Strength characteristics

（1）Hardness is the main technical index of mold steel, in order to maintain the shape and size of the mold under high stress, there must be a high enough hardness. The hardness of cold work die steel at room temperature is generally maintained at about HRC60, the hardness of hot work die steel according to its use conditions should generally be maintained in the range of 40 ~ 55 HRC. The same steel in a certain range of hardness values, its hardness and resistance to deformation is proportional, but in the hardness value of the same but different composition and organization of the steel, its resistance to plastic deformation will be very different.

（2）Red hardness In order to maintain a high enough hardness, the hot work die must maintain the stability of its structure and properties, this characteristic is called red hardness. Carbon tool steel and low-alloy tool steel generally in the temperature range of 180 ~ 250 ℃ can maintain these characteristics, hot work chromium-molybdenum mold steel generally in the temperature range of 550 ~ 600 ℃ can maintain these characteristics. The red hardness of steel mainly depends on the chemical composition of the steel and heat treatment process.

（3）Compressive yield strength and bending strength. Moulds are often subjected to strong pressure and bending during use, so the mould material must have a certain compressive yield strength and bending strength. In many cases, the conditions under which compression and bending tests are performed are close to the actual working conditions of the die (for example, the measured yield strength of the die steel in compression closely matches the resistance to deformation exhibited by the punch during (Surgery). ). Another advantage of the bending test is the large absolute strain values, which can reflect more sensitively the differences in strain resistance for different steel grades and different heat treatment and microstructure conditions.

Force

Molds are subjected to impact loads during operation. In order to reduce damage in the form of breakage and spalling during operation, the steel of the mold is required to have a certain impact strength.

The chemical composition, grain size, purity, quantity, morphology, size and distribution of carbides and inclusions, as well as the heat treatment regime of the die steel and the metallographic organization obtained after heat treatment, are important factors of the steel. Great influence. In particular, the cleanliness and thermal deformation of the steel have a more significant effect on its transverse toughness. The impact strength, strength and wear resistance of steels are often controversial. Therefore, it is necessary to intelligently select the chemical composition of the steel and apply reasonable refining, thermal processing and heat treatment processes to achieve a better combination of wear resistance, strength and toughness of the mold material.

Toughness is the total energy absorbed by a specimen throughout the fracture process of a single impact. However, many tools are subject to fatigue damage under various operating conditions, so the usual toughness does not adequately reflect the damage characteristics of the die steel. The test methods used are low energy multiple impact performance failure or multiple fracture life and fatigue life.

Thermal fatigue resistance

Hot work steel in addition to the cyclical changes in load, in addition to high temperature and cyclical rapid cooling and heating under working conditions, therefore, when evaluating the fracture resistance of hot work steel, should pay attention to its thermal properties. The material undergoes mechanical fatigue damage. Thermomechanical fatigue is a comprehensive performance index, including thermal fatigue performance, mechanical fatigue crack expansion rate and fracture toughness of three aspects.

Thermal fatigue rating reflects the life of the material before the thermal fatigue crack sprouts Materials with high resistance to thermal fatigue have more thermal cycles to initiate thermal fatigue cracks Mechanical fatigue crack growth rate As the crack expands inward, the amount of expansion per stress cycle under pressure, fracture toughness reflects the resistance of the material to the unstable expansion of existing cracks. For materials with high fracture toughness, they must have a sufficiently high stress intensity factor at the crack tip if the cracks are to be unstable and expand, i.e. they must have a long crack length. At constant voltage

Wear resistance

The more important factor in determining the life of a mold is often the wear resistance of the mold material. The mold is subjected to high compressive stress and friction during operation, requiring the mold to maintain dimensional accuracy under high friction. There are three main types of mold wear: mechanical wear, oxidation wear and melt wear. In order to improve the wear resistance of die steel, it is not only necessary to maintain the high hardness of the die steel, but also to ensure that the composition, shape and distribution of carbide or other hardened phases in the steel are reasonable. For dies working under heavy load and high speed wear, it is required that the surface of the die steel can form a thin and dense oxide film with good adhesion to maintain lubrication and reduce wear by fusion, such as adhesion and welding between the dies and workpieces. It can reduce the oxidation wear caused by oxidation on the surface of the mold. Therefore, the working conditions of the mold have a great influence on the wear of the steel.

Wear resistance can be measured by simulation tests, and the relative wear resistance index є can be used as a parameter to characterize the level of wear resistance for different chemical compositions and tissue states. To show the resources to a given burr height, the level of wear resistance of various steel grades was reflected; for comparison, tests were performed on the basis of Kh12MF steel (є = 1).

Bite resistance

The bite resistance is actually the resistance when "cold welding" occurs. This characteristic is important for forming materials. In the dry friction test, the tool steel under test is subjected to double friction with a material that has a tendency to bite (e.g. austenitic steel) at a constant rate, and the load is gradually increased at a certain rate. Accordingly, the moment also increases, this load is called "bite critical load", the higher the critical load, the stronger the bite resistance. (German Saarland)