Fracture Analysis And Research On Fatigue Life Of High Speed Steel W6Mo5Cr4V2 Cold Ring-Rolling Mandrel

Ring rolling is an advanced and highly efficient manufacturing technology which have many characteristics such as low cost, saving forming force, high quality of product, improving productivity, increasing utilization ratio of energy and material and so on. Generally it is used to produce seamless ring, for example bearing ring, gear ring and etc. It is empolyed broadly in many industrial domains.Cold ring-rolling mandrel is one of the important and damageable parts of ring-rolling equipment. And the life of cold ring-rolling mandrel has a direct influence on the efficiency and the cost of products. Except that, cold ring-rolling play a cruial role on the quality and the accuracy of the products. So cold ring-rolling mandrel is one of the symbols of the level of cold ring-rolling.The fracture mode of cold-rolling mandrel is mainly early fatigue fracture. There is a regularity in life distribution of the broken mandrels that the majority of mandrels concertrated in two zones, longer life zone(>8000) and shorter life zone( 1000-3000) in stead of equal distribution.In this article, the fracture of the W6Mo5Cr4V2 cold ring-rolling mandrels with different life was analyzed, which are used to product GCrl5 bearing ring. The macro/micrograph, fatigue sources and surface defects of the mandrels with different life were analyzed and compared, it can be concoulded as follows:(1) The main part of the life of W6Mo5Cr4V2 steel cold ring-rolling mandrel is crack initiation life under the condition of rotating bending fatigue. According to the mechanic properties of W6Mo5Cr4V2 steel after heat treatment, crack initiation life is 70-90% of the fatigue life.(2)Most of the crack sources are different surface defects inclouding residual machining spurs, non-metal inclosions, surface damage, carbid defects etc.(3) W6Mo5Cr4V2 mandrel has an appearant selectivity for different crack sources, and different crack source lead to differerent life. Residual machining spur is the main reason for early fatigue fracture of mandrels, and surface or sub-surface carbid is often the crack source of the mandrel with long life . That type of fatigue frature is studied deeply.The mechanics of the working W6Mo5Cr4V2 mandrel was analyzed with the ANSYS software. The result indicated that the stress exerting on working groove of the mandrel is close to the W6Mo5Cr4V2 fatigue limit. According tothe results of fatigue fracture study and mechanical analysis, some parameters of heat treatment was designed again with the aim to obtain the best combination of strength and toughness, and at the same time the microstructure of mandrels was studied and compared after heat treatment. After trial application the result showed that low temperature quenching can improve the microstructure, enhance toughness of W6Mo5Cr4V2 steel. So the average life of W6Mo5Cr4V2 cold ring-rolling mandrel after low temperature quenching increased three times than before, and some of them are up to nearly 20000.