The Effect Of Rare Earth Ce On Thermal Processing Performance And Corrosion Performance Of AF-3 Steel

With the development of society and increasing of electricity demands,the capacity of steam turbine units become larger and larger.The grommet is the core component of the steam turbine unit.It withstands superimposed load from the rotor rotation and is used in damp and corrosive environments.Therefore the grommet should have sufficient strength,good toughness,small residual stress and austenitic nonmagnetic.Its performance directly impacts safe and stable operation of steam turbine.However,retaining ring steel in production and using process is facing two major problems.For one hand,forging billet cracks seriously in hot forging process,which results in low yield.On the other hand,it has poor corrosion resistance.It leads to incalculable consequences due to poor corrosion resistance in the service process.In this paper,the research object is AF-3 steel which added rare earth Ce.We used hot simulation compression test,metallographic observation,scanning electron microscopy,electrochemical experments and other methods to analyze microstructre,pitting corrosion and intergranular corrosion behavior.The effect mechanism of rare earth Ce on the hot workability and corrosion performance of Af-3 is emphatically discussed.Researching indicates:(1)Ce has the ability of deep deoxygenation and desulfurization to achieve purpose of purification of molten steel.Ce can change the morphology of inclusions,which transitions irregular angular inclusions to matrix spherical or nearly spherical rare earth inclusions.It can reduce the probability of corner cracking during the hot forging process effectively.Rare earth plays the role of dispersion strengthening,making inclusions small and diffuse,which reduces the anisotropy of the material.(2)The true stress-strain curves were obtained by single-pass thermal simulation compression experiments.The results show that the flow stress increases repidly during initial stage of hot working,and then the flow stress into a stable stage due to combined effect of dymamic recrystallization and working hardening.When the deformation rate is same,the flow stress decreases with temperature increasing.When the deformatin temperature is same,the flow stress increases with the strain rate increasing.(3)The addition of Ce at low deformatin temperature improves the flow stress in the process of thermal deformation,which has the effect of strengthening grain boundaries and refining the grains.The thermal deformation activation energy decreases with the increasing of true strain.When true strain is less than 0.35,Ce increases thermal deformation activation energy of the material.Whe true strain exceeds 0.35,Ce decreases the thermal deformation activation energy and improves the thermal processing of material performace.the thermal deformation constitutive equation for AF-3+Ce steel is:?=1.805 ×1018[sinh(0.00993?P)]4.984 exp(-528523/ET)Corresponding rheoogical stress equation:(?)(4)The predicting model of AF-3+Ce steel was established,and the flow stress equation was used to predict the flow stress.The mean relative error ? was 6.742%,which indicates that the prediction model established in this paper can effectively predict the rheological behavior of AF-3+Ce steel under different deformation conditions.The deformation temperature is higher,the strain rate is smaller,the degree of dynamic recrystallization is higher,and the hot workability is better.(5)The region with higher power dissipation factor corresponds to the region with high deformation temperature and low strain rate,and higher degree of dynamic recrystallization is happened in this region.Ce can increase the power dissipation factor,enlarge the area of machinable area and complete the dynamic recrystallization process,which improves the hot workability of AF-3 steel.The optimum hot working window of AF-3 steel is deformation temperature 1050?1200?,strain rate 0.01?0.1 s-1.(6)Ce can reduce the corrosion current density,improve the self-corrosion potential and pitting potential,increase the passivation film resistance of AF-3 steel because rare earth has the effect of purifying steel and reducing the inclusion-based pitting Sources.In addition rare earth oxides in the sample surface forms dense passivation film impede chloride ion erosion.The reactivation rates of AF-3 steel measured by EPR method showed the trend of first increasing and then decreasing with the aging time.It reached the maximum resistance in the aging time of 30 min which indicates intergranular corrosion is the worst.The reactivation rate increased with the addition of Ce,indicating that Ce reduced the intergranular corrosion resistance of AF-3 steel,which was mainly due to the strengthening of the matrix by Ce,leading to the speed decreasing of Cr moving to grain boundary.