Study On The Microstructure And Properties Gradient Nanostructured High-energy Ion Implantation Silicon Nitride Layers On The Surface Of 8407 Steel

Mould forming,as a high frequency material forming process,is widely recognised in the modern machining industry for its high processing accuracy and ease of processing.In the mould forming process moulds are usually in high frequency and high temperature environment for long cycle service,so it is of high research value to enhance the frictional wear and thermal fatigue properties of mould surfaces,and strengthening the surface properties of moulds can ensure processing accuracy and extend service life,which has high economic significance in engineering practice.8407 die steel is made from a chromium-molybdenum-vanadium alloy system,which gives it good resistance to high temperatures and frictional wear.The initial properties of 8407 die steel are not sufficient for use as an extrusion die or hot work die,so surface strengthening of 8407 die steel needs to take into account the effect of the strengthening method on the accuracy and service life of the die.This paper investigates the use of ultrasonic surface tumbling as a pretreatment(USRP)process combined with high energy ion infiltration(HEII)of silicon nitride to produce a functional surface modification layer.8407 die steel is ultrasonically surface tumbled to cause plastic deformation of the surface grains,resulting in grain refinement,on the basis of which silicon nitride is infiltrated with high energy ions to form a gradient nanostructure modification layer with excellent properties.The metallographic organisation,chemical composition,physical composition and microhardness of the gradient nanostructure modified layer were analysed,and its frictional wear performance under different friction conditions and thermal fatigue performance in the temperature range from 25°C to 600°C were tested,and the following conclusions were obtained.(1)Ultrasonic surface tumbling pretreatment caused plastic deformation of the surface layer of 8407 die steel resulting in nanosized surface grains with a plastic deformation layer thickness of approximately 150 μm.The depth of infiltration of USRP+HEII specimens increased by 67.6% compared to HEII specimens,the grains on the material surface were proliferated by grain boundaries and dislocations after high frequency tumbling,the activation energy of the die steel surface was reduced by these defects,and The activation ions are facilitated to enter the material at the defects,providing a stable location for their presence.(2)The hardness level of the USRP+HEII specimen is increased by approximately 22.2% compared to the HEII specimen,the depth of the hardened layer is increased by 150 μm and the strengthening phase is mainly in the form of iron compounds with silicon nitride.The residual compressive stress on the surface of the specimen after ultrasonic surface tumbling was detected by the blind hole method to be approximately-530.3 MPa.The residual compressive stress from ultrasonic surface tumbling was reduced by high energy ion infusion.(3)The USRP+HEII specimens have a low coefficient of friction and a low wear volume under dry sliding friction conditions,demonstrating excellent scratch resistance and a slight abrasive and adhesive wear mechanism.(4)Oil-lubricated friction wear experiments at different frequencies and temperatures under alternating loads show that the coefficient of friction decreases as the load rises,the USRP+HEII specimen has a small wear volume at medium to high frequencies,and the wear volume increases at different temperatures with increasing temperature.The wear mechanism of both specimens contains both abrasive and adhesive wear,but the adhesive wear of HEII specimens is more serious,with the phenomenon of laminar spalling.(5)The thermal cracking of the USRP+HEII specimen was reduced by approximately 47.5% compared to the HEII specimen in terms of crack length,crack expansion was impeded,and the crack width was narrower and the surface cracking was finer,with no obvious pit spalling and good surface integrity.(6)The USRP+HEII specimens have a better ability to bond to the infiltrated elements,with less element loss after thermal fatigue.The effective hardening layer depth after the test is approximately 175 μm,and the remaining infiltrated elements can still provide good strengthening of the specimen surface layer.