④ Surface A is matched with other parts to improve the smoothness and ensure the sealing performance. 17-4PH steel has a high hardness after aging treatment, so it can be reduced when used as friction parts such as valve seat and disc.; Locked death; Occurrence of phenomenon. After heat treatment, 17-4PH steel has achieved martensite+copper rich phase strengthening, reaching the mechanical properties of the valve seat. Its hardness reaches 329HB or below, meeting the requirement of friction pair hardness difference of more than 50HB.

Air close type: the action direction is exactly opposite to that of air open type. When the air pressure increases, the valve acts in the closing direction. When the air pressure decreases or does not exist, the valve acts in the opening direction until it is fully opened, so it is sometimes called fail to open FO. Air opening or air closing of pneumatic control valve is usually realized by different assembly methods of valve body internal structure and actuator.