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Fatigue characteristics of common live working tool materials
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Fatigue characteristics of metal materials: there are many factors that affect the fatigue characteristics of metal materials, besides the influence of composition, inclusions and heat treatment, there are other factors.
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The influence of surface processing, dimension and stress concentration of metal materials: the surface stress of tool parts under bending and torsion load is the largest, and most of the fatigue damage starts from the surface. The rough surface state is equivalent to the existence of many micro notches, which lead to the reduction of fatigue strength due to the stress concentration of the notches. The rougher the surface is, the more serious the fatigue strength is. The fatigue strength of parts with round corner, oil hole, thread keyway and other notches is reduced due to stress concentration. The sharper the notch, the more concentrated the stress, the more serious the fatigue strength reduction.
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The coating on the surface of parts also has some influence on its fatigue strength: the coating on the surface of tools is mainly chromium (CR) and nickel (Ni). The main reason for the great reduction of fatigue limit caused by the plating of these two metals is the large residual tensile stress in the coating. Because there are many micro tortoise cracks in the coating, it acts as a notch to make the stress concentration. For example, the fatigue limit of 30CrMo material before plating is 72.5mpa. After plating 0.25mm Cr, the fatigue limit is 50.6mpa, only 68% of that before plating.
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The influence of heat treatment on its fatigue strength: the technical means to adjust the internal structure of metal and improve the performance of material through heating, heat preservation and cooling, is called heat treatment. Heat treatment can be divided into annealing, sub fire, tempering and quenching. Annealing is cooling with furnace, sub fire is cooling with air, quenching is cooling suddenly with water, tempering is heating again. Annealing and tempering can improve mechanical properties, reduce hardness, improve plasticity and eliminate internal stress. Quenching is to obtain high hardness and high wear resistance, so as to obtain a good combination of strength and plasticity. Tempering is to prevent internal stress and brittleness caused by quenching.
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