Cracking and fracture of valve shafts in the secondary steam circuit of a nuclear power plant. A case investigation.

The failure of a secondary steam circuit valve was investigated. Several fatigue cracks were initiated on valve shafts in the angle of the groove supporting the puppet. Cracks propagated obliquely toward the shaft end. The final shaft fracture occurred by shearing in valve position closed, when the shaft bearing section was diminished sufficiently. The surface of the split ring transmitting the puppet weight to the shaft was worn irregularly. Fatigue cracks initiated and propagated in the split ring groove angle due to resonance stresses at the start of the operation, when the temperature of the valve was low and the shaft steel notch toughness was also low.     

An Evaluation of Steel Embrittlement Caused by Welding

The fracture toughness of the HAZ in welds on a HSLA structural steel was assessed by using two types of precracked small-scale fracture-mechanics specimens. The microstructure of the single- and double-pass HAZs was simulated on a Gleeble machine at two different cooling rates. A direct measurement of K_Ic was not possible due to the limited size of both kinds of specimens. A suitable adaptation of the loading diagrams was performed in order to produce relevant data about the fracture properties of the investigated sub-zones of the HAZ.     

The effect of ageing temperature and time on the mechanical properties of Fe-NiCrMo alloys with different contents of δ ferrite

Laboratory cast alloys with 2–27% of δ ferrite were aged for up to 17,520 h in the temperature range 290–350 °C. Tensile and Charpy tests were performed at 22 and 290 °C on specimens aged for different times, and the microhardnesses of both constituents of the microstructure were determined for the alloy with 27% of δ ferrite. The effects of the content of δ ferrite, the ageing and testing temperature, and the ageing time on mechanical properties and notch toughness are presented and discussed.