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 maintains hardness at around 60 HRC at room temperature, and hot work die steel is generally required to stay in the HRC 40 ~ 55 range according to its working conditions. For the same steel type, within a certain range of hardness values, the hardness is directly proportional to the resistance to deformation; however, the plastic deformation resistance may be significantly different between steel types with the same hardness value but different compositions and structures.

(2) Red-hardness Hot-working molds that work at high temperatures require the stability of their structure and properties to maintain a sufficiently high hardness. This property is called red-hardness. Carbon tool steel and low alloy tool steel can usually maintain this performance in the temperature range of 180 ~ 250 ℃, and chromium-molybdenum hot work die steel generally maintain this performance in the temperature range of 550 ~ 600 ℃. The red hardness of steel mainly depends on the chemical composition and heat treatment process of the steel.

(3) Compressive yield strength and compressive flexural strength The mold is often subjected to high pressure and bending during use. Therefore, the mold material should have a certain compressive strength and flexural strength. In many cases, the conditions for compressive and bending tests are close to the actual working conditions of the mold (for example, the measured compressive yield strength of the mold steel is more in line with the deformation resistance shown when the punch is working) . 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 states.

2 toughness

In the working process, the mold is subjected to impact loads. In order to reduce damage such as breakage and chipping during use, the mold steel is required to have certain toughness.

Factors such as chemical composition, grain size, purity, carbides and inclusions, morphology, size, and distribution of the mold steel, as well as the heat treatment system of the mold steel and the metallurgical structure obtained after the heat treatment, etc. The toughness has a great impact. In particular, the influence of steel purity and hot working deformation on its transverse toughness is more obvious. The toughness, strength and wear resistance of steel are often contradictory. Therefore, the chemical composition of the steel must be reasonably selected and reasonable refining, hot working and heat treatment processes must be used to achieve the best combination of wear resistance, strength and toughness of the mold material.

Impact toughness refers to the total energy absorbed by a characteristic material during the entire fracture process during one impact. However, many tools are fatigue fractured under different working conditions. Therefore, the conventional impact toughness cannot fully reflect the fracture performance of mold steel. Small energy multiple impact fracture work or multiple fracture life and fatigue life testing techniques are being used.

3 abrasion resistance

The most important factor in determining the service life of a mold is often the wear resistance of the mold material. The mold is subject to considerable compressive stress and friction during work, which requires the mold to maintain its dimensional accuracy under strong friction. Mold wear is mainly of three types: 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's high hardness and ensure that the composition, morphology and distribution of carbides or other hardened phases in the steel are reasonable. For molds that are 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 lubrication and reduce melt wear such as seizure and welding 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 simulated test method to measure the relative abrasion resistance index as a parameter to characterize the abrasion resistance level under different chemical composition and microstructure state. The life before the specified burr height reflects the wear resistance of various steel types; the test is based on Cr12MoV steel for comparison.

4 thermal fatigue resistance

In addition to the cyclical changes in load under service conditions, hot work die steel is also subject to high temperature and periodic rapid cooling 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 indicator of comprehensive performance, which includes three aspects: thermal fatigue performance, mechanical fatigue crack growth rate, and fracture toughness.

The 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 of thermal fatigue crack initiation is large; the mechanical fatigue crack propagation rate reflects the material after thermal fatigue crack initiation, during The amount of expansion of each stress cycle when a crack propagates to the inside under the action of pressure; fracture toughness reflects the material's resistance to the instability propagation of an existing crack. For materials with high fracture toughness, the cracks in the cracks must have a sufficiently high stress intensity factor at the crack tip, which means 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 to cause instability propagation.

In other words, the thermal fatigue resistance determines the part of the life before the fatigue crack initiation; and the crack growth rate and fracture toughness can determine the part of the life that occurs when the crack starts subcritically. Therefore, in order to obtain a high life for hot work molds, the mold materials should have high thermal fatigue resistance, low crack growth rate and high fracture toughness values.

The index of thermal fatigue resistance performance can be measured by the number of thermal cycles that initiate 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 occlusion resistance is actually the resistance when "cold welding" occurs. This property is important for mold materials. During the test, usually under the condition of dry friction, the test tool steel specimen and the material with a tendency to occlude (such as austenitic steel) are subjected to a constant-speed dual-friction motion to gradually increase the load at a certain speed. The moment is also increased accordingly. This load is called the "occlusion critical load". The higher the critical load, the stronger the occlusal resistance.