Study On Grain Evolution Mechanism And Thermostability Of M50NiL Steel

Aeroengine bearings need to be operated at high speeds,heavy loads,and high temperatures for a long period of time,which places high demands on the mechanical properties and organizational stability of bearing steels.As a new type of aeroengine bearing steel,M50NiL steel has a long hot working and heat treatment process,high temperature,and unpredictable grain size,which is detrimental to the consistency and stability of product quality.Therefore,the research of M50NiL steel grain refinement mechanism and thermal stability is important for improving the comprehensive mechanical properties of the steel and increasing its useful life.Firstly,in this paper,the variation of austenite grain size and properties under different heating regimes of M50NiL steel was analyzed by using OM,SEM,MIAPS image processing software and Vickers hardness tester The results show that the austenite grain size of M50NiL steel grows faster with the increase of heating temperature,and it has an exponential relationship with the grain size.With the increase of holding time,the austenite grain size of M50NiL steel increases slowly.Approximate parabolic relationship with grain size.When the holding time is10⁵0min,the content of martensite and solid solution alloys plays a leading role in the change of hardness.When the heating temperature temperature is 1050?for50min,The hardness of the sample reaches 452.9 HV.Secondly,starting from the thermodynamic principle of grain growth,taking into account the solute drag effect and pinning effect,a prediction model of austenite grain growth of M50NiL steel was established in this paper.the effect of solid-solution Mo,Cr and V atoms and M₆C particles on the austenite grain growth behavior of the steel was quantitatively analyzed with this model.The results show that the solute atoms mainly affect the grain boundary mobility,while the M₆C particles mainly affect the driving force of grain boundary movement.Compared with the pinning effect of M₆C particles,the drag effect of Mo,Cr,V solid solution atoms is more significant for the austenite grain boundary migration,and the drag effect in the solid solution Mo atom is the maximum.The results of model prediction are in good agreement with the austenite grain size measured in the experiment.Finally,in this paper,the effects of holding time and quenching times on the austenite grain size stability of M50NiL steel were studied.The results show that when the heating temperature is 1050?,holding time is less than 70min,the grain size of M50NiL steel is relatively stable.When the holding time is higher than 70min,the grain grows faster.The cycle quenching can refine the grain size of M50NiL steel effectively.When repeated four times,the grain refinement effect is better and the hardness reaches 451.6 HV.