Key Technologies Of Surface Self-nanocrystallization And Vanadizing In Salt-bath On Die Steel

Moulds are easy to be worn and cracked under heavy load and high impact conditions.It is necessary to improve the service life of moulds.Strengthening the surface of die steel is one of the effective ways to improve its hardness and wear resistance.High temperature salt-bath is a traditional surface strengthening method for die steel.It has many problems,such as high treatment temperature,high energy consumption,long treatment time and bad environment on site.This paper puts forward a process method combining nano-technology with salt bath vanadizing technology because surface nano-technology can reduce the temperature of chemical heat treatment.This process can reduce the temperature of salt bath treatment and increase the growth rate of coatings.It is of great significance for energy saving and emission reduction and environmental improvement.By means of metallographic,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,microhardness tester and friction and wear machine,the mechanism of self-nanocrystallization on the surface of 45#steel and Cr12MoV steel,the microstructure and composition of Vanadizing layer in salt bath,the hardness gradient of cross section,the mechanical properties of nano-layer and coating were systematically studied.The main research contents and conclusions are as follows:（1）According to the principle of self-nanocrystallization in surface mechanical abrasion,the influencing factors of self-nanocrystallization in existing equipment are analyzed,and the key control parameters are determined.The characteristics and propagation mechanism of pressure transient flow wave are studied.Based on this,the construction principle of hydraulic vibration based on transient flow is proposed.On this basis,a rotating hydraulic vibration generator is developed,and a self-nanometer hydraulic vibration system for surface mechanical grinding is constructed.（2）Based on the principle of pressure transient flow dynamics,the design method of high frequency hydraulic vibration system is established.According to this theory,the whole surface mechanical grinding self-nanometer power system is designed,and the dynamic characteristics of the system are simulated and verified by experiments.The simulation results and experiments show that the frequency of the surface mechanical grinding self-nanometer hydraulic vibration system is controlled by the hydraulic vibration generator.Its frequency is consistent with the opening and closing frequency of the hydraulic vibration generator’s opening and closing port,and its amplitude is controlled by the flow rate of the system,and increases with the increase of the flow rate.Experiments show that the system can output high frequency hydraulic vibration of 50Hz⁸0Hz,the vibration speed can reach0.45m/s⁰.6m/s,and the initial impact speed of projectile can reach 27m/s-37m/s,which meets the requirements of surface mechanical grinding self-nanometer.（3）45#steel and Cr12MoV can obtain a 80μm-thick nano-layer by mechanical grinding self-nano-treatment.The average grain size of nano-layers ranges from 45 to 70 nm,the surface microhardness increases by 25.7%and 44.8%respectively,the hardness changes gradiently along the depth direction,the wear rate and friction coefficient of nano-layers are lower than those of untreated surfaces.（4）The annealed 45#steel and Cr12MoV steel after surface mechanical abrasion treatment have a great relationship with the mechanical abrasion process parameters,such as the depth of the surface nano-layer and the size of the grain.When the distance between the sample and the projectile is fixed,the depth of nanocrystalline layer increases with the increase of vibration time,decreases with the increase of frequency,and the grain size and depth refine with the increase of vibration time.（5）Through analysis,the formula of Vanadizing in salt bath is 73%Na2B₄O₇+13%V₂O₅+4%B₄C+10%NaF.The 2.5¹0μm-thick vanadium carbide coatings of die steel are prepared by mechanical grinding self-nano-pretreatment at low temperature of 750℃⁹00℃for 4⁸ hours.The thickness of the coating increases by at least 43%compared with that of the uncoated coating at the same salt bath temperature and time.Surface nanocrystallization makes elements infiltrate into each other.The penetration thickness of vanadium carbide in the matrix of 45#steel is up to 3μm under the salt-bath vanadizing parameters of 900℃and6 hours,which enhances the bonding strength between the coating and the matrix.The microhardness of the surface is increased remarkably.The microhardness of 45#steel is more than HV1600,and that of Cr12MoV steel is more than HV2300,and the hardness decreases gradually along the depth of the coating towards the center of the matrix.In summary,the research work in this paper provides a new method for the preparation of self-nanometer metal surface mechanical abrasion,provides a new way for low temperature salt-bath vanadizing,and has important significance for energy saving and emission reduction of surface strengthening treatment in die and mould industry.