Effect Of Molybdenum On Microstructure And High Temperature Tensile Strength Of Cast Iron Brake Drum

Gray cast iron has been playing an indispensable role in the field of heavy truck brake drum.The heavy truck running under special working conditions often cracks or fails to brake due to insufficient high temperature tensile strength of the brake drum in frequent braking,which not only shortens the service life of the brake drum,but also poses a hidden danger to driving safety.Therefore,it is urgent to improve the high temperature tensile strength of brake drum materials to prolong its service life.Adding alloying elements is the most economical and effective means to improve the properties of gray cast iron,in which molybdenum is considered to be one of the most effective elements to improve the high temperature strength of cast iron.To this end,the present paper intends to study how alloy element Mo affects the properties of gray cast iron（especially its high temperature tensile strength）.Firstly,the effect of molybdenum on the microstructure of gray cast iron was studied in preliminary experiments.On this basis,the laboratory experiment was expanded to study the effect of molybdenum on high temperature tensile strength,thermal fatigue property and thermal conductivity of gray cast iron.The preliminary experimental results show that with the increase of molybdenum content,the length and quantity of graphite in gray cast iron become shorter and larger,and the area fraction of graphite decreases slightly;the addition of molybdenum reduced the pearlite content,but refined the pearlite structure.When the addition of molybdenum increased from 0.034%to 0.77%,the average lamellar spacing of pearlite decreased from 395 nm to 229 nm.The vacuum induction furnace was used to expand smelting.The research results show that:（1）the length of graphite is obviously shortened and the area fraction of graphite is slightly reduced by the addition of molybdenum,which are consistent with the preliminary experimental results;with the increase of molybdenum content,the pearlite content decreases,but the pearlite and eutectic cluster are refined.With the increase of molybdenum content from 0.024%to 0.8%,the average lamellar spacing of pearlite decreases from 619 nm to 438 nm,and the distribution density of eutectic cluster increases from 217 cm^-2 to 286 cm^-2.（2）Adding alloying element molybdenum can effectively improve the tensile strength of gray cast iron at room temperature and high temperature.Surprisingly,when the content of molybdenum is 0.8%,the tensile strength of gray cast iron at 500℃and 600℃still reaches the strength standard of HT250 and HT150 at room temperature respectively.（3）The addition of alloying element molybdenum can effectively improve the thermal fatigue properties of gray cast iron.The samples with molybdenum content of 0.51%and 0.8%show greater thermal fatigue resistance.The addition of molybdenum reduces the thermal conductivity of gray cast iron at room temperature,but has a little effect on the thermal conductivity of gray cast iron at high temperature.In conclusion,the alloy element molybdenum can effectively improve the high-temperature tensile strength and thermal fatigue resistance of gray cast iron,but does not affect its thermal conductivity at high temperature.In particular,the gray cast iron sample with molybdenum content of 0.8%shows excellent mechanical properties and meets the material standard of HT350.Therefore,adding about 0.8%molybdenum to gray cast iron brake drum is undoubtedly an effective means to improve its high temperature strength,high temperature braking safety and prolong its service life.