Strength performance

(1) Hardness and hardness are the main technical indexes of die steel. The mold must have a sufficiently high hardness to maintain its shape and size under the action of high stress. The cold work die steel generally maintains a hardness of about HRC60 at room temperature. The hot work die steel is generally required to remain in the range of HRC40~55 according to its working conditions. For the same steel grade, the hardness is proportional to the deformation resistance within a certain range of hardness values; however, the plastic deformation resistance between steel grades with the same hardness value and different composition and structure may have significant differences.

(2) Red hard molds that work at high temperatures require a stable structure and performance to maintain a sufficiently high hardness. This property is called red hardness. Carbon tool steels and low alloy tool steels typically maintain this performance over a temperature range of 180 to 250 ° C. Chromium molybdenum hot work die steels typically maintain this performance over a temperature range of 550 to 600 ° C. The red hardness of steel depends mainly 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, so the mold material is required to have a certain compressive strength and bending 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 die steel is in good agreement with the deformation resistance exhibited by the punch). . Another advantage of the bending test is that the absolute value of the strain is large and can more sensitively reflect the difference in deformation resistance between different steel grades and in different heat treatment and microstructure states.

2. Resilience

During the working process, the mold is subjected to impact load. In order to reduce the damage in the form of breakage or chipping during use, the mold steel is required to have certain toughness.

The chemical composition, grain size, purity, carbide, inclusions, etc. of the die steel, the heat treatment system of the die steel and the metallographic structure obtained after the heat treatment are all on the steel. The resilience has a big impact. In particular, the purity of the steel and the deformation of the hot work have a more pronounced effect on the lateral toughness. The toughness, strength and wear resistance of steel are often contradictory. Therefore, the chemical composition of steel should be reasonably selected and reasonable refining, hot working and heat treatment processes should be adopted to achieve the best fit of the wear resistance, strength and toughness of the mold material.

Impact toughness is the total energy absorbed by a sample during the entire impact process during a single impact. However, many tools are fatigue fractured under different working conditions. Therefore, the conventional impact toughness cannot fully reflect the fracture properties of the die steel. Test techniques such as small energy multiple impact fracture work or multiple fracture life and fatigue life are being employed.

3. Wear resistance

The most important factor in determining mold life is often the wear resistance of the mold material. The mold is subjected to considerable compressive stress and friction during operation, and the mold is required to maintain its dimensional accuracy under strong 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 die steel, it is necessary to maintain the high hardness of the die steel, and to ensure that the composition, morphology and distribution of carbides or other hardened phases in the steel are reasonable. For heavy-duty, high-speed wear conditions, it is required to form a thin, dense and adherent oxide film on the surface of the mold steel to maintain lubrication and reduce the occurrence of adhesive wear such as sticking and welding between the mold and the workpiece. It can reduce the oxidation of the mold surface and cause oxidative wear. Therefore, the working conditions of the mold have a great influence on the wear of the steel.

Abrasion resistance can be measured by a simulated test method to determine the relative wear resistance index as a parameter for characterizing the wear resistance levels of different chemical compositions and microstructures. The life before the specified burr height is reflected, reflecting the wear level of various steel grades; the test is based on Cr12MoV steel.

4. Resistance to thermal fatigue

In the service conditions, the hot work die steel is subjected to the high temperature and periodic quenching and rapid heat in addition to the cyclical changes of the load. Therefore, the evaluation of the fracture resistance of the hot work die steel should pay attention to the thermomechanical fatigue fracture properties of the material. . Thermomechanical fatigue is an indication of comprehensive performance, including thermal fatigue properties, mechanical fatigue crack growth rate and fracture toughness.

The thermal fatigue performance reflects the working life of the material before the thermal fatigue crack initiation, and the material with high thermal fatigue resistance has more thermal cycles than the thermal fatigue crack. The mechanical fatigue crack growth rate reflects the material after the thermal fatigue crack initiation. The amount of expansion of each stress cycle when the crack propagates to the inside under pressure; the fracture toughness reflects the resistance of the material to the instability of the existing crack. For materials with high fracture toughness, the cracks must have a sufficiently high stress intensity factor at the crack tip if they are to be unstable and expand, that is, they must have a large 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 extended deeper to cause instability and expansion.

That is to say, the thermal fatigue resistance determines the life of the part before the fatigue crack is initiated; and the crack growth rate and fracture toughness can determine the part of the life where the subcritical expansion occurs after the crack initiation. Therefore, in order to obtain a high life of the hot working mold, the mold 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 of thermal fatigue cracking, or by the number of fatigue cracks and the average depth or length after a certain thermal cycle.

5. Occlusal resistance

The occlusion resistance is actually the resistance when "cold welding" occurs. This property is important for mold materials. During the test, the test tool steel sample and the material with occlusal tendency (such as austenitic steel) are subjected to constant speed dual friction movement under dry friction conditions, and the load is gradually increased at a certain speed. The moment also increases accordingly. This load is called the “biting critical load”. The higher the critical load, the stronger the bite resistance.

Ferrite Magnet Rotor

Ring Magnet,Ferrite Core,Ferrite Ring Magnets

Sunxal Magnetics Co., Ltd. , http://www.hlmagnets.com