At a glance, the number of valve parts appears to be small, and with structurally simple characteristics, generally at low precision, they seem to be some of the most straightforward parts in the mechanical industry. However, the core sealing areas of the valve have exceptionally high standards. The manufacturing process of a valve is complicated with considerable technical challenges. What features of processes should be attended to?
Valve Manufacturing Materials
The wide variety and specifications of valves, used across the board in all fields of the national economy, determine a big difference between their conditions of application: high-temperature and high-pressure, cryogenic, flammable and explosive media, and also toxic media with high corrosion activity. Such a wide range of operating conditions puts high demands on valve materials.
In addition to cast iron, carbon steel, and alloy structural steel, many other materials are in use, among which are CrNi stainless steel, CrMoAl nitrided steel, CrMoV heat-resistant steel, CrMnN acid-resistant steel, precipitation-hardened steel, duplex stainless steel, cryogenic steel, titanium alloys, Monel alloys, Inconel alloys, Hastelloy, and G0CrW hard alloys.These materials’ casting, welding, and machining properties are poor, which complicates the manufacturing process immensely. In addition, the material selection, preparation, and procurement are greatly complicated because these materials usually happen to be high-alloy, high-strength, high-hardness premium materials. Some kinds of them have difficulties in sourcing and supply resulting from small usage volumes.
Structure Of Valve Casting Blanks
The structurally complex, thin-shell casting is usually used for most valve blanks. There are not only excellent external qualities but also dense internal quality and a well-formed metallographic structure. The defects such as porosity, shrinkage cavities, sand inclusions, or cracks are not allowed to occur. Therefore, their casting process is complicated, and the techniques of heat treatments are very tough.
In the mechanical industry, the casting of thin-shell blanks for pressure-bearing valves is much more complex and difficult than castings for other mechanical components.
Machining Process
Most of the high-strength, high-hardness, and highly corrosion-resistant materials have poor machinability. For example, high-alloy stainless steel and acid-resistant steel have disadvantages in high toughness, high strength, poor heat dissipation, high chip adhesion, and a strong tendency toward work hardening. All these factors bring great difficulties to tools, methods, and equipment in the machining process to achieve the required dimensional accuracy and surface finish.
At the same time, it has high operating requirements with regard to machining accuracy, mating angles, and surface finish in the application of valve sealing surfaces fitting with sealing pairs.
Process Arrangement For Valve Components
Valves are usually composed of no more than a few pieces of main parts, and most of their structures are relatively simple. Most of the dimensions do not need high machining accuracy, and the external surface is comparatively rough; therefore, people get the impression of a simple mechanical part. However, the core sealing areas of valves are extremely precise. The “three degrees” of the sealing surface-flatness, smoothness, and hardness-are held to very high standards. Also, the two sealing surfaces of the sealing pair should achieve perfect zero-to-zero alignment to make sure zero leakage during air-tightness testing.
The rough baselines used to guarantee the precision of the core zero-to-zero sealing requirements are one of the biggest process difficulties in valve machining.
Valve Manufacturing Process
Manufacture of valves involves a number of complicated and precise stages with the goal of ensuring the finished product will stand strict requirements for performance, safety, and reliability.
Step 1: Production Of Valve Body
Valve Body (Casting, Sealing Surface Cladding)
The process for manufacturing the valve body begins with procurement in accordance with stipulated standards. Incoming inspection is then executed to ensure that these standards have been followed. Next, the cladding groove is prepared. In this regard, an ultrasonic test has to be performed in order to check on the quality of the casting according to the drawings. Thereafter, cladding is performed together with post-weld heat treatment in order to improve the material characteristics. Afterwards, finishing operations are carried out such as polishing the sealing surfaces. Sealing surface hardness is then inspected, and dye penetrant testing is done to find defects that may appear on the surface.
Step 2: Valve Internal Components Manufacturing Process
For internal components that are to be clad on their sealing surfaces, such as valve discs or valve seats:
The first process in manufacturing is material purchase according to the standard requirements. Then, after material arrival, an incoming inspection is necessary to check if the materials meet all the standards specified. The subsequent process is making blanks from round steel or forgings, which is prepared according to drawing and process requirements. Rough machining is carried out along with ultrasonic testing of surfaces, wherever the drawings specify. Afterwards, cladding grooves are machined and the cladding process along with post-weld heat treatment is carried out. Finishing of the components is carried out, which includes polishing of sealing surfaces, followed by sealing surface hardness inspection and dye penetrant testing.
Valve stem:
Valves’ stem material procurement and incoming inspection are exactly the same as the other materials, and all should meet the requirements. Blanks are made from round steel or forgings according to drawings. Cladding grooves are roughly machined before cladding and post-weld heat treatment. Finish-machining is done after that, with the outer diameter of the valve stem ground. Surface treatments such as nitriding, quenching, or chemical plating are made based on improving durability. The final steps include polishing, grinding, and polishing of sealing surfaces, sealing surface hardness inspection, and dye penetrant testing.
For internal components not requiring cladding of the sealing surfaces:
Material procurement and incoming inspection of internal components that do not require cladding on sealing surfaces are the same as general stipulations. Blanks are manufactured by round steel or forging according to drawing and process requirements, respectively. After the completion of rough machining, if specified in drawings, ultrasonic testing of surfaces is implemented, followed by finishing of all parts to guarantee that every component meets the required specification for assembly.
Step 3: Final Assembly
The process starts with the retrieval of the required parts, which need to be assembled, cleaned, and washed to remove all the contamination from the components. Then, a rough assembly is done based on the drawing provided. Hydrostatic testing is done on assembled components, ensuring the assembled parts meet the standard requirements. Once this test passes, it is disassembled for cleaning. The final assembly is followed by adjustments with electrical components or actuators, especially for electric valves. Lastly, the valves are painted, packaged, and ready to be shipped.
Valve Testing And Inspection
The manufacturing of the valve includes some painstaking processes of inspection and testing to attain good quality and functionality of each component. First, the inspection of raw materials checks on the diameter for proper cutting and length, then sends the materials to the forging workshop for shaping. After forging, the dimensions of the blanks are checked for accuracy, and excess edges are cut off by the workers. Then, the blanks will go through surface treatment by sand blasting and inspection after treatment. The second last step involves machining work, such as sealing thread machining of the valve body, with workers undertaking self-inspection and the final inspection by designated staff.
The entire product is inspected after the completion of machining. The semi-products are then put into stock in the warehouse for electroplating and surface treatment inspections. The inspection of key components includes that of the balls, valve stems, and sealing valve seats. The final assembly consists of pressure testing, drying, and packaging, where inspectors verify the sealing, appearance, and torque. Only those products that meet strict standards are permitted to package and ship.
Conclusion
Machining a valve is a very complex and precise operation, whereby material selection, machining techniques, and quality control are very important to be considered and carried out responsibly in order for the final product to meet the demanding standards. Every stage follows a critical role in high-quality valve manufacturing, from the procurement of raw materials to the final assembly and testing itself. Precision machining encompasses threading, sealing surface finishing, and the assembling of components to guarantee that valves can operate with predictability even in critical working environments. Rigorous testing and inspection, such as pressure tests and surface checks, further ensure functional ability and durability of the valve. The well-performed process of valve machining is an indispensable way to create components to support various industrial applications under pressure safely and efficiently.
Final Thoughts
As a professional with 15 years of CNC machining service experience, I understand the complexity and high precision required in the valve machining process. In a recent aerospace project I worked on, we machined a batch of high-performance valves using precise CNC machining techniques, ensuring the accuracy of sealing surfaces and connecting parts to meet stringent pressure test requirements. Through proper process planning and strict quality control, we not only improved production efficiency but also successfully reduced scrap rates. This experience highlighted the importance of well-planned processes and precise machining equipment in ensuring valve performance.Ultirapid, has extensive experience and advanced equipment, enabling us to provide high-quality valve machining solutions, ensuring the successful delivery of each project.
