Study On Optimization Of Heat Treatment Process And Corrosion Resistance For Mold Steel LJ338ESR For Plastics

This article describes the service conditions and basic performance requirements of mold steel for plastics,and focuses on the development status of its corrosion resistance in China and abroad.A new type of corrosion-resistant mold steel for plastics,LJ338 ESR,was taken as the research object and the metallographic microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and attached energy spectrometer(EDS)were used to observe and measure the microstructure and morphology for matrix,and precipitates and their chemical compositions after different heat treatment.The phase constitutent and its content of the samples after different heat treatment processes were estimated by X-ray diffraction(XRD)method.The optimized heat treatment process was determined by comparing the microstructure and hardness.A series of electrochemical corrosion experiments were performed using an electrochemical workstation.The corrosion resistance of the samples after different heat treatment processes was studied through comprehensive analysis of nyquist plots,AC impedance spectroscopy,hardness and microstructure.The results show as follows:(1)The best heat treatment process of LJ338 ESR is holding at 1090°C for 0.5h followd by water cooling and holding at 450°C for 8h followed by water cooling.(2)When the solid solution temperature is during 1000-1090°C,the second phase dissolves more in the matrix,the amount of martensite increases,the hardness rises,and the corrosion resistance enhances as the aging time increases.(3)When the solution temperature is 1090°C and the aging temperature is during300-450°C,the hardness increases and the corrosion resistance decreases as the aging temperature rises.When the aging temperature is during 450-600°C,the hardness decreases but the corrosion resistance increases as the temperature rises.(4)The solution temperature is 1090°C,the aging temperature is 300°C and450°C,the aging time is during 1-8h,the hardness increases and the corrosion resistance weakens as the aging time prolongs;when the aging time is more than 8 h up to 10 h,the hardness decreases and corrosion resistance enhances;agingtemperature is 600 °C and aging time is during 1 ~ 6h,hardness increases and corrosion resistance weakens as the aging time prolongs.when the aging time is more than 6h up to 10 h,the hardness decreases sharply and corrosion resistance enhances.(5)The peak phenomena of hardness at different aging temperatures and aging times are related to the formation of Cu-rich precipitates(or inverse austenite).The Cu-rich precipitates that are coherent with the martensite matrix are continuously nucleated,precipitated,and grown on "underageing".The coherent strain energy increases continuously,and maximum at the "peak aging".On “overaging”,the Cu-rich precipitates continue to precipitate,grow,and gradually change from coherent to non-coherent state and the coherent strain energy decreases continuously.Coherent strain can reduce corrosion resistance.