| X12CrMoWVNbN10-1-1 steel is widely used as feedstock of rotor shafts because of its high temperature strength, high temperature corrosion resistance and low thermal expansion coefficient. In recent years, the rotor shaft size increases due to the size increase of generating units in order to satisfy human’s dramatic need in energy. Although, the larger rotor shaft enhances the safety and reliability of the power stations, it also brings great challenge to the manufacture which is a complicated process merging with casting, machining, heat treatment and nondestructive testing. Therefore, to produce sound larger rotor shaft, an ultra-large ingot with homogeneous and fine grain is preferred.In this dissertation, the X12CrMoWVNbN10-1-1 ingots were successfully grain refined by thermal control in two different directions. The main results are the follows:By varying the casting parameters, i.e. mold temperature, melt pouring temperature and melt superheating temperature, the structure of the X12 steel ingot was successfully refined in sand mold. The experiment results indicated that the optimized parameters for the sound ingot were 1650°C for the melt superheating temperature, 800°C for the mold temperature and 1600°C for the melt pouring temperature. Under this condition, the ingot shows fine equiaxed grain with a radius of 1.1mm and the fraction of the delta ferrite in the microstructure was less than 2%. The melt superheating temperature and the pouring temperature are the main factors that affected the grain size of the ingot. While the mold temperature is the main factor that affected the proportion of the equiaxed grain zone. Apart from that, the melt superheating temperature is also the main factor that affected the fraction of the delta ferrite in the microstructure.The effect of the melt superheating temperature on the structure of the X12 steel was investigated systematically and the related mechanism was discussed. It is found that the macrostructure of X12 steel was composed of fine equiaxed grain after superheated at 1650°C for 3 minutes and the cooling rate has limited effect on that(In the experiment, the samples were solidified in steel-mold and silica-mold and the cooling rate of them are 10°C/s and 100°C/s respectively). The analysis of microstructure and XRD date revealed that there is no high melting particle and carbide in the sample. Macrostructure comparison between the samples with different cooling rates revealed that the undercooling was not responsible for the grain refinement. Considering of the solidification path of X12 steel, which is peritectic reaction(L+δ(ferrite)â†'γ(austenite)), the grain refinement mechanism of the X12 steel caused by melt superheating treatment was proposed. It is considered that the liquid-liquid phase transition plays an important role during this process. When the liquid was superheated to 1650°C, the melt was located in two phase region and the liquid structure of the melt contains two types of liquid clusters, i.e., δ-like cluster and γ-like cluster. The δ-like clusters which could be the nucleation sites for δ ferrite reduce the nucleation barrier, the γ-like clusters reduce the peritectic reaction barrier. Therefore, the peritectic reaction speed was improved greatly and the grain was dramatically refined.A new solidification named non-interfacial permanent casting was invented, which could improve the cooling rate during solidification by eliminating the air gap between casting and mold. The key point of this method is the introduction of a melt-mold, which is made of low-melting-point materials, next to the permanent mold. The X12 steel was cast at the pouring temperature of 1600°C by conventional permanent casting(CPMC) and non-interfacial-gap permanent casting(NIGPMC). Thanks to the improvement of cooling rate, the columnar-to-equiaxed(CET) did not occur during NIGPMC and the average primary dendrite was greatly refined. Based on the temperature curve of the melt-mold and the numerical modeling result, the mechanism for the improvement of cooling rate of NIGPMC was explained. Once the melt was poured into mold, a thin solidification shell was formed adjacent to the mold and the heat transferred through this shell to melt-mold. The melt-mold was then melted into liquid restricted between the casting and the permanent mold. During the latter solidification process, the liquid metal still existed until the solidification of the entire ingot was finished and which subsequently eliminated the interfacial gap of casting and mold. Therefore, the cooling rate of the casting was greatly enhanced and the primary dendrite was refined.Based on the numerical simulation results of the temperature field and the solid fraction of X12 steel ingot, the main factor affected the cooling rate during non-interfacial-gap permanent casting was investigated and the optimization casting parameter was obtained. The simulation results indicated that the physical properties of the melt-mold materials had an important effect on the cooling rate. The melt-mold material with lower melt point, higher thermal conductivity and lower latent heat of fusion is preferred which is capable of increasing the melting of the melt-mold and subsequently inducing a higher cooling rate of the casting. Although a higher melt pouring temperature could lead to a higher cooling rate for casting, the solidification of the X12 ingot was delayed because excess heat was introduced to the system. The simulation results also showed that the optimized casting parameter for the X12 steel is 1560°C for the pouring temperature with Aluminum alloys AA6061 as melt-mold material. The ingot cast under optimized conditions shows that the NIGPMC could lead to a homogeneous solidification structure with finer grain size. The final macrostructure of the ingot is composed of about 100 μm fine equiaxed grain with martensite only. |
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