Theoretic And Experimental Study Of High Temperature Diffusion In Large Forgings

The high temperature diffusion, originating in the 1970s, is a significant process in the production of large forgings. It aims at mitigating or eliminating the dendritic segregation in steel ingots to improve the capability of large forging products by the way of heating up steel ingots in high temperature. Although the use of high temperature diffusion has a history of dozens of years in China, we are short of theoretical analysis and experimental research foundation in confirming the processing parameters because of the sparsity of reference materials. With the rapid development of electric power, metallurgy, petrochemical engineering, shipbuilding and other industries, more large forgings are highly demanded. The increasing demand causes the severe disproportion between the output and productivity of large forgings. Together with the pressure from the cost, the energy-saving policy of reducing discharge and environmental legislation, the technology of high temperature diffusion is facing a series of problems. With 2.25Cr-1Mo-0.25V Steel used in hydrogenation forgings as a research object, this paper explores the law of impact of the high temperature diffusion on its ingredient distribution, organization, and mechanical property under different deformation conditions, and puts forward some new views and conclusions through theoretical analysis and all kinds of experiments.The paper suggests the dendritic segregation homogenization mathematic model based segregation ratio and shaped Guass Solution.The least square experimental fitting method is used to determine the diffusion coefficient of Cr and Mo in 2.25Cr-1Mo-0.25V Steel under 1100oC~1200oC, and further determine their diffusion constant and diffusion activation energy. Finally, a mathematic model is created to forecast the homogenization degree of Cr and Mo in 2.25Cr-1Mo-0.25V Steel Ingot.Relying on experiments, the paper gives a comprehensive research about the impact of heating and holding time on dendrite, dendritic structure, retained austenite and microhardness. The decision rule for processing parameter of the high temperature diffusion is found on the basis of segregation ratio and experiment results.With mechanical property experiments, the paper painstakingly explores the impact of holding time on as-forged force and the impact of high temperature diffusion on as-forged impact-toughness under different forging ratios. The result of mechanical property shows that the pre-forging high temperature diffusion is not necessary for hydrogenation reactor tube forging with 2.25Cr-1Mo-0.25V Steel Ingot as stock.The paper advances the idea of predeformation high temperature diffusion for the sake of shortening the holding time and optimizing the craft. The homogenization mathematic model of predeforming alloying agents is created on the basis of the dendritic segregation homogenization mathematic model. The data from experiments tally with the model curve. With the method of experiments, the paper also researches the impact of the predeformation high temperature diffusion on dendrite and dendritic structure. By theoretical analysis and results from experiments, the paper presents the predeformation high temperature diffusion processing parameter of 2.25Cr-1Mo-0.25V Steel.