Forging Knowledge of Valves
Jan 15, 2024
Forging Process is a method of processing metal blanks using forging machinery to apply pressure, resulting in plastic deformation to obtain forged parts with specific mechanical properties, shapes, and sizes. It is one of the two major components of forging (forging and stamping). Valves, when classified by the forging process, can be divided into forged valves and cast valves. This article will share knowledge about forged valves from different perspectives.
The starting recrystallization temperature of steel is approximately 727°C, but 800°C is commonly used as the dividing line. Above 800°C is hot forging, and between 300 to 800°C is referred to as warm forging or semi-hot forging.
Based on the movement of blanks, forging can be categorized as free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting.
Free Forging: Using impact force or pressure to deform metal between two anvils to obtain the desired forged piece. It includes hand forging and machine forging.
Die Forging: Divided into open die forging and closed die forging. Metal blanks are compressed and deformed within a specific die to obtain the forged piece. It includes processes like cold heading, roll forging, radial forging, and extrusion.
Closed Die Forging and Closed Upsetting: These methods have high material utilization due to no flash. Complex forged pieces can be finished in one or a few steps. However, it is essential not to completely restrict the blank and control its volume strictly.
Depending on the movement of forging dies, forging can be further divided into swing forging, swivel forging, roll forging, wedge cross rolling, ring rolling, and skew rolling. Computer-controlled die movement can achieve lower forging forces for complex and high-precision products, such as large turbine blades.
Forging equipment can be classified into four forms based on the characteristics of the dead point and deformation limits: limiting forging force form, quasi-stroke limiting form, stroke limiting form, and energy limiting form. Considerations such as avoiding overloads at the dead point, controlling speed and die position, and adjusting slider guide clearances are crucial for maintaining accuracy.
Sliders can move vertically and horizontally (used for forging long and slender pieces, lubrication cooling, and high-speed production of parts). The use of compensatory devices allows smooth forging in various directions. The different methods mentioned have varying requirements for forging force, processes, material utilization, production output, size tolerance, and lubrication cooling methods, influencing the level of automation.
Forging production is one of the main methods for providing rough mechanical parts in the mechanical manufacturing industry. It not only shapes mechanical parts but also improves the internal structure of metals, enhancing their mechanical and physical properties. Important mechanical parts with high loads and requirements often use forging production methods. For example, turbine generator shafts, rotors, blades, casings, large hydraulic press columns, high-pressure cylinders, steel rolling mill rolls, internal combustion engine crankshafts, connecting rods, gears, bearings, and defense industry components like cannons are mostly manufactured using forging.
Therefore, forging production is widely applied in metallurgy, mining, automotive, tractor, harvesting machinery, petroleum, chemical, aviation, aerospace, and defense industries. In daily life, forging production also holds a significant position. The annual production of forgings, the proportion of closed die forgings in the total forging production, and the size and quantity of forging equipment reflect a country's industrial level to some extent.
Forging materials mainly include various carbon steels and alloy steels, followed by aluminum, magnesium, copper, titanium, and their alloys. The original states of materials include bar stock, ingots, metal powder, and liquid metal. The ratio of the cross-sectional area before deformation to the cross-sectional area after deformation is called the forging ratio. Properly selecting the forging ratio, reasonable heating temperature and holding time, appropriate initial forging temperature, final forging temperature, deformation amount, and deformation speed are crucial for improving product quality and reducing costs.
General small to medium-sized forgings use round or square bar stock as blanks. Bar stock has uniform and good grain structure, mechanical properties, accurate shape and size, and good surface quality, making it suitable for mass production. Large forgings use ingots, which require significant plastic deformation to break down large columnar grains and compact the central area to obtain excellent metal structure and mechanical properties.
Powder metallurgy preforms, pressed and sintered from metal powder, can be forged into forgings without flash using non-flash dies. Forgings made from powder have density close to that of general forgings, high mechanical properties, and high precision, reducing subsequent machining. However, powder prices are much higher than regular bar stock, limiting its application in production. Liquid metal forgings involve applying static pressure to molten metal cast in molds, achieving the desired shape and performance. Liquid metal forging is a forming method between die casting and die forging, especially suitable for complex thin-walled parts that are difficult to forge with general die forging.
In addition to conventional materials, forging materials also include iron-based high-temperature alloys, nickel-based high-temperature alloys, and cobalt-based high-temperature alloys. Deformation alloys are also forged or rolled, but the narrow plastic zone of these alloys makes forging more challenging. Different materials have strict requirements for heating temperature, initial forging temperature, and final forging temperature.
Different forging methods have different processes. The process flow for hot die forging is the longest, generally including: forging blank cutting, forging blank heating, roll forging preparation, die forging forming, trimming, punching, correction, intermediate inspection for dimensional and surface defects, forging heat treatment for stress relief and improvement of metal cutting performance, cleaning to remove surface oxide scale, re-correction, and final inspection, including visual and hardness checks. Important forgings may undergo additional tests such as chemical composition analysis, mechanical properties, residual stress, and non-destructive testing.
Compared to castings, metal processed through forging has improved structure and mechanical properties. The recrystallization structure and mechanical properties of casting structures are improved through hot working, converting coarse dendrites and columnar grains into fine, uniformly sized equiaxed recrystallized structures. This compacts and welds the original segregation, looseness, air holes, and inclusions, resulting in a denser structure and improved metal plasticity and mechanical properties. The mechanical properties of castings are lower than those of the same material forgings. Additionally, forging processing ensures the continuity of metal fiber structures, maintaining consistency between the fiber structure and the external shape of the forging. This integral flow of metal ensures good mechanical performance and a long service life. Precision die forging, cold extrusion, warm extrusion, and other processes produce forgings that cannot be matched by castings. A forging is a metal object shaped by applying pressure to achieve the required shape or suitable compression force. This force is typically applied using a hammer or press. The casting process constructs a refined granular structure, improving the physical properties of the metal. In practical use, correct design allows particle flow in the direction of the main pressure. Castings are metal-shaped objects obtained through various casting methods, injecting refined molten metal into prepared molds through pouring, injection, suction, or other casting methods. After cooling, followed by sand removal, cleaning, and post-treatment, the resulting objects have specific shapes, sizes, and properties.
Forging is a crucial process in the production of various mechanical components, ensuring high-quality, durable, and reliable products. The choice of forging process, materials, equipment, and parameters significantly influences the final product's properties and performance. Advances in forging technology continue to enhance the efficiency and precision of this manufacturing method, contributing to the development of industries worldwide.
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