Research On Heat Transfer And Mass Transportation In Heavy Steel Ingotsand Castings

The equipment manufacturing industry is the key support of the national economy and a symbol of industry capability. Heavy ingots and castings play a very important role in the manufacture of equipment for nuclear power plants, hydro power plants and etc. However, there are still many problems in the production of heavy ingots and castings, which directly hinders the domestic making of key equipment.The heat transfer and mass transportation are two basic physical phenomena in foundry, and are directly related to defects such as macrosegregation in heavy ingots and shrinkage porosity in steel castings. Specifically, this thesis focuses on the mass transportation during the multi-concentration pouring process of heavy ingots and the replacement of the metal pads with insulating layer for large steel castings.Multi-concentration pouring process is applied to reduce the macrosegregation in heavy ingots, in which the concentrations of ladles vary from each other to realize initial reverse macrosegregation after pouring so as to offset the macrosegregationoccurred during solidification. However, it’s still not clear how the alloy element carbon distribute in ingot during pouring process. In this thesis, a water analogy experimental method based on similar principles is adopted. In this method, methylene blue dye is used as solute to simulate the alloy element carbon in the melt steel. And a measurement method was proposed to measure the concentration of this solute, in which the laser reflection intensity detection method is used. The water analogy experiment platform for the carbon transportation during pouring process for heavy steel ingots was constructed. Moreover, a method to predict the concentration of carbon at the tundish outlet was proposed based on the RTD curve of the tundish.Based on the established water analogy experimental platform for the mass transportation, a 5 ladle multi-concentration pouring process for a 585 t steel ingot of the low-pressure rotor of nuclear power steam turbine was investigated. The carbon concentration variation at the outlet of tundish and its distribution in the mold were acquired and analyzed. The results show that mixing of the steel melt from different ladles occurs in both tundish and mold, therefore, the concentration at the outlet of tundish changes continuously and irregular negative concentration gradient from the bottom to top in the mold is achieved at the end of pouring. The concentration at top of ingot is close to the concentration of the last ladle, while the concentration at bottom is much lower than that of the first ladle. These results will be helpful for the further research and the control of macrosegregation occurring in the heavy ingots.The metal pads are widely used to facilitate the feeding of risers in large steel castings by extending feeding distance. However, it leads to material waste and the pads are difficult to dispose. In this thesis the replacement of metal pads with insulation layers was investigated. The mechanical properties and thermal parameters of several insulation materials were measured and then the proper insulation material was chosen. Based on the modulus theory, the insulation pads was designed for a thick plate steel specimen. The solidification process under metal pads and insulation layers were simulated and compared, too. The results show that the insulation layer can successfully replace the metal pads to achieve a sound casting.