Analysis of internal crack in a six-ton P91 ingot

Abstract

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.

Simulation of As‐Cast Steel Ingots

“…By analyzing the local extremes of the tensile or compressive stress in some critical regions and at some critical moments, the knowledge about the formation of the mid‐face and off‐corner panel cracks was obtained, and on this base some counter measures were suggested to reduce this kind of cracks in as‐cast ingots. Cracks as formed in/near the ingot center are mostly regarded as examples of hot tearing, which arises from a complex combination of thermal mechanical and solidification phenomena . This kind of defect is basically associated with the incomplete melt feeding and tensile deformation as generated in a coherent region of the mushy zone (solid fraction f s between 0.9 and 0.99), which is also evaluated by a so‐called brittle temperature range (BTR), i.e., the temperature range between the coherent temperature and the zero ductility temperature.…”

“…a) Schematic of the ingot geometry; b) the as‐cast ingot was sliced longitudinally near the casting center; c) the sliced samples were examined by a high energy 3D X‐ray computer tomography for the internal crack and pores; d) numerical calculation of the equivalent stress, as built during cooling through the BTR; e) CD (Clyne‐Davies) criterion function, as susceptibility indicator for the hot tearing (internal crack). The simulations were performed with a commercial software ProCAST, figures were reproduced from refs …”

“…The major challenges for calculating thermal stress and strain during solidification are the broad temperature range, which spans different material laws (pure fluid mechanics, visco‐plastic, elasto‐viscoplastic), and the shortage of material data at an elevated temperature. What makes the problem more complex is solid state phase transformation, nucleation of cracks or tears from other defects like pores, and the feeding flow through the coherent dendrite network. A thermal mechanical model considering different material laws and incorporating above solidification related phenomena does not exist and further research is needed.…”

Structural and electronic properties of the Al4Sr(001) surface: A first‐principles study

“…At present, the idea that the addition of Sr to Mg alloys can effectively modify and refine the Mg 2 Si phase is widely accepted. [9][10][11] According to the lattice mismatch theory proposed by Bramfitt,12 the mismatch of the Al 4 Sr(001)// Mg 2 Si(100) interface is less than 6%, indicating that Al 4 Sr particles can be used as heterogeneous nuclei of Mg 2 Si phases. However, the deeper identification of interfacial structures at the atomic scale and theoretical calculations of the interfacial energy are necessary to demonstrate the nucleation ability of Al 4 Sr particles for the Mg 2 Si phase.…”

Volume-Averaged Modeling of Multiphase Flow Phenomena during Alloy Solidification

“…This work was recently extended by Ludwig et al to cover the entire range of solidification from pure liquid melt to complete solidification [193,194]. Note that some other thermal mechanical models (mostly finite element based) were used to analyze the surface cracks [195][196][197], hot tearing [198][199][200] and other related phenomena of different castings. In those models, however, the nature of multiphase flow during solidification is ignored or simplified; hence, the formation of structural and compositional heterogeneities (mixed columnar-equiaxed structure, macrosegregation, porosity) cannot be accounted.…”