An automatic loading system for rock core testing with an industrial CT scanner

A new type of a loading and measuring system was developed for testing failure and deformation of rock core samples with an industrial CT （ICT） scanner.The loading and measuring system consisted of a loading system and a computer control system.The maximum servo-controlled force was 2 tonnes.The new system was a high-stiffness system with a small size.During ICT tests,rock core samples could be easily loaded in the axial direction.So the initiation,propagation,and coalescence of cracks in core samples were observed on ICT images.

Analysis of internal crack in a six-ton P91 ingot

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.

Internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio

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.