Effect Of Boron On Microstructure And Mechanical Properties Of Austempered Ductile Iron

ADI(Austempered Ductile Iron) is a kind of ductile iron with acicular ferrite and high carbon austenitic as its main base structure, which is obtained by the austempering process. This material is characterized by high strength, good toughness, excellent wear-resistance. Besides, with its low ratio of density to tensile strength, as well as low cost, and high economic benefit and some other unique advantages, it has become one of the hot spot of the21st century materials.In this paper, we studied the effect of different boron content and heat treatment process on the microstructure and properties of ADI through metallographic observation, SEM, EDAX, spectrum analysis, XRD, hardness measurement, tensile strength and wear test.In the research of boron range (0～0.077%), with the increase of the content of boron, graphite spheroidization level reduced, graphite size level increased, the number and the roundness of the degrees declined:when the boron content was below0.008%, the graphite size class was reached the7level, and the spheroidization class was reached the2level. When the Boron content was reached0.012%, the graphite size class was6degree and the spheroidization class was3degree, when the boron content was reached0.020%, the graphite size class was6level and the spheroidization class was4level, when the Boron content was reached0.049%, the graphite spheroidization worst effect, the graphite size class was5level and the spheroidization class was5-6level.The microstructure of as-cast samples was graphite nodule, pearlite, ferrite and some certain amount of carbide. When the boron content was low, boron existed in matrix with the form of solid solution. When the boron content was higher, boron was mainly dissolved out in the form of boron carbide, and with the increased amount of boron content increased:when the boron content was below0.008%, boron existed in ferrite with the form of solid solution, the boron carbide content was low, when the boron content was0.012%, some boron existed in matrix with the form of solid solution and some boron formed boron carbide, the number of boron carbide increase, the carbide was blocks and distributed uniformity, when the boron content was0.020%, boron carbide content continue to increase, boron was mainly dissolved out in the form of boron carbide, when the boron content was0.049%, boron was mainly dissolved out in the form of boron-ledeburite, and the shape was a larger block or fish bone, when the boron content was0.077%, boron carbide content continue to increase, this carbide with strong directional and rows distributed in the matrix.The microstructure of austempering samples was graphite nodule, needle ferrite, retained austenite and some certain amount of carbide, the form of boron was Fe2B、 Fe3B and Fe23(CB)6. With boron content increased from0%to0.020%, and the bainite increased and gradually thinning, white area to reduce, carbide content increase; the brinell hardness and tensile strength increased. The elongation and wear rate decreased, wear resistance rised. After austempering at280℃, the brinell hardness was from455HB to484HB, the tensile strength was from1410MP to1482MP, the elongation was from2.5%to1%and the wear rate was from1.39mg/m to1.12mg/m, wear resistance rised.In the research of austempering temperature range (240℃～32℃), with the rise of austempering temperature, the amount of retained austenite increased and the bainite form was from needle into featheriness and organization gradually coarsening. The brinell hardness and tensile strength decreased. The elongation and wear rate increased, wear resistance reduced. When the boron content was0.008%, with the rise of austempering temperature, brinell hardness was from481HB to394HB, the tensile strength was from1465MP to1160MP, the elongation was from2%to2.5%and the wear rate was from1.05mg/m to1.29mg/m, wear resistance reduced.The effect of different boron content and heat treatment process on the microstructure, properties were studied in this paper. The results show that:when the boron content was0.008%, after austeniting at900℃for1.5h and austempering at280℃for2h, which had good properties. The finally organization of samples was needle ferrite, graphite nodule, a small amount of carbide and retained austenite, the brinell hardness was467HB, the tensile strength was1435MPa, the elongation was2%and the wear rate was1.244mg/m.