Study On Microstructure And Properties Of High Speed Steel By Electromagnetic Centrifugal Casting

In the iron and steel industry, roller is a main instrument to deform the rolled productsplastically. During the rolling process, rollers contact with rolled products directly, thusmanifesting the failure mode of abnormal wear, such as crack and indentation on the surface.In order to improve the surface abrasion resistance, materials scientists have developed rollmaterials from chilled cast iron to high chromium cast iron and high carbon high speed steel.Vast attention was put into high chromium cast iron in the1990s due to its good match ofhardness and toughness, however, the defect lies in that the wear resistance of high chromiumcast iron is very sensitive to the working temperature. With good thermal stability andexcellent mechanical properties, the high speed steel has received extensive attention recently.Therefore, it is of great significance to study on the preparation process and mechanicalproperties of high speed steel for promoting the roll preparation techniques.High speed steel was prepared in this paper with a homemade electromagneticcentrifugal casting （EMCC） device. The effect of process parameters of sand casting,centrifugal casting （CC） and ECC on the microstructures of as-cast and heat treatment of highspeed steel was studied by means of optical microscopy, scanning electron microscopy,transmission electron microscope, energy dispersive spectrometer and X-ray diffraction.Furthermore, the Fluent software was applied to analyze the heat flux magnetic coupling inthe molding process of high speed steel by EMCC. The result shows that the structure of highspeed steel is mainly composed of carbides （MC+M₂C+M₇C₃）+Marten site+Austenite.During the course of CC, the size of granular carbides decreases gradually with the increaseof centrifugal speed, and the distribution of carbides becomes uniform and dispersion, whilelath of carbides becomes small. The electromagnetic stirring force can further reduce the sizeof carbides in the high speed steel, by which the sharp corners of angular carbides roundgradually, and the distribution of alloying elements becomes more uniform. The secondaryfined carbides, the size of which ranges from220to260nm, are separated out of the highspeed steel matrix under the condition of1200℃+560℃double tempering, their quantitygrows and distributes homogeneously in the grains with the increase of quenchingtemperature, which lays the structural foundation to improve the red-hardness of high speedsteel.Meanwhile, the effect of process parameters of sand casting, CC and EMCC on themechanical properties of high speed steel after heat treatment was investigated throughhardness, impact and wear test. The results indicate that the hardness value of high speed steel increases and then decreases with the increase of centrifugal speed, it reaches the peak whencentrifugal speed is1100r/min, while no obvious effect of centrifugal speed on impacttoughness is detected. Under the same circumstance of heat treatment, the hardness of highspeed steel increases first and then decreases with the increase of magnetic strength; when themagnetic strength is0.1T, the high speed steel has the highest hardness as well as impacttoughness. The abrasion loss of high speed steel reduces with the increase of quenchingtemperature, and the minimum loss value appears at1200℃, with560℃double tempering.Heat flux magnetic coupling numerical simulation analysis results show that the liquidmetal flows into the gate of high-speed spinning mold by a parabolic form, and moves alongthe direction of axial by the form of spiral close to the inner surface of mold, with itsmaximum speed being same to the rotational speed of the inner layer of the mold. During thesolidification process of high speed steel, the temperature distribution is non-uniform; theliquid metal solidifies from two ends to the middle on the direction of axial, and from outer toinner layer on the direction of radial, respectively. At a pouring time in the same position, thepressure of internal liquid and the flow rate are all different between CC and EMCC,indicating that the electromagnetic force plays a role in stirring molten metal.