| Because of its outstanding mechanical properties and corrosion resistance,super ferritic stainless steel is widely used in power plant condensers,heat exchanges,petrochemical,geothermal and desalination.It has replaced copper,titanium,and super austenite,and been established as the preferred material for sea water cooled heat exchanger tubing in coastal power plants since 1980.Effect of heat treatment on microstructure,inclusion,vickers hardness,tensile strength,corrosion resistance and erosion-corrosion resistance of super ferritic stainless steels bearing different amount of Ti and Nb,have been disscussed.Precipitation in super ferritic stainless steels during heart treatment process has been calculated by Thermo-Calc in order to formulate the best heat treating regime.The results indicated that the mainly precipitated phase are Nb,Ti carbo-nitride,in addition,the ?,?,Laves phases will precipitate at the same time.For instance,the temperatuer of heat treament should exceed 980?.In terms of inclusions,the Cr carbo-nitride precipitate phase and ? and other intermetallic compound were disappeared after heat treatment,as a result,the main inclusion was Nb,Ti carbo-nitride with the size located in micron level.On the other hand,Nb,Ti could refine the grain,and the better effect have been observed with the Nb/Ti increased.This is due to the fact that heat treatment could promote the super ferritic stainless steels recrystallized.In details,recrystallization degree had induced with the temperature and holding time increased,as a result,the refined equiaxed crystal well-distributed after banded tissue disappeared.1#and 2#steel samples will get the best recrystallization degree under 1030? and 1050? with 5 minutes respectively.The holding time should control less than 30 minutes in order to avoid the large size second recrystallized tissue.In addition,heat treatment could improve the mechanical properties by eliminating the harmful precipitated phases,removing leftover internal stress,refining well-distributed crystal.As a result,the hardness,tensile strength and yield strength decreased sharply,contrarily,the ductility increased.The effect of heat treatment on the hardness of steels is mainly embodied in solid solution strengthening and fine grain strengthening,the solid solution strengthening occupies the dominant role when the holding time locate in low level,and the hardness will increase gently with the holding time raising.On the contrst,the hardness will decrease gently with the holding time increase due to the fine grain strengthening occupies the dominant role when the holding time locate in high level.The precipitation of Nb,Ti carbo-nitride could fine grain Nb,Ti solid solution in steel matrix could cause strengthening,and fine grain boundaries hinder dislocation motion of carbonitride niobium,all of these factors could have mostly effect on the mechanical properties of steel samples.Further more,heat treatment could have serious effect on pitting corrosion resistance.After a sharp rise,the pitting potential reaching the peak at 1050 ? and followed by a slightly decrease.The same laws have been observed in passivation film impedance value.The value of Nb/Ti could also have serious effect on pitting corrosion resistance,the large size TiN inclusions will be caused and become the source of pitting corrosion when the Nb/Ti is small.Finally,the heat treatment could have serious effect on erosion-corrosion.With the increase of the angle of attack,the weight loss of the super ferrite stainless steel increases and reaches the maximum at the angle of 90°.With the increase of heat treatment temperature,the loss of weight is first reduced and then increased.The loss of weight is the lowest when the heat treatment temperature is 1050?,and the best corrosion resistance is the best.Erosion accelerated the corrosion rate of super ferrite stainless steel,compared with the static open circuit potential,open circuit potential erosion is more negative,the potentiodynamic polarization curve in passive current density than static high two orders of magnitude,the corrosion potential move to the negative direction. |
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