Hydraulic valves are subjected to complex and variable loads during their operation. Therefore, the strength of the valve body must meet the specified requirements, i.e. it has sufficient resistance to damage.Valve body failure occurs during operation, such as cracks (visible cracks due to insufficient strength, valve body cracks), plastic deformation (permanent deformation due to allowable stress exceeding elastic limit), leakage, etc.There are many reasons for valve body failure, which can be summarized as follows:
(1) Material properties of valve body
When the tensile strength of the valve body material is lower than the specified value, the bearing capacity of the valve body decreases and eventually causes the valve body to crack.In the chemical composition C, Mn, Si, S, P, Cr, Ni, Mo and other elements of the material, if the content of P and S is high, this will lead to brittle phosphorus eutectic in the metallographic structure, thus reducing the mechanical properties of the material; if the content of Si and C is high, this means that the plasticity and toughness of the material are weakened; if the content of C is high, it is easy to cause the material to chill during casting.The grain size in the metallographic structure of the material is also one of the important factors affecting the mechanical properties and corrosion resistance of the material. The larger the grain size, the lower the toughness, plasticity and hardness of the material, the easier cracks will occur and the better the crack propagation.Non-uniform grain size and non-metallic inclusions will degrade the mechanical properties of the valve body material, while a large number of inclusions will degrade the mechanical properties of the material seriously and accelerate the possibility of intergranular corrosion of the material.
(2) Valve body wall thickness
When the wall thickness of the valve body is less than the minimum wall thickness required by the design, the area under load will be reduced, the bearing capacity will be reduced, and the strength requirement will not be met. This is another cause of valve body failure .However, when the wall thickness of the valve body is too large, it is easy to cause shrinkage defects in the body during casting, thus weakening the strength of the valve body.Therefore, the thicker the body wall is not the better.
(3) Casting defects of valve body
When casting defects exist in valve body castings, the compactness of valve body material is destroyed and the mechanical properties of valve body are reduced, which is one of the main reasons for valve body failure.During actual production and manufacturing of hydraulic valve body castings, there is non-uniformity between graphite of cast iron itself and the interface between graphite and matrix. Graphite will break down locally when it is subjected to relatively high pressure, resulting in leakage of valve body . Therefore, shrinkage holes, slag inclusions, air holes and sand holes in valve body castings will cause leakage.When some parts of the valve body are heavily sanded and cannot be removed, the castings do not meet the surface requirements. When oil flows through this part, excessive energy loss occurs and the output characteristics of the hydraulic valve do not meet the design requirements.After finishing, when casting defects are found on the mating surface of the hydraulic valve body, not only the sealing performance of the hydraulic valve will be reduced, but also the radial force acting on the valve spool will be unbalanced and the hydraulic clamping will occur.
(4) Effect of operating conditions
Hydraulic valve body works under high and variable pressure for a long time. Therefore, the valve body endures alternating loads caused by opening, closing and reversing for a long time, which makes the stress of the valve body less than the strength limit, but does not meet the fatigue strength requirements.
When the hydraulic valve is in operation, when oil flows through the valve port, the temperature of the valve port rises sharply due to throttling, which will cause plastic deformation and irreversible accumulation of the valve body in local position.
Hydraulic valve, as a precision machine with high load, works in harsh environment and varies loads, requiring high bearing capacity of hydraulic valve body castings.However, the casting process of the hydraulic valve body is complicated and versatile, casting defects often occur in the casting, which destroy the compactness of the valve body and cause leakage when the valve body is under high pressure; due to casting defects, the bearing capacity of the valve body is reduced, the strength of the valve body is weakened, and the hydraulic clamping phenomenon is also caused; all of these phenomena will lead to the failure of the hydraulic valve.Therefore, it is of great significance to study casting defects of hydraulic valve body and their influence on strength.