Investigation On Heat Treatment Process And Wear Mechanisms Of Low-carbon Nano-bainite Wear Resistant Steel

Low-carbon bainitic wear resistant steels have been proved to have excellent wear resistance and great weldability in many applications.Moreover,bainite wear resistant steels shows both high strength and toughness due to its extremely fine bainitic lath,which makes it possible for bainitic steels to replace traditional tempering martensite wear resistant steels in high stress wear conditions.In recent years,in order to produce bainitic wear resistant steels on a larger scale,the designation of chemical composition of bainitic wear resistant steels and the optimization of rolling,cooling,heat treatment have gain extensive attention all over the world.However,there are still few systematical and profound researches on the production process of bainitic wear resistant steels.Furthermore,it is still in controversial whether bainitic wear resistant steels are superior to martensitic wear resistant steels.Hence,the optimization of the chemical composition and production parameters of bainitic wear resistant steels are of great significant to balance the production cost and mechanical properties to replace the traditional martensitic wear resistant steels.In this study,a new chemical composition of bainitic steel was designed according to the previous researches.The thermal simulation test machine,scanning electron microscope,transmission electron microscope and impact wear test machine were used to analyze the optimization of rolling,cooling and heat treatment process.On the other hand,different bainitic and martensitic microstructure obtained by isothermal process,direct cooling and tempering process,quenching and partitioning process were systematically analyzed.The main conclusions are as follows:（1）The volume fraction of bainite increases at 1050℃ under conditions of all deformation strains（comparisons were made with respect to the undeformed sample）.The bainite amount and transformation rate decreased at both 850 and 950℃ at the deformation strain of 50%.In addition,a small deformation of 5%at high temperature results in an acceleration of the bainite transformation rate and an increase in the bainite amount compared with the undeformed sample.The promoting effect increases with a decrease in the deformation temperature.But a large deformation of 50%retards the bainite transformation rate at all temperatures,and the inhibiting effect increases with a decrease in the deformation temperature.（2）With the increase of isothermal time,the yield strength and hardness both decrease,whereas the tensile elongation slightly increases;thus,the wear performance decrease due to the obvious decrease of hardness.With the increase of isothermal temperature,the yield strength,hardness and toughness all obviously decrease.Hence,the wear performance was better in samples austempered below Ms than above Ms.In the present study,the best wear performance was obtained in the sample isothermally treated at 300℃ for 200s.In addition,the bainite transformation was accelerated by the formation of athermal martensite（AM）.（3）At the final cooling temperature of 380℃ after rolling,the microstructure of samples consists of bainite,martensite and retained austenite,and the martensite lath is obvious thicker than bainite lath.At the final cooling temperature of 380℃ after rolling,the microstructure of samples is all bainite,and two different morphology of lath bainite and granular bainite is observed.Compared to the rolled state,the samples after tempering for 30 min show better hardness,toughness and wear performance at the final cooling temperature of both 380 and 650℃ after rolling.However,the wear performance is worse than the rolled state after tempering for 60 and 90 min.The wear properties of samples at different tempering temperatures at the final cooling temperature of 650℃are better than those at 380℃.（4）The correlation of wear loss,hardness,and fracture toughness（KIC）is modeled as W（?）(mK_IC^3/4+H)/(HK_IC^3/4).Qualitatively,the model can be used to compare the wear resistances of the test steel under different heat treatments without calculating the m values for wear tests.Quantitatively,the value of m is calculated as 15,and the fitting curves of the Q&P samples match well with those of the Q&T samples.Hence,the fitting model of several groups of wear results can predict the wear loss of other heat treatment samples.（5）In comparison to the Q&T samples,the sample quenched at 220℃under Q&P process manifested better wear performance.The optimum amount of RA and the finer size of martensite corresponding to the minimum wear loss is obtained at 220℃.Unstable blocky RA decreased the wear resistance of the sample quenched at 190℃under Q&P process,and stable film-like RA and fine martensite laths increase the wear resistance of the sample quenched at 220℃under Q&P process.The increase of bainite amount leads to the decrease of hardness and wear performance in the sample quenched at 250℃under Q&P process.In addition,the wear performance of the high fracture toughness steel is better at the higher impact energy.（6）For the samples with similar hardness and fracture toughness,the wear performance of the carbide-free bainitic steels outperforms the tempered martensitic steels at both high-stress abrasive and low-stress abrasive wear conditions.For the samples of the same steel,the bainitic samples have a better wear performance at high-stress abrasive wear condition,while the martensitic samples show a better wear resistance at low-stress abrasive wear condition.（7）Depending on wear conditions,the stability of retained austenite has two-sided effects on wear performance of bainitic steels.At high-stress abrasive wear condition,the higher mechanical stability of RA results in better wear performance in the bainitic steels.However,at low-stress abrasive wear condition,the wear performance becomes worse when the stability of RA is sufficient to retard the transformation of RA into martensite.