Effect Of Boron On The Microstructure And Properties Of Chromium White Cast Irons

Due to chromium white cast iron has excellent hardness and anti-wear performance, these alloys are widely used in mining, cement, power and other industries. In recent years, chromium white cast iron can not meet the higher requirement of wear resistant material because these alloys are likely to be happen unreliable and unsafe phenomenon such as fault, broken, et al. It restricts the application of high performance wear-resistant material. From the view of improving performance, the dissertation studies chromium series wear-resistant cast iron containing boron which wear-resistant skeleton are boron carbides and borides.The chromium white cast iron include low chromium (2wt.%Cr) white cast iron, medium chromium (8wt.%Cr) white cast iron and high chromium (15wt.%Cr) white cast iron. The following content of boron is used:0.5wt.%,1.0wt.%,1.5wt.%,2.0wt.%and2.5wt.%. The solidification microstructure and properties, wear resistance and wear mechanism, the microstructure and properties after heat treatment have been researched by means of electron microscope, electron probe micro analysis, energy disperse spectroscopy and X-ray diffractometer. The influence of boron on the microstructure and properties of chromium white cast iron are investigated.The results showed as follow, boron promote the formation of M3(C,B), M7(C,B)3, M2B, M23(C,B)6and martensite. The number of boron carbides, borides and martensite increases with the increase of the content of born in alloys. With the increase of the content of boron, the eutectic point of alloy turns to left, causing the eutectic composition of the alloy changes to eutectic alloy. When the boron concentration rises to2wt.%in low and medium chromium white cast iron, the primary phase changes from dendrite a-Fe phase to polygonal M3(C,B), but the change will be occurred when the boron content is more than2.0wt.%and less than2.5wt.%. Boron improves the hardness of chromium white cast iron. When the content of boron reaches2.5wt.%, the hardness of low chromium white cast iron reach the maximum. When the boron concentration reaches1.0wt.%, the hardness of high chromium and medium chromium white cast iron reached the maximum. The research shows that the hardness of alloys not only depends on the kinds and number of the boride carbides and borides, but also closely relates to the morphology and distribution of them.After heat treatment, the hardness of low chromium and medium chromium white cast iron containing boron is higher than their as-cast hardness, but the hardness of high chromium white cast iron containing boron is lower than their as-cast hardness. And austenitizing temperature has a great influence on the microstructure and properties of alloys. After heat treatment, the carbide boron in the organization of alloys appears to be refined or be dissolved, and the mesh structure is damaged. With the increase of austenitizing temperature, the hardness of low chromium white cast irons containing boron decreases, the hardness of medium and high chromium white cast irons containing boron increases. Low chromium white cast irons containing boron have the highest hardness after950℃heat quenching, the medium chromium white cast irons containing boron have the highest hardness after1050℃heat quenching.Through the analysis of the wear surface of chromium white cast iron containing boron, we can know that the main wear mechanism of chromium white cast iron containing boron have abrasive wear, adhesion wear, oxidative wear and spalling wear. Generally, the smaller friction coefficient means the better wear resistance. With the increase of boron content, the friction coefficient of low chromium boron alloy has been reduced, the friction coefficient of medium and high chromium boron alloy first decreases then increases. Obviously, the changes of the friction coefficient and hardness of alloys is opposite. The scratch of low chromium boron alloy turns from wide and deep to narrow and shallow, and the scratch of medium and high chromium boron alloy turns from wide and deep to narrow and shallow then changes to wide and deep. Therefore, the change of friction coefficient, hardness and wear resistance of alloy is the same.My study improves the microstructure and properties of chromium white cast iron using B so long as adding proper amount of boron, and adopting suitable processing of heat treatment.