Experimental Study And Numerical Simulation Of Hot Cracking In Steel Ingots

During the period of the Thirteen-Five strategic planning,a large number of high-end equipments are in demand in China’s energy,petrochemical and other areas.Steel ingot is the raw material for the production of these key components.The hot cracking defect in the steel ingot usually lies in the ingot core,which is not easy to find by appearance and difficult to detect.It is often revealed in the subsequent processing,seriously affecting the production.Studying the hot cracking mechanism and criterion in steel ingot solidification by means of experiment and numerical simulation and using certain methods to restrain the formation and development of hot cracking have great theoretical significance and practical value.In this paper,the viscous-elastic-plastic constitutive relation was constructed by high temperature mechanical properties experiment.The interface heat transfer parameters of the air gap model and the solidification structure characteristics were obtained by the steel ingot continuous temperature measurement and dissection experiment.At the same time,a hot cracking prediction program was developed.And the first principal stress and the mushy zone dynamic yield strength in the brittle temperature range were obtained based on the thermo-mechanical coupling simulation results of ProCAST.Considering the elements segregation and grain size in the steel ingot solidification,a hot cracking potential(HCP)criterion was proposed.This hot cracking prediction method and criterion were applied fairly well in the 6~12 ton P91 heat-resistant steel ingots.A 6 ton conventional P91 steel ingot was cast,dissected and studied by experiment.The hot cracking defect samples were further fetched out and examined by 3D CT.A connected membranous structure was observed.The infrared carbon and sulfur spectrum analysis revealed the carbon and sulfur segregation near the hot cracking defect,and the segregation degree of sulfur was higher than that of carbon.Numeical simulation and the HCP criterion were used to assess the steel ingot hot cracking potential,which was consistent with the actual results.A new riser design with a removable part for the 6 ton conventional P91 steel ingot was proposed,which can reduce the shrinkage porosity and hot cracking defect in the ingot at the same time.A 10 ton slender watercooling P91 steel ingot was cast,ultrasonic examined,dissected and studied by experiment.A long hot cracking defect zone and starlike hot cracking feature were found in the ingot center.Through SEM inspection of the samples,bridge fractured dendrites and bare dendrite arms were observed and intergranular inclusions and microsegregation of sulfur were found.The grain size at different cooling rates in the steel ingot was obtained by high temperature confocal microscopy experiment.Numerical simulation and the HCP criterion were used to assess the steel ingot hot cracking potential.A proper progressive cooling method for the 10 ton watercooling P91 steel ingot was proposed,which can greatly reduce hot cracking potential in the slender ingot center.The HCP criterion was further applied to predict the hot cracking in a 234 ton vacuum 12Cr-2Mo-1V steel ingot.Due to the great difference of the ingot material,tonnage and casting process,the prediction accuracy was not as good as the P91 steel ingot.Therefore,additionally considering the influence of shrinkage porosity,a shrinkage cracking potential criterion(SCP)was proposed and successful in predicting the ingot center hot cracking’s location.Accordingly,the SCP criterion was used to access the casting process of the 234 ton vacuum steel ingot.It was found that higher casting speed,proper mould preheating and good riser insulation can reduce hot cracking potential in the ingot.This research on hot cracking in steel ingots has provided technical support for the actual production.