Causal Analysis on Fracture of High-pressure Bypass Desuperheating Water Control Valve Stem of Steam Turbines
By means of visual test,chemical composition analysis,metallographic inspection,hardness test,SEM fracture analysis and comprehensive analysis,the fracture cause of high-pressure bypass desuperheating water control valve stem made from the 17-4PH in a steam turbine of a power plant was analyzed.The results show that the fracture mode is fatigue fracture.The internal cause of the fracture is the existence of"band-like structure"along the axial direc-tion in the valve stem structure,and the banded segregation structure reduces the mechanical properties of the valve rod a-long the transverse direction.The external cause is that the valve rod not only bears normal axial load,but also bears addi-tional bidirectional bending load.Because there are many diameter changes in the valve stem structure,under the com-bined action of internal and external factors,the stress concentrates at the smallest part of the cross section,where the in-itial fatigue crack source is initiated on the outer surface.The crack spreads from the outer surface on both sides to the in-side of the valve stem,and finally leads to fracture at the shaft center.
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