Resistant to thermal fatigue Hot work die steel under the conditions of use in addition to withstand periodic load changes, but also by high temperature and periodic hot and cold, therefore, assess the hot work die steel anti-fracture properties should pay attention to the material thermal mechanical fatigue fracture properties. Thermal mechanical fatigue is an indicator of overall performance, including three aspects of thermal fatigue properties, mechanical fatigue crack growth rate and fracture toughness. The thermal fatigue properties reflect the material life before the thermal fatigue crack initiation, the thermal fatigue resistance of the material, thermal fatigue crack initiation thermal cycles are more; mechanical fatigue crack growth rate reflects the material after thermal fatigue crack initiation, the expansion of each stress cycle forging The amount increases as the crack propagates inward under pressure. Fracture toughness reflects the resistance of the material to existing crack instabilities. In materials with high fracture toughness, cracks propagate unstably, requiring a sufficiently high stress intensity factor at the crack tip, ie, a larger crack length. Under constant stress conditions, fatigue cracks already exist in the mold. If the mold material has a higher fracture toughness value, the crack must expand deeper to cause unstable expansion.
The most important factor that determines mold service life is often the wear resistance of the mold material. Die in the work to withstand considerable compressive stress and friction, the mold can still under strong friction to maintain its dimensional accuracy. Die wear is mainly mechanical wear, wear and oxidation wear and tear of the three types. In order to improve the wear resistance of mold steel, it is necessary to maintain the mold steel with high hardness, but also to ensure that the composition of steel carbides or other hardening phase, morphology and distribution more reasonable. For heavy duty, high-speed wear service mold, the surface of the mold steel to form a thin and dense adhesion of the oxide film, to maintain the lubrication, reducing the mold and the workpiece between the sticky bite, welding and other melting wear, and Reduce the oxidation of the mold surface oxidation wear. Therefore, the working conditions of the mold have a greater impact on the wear and tear of steel.
Bite resistance is actually the occurrence of "cold welding" when the resistance. This property is more important for mold materials. The test is usually under dry friction conditions, the tool steel to be tested with the tendency to bite the material (such as austenitic steel) friction at constant speed dual-friction, with a certain speed gradually increase the load, then turn Moment also increased accordingly, the load is called "bite critical load", the higher the critical load, marking the bite resistance is stronger.