The Effect Of Niobium Microalloying On High Temperature Strength And Microstructure Of Cast Iron Brake Drum

The gray cast iron brake drum is one of the most critical components for the vehicle of domestic and foreign trucks.However,the existing gray cast iron brake drums face severe challenges.When the car performs emergency braking or frequent braking,the temperature of the inner surface of the brake drum will rise to 500-700 ℃,resulting in a large temperature gradient and temperature difference stress.Ultimately,the high temperature cracking of the brake drum and even the failure of the brake drum are caused,which reduces the safety and reliability of the cast iron brake drum.Therefore,cast iron brake drums should have higher high temperature mechanical properties and thermal fatigue resistanceIn order to improve the high temperature strength of cast iron brake drum,this paper selects niobium microalloying as the strengthening method.Firstly,the main existence form,distribution and related strengthening mechanism of niobium in gray cast iron were studied by thermodynamic calculation and Jmat Pro software simulation calculation;Secondly,on the basis of niobium microalloying experiment,the effect of niobium microalloying on microstructure was studied Refinement and related mechanisms;Finally,through the detection and analysis of the high temperature strength and thermal fatigue strength of the cast iron,the strengthening mechanism of the high temperature strength of the cast iron brake drum is proved.The main conclusions are as follows:(1)There are three main forms of niobium in cast iron: solid solution,the strip-shaped niobium-rich phase and the bulk Nb C.The solid solution plays the role of solid solution strengthening,while the strip-shaped niobium-rich phase and the bulk Nb C play the role of precipitation strengthening of the second phase.(2)When the niobium content is 0.10%,the existing form of niobium is mainly the strip-shaped Nb C niobium-rich phase precipitated during the solidification process;when the niobium content is higher(Nb is 0.22% and 0.30%,respectively),it is also found that the precipitation in the liquid phase is of the bulk Nb C.With the increase of niobium content,the number of precipitated carbides increases and the particle size increases.(3)With the increase of niobium content,the graphite grade changes from A3 to A4,and the average graphite length also decreases continuously.The proportion of graphite larger than 30 μm is constantly decreasing.The effect of niobium microalloying on the pearlite structure is to refine the pearlite lamellar spacing and reduce the precipitation of pearlite with larger lamellar spacing.With the increase of niobium content,the average diameter of eutectic clusters in gray cast iron gradually decreased,and the number increased gradually.(4)With the increase of niobium content,the tensile strength of gray cast iron in each temperature section is gradually increased.Among them,when the content of niobium is 0.30%,the tensile strength has the maximum value.The tensile strengths of grey cast iron increased from 251.0 MPa,216.2 MPa,132.7 MPa and 50.1 MPa to299.3 MPa,248.9 MPa,164.9 MPa and 70.0 MPa at room temperature,200 ℃,500 ℃and 700 ℃,respectively.(5)With the increase of temperature,the lamellar spacing of lamellar pearlite keeps increasing;when the temperature rises to 600 ℃,pearlite begins to spheroidize;at700 ℃,pearlite has been completely spheroidized.Due to the high temperature oxidation,the increase of atomic diffusion ability and the spheroidization reaction of pearlite,the initiation and unstable expansion of internal cracks in gray cast iron are aggravated,resulting in the decrease of high temperature strength of gray cast iron.(6)Due to the refinement of the lamellar spacing between graphite and pearlite,it is difficult for cracks in gray cast iron to initiate and expand instability.At the same time,the precipitation of the strip-shaped niobium-rich phase and the bulk Nb C will not only hinder the slip of dislocations,but also slow down the diffusion of carbon atoms,improve the high-temperature stability of the cast iron structure,and thus significantly improve the high-temperature strength and thermal fatigue of gray cast iron.