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Mold steel performance
Nov 02, 2017

In general, the higher the temperature, the higher the impact toughness value, and some steel toughness at room temperature is very good, when the temperature dropped to a 20 ~ 40 ℃ will become brittle steel.

    (11) Thermal stability The thermal stability characterizes the stability of the steel during the heating process to maintain the microstructure and properties. In general, the thermal stability of the steel is followed by tempering for 4 h and the maximum heating temperature when the hardness is reduced to 45 HRC. This method is related to the original hardness of the material, the data will reach the expected strength level of steel heating, heat 2h, so that the hardness of the general hot forging die failure hardness 35HRC maximum heating temperature as the steel thermal stability index. For the hot die which is accumulated due to insufficient heat resistance, the life level of the mold can be predicted according to the thermal stability.

    (12) tempering stability Tempering stability refers to the steel with the tempering temperature increases, the strength of the material and the degree of rapid decline in hardness, also known as tempering resistance or resistance to tempering softening. Usually the tempering temperature of a steel to show a hardness curve, slow down the hardness, said the tempering stability or tempering resistance. The stability of the tempering is also related to the change of the tempering structure. It is consistent with the thermal stability of the steel to characterize the stability of the steel at high temperature and characterize the deformation resistance of the mold at high temperature.

    (13) high temperature wear and oxidation resistance High temperature wear is the main form of hot die one of the main failure, under normal circumstances, the vast majority of hammer forging die and press die are due to wear and failure. Heat-resistant wear is the use of hot work on the requirements of the mold, is a comprehensive manifestation of a variety of high-temperature mechanical properties. Now the domestic units have been in the self-made thermal wear machine for mold thermal wear test, received a better test results.

    The actual use shows that the anti-oxidation properties of mold materials, the great impact on the life of the mold. Due to oxidation will exacerbate the mold during the work of the wear and tear, resulting in mold cavity size tolerance and scrapped. Oxidation will also produce corrosion surface of the mold surface, a thermal fatigue crack origin, exacerbate the mold thermal fatigue crack initiation and expansion. Therefore, the requirements of the mold with a certain degree of antioxidant properties.

    (14) anti-occlusal ability to engage the actual resistance is the occurrence of "cold welding" when the resistance. This performance is more important for mold materials. Tests are usually carried out under dry friction conditions, and the tested tool steel samples are subjected to constant-speed dual friction motion with materials with bite-like tendons (such as austenitic steels) to gradually increase the load at a constant speed. The moment is also increased accordingly, the load is called "occlusal critical load". The higher the critical load, the stronger the occlusal resistance.

    (15) anti-softening ability to soften the ability to characterize the mold in the load, because the steel temperature rise on the hardness, wear resistance resistance. ,

    (16) compressive yield strength and compression bending strength of the mold in the course of the process is often subject to high pressure and bending effect, therefore requires the mold material should have a certain compressive strength and bending strength. In many cases, the conditions under which the compression test and the bending test are carried out are close to the actual operating conditions of the mold. For example, the measured compressive strength of the die steel is more consistent with the deformation resistance shown in the work of the punch. The advantage of the bending test is that the absolute value of the strain is large and can reflect the difference of the deformation resistance between the different steel grades and the different heat treatment and organization.

    (17) wear resistance determines the useful life of the mold is often the most important factors are wear resistance of mold materials. Mold in the work to withstand considerable pressure and friction, the mold can be in a strong friction conditions still maintain its dimensional accuracy. The wear patterns of the mold are mainly mechanical wear, oxidation wear and melt wear three types. In order to improve the wear resistance of mold steel, it is necessary to maintain both the high hardness of the mold steel, but also to ensure that the steel carbide or other hardened phase composition, morphology and distribution is reasonable. For heavy duty, high-speed wear conditions under the conditions of the mold, the surface of the mold steel surface to form a thin and dense, good adhesion of the oxide film to maintain lubrication and reduce the mold and the workpiece between the sticky, welding and other melt wear, and Can reduce the mold surface oxidation caused by oxidation wear. So the working conditions of the mold on the steel '

    Wear has a greater impact.

    Wear resistance can be used to simulate the test method to measure the relative wear index e, as the characterization of different chemical composition and organization under the state of the wear resistance of the parameters. To show the pre-burr height before the life, reflecting the wear and tear of a variety of steel

    The test is based on Crl2MoV steel as a benchmark (e-1), reflecting the wear resistance of the tool steel under abrasive wear conditions.

    (18) Thermal fatigue resistance and fracture toughness thermal fatigue resistance of the material thermal fatigue crack initiation before the working life and the rate of expansion after initiation. The thermal fatigue resistance is usually determined by repeated heating and cooling at a temperature of 20 to 750 ° C, the number of cycles of occurrence of cracks, or the length of the crack after a certain number of cycles. Thermal fatigue resistance of the material is not prone to thermal fatigue crack, or when the crack initiation, the expansion of small, slow expansion. Fracture toughness is the characterization of the crack instability expansion resistance, fracture toughness is high, the crack is not easy to instability expansion.

    In addition to conventional mechanical properties such as impact toughness, compressive strength, bending strength and other one-time fracture resistance index, the small energy multiple impact fracture resistance is more in line with the actual performance of cold work die performance, as the mold material performance index package

    Including compressive fatigue strength, contact fatigue strength. The fatigue fracture resistance index is characterized by the number of fracture cycles measured under a certain cyclic stress, or the load that causes the fracture under a certain number of cycles. Whether the fracture toughness is an important performance index of the cold mold material, To be studied and discussed. Hot work die steel in the service conditions in addition to bearing

    In addition to the periodic changes in load, but also by the high temperature and cyclical quenching and emergency role, therefore, the evaluation of hot work die steel fracture resistance should pay attention to the material thermal mechanical fatigue fracture performance. Thermomechanical fatigue is an integrated performance index, which includes three aspects: thermal fatigue resistance, mechanical fatigue crack growth rate and fracture toughness.

    The thermal fatigue performance of the material reflects the working life of the material before the thermal fatigue crack initiation, the thermal fatigue resistance of the material, the number of thermal cycles of the initiation thermal fatigue crack is more; the mechanical fatigue crack propagation rate reflects the material after the thermal fatigue crack initiation Under the action of forging force, the expansion of each stress cycle when the crack is extended to the inside; the fracture toughness reflects the resistance of the material to the existing crack instability. Fracture toughness of the material, the crack to the occurrence of instability expansion, must be at the crack tip with a high enough stress intensity factor, that is, must have a larger crack length. Under the premise of constant stress, there is already a fatigue crack in a mold. If the fracture toughness value of the mold material is high, the crack must be expanded so that the instability can be extended.

    Therefore, the thermal fatigue resistance determines the part of the life before the fatigue crack initiation; and the mechanical fatigue crack growth rate and fracture toughness, can determine the subcritical expansion after the crack initiation of that part of the life. Therefore, if the hot work die to obtain high life, mold materials should have high thermal fatigue resistance, low mechanical fatigue crack growth rate and high fracture toughness value. The index of thermal fatigue resistance can be expressed by the number of thermal cycles of initiation thermal fatigue cracking, or by the number of fatigue cracks and the average depth or length after a certain thermal cycle

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