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Performance requirements of die steel

Release time:2020-06-24
1. Strength performance
(1) Hardness Hardness is the main technical index of mold steel. The mold must have a sufficiently high hardness to maintain its shape and size under the action of high stress. Cold work die steel generally keeps its hardness around HRC60 at room temperature. Hot work die steel generally needs to be kept in the range of HRC40~55 according to its working conditions. For the same steel grade, within a certain range of hardness values, the hardness is proportional to the deformation resistance; but between the steel grades with the same hardness value and different compositions and structures, the plastic deformation resistance may be significantly different.
(2) Red hardness. Hot work molds working under high temperature conditions are required to maintain the stability of their structure and performance, so as to maintain a sufficiently high hardness. This performance is called red hardness. Carbon tool steel and low-alloy tool steel can usually maintain this performance in the temperature range of 180 ~ 250 ℃, chromium molybdenum hot work die steel generally maintains this performance in the temperature range of 550 ~ 600 ℃. The red hardness of steel mainly depends on the chemical composition of the steel and the heat treatment process.
(3) Compressive yield strength and compressive bending strength The mold is often subjected to high-strength pressure and bending during use. Therefore, the mold material should have certain compressive strength and flexural strength. In many cases, the conditions for the compression test and the bending test are close to the actual working conditions of the mold (for example, the measured compressive yield strength of the mold steel is in good agreement with the deformation resistance shown by the punch during work) . Another advantage of the bending test is that the absolute value of the strain variable is large, which can more sensitively reflect the difference in deformation resistance between different steel types and under different heat treatment and microstructure conditions.
2. Resilience
During the working process, the mold is subjected to impact load. In order to reduce the breakage, chipping and other forms of damage during use, the mold steel is required to have a certain toughness.
The chemical composition, grain size, purity, number, morphology, size and distribution of carbides and inclusions in the die steel, as well as the heat treatment system of the die steel and the metallographic structure obtained after the heat treatment are all factors that affect the steel. The toughness has a great impact. In particular, the influence of steel purity and hot working deformation on its transverse toughness is more obvious. Steel toughness, strength and wear resistance are often contradictory. Therefore, it is necessary to reasonably select the chemical composition of the steel and adopt reasonable refining, hot working and heat treatment processes to achieve the best match of the wear resistance, strength and toughness of the mold material.
Impact toughness is the total energy absorbed by the sample during the entire fracture process of a characteristic material during a single impact. However, many tools are fatigue fractured under different working conditions. Therefore, the conventional impact toughness cannot fully reflect the fracture performance of die steel. Small energy multiple impact fracture work or multiple fracture life and fatigue life test techniques are being adopted.
3. Wear resistance
The most important factor that determines the service life of the mold is often the wear resistance of the mold material. The mold is subjected to considerable compressive stress and friction during work, requiring the mold to maintain its dimensional accuracy under intense friction. The wear of the mold is mainly three types of mechanical wear, oxidative wear and melt wear. In order to improve the wear resistance of the mold steel, it is necessary to maintain the mold steel with high hardness, and to ensure that the composition, morphology and distribution of carbides or other hardened phases in the steel are relatively reasonable. For molds in service under heavy load and high-speed wear conditions, it is required that a thin and dense oxide film can be formed on the surface of the mold steel to maintain lubricating effect and reduce the occurrence of melt wear between the mold and the workpiece. It can reduce the oxidation wear caused by the oxidation of the mold surface. Therefore, the working conditions of the mold have a greater impact on the wear of the steel.
The abrasion resistance can be measured by a simulated test method, and the relative abrasion index can be measured as a parameter to characterize the abrasion resistance level under different chemical compositions and tissue states. To show the life before the specified burr height, reflect the wear resistance level of various steel types; the test is based on Cr12MoV steel for comparison.
4. Resistance to thermal fatigue
In addition to the periodic changes of load under service conditions, hot work die steel is also subjected to high temperature and periodic quenching and heating. Therefore, the evaluation of the fracture resistance of hot work die steel should pay attention to the thermomechanical fatigue fracture performance of the material . Thermomechanical fatigue is an index of comprehensive performance, which includes thermal fatigue performance, mechanical fatigue crack growth rate and fracture toughness.
Thermal fatigue performance reflects the working life of the material before thermal fatigue crack initiation. For materials with high thermal fatigue resistance, the number of thermal cycles in which thermal fatigue crack initiation is more; mechanical fatigue crack growth rate reflects the material after thermal fatigue crack initiation. The expansion of each stress cycle when the crack propagates inward under the effect of pressure; the fracture toughness reflects the resistance of the material to the instability of the existing crack. For materials with high fracture toughness, if the cracks are to propagate instability, they must have a sufficiently high stress intensity factor at the crack tip, that is, they must have a large crack length. Under the premise of constant stress, a fatigue crack already exists in a mold. If the fracture toughness value of the mold material is high, the crack must propagate deeper before the instability can occur.
That is to say, the thermal fatigue resistance determines the part of life before fatigue crack initiation; and the crack growth rate and fracture toughness can determine the part of life that occurs when the crack initiates subcritical expansion. Therefore, in order to obtain a long life for a hot work die, the die material should have high thermal fatigue resistance, low crack growth rate and high fracture toughness value.
The index of thermal fatigue resistance can be measured by the number of thermal cycles that spawn thermal fatigue cracks, or by the number of fatigue cracks and the average depth or length after a certain thermal cycle.
5. Bite resistance
The bite resistance is actually the resistance when "cold welding" occurs. This property is more important for mold materials. During the test, under dry friction conditions, the tool steel sample to be tested and the material with a tendency to bite (such as austenitic steel) are subjected to a constant speed dual friction movement, and the load is gradually increased at a certain speed. The moment also increases accordingly. This load is called the "critical bite load." The higher the critical load, the stronger the bite resistance.