Study On Supersaturated Carburization Process And New Supersaturated Carburization Steel

For conventional carburizing steels, the carbon content in carburized layer is usually controlled within 0.8%¹.0%. After quenching and low-temperature tempering, the microstructure of the carburized layer comprises the matrix composed of tempered martensite and retained austenite and dispersed carbides. The existence of the carbides and their amount, shape, distribution and size have a great effect on the hardness, strength and wearing resistance of the surface of the workpiece. In order to improve mechanical properties of the workpiece, carburization is commonly performed under higher carbon potential to obtain high content of carbides. However, network carbides will precipitate along the grain boundary of the austenite which deteriorates severely the mechanical properties of materials. In order to make full use of the potential of the material and obtain superior microstructure and mechanical properties to those by conventional carburizing, this paper studies supersaturated carburization in term of the carburizing process and carburizing steel.The supersaturated carburizing process characterized by pre-carbonizing and circular carbonization was developed for the common low-alloy carburizing steels. Microstructural observation of 20CrMnTi steel after supersaturated carburizing, quenching and low-temperature tempering by XJL-02A optical microscope reveals that the carbides distributed dispersedly in the matrix of tempered martensite and retained austenite. The hardness of supersaturated carburized layer is up to HV0.1958 about 1.1 times to that of conventional carburized layer. Moreover, wearing resistance of f supersaturated carburized layer increases by 20% compared to that of conventional carburized layer in the same abrasion condition, and the mechanical properties are improved significantly.Focused on the mold of cold drawing steel pipe produced by Stainless Steel Plant of Taiyuan Iron & Steel Group Co.,Ltd., valence electron parameters, statistical distributed probability, real distributed probability, phase structure factor and macroscopic structure factor are calculated by EET theory with 43Si2CrNi2MoV as the example, and empirical formulas on how to design composition of low alloyed steel in multielement system are established with Visual Frotran6.5 language and matrix theory , based on which the new-type supersaturated carburized steel is design. The carbon content of carburized layer of 35Cr3SiMnMoV steel after supersaturated carburizing and furnace cooling is 1.8% by peeling method. SEM analysis results reveal that the microstructure of 35Cr3SiMnMoV steel after quenching and low temperature tempering comprises the matrix composed of tempered martensite and remained austenite and dispersedly fine carbides. XRD analysis reveals that carbides in carburized layer are composed of （Fe.Cr）₇C₃ and Mo₂C. Further, formation mechanism of such dispersed carbides is analyzed and discussed. The hardness of carburized layer of 30Cr4SiMoRE steel after supersaturated carburizing, quenching and low-temperature tempering reaches HV0.11025, 20% higher than those of 20CrMnTi and 20Cr2Ni4 by conventional carburization. And its wear resistance increase by 60% compared to those of 20CrMnTi steel and 20Cr2Ni4 by conventional carburization process. 35Cr3SiMnMoV steel contains appropriate amount of strong carbide forming elements including Cr, Mo, V and non-carbide forming element Si, which meets the requirement of supersaturated carburizing process. As a result, a simple carburized process without special equipment may by used to produce satisfied supersaturated carburized layer.Based on the developed 35Cr3SiMnMoV steel, a few amount of rare earth is added and the alloying elements are further adjusted to develop new-type supersaturated carburized steel. SEM analysis results reveal that the microstructure of 30Cr4SiMoRE steel after quenching and low temperature tempering comprises the matrix composed of tempered martensite and remained austenite and dispersedly ultrafine carbides. The study suggests that adding rare earths leads to the increase of carburization speed and carburized layer depth, and the carbon content in the carburized layer is as high as 1.9%, and finer carbides than that of 35Cr3SiMnMoV is obtained owing to the coeffect of RE, Cr and Mo. After quenching and low temperature tempering of 30Cr4SiMoRE steel , the hardness of carburized layer reaches HV0.11057, approximately 20% higher than that of conventional carburized layer of 20CrMnTi and 20Cr2Ni4. Carburized layer of supersaturated carburized 30Cr4SiMoRE steel has remarkable wearing resistance which increase by 60% compared to thoes of conventional carburized layer of 20CrMnTi steel and 20Cr2Ni4, and therefore the mechanical property of the workpiece was improved significantly. 30Cr4SiMoRE steel contains appropriate amount of Cr, Mo, V and Si, and its composition meets the requirement of supersaturated carburized process. So a simple carburized process without special equipment may by used to produce satisfied supersaturated carburized layer.The developed supersaturated carburized process and the prepared workpieces in this paper not only meet the technical requirement but also are good for miniaturization and high performance of the machine. If the dimensions of the workpieces are the same, the machine will have a longer service life. 45# steel （hardening and tempering+ cyaniding） originally used by Taiyuan Iron & Steel （Group） Co., Ltd gets limited due to its low service life and environmental concerns. Now, Cr12MoV （quenching+ tempering） for 45# steel is applied. However, the its service life is still not so ideal and the cost is too high. Thus, Taiyuan Iron & Steel （Group） Co., Ltd has tried to employ 30Cr4SiMoRE supersaturated carburized steel as the mold of cold drawn steel tube and has produced more than 80tons of cold drawn steel tube. The dimensional precision of mold which is made of 30Cr4SiMoRE supersaturated carburized steel doesn’t change any more and only slight abrasion occurs. Compared with Cr12MoV and 45#steel, 30Cr4SiMoRE supersaturated carburized steel not only has excellent mechanical properties but also overcomes fatal defects of high costs and cyaniding pollution. The service life of the mold made of 30Cr4SiMoRE supersaturated carburized steel in practical production raised by 400% and over 800% compared to those of Cr12MoV and 45#steel, respectively, which brings good economic and social benefit.