Research Of Key Technique And Application Of Materials Used In Spring Elements Of Load Cells

Spring elements are key component in load cells.Anelasticity of spring elements have an impact on the metrology and precision of load cells,which is the crucial technology in load cells’ design.Invesitigation of anelasticity of spring elements play a crucial role in high precison load cells development.It is the first time to use double-frequency laser interferometer device combined with high precison temperature chamber and loading setup for nanometer scale anelasticity eveluation of 17-4PH steel spring element.Test resolution of this measurement system reaches 0.1nm.Strain gage based load cells were adopted to qualitatively evaluate 40CrNiMo steel anelasticity induced by different substructures.In combination with transmission electron microscopy（TEM）,internal friction,finite element analysis（FEA） and X-ray diffraction,etc.,systematically studied the anelasticity of 17-4PH steel and 40CrNiMo steel spring elements.The root cause of 2Cr13 steel spring element’s susceptibility to cracking failure was analyzed and solved,which substantially promotes 2Cr13 steel spring elements’ application in different fields.Key points of this paper are outlined as follows:Impact of test interval time on the anelasticity performance of spring element had been researched.It shows when test waiting time is less than about 12 minitues, hysteresis and zero return of spring elements are no longer changing.When waiting time is longer than 12 minitues,hysteresis and zero return increase with prolonging waiting time.It shows that stress-induced reorientation induced by interstice atoms accounts for pre-mentioned phenomenon in 17-4PH spring element.Hysteresis produced by stress-induced reorientation of 17-4PH steel is about 4～7%of its total hysteresis.Magnetism field has obvious influence on 17-4PH steel’s hysteresis. Hysteresis of 17-4PH steel spring element with saturated magnetization is about 20～30%reduction compared with that of no magnetized spring element.Noteworthy, hysteresis of 17-4PH steel spring element with partial magnetized is bigger than that of no magnetized.It indicates that no magnetication-induced hysteresis（magnetostriction and counter- magnetostriction） produces in magetic saturation spring element.As a result,hysteresis of spring element goes down.Macroscopic eddy flow loss maybe present in partial magnetization spring element,which makes hysteresis pronounced. Anelasticity of 17-4PH steel spring elements with same solution heat treatment and different ageing process were systematically studied.It shows sample with solution treatment（without being aged） has the worst anelasticity,especially elasticity creep performance.With the same hardness,anelasticity of samples with low temperature ageing is better than that with high temperature ageing.Over-ageing worsens anelasticity of 17-4PH steel.There exists non-linearity relationship between hardness and anelasticity of 17-4PH steel spring elements:It shows the strenghening of 17-4PH steel stems from copper particles dispersedly precipitate in substrate.With escalating ageing temperature,copper particle size increases,copper particle dispersibility and dislocation density of substrate decrease,and recovery of lath martensites well develops,all these evolutions make anelasticity of 17-4PH steel poorer.Anelasticity of spring elements can be substantially influenced by load stress and test temperature.Elasticity creep and creep recovery have same change trend.Generally,anelasticity value of spring elements increases with escalating test temperature and growing load stress.Load stress has bigger effect than test temperature on hysteresis behavior of spring elements.Absolute value and relative value of anelasticity of spring elements have no intrinsic relationship. Internal friction test of 7-4PH steel indicates that internal friction goes up with growing test temperature and amplitude of vibration.Dynamic modulus has opposite change of internal friction.Internal friction and hysteresis of sample have similar change tendence. It is found internal friction and hysteresis take place when strain is well below 10^-6. Pre-load had performed on solutioned 17-4PH steel（before ageing） to evaluate pre-load effect on hysteresis performance.Result shows when pre-load direction is the same as that of load cell’s test,then hysteresis goes down,otherwise,hysteresis goes up.This phenomenon is related to Bauschinger effect.Anelasticity of 40CrNiMo steel with same quenching temperature and different tempering have been investigated.It shows that tempering troosite has excellent anelasticity performance compared with tempering martensite and tempering sorbite. When low temperature tempered,about 4.1%retained austenite propagates along lath martensites,combined with residual tensile stress and unstable substructures,tempering martensite has relatively poor anelasticity property.Tempered at medium temperature, most of twins substructures and retained austenites formed after quenching disappear, meanwhile,residual tensile stress eliminates.All these lead to tempering troosite owns nice anelasticity.When tempered at high temperature,carbides coarsen,and recovery and recrystallization take place.Tempering troosite has low strength,as a consequence, poor anelasticity.Anelasticity of 40CrNiMo steel with similar hardness（44～46HRC） induced by different heat treatment has been investigated.Result indicates that lower troostite owns the best anelasticity,sub-temperature quenching and over-temperature quenching all have negative impact on anelasticity of spring elements.When heated with sub-temperature,retained ferrite presents along pre-austenite grain boundaries,retained ferrite impairs anelasticity.Substructure coarsens by over-temperature quenching,which leads to phase-interfaces reduction,and slip resistance along grains boundaries decrease. So,anelasticity is pronounced.Lower bainite is composed of lath-like ferrites and even rod-like carbides.This special substructure as well residual compressive stress induced by isothermal heat treatment for lower bainite’s the best anelasticity.It shows that 2Cr13 steel has stress corrosion-cracking（SCC） susceptibility.The initial 420℃tempering temperature of spring elements is just in the temperature range of sensitization and tempering embrittlement.From 270℃to 420℃,SCC resistance of 2Cr13 steel substantially decreases with escalating tempering temperature,and corresponding sensitive index of SCC from 9.2%up to 71.8%.During above tempering range,mechanical strength of 2Cr13 steel slightly grows with escalating temperature, meanwhile,impact property evidently goes down.Hysteresis error slightly goes down with increasing tempering temperature.About 150MPa residual tensile stress was detected in Ni-P coating layer on 2Cr13 steel spring element surface.Ni-P plating obviously degrades SCC resistance of 2Cr13 steel.About 650MPa residual compressive stress imposed by shot-peening is introduced on spring element surface,which dramatically enhances SCC resistance of 2Cr13 steel spring element.