Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the int...Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.展开更多
Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice.Water cooling is a usual way to enhance production efficiency.However,the combination of the two fact...Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice.Water cooling is a usual way to enhance production efficiency.However,the combination of the two factors will generate internal defects,such as shrinkage porosity and hot crack.The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test.A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted.The billets were examined by X-ray high energy industrial CT,and the compactness was reconstructed in three dimensions.Microstructure near the crack was observed using scanning electron microscopy,and the solidification process and grain size were studied by high temperature confocal microscopy.Moreover,thermo-mechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack.A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot.The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio,and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities.Then,hot cracks begin to propagate along grain boundaries.The grain size in the upper and center of the ingot is large,which leads to an inverted cone shape defects zone in the ingot center.展开更多
P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack...P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are f ive main portions of defects larger than 200 mm^3, four of which are interconnected. These initiated from continuous liquid f ilm, and then were torn apart by excessive tensile stress within the brittle temperature range(BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies' criterion and Katgerman's criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.展开更多
基金supported by the NSFC-Liaoning Joint Fund(U1508215)the project to strengthen industrial development at the grass-roots level of MIIT China(TC160A310/21)
文摘Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.
基金financially supported by the NSFC-Liaoning Joint Fund(Grant No.U1508215)the project"To Strengthen Industrial Development at the Grass-Roots Level of MIIT China"(Grant No.TC160A310/21)assistance from the Special Material Institute of Inner Mongolia North Heavy Industries Group Co.,Ltd.is acknowledged。
文摘Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice.Water cooling is a usual way to enhance production efficiency.However,the combination of the two factors will generate internal defects,such as shrinkage porosity and hot crack.The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test.A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted.The billets were examined by X-ray high energy industrial CT,and the compactness was reconstructed in three dimensions.Microstructure near the crack was observed using scanning electron microscopy,and the solidification process and grain size were studied by high temperature confocal microscopy.Moreover,thermo-mechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack.A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot.The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio,and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities.Then,hot cracks begin to propagate along grain boundaries.The grain size in the upper and center of the ingot is large,which leads to an inverted cone shape defects zone in the ingot center.
基金financially supported by the National Basic Research Program of China(No.2011CB012900)the National Science NSFC-Liaoning Joint Fund(U1508215)
文摘P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are f ive main portions of defects larger than 200 mm^3, four of which are interconnected. These initiated from continuous liquid f ilm, and then were torn apart by excessive tensile stress within the brittle temperature range(BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies' criterion and Katgerman's criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.
