The easy fracture of forged steel gate valves is usually caused by multiple factors, mainly including material problems, process defects, environmental impact, and improper operation.
Firstly, from the perspective of materials, if the quality of the selected steel does not meet the standards, such as metallurgical defects such as inclusions and pores, or uneven chemical composition (such as high sulfur and phosphorus content), it will significantly reduce the toughness and plasticity of the material, making it more prone to cracking during forging or use. In some cases, there may even be the misuse of materials, such as the misuse of 20Cr2MoVCu steel for the high-pressure main steam valve stem in a power plant, replacing the design requirement of 2Cr12NiMoWV steel, resulting in coarse grains and substandard mechanical properties. Long term service in high temperature environments can also deteriorate the material structure, with the precipitation of network carbides near grain boundaries, further weakening the material strength.
Manufacturing process defects are another important reason. Improper control of forging temperature (too high leads to coarse grains, too low leads to decreased plasticity), excessive deformation, or too fast cooling rate can all result in residual stress inside. Poor processing quality of the sealing surface (such as sand holes and scars) or incomplete heat treatment process (such as increased brittleness in the variable diameter area due to nitriding layer) can also pose hidden dangers. The valve of a certain natural gas processing station has broken multiple times during operation due to the substandard stem material and heat treatment of outsourced processing. Microcracks, burrs, and other defects in the casting process can accelerate their propagation in high temperature and high pressure environments, leading to valve body cracking.
The impact of the usage environment cannot be ignored. High temperature and high pressure conditions can cause material creep, corrosive media (such as acidic fluids) can reduce material strength, and thermal shock stress caused by sudden temperature changes is the direct culprit leading to cracks. The main feedwater gate valve of a power plant often experiences abnormal pressure rise in the middle chamber (due to thermal expansion or sealing compression of the fluid), causing the pressure to exceed the design limit and ultimately causing the valve stem to break. In addition, the alternating load generated by frequent opening and closing can cause fatigue fracture, which is particularly common in regulating valves.
Improper operation can also cause fracture. Uneven tightening force of bolts during installation can lead to local stress concentration, excessive force on opening and closing valves can cause mechanical damage, or failure to maintain according to specifications (such as not regularly replacing aging fillers) can accelerate damage. The pressure reducing valve of a certain power plant was repeatedly broken at the root of the thread due to the excessive torque of the newly installed electric actuator and the mismatch between the valve and the valve.
It is worth noting that the structural characteristics of forged steel gate valves, such as the double sealing surface design, also make them more prone to failure due to friction and abrasion.