Power plant main steam gate valve is the main control element used to control the main steam at the outlet of boiler superheater, which plays a very important role in power plant production. The valve casting of a power plant is installed at the outlet of boiler superheater (the rated working pressure of boiler main steam is 9.8 MPa and the rated working temperature is 540 ℃), the model of valve casting is z962y-p54 100V DN225, and the design material of valve body is ZG15Cr1Mo1V. After the valve casting was put into operation for 1a, several cracks appeared in the valve body through magnetic particle testing. Through metallographic examination, the metallographic spheroidization level is level 4, which seriously exceeds the standard. Through hardness test, the hardness value is also in the lower limit of the standard value. Bring hidden dangers to the operation of the equipment.
ZG15Cr1Mo1V alloy heat-resistant steel valve castings should reach grade 1 metallographic spheroidization. First, strictly implement the heat treatment system, especially pay attention to ensure the normalizing cooling rate. If the cooling rate is unreasonable, the required metallographic structure cannot be achieved. At the same time, the mechanical properties cannot be guaranteed. Second, the valve casting method also has a certain impact. For example, the metallurgical structure of electric arc furnace casting is easier to ensure than that of medium frequency furnace casting. Third, during metallographic inspection, if a portable metallographic detector is used for direct inspection on the valve casting body, the inspection shall be less than 2mm from the surface, and the seamless steel pipe shall also be less than 1mm from the surface. There is no restriction on metallographic sampling.