Effects Of Plasma Nitriding On Microstructure And Properties Of Gray Iron Brake Discs

As the key component of automobile brake system,the properties of brake discs determine the braking performance and safety of automobiles.Automobile brake discs require high hardness,wear resistance,thermal conductivity,thermal fatigue resistance and corrosion resistance.Although the traditional non-alloyed gray iron has good thermal conductivity and friction reduction,its hardness and wear resistance are poor,it is easy to deform during the wear process,produce jitter,noise,and influence the use;the corrosion resistance is poor,and it is easy to rust and affect the beauty.Nowadays,with the harsh service conditions of the automobile brake disc,the traditional gray iron brake disc has been difficult to meet the stringent performance requirements.It is imminent to find new kinds of brake disc to replace the traditional gray iron brake disc.Through the plasma nitriding of the gray iron brake disc,the hardness,wear resistance,fatigue strength and corrosion resistance of the working surface of the brake disc can be greatly improved,and the safety and service life of the brake disc can be improved.Compared with other nitriding processes,plasma nitriding has short cycle,energy consumption and gas source consumption,high surface quality,small deformation,thickness of nitriding and tissue control,which are more suitable for brake disc treatment.In this paper,the plasma nitriding of the HT250 brake disc was carried out.OM and SEM were used to observe and analyze the microstructure of HT250 after plasma nitriding at the same temperature(520?)and different time(20h?25h?30h).The surface hardness and nitriding depth of HT250 after nitriding at the same temperature and different time were compared.The wear resistance of HT250 specimens after nitriding at different times at low load(100N)and high load(300N)was compared,and the wear mechanism was analyzed.Finally,the plasma nitriding of HT250 brake discs before and after destressing was carried out at 520? and 25h.And the effect of residual stress on the shape and size of plasma nitriding of brake discs is studied.The following conclusions are drawn:(1)After nitriding at 520? and different times,the surface nitriding layer of HT250 brake disc is composed of compound layer and diffusion layer and the diffusion layer plays the main wear resistance.After 20h nitriding,the surface compound layer is thin and inhomogeneous,and the thickest part is about 6?m.the thickness of the compound layer of 25h is about 9?m,and 30h reaches 13?m.The nitriding thickness of 20h,25h and 30h after nitriding is 190?m,240?m and 240?m respectively,among which the effect of 25h plasma nitriding is the best.(2)After HT250 samples were plasma nitrided,wear resistance gradually increases with the increase of nitriding time.Under low load(100 N),the wear resistance of 20 h,25 h and 30 h increased respectively by 4.14 times,5.12 times and 5.80 times.The wear mechanism was mainly slight adhesive wear;under high load(300 N),the properties of wear resistance of 20h,25h and 30h were respectively increased by 3.16 times,3.54 times and 3.93 times,and the wear mechanisms were mainly adhesive wear and abrasive wear.(3)After ion nitriding,the compound layer is mainly composed of ?-Fe2-3N phase??'-Fe4N phase and Si3N4 phase,and the diffusion layer is composed of a phase.Nitrogen atoms occupy the positions of ferrite lattice gaps during nitriding.When the nitrogen concentration of the matrix reaches the limit solubility of the a phase,it precipitates from the phase of the ?-Fe2-3N phase and ?'-Fe4N phase.Due to the splitting effect of graphite on the matrix,the nitriding layer is discontinuous and the nitriding layer grows along flaky graphite.(4)The residual stress of cast gray iron brake disc is reduced by 31.7%after stress relief annealing.The dimensions and roughness of the brake disc before and after stress relief annealing were changed after plasma nitriding,but all were within the require ranges,among which the size increment of the annealed brake disc after nitriding is reduced by more than 50%that of the unannealed brake disc.