| P92 heat-resistant steel is a new type of ferritic heat-resistant steel developed by Japan in the late 1980s on the basis of P91 heat-resistant steel by adding W and Mo.Because of solution strengthening,dislocation strengthening and precipitation strengthening.P92 heat-resistant steel has excellent heat resistance,oxidation resistance and long-term durability in high temperature.In addition,it also has excellent weldability,thermal conductivity and processing performance.It has more excellent creep resistance and thermal stability than P92 heat-resistant steel above 600?.In the increasing steam parameters of power units,its wide use is imperative.The properties of domestic P92 heat-resistant steel are far inferior to that of the import steel.In order to accelerate the pace of P92 steel applied to industrial production in large quantities,this paper carried out the following studies:(1)The composition of P92 heat-resistant steel was designed based on the function of its element and in order to obtain martensite.A series of researches on the test steel has been carried out.A suitable rolling and heat treatment scheme was carried out according to the continuous heating and cooling process which shows that with the increase of heat rate,the transformation points Acl,AC3 and the autensite transformation rate are continuously increased.With the increase of cooling rate,the transformation points Ms and Mf decrease continuously.Martensite microstructure can be obtained in a large cooling rate range and the CCT curve was drawn then.After a series of experiments the test steel which was normalizing at 1060? for 130min and tempering at 760? for 120min has the best mechanical properties.(2)The analysis of scanning electron microscopy,electron probe,transmission electron microscopy and so on show that the microstructure at room temperature of test steel has a structure of martensite lath.In the boundary of these martensite lath there are distributed rod-like precipitates M23C6 which is Cr-containing carbide.Within the martensite lath there are distributed small granular precipitates MX which is carbonitride of Nb and V.(3)After of the tensile experiments of high temperature on the Thermal Simulator,the result shows that the recrystallization behavior do not occur at 600?and 700 ?,but occurs at 900?-1100?.The high-temperature tensile test at 200?-600? was conducted in the tensile machine.The obtained mechanical properties are better than those of GB5310-2008 and ASME CODE CASE 2179.The research of the oxide film of the test steel at 600? and 650? showed that the surface of the test steel would produce a dense oxide film at the initial stage of oxidation.The oxide film is composed of the granular oxide which is composed of Fe,Cr,and Mn.(4)The test steel which is normalizing at 1060? for 130min and tempering at 760? for 120min was aged at 650? for 25h,50h,100h,150h and 240h respectively.The results show that during the ageing process,the mechanical strength of the test steel fluctuate within a small range,which is the result of the solution strengthening,dislocation strengthening and precipitation strengthening together.After the ageing process,the martensite lath of the test steel is widened and the dislocation density is also reduced to a certain extent.The precipitated phases in the aged test steel are similar to those in the original condition.There are mainly two kinds of precipitated phases.One is the rod-like precipitated phase M23C6 which is Cr-containing carbide.It is formed by heterogeneous nucleation and distributed along the martensite boundaries.Compared with the original state,after 240h ageing,the precipitation grow up obviously,but its number decreases.Therefore,with the increase of ageing time,the strengthening effect of the precipitation on the test steel will decrease gradually.The other is the granular precipitation MX which is carbonitride of Nb and V.They are dispersed in the martensite lath.After 240h ageing,the composition,quantity and size of the precipitation did not change compared to the original state,indication that the precipitation has excellent long-term stability. |
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