| A disc spring,also known as a Belleville spring,is a conical shell which can be loaded along its axis either statically or dynamically.It can generate a high force in a very short spring length with minimal movement when compressed and based on the above points,it has been widely used in military,machinery,energy and other fields recently.However,the domestic independent production of disc springs,compared with International level in strength and toughness,are still lack of performance.Attention has been given to ahvanced high strength steels(AHSS)due to the combination of high strength and toughness.So a novel multi-step austempering(Q-P-A)heat treatment process which is inspired by the AHSS is developed to improve the both hardness and strength of a conventional 60Si2Mn disc spring.In this case,the disc spring is austenitized at 900? for 0.5h,control-quenched to a temperature below Ms(the starting temperature of martensite transformation)for a very short time,subsequently heated to the different temperature and holding for different times,and finally air cooled to the room temperature.Then,OM(Optical Microscope),SEM(Scanning Electron Microscope),TEM(Transformation Electron Microscope),EBSD(Electron Backscattered Diffraction)and Image-pro Plus are used to investigate the microstructure evolution with different heat treatment parameters.Hardness,tensile,compression and fatigue tests are conducted to discuss the relationship between mechanical properties and multiple phase microstructure.After that,the microstructure and properties of disc springs under Q-P-A process are compared to another two,which are recognized as high performance disc spring.Furthermore,phase transformation,as well as systematic strengthening effect to obtain high strength 60Si2Mn disc spring are discussed.It is found that the resulting multiphase microstructure consists mainly of prior lenticular martensite formed during controlled quenching(PM),needle bainitic ferrite(BF),and high carbon enriched retained austenite(RA).Further observation shows that a nanostructured(BF+RA(+C))nano phase including lath BF and film RA with a width below 100nm nucleates around the PM.The number of BF increased with time while RA shows a decreasing trend and changed from blocky RA to film RA.Such a microstructure results in uniform compression behavior,and significantly higher strength and hardness than for a conventional 60Si2Mn disc spring.And disc springs own better strength at lower temperature(230?200?),compared with 260?.While the compression performance shows a dreasing trend at 200? comparing with 230?,which does not meet the standard.Overall,the disc spring own the best properties at 230? for 12h.This controlled multi-step austempering process is a promising solution for enhancing the disc spring properties for those applications involving higher loading and fatigue conditions(like over 58.5 HRC and 1500 fatigue cycles),compared with another 2 disc springhs with high performance(DH-2,DH-3).The good combination of strength and toughness of the disc spring under Q-P-A process result from multiple enhancement mechanisms.The multi-phase synergistic strengtheningtoughening is one of the reasons,as well as the crack toughening.In conclusion,the high strength disc spring can be attributed to the synergistic strengthening effect of grain refinement,carbides precipitation,dislocation,work hardening and filmy microstructure strengthening mechanisms. |
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