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I nternal crack of an ingot is frequently encountered in casting practice, which directly leads to scrapping the whole product. From many studies [1][2][3][4][5][6][7] , it appears that hot crack initiates in the brittle temperature range and propagates in the interdendritic liquid film. An extension of the two-phase model, which includes plasticity of the porous network, is reported, considering the void as the crack nucleus, although the pores should not necessarily develop into a crack [8] . The crack may nucleate or develop from other defects and then propagate through a chain of pores. The excessive stress and strain developing during the brittle temperature range are the main reasons for hot crack formation. A coherent dendritic network can sustain and transmit stress. Above the coherency temperature, continuous liquid film still exists as the solid dendrite arms have not yet coalesced. Deformation induced by thermal stress can pull these Abstract: 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 five main portions of defects larger than 200 mm 3 , four of which are interconnected. These initiated from continuous liquid film, 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.

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Abstract: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.