Study On Microstructure And Mechanical Properties Of12%Cr Heat-resistant Steel

At present, our country is not only the world’s largest coal producer and consumer,but also the world’s largest carbon dioxide emission country. To accelerate thedevelopment of ultra-supercritical (USC) thermal power generation technology andimprove thermal efficiency of coal-fired plants, is considered to be the most realistic andeffective approach of reducing the CO2emission. The technical viability of USC is provedto mainly depend on high temperature materials, however, higher steam parameters ofthermal power units are always limited by the performance of high temperature materials.As the key material of USC units,12%Cr heat-resistant steel is widely applied to steamturbine high pressure/intermediate pressure rotors and blades. Now our country has a largedemand for this material but it has been mainly imported from foreign countries, whichcauses a significant increase in manufacturing cost of thermal power units. Therefore itcan be expected that, making related experiments and technology research on12%Cr steeland realizing its localization manufacture as soon as possible, has become the urgent taskat the moment.In this paper, the effects of long-term aging, high temperature overaging and hotcompressive plastic deformation on microstructure and mechanical properties have beeninvestigated. By advanced analysis methods of SEM-BSE, EBSD, TEM and XRD, theintrinsic relationships of microstructure including precipitates, dislocation, subgrains andgrain boundary was discussed. Moreover, the correlation between microstructure andmechanical properties was established.The influence of long-term aging at650°C on microstructure and mechanicalproperties was determined. Most of M23C6carbides precipitate at grain boundaries with amisorientation angle of40°-60°, which obeys the Kurdjumov-Sachs orientationrelationship (KS-OR) with α-Fe. Most of Laves phases appear on grain boundaries with40°-60°, which obeys the Burgers orientation relationship (Burgers-OR) with α-Fe. Withincreasing aging time, the Laves phase formed in the regions adjacent to M23C6particleswill gradually swallow the M23C6carbides in close vicinity. Meanwhile, carbon atoms will segregate on the vicinity of phase boundary between Laves phase and α-Fe in process ofdecomposition of M23C6carbides and formation of Laves phases. During short-term aging,the reduction of room temperature hardness and yield strength are mainly caused by thedecline of dislocation density. However, under long-term aging, it is directly related to thecoarsening of subgrains.The effect of high temperature overaging on microstructure and mechanicalproperties was studied. Comparing with long-term aging at650°C, changes of precipitates,subgrain, fraction of low angle boundaries and hardness are more remarkable at700°C,however, the KS-OR between M23C6and α-Fe is very stable even under conditions of700°C.The effect of different hot compressive deformation at650°C on microstructure wasinvestigated. With the increase of hot compressive deformation, the number of subgrainsincreases significantly and the size of them gradually decreases; after50%compressivedeformation, dynamic recrystallization (DRX) takes place. When the deformation reaches8%,<111> and <001> fiber textures occur in microstructure and the intensity at <111> isobviously higher than that at <001>, which all increase with increasing the compressivedeformation. With the increase of the compressive deformation, the deviation angle fromthe initial KS-OR gradually increases; after6%compressive deformation, the KS-ORbetween M23C6and α-Fe begins to be disrupted; after20%compressive deformation,M23C6completely loses their orientation relationship with α-Fe.