Study On The Preparation, Heat-treatment, Microstructure And Properties Of High Speed Steel For Rolls

With the development of modern industry, the demand of mill bar with high performance increases rapidly, the service performance of traditional high-chromium cast iron roll can not meet the requirement of service. Develop a new kind of steel for rolls in order to elevate the rolling life and quality of mill bar is a new question for the persons who study on rolls. During recent years, high speed steel was used to manufacture the new type composite rolls. It is possible that the high-chromium cast iron rolls widely used at present will be replaced by high speed steel rolls gradually, because the latter has longer service life, better red-hardness and wear property.This thesis starts from the theory of materials strengthening, design and prepare four kinds of high speed steel with different composition. The microstructure and properties of high speed steel with different composition and heat-treatment process were studies by the way of metallograph, XRD, SEM, EDS and hardness measurement etal. The influence of tungsten equivalent, quenching process and tempering process on microstructure and properties of high speed steel was investigated emphatically.The results show that the as-cast microstructure of high speed steel contains martensite matrix, retained austenite and different kinds of carbides, for instance, granular MC carbides, network M₇C₃ carbides, fishbone M₆C carbides and other complex carbides. Among them, MC carbides mainly contain V and Nb, M₇C₃ carbides contain Cr and M₆C carbides mainly contain W and Mo. The hardness of as-cast high speed steel is proportional to the tungsten equivalent, while the impact toughness is not changed obviously with different composition. The size of grain and carbides shows a decreasing trend with higher tungsten equivalent in research range. The quenching hardness of high speed steel increase and then decrease with the quenching temperature rising. The temperature when the peak quenching hardness appears is proportional to the tungsten equivalent. The tempering hardness of high speed steel increase and then decrease with the tempering temperature rising. But the temperature when the peak tempering hardness appears is inverse proportion to the tungsten equivalent. The impact toughness of high speed steel increases when the tempering temperature rising.The proper quenching tempering of high speed steel is between 1050℃and 1100℃, while the proper tempering temperature is between 500℃and 550℃. There are lots of fine granular carbides and the network of carbides is eliminated on the heat-treated microstructure of high speed steel. The hardness and impact toughness of high speed steel can reach HRC 62 and 6J/cm² respectively, and can meet the demand of manufacture rolls primely.