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Tempering of mold steel
Tempering temperature and tempering principle of mold steel
Cr12MoV, SKD11, D2, DC53, SKD61 and high-speed steel, etc. When the mold requires higher hardness, it has the phenomenon of secondary hardening of tempering. Therefore, tempering is an indispensable subsequent step after quenching of the die steel. The selection of the tempering temperature of the mold steel is completely determined by the requirements of the mold parts and the mechanical properties.
1.Mold steel is tempered at low temperature. Generally it is below 250 °C. The purpose is to reduce the microstructure transformation of quenched martensite to tempered martensite under the condition of retaining high hardness as much as possible, eliminating and reducing the quenching stress of the die steel, and the quenching of the microstructure stress is partially relieved. It can maintain the high hardness of quenching; advantages: the color of the mold after tempering is good, can maintain vacuum quenching white (below 180 ° C), or turn into pale yellow (200 ~ 250 ° C); Disadvantages: internal stress elimination is not complete, The mold is more brittle and is more suitable for molds with compressive stress working conditions (such as punches, cores, etc.). Generally, molds requiring wire cutting or spark machining are not suitable for low temperature tempering; retaining more retained austenite, mold The dimensional stability is slightly poor; the tempering time is long and the delivery is slow; the cold-working mold parts requiring high wear resistance are used.
2. Mold steel medium temperature tempering. Generally, it is 300~500 °C, so that the hardened steel has certain strength and elasticity, and has sufficient toughness and plasticity, and is used by impacted mold parts.
3. Mold steel is tempered at high temperature. Generally it is between 500~650 °C or higher. The microstructure of high temperature tempering is transformed into retained austenite and transformed into martensite. At the same time, fine carbides are precipitated in the tempered martensite, which causes secondary hardening of the material to meet the requirement of high hardness. At this time, the residual stress of the mold is small. The purpose is to adjust the toughness of the die steel (both comprehensive mechanical properties) to achieve the best fit. When the mold steel is previously heat-treated, it is also subjected to surface quenching in a subsequent step, and nitriding or the like is prepared for the structure to improve workability. Also. For some high-alloy steels, secondary hardening can be obtained, hardness, wear resistance and dimensional stability are improved, and retained austenite is eliminated. Hot work and plastic mold parts are used to improve toughness and wear resistance at mold operating temperatures.
Advantages: Structural transformation is more sufficient, internal stress is smaller, so generally large-scale templates and dies requiring WEDM or EDM are tempered at high temperature; dimensional stability and wear resistance are better than low-temperature tempering; tempering time is shorter;
Disadvantages: Poor color, can not maintain the true color of vacuum quenching, generally dark brown or dark blue after high temperature tempering, some materials with lower alloy content will also have a thin oxide layer, so high temperature tempering is not applicable Finished mirror mold;
The tempering time of mold steel is based on the principle that the holding time should be sufficient. Insufficient insulation often causes adverse consequences. Sometimes it takes two or more tempering, and it should not be short in the holding time. In particular, the so-called rapid tempering is simply not necessary to shorten the time and damage the quality.
Usually the minimum requirement is not less than 1h. Like tempering back to heat (self-tempering), partial tempering is a last resort and is not desirable. High-speed steel or high-alloy die steel has a secondary hardening effect and must be tempered 2-4 times. The slow cooling effect is better because fine secondary carbides precipitate during this process.
Learn More : Vacuum Tempering Furnace