Study On The Self-Nanocrystallization Of Mould Steel And The Surface Modification Of Vanadium Infiltration In Salt Bath

Mould is an important part of industrial production,usually works in high load and high impact environment.It is very necessary to improve the performance of the mould and extend its service life.Therefore,it is an effective method to strengthen the surface of the mould steel.The surface nanocrystallization method can effectively reduce the chemical heat treatment temperature,optimize the structure and performance of metal surfaces,and solve the problems of high energy consumption,high processing temperature,and poor experimental environment in traditional salt baths.In this paper,the combination of surface nanocrystallization and salt bath vanadium infiltration technology is used to obtain a nanostructured surface layer on the surface of 45#steel using SMAT,and vanadium carbide coatings are obtained at low temperatures by using the characteristics of nanocrystalline steel.By means of optical microscope?OM?,scanning electron microscope?SEM?,X-ray diffractometer?XRD?,HV-1000 hardness tester,friction and abrasion machine and other characterization methods,the structure and mechanical properties of 45#steel after SMAT treatment were analyzed.The analysis is to analyze the microstructure and microhardness of the nano-surface vanadium layer.The main conclusions are as follows:?1?After SMAT treatment,a deformed layer with a thickness of more than 80?m was obtained on the surface of 45#steel,and the grain size of the surface layer was between 48nm and 70 nm;the microhardness of the surface layer increased by 25.7%,and the hardness changed in gradient from the surface to the center along the depth direction;the depth,width and friction coefficient of the surface wear mark were smaller than those of the surface without SMAT treatment.?2?The parameters of mechanical grinding?vibration time and frequency?affect the depth and grain size of nano layer.When the distance between the projectile and the workpiece is constant,the depth of the nanocrystalline layer increases with the increase of vibration time and decreases with the increase of vibration frequency;the grain size decreases with the increase of vibration time.?3?Determine the formula of salt bath vanadium infiltration:73%Na₂B₄O₇+13%V₂O₅+4%B₄C+10%NaF,and obtain a dense and uniform vanadium carbide coating.The SMAT-pretreated 45#steel specimen was subjected to vanadium treatment in a salt bath?treatment temperature 750⁹00°C,holding time 4⁸ h?to obtain a vanadium carbide coating of 2.58¹0.42?m.The coating thickness of the test piece is 0⁷.18?m.At a processing temperature of 850°C and a holding time of 4 to 8 hours,the thickness of the coating increased by 44.4 to 68.1%compared to the test piece without SMAT treatment.At a holding time of 4 hours and a processing temperature of 800 to 900°C,the thickness of the coating is 1.38 to 2.51 times the thickness of the specimen without SMAT treatment.?4?There is a certain relationship between the coating thickness and the mechanical grinding parameters:under the same vanadiumizing process,the thickness of the vanadiumizing layer increases first and then decreases with the increase of the vibration frequency,and increases with the increase of the grinding time.?5?Under the vanadium infiltration process with a processing temperature of 850°C and a holding time of 4 to 8 hours,the surface hardness of the coating is higher than HV1600,an increase of 3.86 to 4.72 times,and the hardness distribution along the depth decreases sharply from high hardness to low matrix Hardness,there is a sudden change near the interface.This study shows that the combination of surface nanocrystallization and salt bath vanadium infiltration technology has a significant effect on improving the surface properties of the mold,which is of great significance for saving energy,reducing emissions and improving the environment.