Comprehensive analysis by scanning electron microscopy, electron probe inspection and metallographic analysis showed that the broken turbine bolt material contained a large amount of non-metallic inclusions. Its composition is oxides and silicates. These brittle inclusions have low deformation rate. During the process of steel deformation, the base metal which is relatively easy to deform plastically flows around the hard-to-deformed inclusions, causing a large tension. The connection between the metal and the inclusion interface breaks to form a void; or the inclusions break up due to the stress around the inclusion to form a void, which becomes a weak link in the material.
Generally, the inclusions have little effect on the longitudinal ductility of the steel, but the effect on the lateral ductility is significant. As the inclusions in the steel increase, the transverse section shrinkage of the steel decreases. At the same time, brittle inclusions with low deformation rates such as oxides, nitrides and silicates tend to act as a source of stress concentration, which lowers the impact value and increases the brittle transition temperature. Therefore, the elongation of the five samples measured in the tensile test meets the standard requirements, while the reduction of the area is far below the standard requirements; the impact toughness values ​​of the five impact samples are also far below the standard requirements.
The non-metallic inclusions present in the steam turbine bolt material also cause stress concentration when the bolt is stressed, that is, the stress rises sharply at the metal matrix adjacent to the end of the inclusion. Precautions Because the bolts are broken because the steel used to make the bolts contains a large amount of non-metallic inclusions, the following precautions are taken: the newly purchased bolts of the power plant should have a certificate of low-inspection inspection provided by the manufacturer. Forged bolts should be used when purchasing new bolts.
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