Toughening And Properties Of NM500 Low Alloy Wear Resistant Steel By Sub-temperature Quenching Heat Treatment

Low-alloy wear-resistant steel is widely used in many industrial fields such as machinery,metallurgy,and building materials.Harsh application conditions of the fields make the steel require not only good wear resistance but also high toughness.The low-alloy wear-resistant steel usually has microstructure of tempered martensite with high hardness,which endows the steel with excellent wear resistance.However,the steel shows insufficient impact.In this paper,firstly the chemical composition of a NM500 low-alloy wear-resistant steel is designed;secondly the experimental steel is made in laboratory.It is measured static CCT curves and dynamic CCT curves of the experimental steel.Sub-temperature quenching was used to toughen the experimental steel,and the toughening effect of ferrite in the microstructure is appraised by detecting the volume fraction,size and distribution of ferrite and wear performance of the steel were studied.According to the designed composition,the experimental steel was melted in a 50 kg vacuum induction furnace,cast into an ingot at 1550°C,and then rolled to sheet in thickness of 4mm by 4-pass controlled rolling.The hot-rolled microstructure of the experimental steel is ferrite,granular bainite and a small amount of pearlite.The hardness and the impact toughness of the steel are 307 HBW and 62.0 J/cm²,respectively.The CCT curves are obtained by thermal simulation test.The static CCT curve of the experimental steel shows that ferrite and pearlite microstructure are obtained in cooling rate of less than 0.5°C/s after austenitizing,while bainite can be obtained in cooling rate of 1°C/s.In addition,the microstructure is composed of martensite when cooling rate is more than 15°C/s.The dynamic CCT curve shows that:ferrite microstructure is obtained in cooling rate of less than 1°C/s;bainite in cooling rate of less than 10°C/s;all martensite in the cooling rate of more than 15°C/s.The critical cooling rate of the experimental steel is 15°C/s.Not only the thermal deformation can refine microstructure of the steel,but also it enhance the diffusive phase transformation and shifts the CCT curve to the upper left in the figure.The microstructure of the experimental steel is tempered martensite and a small amount of ferrite by sub-temperature quenching heat treatment.The ferrite is in the form of blocks distributed in the martensite matrix in band.The martensite is mainly lath martensite and a small amount of plate martensite and the hardness of the microstructure in nearly to that of martensite one.During impacting,the ferrite in the microstructure can passivate the crack tip and make it deflect to increase toughness.The toughness is higher than 45.0 J/cm²,meeting customer requirements.The wear loss volume of the microstructure is 10.35×10^-2mm³ which is little lower than that of all tempered martensite one with wear loss volume of 11.24×10^-2 mm³.After the experimental steel is austenitized and isothermally hold at 800°C in the two-phase zone of Fe-Fe₃C diagram,it is quenched and 250℃tempered.So ferrite in the microstructure is mainly distributed on the grain boundary of original austenite and the impact toughness of the steel is not significantly improved.The values of impact toughness are all around 40.0 J/cm²,which does not meet the customer’s requirements.However,the wear resistance is better.The optimized process is 800℃sub-temperature quenching heat treatment.