogen pressure is about 7mbar. The reason for the residual pressure is to avoid the sublimation of alloy elements, that is, to avoid the loss of alloy elements to vacuum. This low pressure condition is at the end of the heating process and remains constant during the heat preservation period at the selected hardening temperature.

• the cooling will enter the heating chamber through a large injection of inert gas (most commonly nitrogen), changing direction to reach the overpressure previously selected in the furnace programming. Maximum overpressure is the nominal characteristic of each furnace.

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