Effect Of Trace Heavy Metal Elements On Properties Of Alloyed Castiron

Brake discs play a crucial role in the vehicle security system,the failure of brake disc can cause serious traffic accidents.Since cast irons possess good thermal conductivity,good wear resistance,good castability and low manufacturing costs,which make them become the main material of brake discs.Therefore,research and development of high-performance iron-based brake discs have more important scientific and application value.Due to the excellent properties of tantalum,tungsten,niobium and molybdenum,they are usually added into alloys as trace elements to enhance their properties.The effects of tantalum(0.05wt.%)and tungsten(0.35wt.%)on the microstructure and properties of gray cast iron,as well as the influence of addition of niobium(0.09 wt.%)and molybdenum(0.3 wt.%)on the microstructure and properties of compacted graphite iron have been studied in this paper.What's more,besides carbon and silicon being added into gray cast iron and compacted graphite iron,nickel(0.4wt%),copper(0.3wt%),manganese(0.8wt%),tin(0.08wt%)and chromium(0.25wt%)were also added.The main results are as follows:The microstructure and chemical composition of gray cast iron and compacted graphite iron were analyzed by X-ray diffraction(XRD),Optical emission spectrometer(OES),Electro probe microscope analysis(EPMA)and Energy dispersive spectrometer(EDS).The mechanical properties were tested by Universal Test Machine and Impact Testing Machine on the strength and impact toughness of cast iron test.At the same time,the friction and wear properties of cast iron were tested on friction and wear testing machine.The NaCl salt with concentration of 3.5%was used to simulate the salt spray composition.The corrosion behaviors of gray cast iron and compact graphite iron in salt-corrosive environment was studied,respectively.The results show that the addition of 0.05wt.%Ta can effectively improve the tensile strength of gray cast iron(322.92MPa),and it displayed the excellent wear resistant in the small friction condition of 800 mesh sand paper with the lowest wear rate(0.7047×10-7(g/Nm))and the suitable coefficient of friction(0.3538).However,the addition of 0.05 wt.%Ta and 0.35 wt.%W reduced the strength(302.95 MPa)(266.67 J/cm2),and the lowest wear rate(2.16×10-7(g/Nm))was obtained in a 600-mesh friction condition.The properties of impact toughness(266.67J/cm2)and hardnesss(218.5HV)are improved.Adding 0.3 wt.%Mo into compact graphite cast iron could improve the strength(546.67 MPa)and impact toughness(587J/cm')as well as hardness(339.5HV),and exhibited the lowest wear rate(1.13)at a 600-mesh friction condition.Whereas compare with the control group compact gray iron(CGI),CGI with 0.09 wt.%Nb reduced the strength of vermicular iron(468.86 MPa),and increased the impact toughness(390.63J/cm2)as well as hardness(335.9HV),and performed good wear resistant in the 800-mesh small friction condition(wear rate 0.745).In 3.5%NaCl corrosive environment,including Ta gray cast iron corrosion resistance is best(the corrosion current density of 6.900×10-6 A·cm-2),containing Mo vermicular cast iron are 3 ingredients in vermicular iron corrosion resistance performance is most advantageous(since the corrosion current density of 2.985 x 10-6 A·cm-2).By analyzing the microstructure and micro-zone composition,the results show that there is no special new phase in the matrix.The hard phases of tantalum and tungsten in gray cast iron are distributed on the pearlite matrix.The compacted graphite cast iron contains niobium carbides and molybdenum carbide,which also dispersed in the pearlite matrix,play a role of dispersion strengthening.The rest of alloying elements dissolved into the matrix,which have a good effect of solid solution strengthening.Comprehensive performance test and microstructure analysis show that the strength,hardness,toughness and wear resistance of gray cast iron and compacted graphite iron have been effectively improved.The properties of Ta and W-addition gray cast iron as well as Nb and Mo-addition compacted graphite iron are more excellent than that of no addition.Their performance parameters are suitable for brake disc.