Study Of Ultrahigh Carbon Steel With Addition Of Aion The Base Of Thermaldynamic Calculation

Using the vacuum induction melting furnace, 6 kinds of UHCS with increasing content of A1(UHCS-OA1, UHCS-1.0A1, UHCS-1.5A1, UHCS-2.0A1, UHCS-2.3A1, UHCS-3.5A1) and 3 kinds of UHCS with different content of Al and Si (UHCS-0.9Si, UHCS-0.6Al-0.6Si, UHCS-1.6Al-0.6Si) were prepared. The ingots were hot-forged and worked into experimental samples. The FactSage software has been applied to calculate the thermodynamics data of UHCS with different addition of Al. With the help of quenching-hardness test, the phase transformation temperatures were determined. The microstructures processed by different techniques were analyzed by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM), and the mechanical properties were tested.Both the thermodynamic calculation and experiments showed that the main reason for inhibition of network carbide of forged UHCS was that the quantity of proeutectoid carbide decreased with the addition of Al. Meantime, the phase transformation temperatures were improved. The areas of eutectoid transformations were widened and that of proeutectoid carbide precipitation were shrunk with the increasing of Al content so that it was easy to form fine pseudo-eutectoid structure. With the C content under 1.7wt%, the addition of 1.5-2.3wt%Al restrained the carbide precipitation of forged UHCS effectively.The analyses of microstructure evolution showed that by heating the austenite with inhomogeneous carbon concentration, the proeutectoid carbide particles of UHCS with addition of Al precipitated dispersedly in the matrix during cooling, no tendency to nuclear at the grain boundary and no formation of network carbide.Compared with the UHCS without Al, the spheroidization techniques of UHCS withaddition of Al were simplified. UHCS with the addition of Al above 1.5wt% can be processed by the divorced eutectoid annealing technique or quenching-and-high-tempering technique, which both can obtain the desired microstructure with the fine spheroidal carbides dispersed on the ferrite matrix.With high content of Al, the carbide growth tendency of UHCS was decreased during spheroidization, which avail to restrain the growth of matrix. But too much Al (>3wt%) may decrease the ductility of UHCS. The optimal composition with 1.5-1.8wt% C content was the addition of 1.5-2.3wt%Al and 1.5wt%Cr, which exhibited good ambient mechanical properties after spheroidization.The UHCS with addition of Al after quenching can obtain the microstructure with the fine spheroidal carbides dispersed on the martensite matrix. The sub-structure of martensite was the mixture of lath and twin-crystal martensite. The carbides mainly precipitated along and in the lath boundary during tempering. With addition of Al, the tempering stability of UHCS was improved and the changes of microstructure at each stage were deferred to higher temperatures.