Performance of die steel
Thermal fatigue resistance
In addition to the periodic change of load, hot working die steel is also affected by high temperature and periodic quick-cooling and quick-heating. Therefore, the evaluation of the fracture resistance of hot working die steel should pay more attention to the thermal mechanical fatigue fracture property of the material. Thermomechanical fatigue is a comprehensive performance index, 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 the initiation of thermal fatigue crack. The number of thermal cycles of the material with high thermal fatigue resistance is more. The mechanical fatigue crack propagation rate reflects the propagation amount of each stress cycle when the crack propagates inward under the action of forging pressure after the thermal fatigue crack initiation. Fracture toughness reflects the resistance of a material to the instable propagation of existing cracks. For the material with high fracture toughness, the crack must have a sufficiently high stress intensity factor at the crack tip, that is, a large crack length, if it is to have instability propagation. Under constant stress, there is already a fatigue crack in a die. If the fracture toughness value of the die material is high, the crack must propagate deeper before instability can occur.
In other words, thermal fatigue resistance determines the part of life before fatigue crack initiation; The crack growth rate and fracture toughness can determine the part of the life when subcritical growth occurs after crack initiation. Therefore, in order to obtain high life of hot working die, 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 fatigue cracks initiation, the number of fatigue cracks after a certain thermal cycle and the average depth or length.
Wear resistance
The most important factor that determines the service life of the die is often the wear resistance of the die material. The die is subjected to considerable compressive stress and friction force in work, so it is required that the die can maintain its dimensional accuracy under strong friction. Mold wear mainly includes mechanical wear, oxidation wear and melt wear. In order to improve the wear resistance of die steel, it is necessary not only to maintain the high hardness of die steel, but also to ensure that the composition, morphology and distribution of carbide or other hardening phase in steel is reasonable. For the die in service under heavy duty and high speed wear conditions, it is required that the surface of the die steel can form a thin and dense oxide film with good adhesion, maintain lubrication, reduce the melting wear between the die and the workpiece, such as sticking, welding and so on, and reduce the oxidation of the die surface resulting in oxidative wear. So the working condition of die has a great influence on the wear of steel.
The wear resistance can be measured by the simulation test method, and the relative wear resistance index (E) can be used as a parameter to characterize the wear resistance level under different chemical compositions and organizational states. To show the life before the specified burr height, reflect the wear resistance level of various kinds of steel; The test was compared with Cr12MoV steel (Ie =1).
Occlusal resistance
Bite resistance is actually the resistance of "cold welding". This property is more important for die materials. During the test, under dry friction conditions, the tool steel sample under test and the material with occlusal tendency (such as austenitic steel) are carried out constant speed dual friction motion, and the load gradually increases at a certain speed. At this time, the torque also increases correspondingly. The load is called "critical occlusal load", the higher the critical load, the stronger the occlusal resistance.
