Research Of Wear Resistance And Thermal Fatigue With Vermicular Cast Tron Used For Brake Drum

With the rapid development of economy, the speed of development of China’sautomobile industry is also speeding up significantly, the car has become indispensablemeans of transport to our daily life. The brake drum is an important part of automobilebrake system, playing a very important role in safe driving. The brake drum is generallyworking in the complex conditions which faces the strong friction, heat load, alternatinghot and cold effect. At the same time, the service process is also under huge braking andcentrifugal force. Therefore the brake drum material should have high strength, goodplasticity, thermal fatigue and wear resistance.The test adopted commercial spheroidizing agent RE-Mg-Si and self-madeinterference agent, using75Ferrosilicon ladle bottom pouring method in the firstbreeding and the Silicon barium alloy in the ladle to ladle pouring method at the secondtime inoculation for the vermicularizing treatment. In addition, analyzed and detected themechanical properties, microstructure, sectional sensitivity, wear resistance, thermalfatigue resistance of the prepared vermicular cast iron and brake drum after failure.The test results show that, the tensile strength of prepared vermicular cast ironspecimen is greater than380MP, hardness is greater than170HB, the elongation rate isgreater than1%and with a good comprehensive mechanical properties. Metallographicobservation founed, the morphology of graphite of prepared vermicular cast iron samplesis mixed up with vermicular graphite and nodular graphite, vermicular rate is more than85%. Metallographic microscope and scanning electron microscope analysis showed thatthe morphology of graphite in the vermicular graphite cast iron is not alone in the middleof the existence, but connected with the surrounding graphite each other, forming apolymer; vermicular graphite shape is short and thick, rough side, laminated structure,surface uplift and obtuse end.We got the formation process of vermicular graphite through liquid quenching. Thesaturated carbon in the molten iron come into being a small graphite ball core with the temperature decreasing. Because of the elemental composition, temperature and someother external factors, small graphite grow along the a[1010] direction and c[0001]direction alternately, and gradually grow into vermicular graphite. The crystallizationprocess of graphite in hypereutectic vermicular cast iron is: Lâ†'L+Gâ… â†'Gâ… +A+Gâ…¡â†'Gâ… +A+Gâ…¡+Gâ…¢. Wall thickness sensitivity test showed that the wall thickness of thespecimen in the75mm-35mm stage, along with the changing of wall thickness, there isbasically no change from vermicular rate, the morphology of graphite to the amount ofpearlite in the matrix. Wall thickness at the35mm-5mm stage, along with the wallthickness decreasing, vermicular graphite became smaller, the amount of pearlite andvermicular rate decreased gradually.Through the high temperature and room temperature wear test, we got the specimenwear mass loss under different conditions,in the wear condition:1000r/min-180s-20℃,variable load, pearlite amount being20%of the sample weight losses was the least; Wearconditions is as follows:1000r/min-180s-100N, variable temperature, sample pearliteamount being15%lose the least weight; When the wear condition is180s-100N-20℃,variable speed, The weight losses of pearlite volume fraction15%and20%is roughly the same. Therefore, the amount of pearlite in vermicular cast iron for thebest wear resistance is15%-20%.Through the thermal fatigue experiments, The higher the upper limit temperature was,the crack initiate earler and the crack propagate faster. The thermal fatigue resistance ofvermicular iron is superior to that of gray iron, the cyclic temperature difference is bigger,the superiority of vermicular iron is more obvious; Vermicular rate is above85%, whenamount of the pearlite in matrix structure of vermicular cast iron is greater, the crackinitiation earlier, expand more quickly in the thermal fatigue process,so the thermalfatigue performance is worse.After analysing vermicular graphite cast iron after failure we get that the failure formof vermicular graphite cast iron brake drum is mainly the crack, and the crack position areon working face; The direction of the crack is mainly along the axial direction, basicallyparallel; Pearlite in the failed vermicular iron brake drum structure decomposed, phasechanged, became only a small portion of ferrite and pearlite, most was the large size of mixed phase structure of martensite and bainite.