The Rolling Process of the Valve Stem

The Rolling Process of the Valve Stem

Oct 26, 2022


The valve stem is an important part in the valve, and its machining accuracy and quality directly affect the performance of the finished valve, especially the trapezoidal thread and bare stem. Forged valves with small diameters whose valve stems are made from martensitic stainless steel 2Crl3 and austenitic stainless steel OCrl8Ni9 manufactured by a company have the following disadvantages:
(1) They contain a large amount of cf, Ni, Mn and other elements, and have good plasticity, toughness, high-temperature strength, great tendency to work hardening, which increases the load during cutting. In addition, some carbides will be precipitated inside in the processing, which will aggravate the scratches of the tool. 
(2) It is easy to form cutting nodules in the cutting process, which makes the chip and the tool stick seriously and affects the quality of the machined surface. 
(3) The chips are not easy to curl and break, which is easy to cause blockage of chips. 
(4) The thermal conductivity is low, resulting in high cutting temperature and accelerated tool wear. 
(5) The coefficient of linear expansion is great, and under the high temperature of cutting, the valve stem is susceptible to thermal deformation, thus affecting the accuracy of the processing size. 
 
According to the above reasons, it is difficult to ensure the dimensional accuracy of the valve stem and the thread shape of the threaded part by using conventional machining methods like turning, and it is not easy to ensure the quality of the assembly and service conditions of the valve. In view of the above situation, after many tests, use the rolling processing to roll the trapezoidal thread and the bare stem, and good benefits are obtained. 
 
Diameters before thread rolling
The processing diameter of the valve stem’s trapezoidal thread before rolling varies according to the different materials and heat treatment hardness. The valve stem is made from 2Cr13; the heat treatment hardness is 200 to 240HB, and the processing diameter before the valve stem is d0= (d2-0. 3)±0.05, where d0 is the processing diameter of the trapezoidal thread before rolling, mm d2 the pitch diameter of the trapezoidal thread.
 
2. Rolling parameters
(1) Rolling pressure
The thread diameter below 12mm is 60kN and that between 14 and 18mm is 120kN.
(2) Rolling time
Thread diameter below 12mm is 5 seconds and between 14 and 18mm 10 seconds.
 
3. Rolled support plates
The roller type is adopted to reduce the friction between the workpiece and the support plate, reduce the rolling force and improve the service life of the roller.
 
4. Cooling oil
By replacing the rolling wheel and replacing the support plate, the bare stem part can be rolled smoothly. After the valve stem is rolled, a strengthening layer is formed on its outer surface, which improves the surface hardness and increases the fatigue strength and wear resistance of the valve stem. The surface roughness of the bare stem of the valve stem can reach more than R 0.4 m; the tooth shape of the threaded part is guaranteed, and the production efficiency is more than 15 times higher than that of turning.

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