Analysis Of Ultimate Heat Treatment On Microstructure And Property Of Cr5 Steel Back-up Roll

The material used for the specimens in this experiment is Cr5(forged steel after heat treatment), The effect of ultimate heat treatment on the microstructure and tribological properties of Cr5 steel were studied by SEM, TEM, Tribometer, Durometer, Tensile and Impact testing machine, in order to obtain the best heat treatment process route by this method.Differential heat treatment was the main stage of the final heat treatment process. The fist process for Cr5 was preheat treatment—hardening and tempering(thermal refining). The technological parameter was 970 ℃×60 min + 540 ℃×120 min. After tempered, the hardness, tensile strength and impact energy reached to 49 HRC, 1420 MPa and 146 J, the ductility was 2.90 %. The materials had good comprehensive mechanical performance by this time.The matrix of Cr5 is martensite, the carbides that not dissoloved distributed in the martensite after quenching. When quenching temperature is low, austenitizing is not sufficient, before part of ferrite transfered to austenite, made the ferrite existed after the quenching and the hardness of ferrite was lower, so that, the hardness of material was lower, the wear loss increased and the severity of wear was serious. When the samples were quenched between 990 ℃ to 1050 ℃, the hardness increases with the quenching temperature increasing, This was due to carbides started to dissolove in the matrix with the increasing of quenching temperature, which lead to carbon content rise, and a shift from martensite substructure to high-carbon martensite occured, in addition, in this temperature range, coarsening of matrix is not obvious. When quenching temperature reached to 1050 ℃-990 ℃, the microstructure of Cr5 steel was in steady state, and hardness reached maximum, to 55 HRC, mass loss was the smallest, and had the best resistance. When the quenched temperature exceed 1050 ℃-990 ℃, carbon content rise constantly, the point of Ms declined, retained austenite increased, which counteract the rise of martensite hardness. Meanwhile, the growing of crystals make mechanical properties decrease. So the best quenching temperature was 1050 ℃-990 ℃.Cr5 steel was tempered after quenching(the temperature was 1050 ℃-990 ℃), when the tempered temperature was low, the carbide precipitined was less, and the matrix organization has kept quenching martensite, and material of tough degree was higher, with the increase of tempering temperature, carbides precipitate in the Cr5 steel, the residual austenite transformation and the decomposition of martensite reached a steady state. When the tempered temperature was 520 ℃, carbide precipitate almost completely and evenly distributed, tempered organization kept a relatively stable state, the strength, hardness and impact energy of the material slightly decreased, that was 1490 MPa, 52 HRC, 153 J. The material break elongation was 3.16 %. Therefore, in the selected temperature point, the optimum tempered temperature was 520 ℃ during the differential heat treatment.Compared with the parameters after preheat treatment, its performance is improved significantly, and strength, hardness and impact energy respectively increased by 4.95 %, 6.12 % and 4.79 % after final treatment. By TEM analysis, the major type of carbides was M7C3, the size refined and evenly distributed, the microstructure and performance of Cr5 improved distinctly.