The Influence Of Co And Al On The Heat Treatments And Microstructure Of High-carbon Steels

The traditional heat treatment of getting martensite is quenching andtempering(Q-T). On this basis, quenching and partitioning(Q&P)process has beendeveloped by Speer, which can be used to generate microstructures with martensite andretained austenite combinations giving attractive properties. This treatment consists ofcooling to a quenching between martensite start temperature (Ms) and martensite finishtemperature (Mf) to fabricate a controlled amount of supersaturated martensite,followed by a partitioning treatment to accomplish fully diffusing of carbon fromsupersaturated martensite to untransformed austenite, thereby increasing the stabilityof the retained austenite upon subsequent cooling to room temperature. Themicrostructure of this treatment in the room temperature is martensite and retainedaustenite. Martensite can obtain a high strength while the retained austenite cantransform into martensite during straining and thus enhance the ductility of the steel.Previous studies have investigated the influences of carbon content and quenchingtemperature on the heat treatment. This paper mainly investigates the influence of Coand Al on the heat treatments. The contrast experiments are made to study theinfluence of these two elements on the continuous cooling transformation andquenching-partitioning heat treatment.The comparative studies on continuous cooling phase transformation and resultantmicrostructures were investigated in a high carbon steel containing Co and Al in thecondition of slow cooling rates (0.02–2.0℃/s). By means of Thermecmastor thermalsimulator, the phase transformation temperatures were measured and the continuouscooling transformation (CCT) curves were established. Experimental results showedthat an extremely fine fully pearlitic structure with an interlamellar spacing of45nmand the hardness of425HV1were obtained during continuous cooling transformationat a slow rate of0.1℃/s. The addition of Co and Al accelerated the pearlitic reactionand reduced the interlamellar spacing by increasing the free energy of transformation, and nano-scale fully pearlitic structure was obtained at slow cooling rates.In the experiment of the influence of the elements on the Q&P heat treatment, theTTT curves were calculated by MUCG83thermodynamic model. The steels firstaustenitized at950℃for30minutes and then quenched for120s at100℃and190℃respectively, which followed by the partitioning at400℃for0s,300s,1800s andfinally water quenched to the room temperature. Optical microscopy and scanningelectron microscopy were utilized to observe the microstructures. While the XRD wasutilized to measure the volume fraction of retained austenite and hardness tests werecarried out using a standard Vickers diamond pyramid hardness testing machine. Theresults indicated that for the same steel, the content of retained austenite increased withthe increasing time of partitioning while the hardness decreased. The adding of Co andAl improved the Ms temperature of the steel and refined the retained austenite. Theretained austenite was film rather than bulk and the content was increased at the sametime.