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Phase Transformations And Strengthening Processes Of T91 Ferritic Heat-Resistant Steel

Posted on:2008-09-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:B Q NingFull Text:PDF
GTID:1101360272985440Subject:Materials science
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T91(9Cr-1Mo-V-Nb-N)is the representative of high Cr ferritic heat-resistant steels, which has been widely used in high temperature structural components such as main steam pipe, superheater tube and resuperheater tube in advanced power plants in view of good mechanical properties, excellent oxidation-resistant properties, outstanding thermal properties relative to other elevated temperature alloys. So it is regarded as research benchmark for the development of new ferritic heat-resistant steels with higher application temperature. Furthermore,under the pressure of energy shortage and environment pollution,the study on the thermal efficiency of generating station and heat-resistant temperature of the boiler tube is also imperative.There are lots of papers which deal with alloying, processing property, chemistry property and potential engineering applications, etc, while few investigations on phase transformation and structural evolution of T91 steel are developing. In order to clarify the phase transformation process and mechanism, structural evolution and explore new forming process of T91 ferritic heat-resistant steel, the possible influence on the properties of T91 steel, such as austenization, continuous cooling transformation, isothermal holding, and applied stress were systematically characterized by means of high-resolution linear differential dilatometry and modern materials analysis methods. Controlled Rolling and Cooling process of T91 products was simulated in labs to demonstrate the feasibility of improving the mechanical properties through structural refinement and induced precipitation of secondary phase, results as following:(1) Austenization is not only the first stage for the heat treatment of iron and steels,but also the governing factor the state of undercooled austenite, which determines the final structure of steels. The curves of continuous heating transformation state the effect of heating rating on the austenization of T91 steel as following:Heating rate has a significant effect on the Ac1 temperature for the onset of austenization and the Ac3 temperature for the end of austenization. The thus determined temperatures of Ac1 and Ac3 increase with heating rate increasing. In case austenization rate evidently increase, the periods of austenization also shorten remarkably. The relation between the peak of transformation rate curve and the heating rate accords with the formulation:(With transformation rate and V heating rate). It is also found that the dissolution of carbonization is correlative to the heating rate. When heating rate is low, substantive M23C6 carbide dissolves. As a result, the formation of MX carbide profits from C concentration altering. Owing to the separation of MX carbide,the grains of austenite keep fine even at low heating rate. When the heating rate increase to some extent, most M23C6 carbide don't dissolve until isothermal holding segment is adopted. The fast heating rate keeps initial grain size fine,but does harm to uniformity of austenite grains. df /dt(2) The rule of phase transformation of T91 steel during continuous cooling is clarified in this part, the martensite critical cooling speed is determined, and the continuous cooling transformation (CCT) curve is obtained. It follows:There reserve only ferritic and martensite during the process of continuous cooling of the explored T91 steel,10℃/min and 2℃/min are the upper and nether critical cooling rates for the phase transformation from austenite to martensite. The quenching rate has a significant effect on the onset temperature of Martensite transformation Ms. The result is different from the classical theory that claimed the transformation point falls when cooling rate reduces. The quenching rate determines the transform point by affecting the carbon atomic group, quenching vacancy and internal stress. With the quenching rate increasing, the structure has a tendency of fining, whereas the cooling rate beyond 1000℃/min, it has nothing on the morphology of the structure.(3) Study on the isothermal stabilization behaviors of austenite localizing near to Ms temperature during the cooling process of undercooled Austenite of T91 steel indicates:When isothermal holding temperature beyond upper Ms, T91 steel possess obvious anti-stabilization characterization. The tendency of anti-stabilization is strong with holding time prolonging. On the other hand, when isothermal holding temperature just beyond Ms, the martensite start temperature of T91 steel is less than Ms, and it decrease continuously with holding time prolonging, but ultimate structure keeps martensite without any residual austenite remaining,what is called false stabilization. While rest on 400℃under Ms, owing to the high transformation temperature,room temperature microstructure of T91 samples does not contain the residual austenite on action of mechanical stabilization, it can still be attributed to false stabilization. When isothermal transformation takes place under 380℃, austenite stabilization of the samples boosts up, the continued transformation temperature of austenization demand reduces, partial transformation of austenite occurs. There exists residual austenite in the final microstructure. At the moment, stabilization of austenite be enslaved to mechanical and heat stabilization treatments.(4) Study on the undercooled austenite applied external stress during cooling, the results demonstrate:In case of the temperature applied external stress over 850℃, the evidence of martensite formation is not observed in T91 steel, While it under 850℃, the result indicates the applied external stress not only facilitates the formation of martensite, but also enhances the onset temperature of the martensite transformation. It is summarized that there are two transformations occurring at this situation: when the temperature applied external compressive stress is high, the mechanism of strain-induced Martensite transformation acts on,as a result, the microstructure inclines to be fine, the grain boundary tends to be irregular. On the other hand, when the temperature applied external compressive stress is low, stress-induced Martensite transformation takes place, the morphology is similar to that of heat-induced martensite. The experiment data reveal that 200MPa is the critical stress for the explored T91 steel and 440℃is critical temperature of the start of the stress-induced martensite transformation.(5) The experiment of Thermal-Mechanical Control Process simulation have been preformed in the wide austenite non-recrystallization region of the explored T91 steel. The principle are verified the induced carbonitride precipitates by thermomechanical treatment can improve the properties of ferritic heat-resistant steel due to abundance Nb, V elements in T91 steel, the results show:The microstructure of T91 steel appears to be refined after thermomechanical treatment, furthermore, the process offers more nucleation positions that facilitate the formation of more nano-sized dispersing MX carbonitride precipitates.In addition, the tension experiment indicated thermomechanical treatment has a significant influence in improving the strength of T91 steel, all of these prove the tentative using thermomechanical treatment to enhance the working temperature of T91 steel is feasible.
Keywords/Search Tags:Ferritic heat-resistant steels, Phase transformation, External compressive stress, thermomechanical treatment, Martensite, Strain-induced separation
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