Study On The Effect Of Modifying Modes On Microstructures And Mechanical Properties Of Cr15 High Chromium Cast Irons

High chromium cast irons are a family of excellent wear resistant materials. But the cast irons have a main problem of lack of enough impact toughness. In this regard, this thesis studied the effect of three different modifying modes, i.e., the addition of modifying agents into furnace, tapping spout and ladle respectively on the microstructures and mechanical properties of Cr15 high chromium cast irons using RE-V-Ti-B and K/Na combined modificators, to find out the most effective modifying way. The microstructural characteristics of the modified cast irons were observed by optical microscope and scanning electron microscope, and XRD was tested to examine the existed phases. The impact toughness and hardness of the cast irons were also measured. The effect of austeniting and tempering temperature on the microstructure and mechanical properties of the high chromium cast iron was investigated. On the basis of these investigations, the mechanism of the effect of different modifying modes and heat treatment on the microstructure and mechanical properties was explored. In general, the main results obtained in this thesis are as follows.The primary austenitic grains of the modified Crl5 high chromium cast irons were fined and the eutectic carbides converted from continuous network to broken, isolated block-like and strip-like carbides. More specifically, the best results were obtained in the ladle modifying mode, better results in the furnace modifying mode and general results in the tapping spout modifying mode. The impact toughness of the modified high chromium cast irons reached at 8.2J/cm2 for the ladle modifying mode,7.8J/cm2 for the furnace modifying mode and 5.2J/cm2 for the tapping spout modifying mode, while only 4.8J/cm2 for the no-modification. Thus, it can be seen that the impact toughness of the modified cast irons is remarkably improved by the modification.After austenitizing at 1050℃ followed by air-cooling, the microstructures of high chromium cast irons were consisted of martensite, retained austenite undissolved carbides and eutectic carbides. The morphology of the matrix did not obviously change comparing with the as-cast cast irons. However, there was a significant change in the network of eutectic carbides. After austenitizing at high temperature, the network of eutectic carbides was broken and eutectic carbides tended to become many pellets. The changes of eutectic carbides for the modified cast irons were more obvious than these for no-modified one. When the tempering temperature was lower than 300℃, the microstructure of high chromium cast iron was not obviously affected by the tempering treatment. After the tempering temperature was higher than 400℃, the fined and dispersed carbide particles were found to precipitate on the martensite matrix, and the martensite gradually transformed into tempered martensite. Increasing the tempering temperature at 500℃, the number of the precipitated carbides reached a peak value. Continually increasing the tempering temperature, the precipitated carbide particles began to grow up, the number of the carbides decrease and the undissolved carbide particles also grow up. The biggest carbide particles distributed in the matrix were obtained at the highest tempering temperature 700℃After austeniting at 1050℃ followed by air-cooling, the highest hardness was obtained in the ladle modifying mode, better values in the furnace modifying mode, general values in the tapping spout modifying mode and the lowest values obtained in no-modification. With the increasing tempering temperature, the hardness of high chromium cast irons decreased uniformly. Over 550℃, the hardness greatly decreased.After austeniting at 1050℃ followed by air-cooling, the highest impact toughness was obtained in the ladle mode, better values in the furnace mode, general values in the tapping spout mode and the lowest values obtained in no-modification. With increasing tempering temperature, the impact toughness increased uniformly. Austenitizing at 1050℃ for 50min followed by air-cooling and then tempering at 400℃ for 50min, the optimum combined mechanical properties of 64.9HRC in hardness and 10.1J/cm2 in impact toughness for the high chromium cast irons with modifying by ladle mode were obtained.The ladle modifying mode with RE-V-Ti-B and K/Na combined modificators was applied to produce high chromium cast iron grinding balls. The modified grinding balls were tested in a grinding mill with diameter 1.83m and the results indicated the test was successful.