The characteristics of the casting flange is easy to obtain the workpiece other methods is not easy to obtain the shape of complex casting flange; low cost; precision castings can be obtained using a special process, the surface without processing has ideal finish; casting forging flange than simple, low price; but prone to defects and flange plate castings non dense area, ensure the quality of forgings in strong corrosion and high pressure situations of domestic technology cannot in general.
Forging flange is used for forging equipment for bar forging, forging workpiece to play more complex, need a large amount of processing, but the structure of forging flange is compact, not prone to internal defects, so it is widely used in machining high requirements, such as valve seat, valve, valve stem, in high pressure and strong corrosion of alloy valve, forging valve also widely used.
Although the casting technology has made tremendous development, and the use of computer aided geometric design of fluid structure optimization design and casting process, but to achieve the 1 class or 2 class X ray /MT or PT quality acceptance standard is still extremely difficult, which are nuclear power plants, thermal power stations or in petrochemical industries the harsh environment of the required standard. Therefore, it is necessary to improve the welding. However, after welding, the overall quality and reliability of the casting valve becomes difficult to guarantee. Sometimes all of these problems are left in the metal frame of the castings. The test rod is usually directed at each temperature, but their analysis may be uncertain. Even if the circular bar indicates that the chemical and physical properties are acceptable, it may still be difficult to detect internal defects that are detrimental to strength or corrosion resistance.
Some other defects in the valve or flange of castings are clear cracks and thermal tears during the solidification process under the combined action of the stress concentration caused by uneven shrinkage and the low strength of metals near melting point temperature. Low casting temperatures can cause cold scars, and the accumulation of sand grains or slag from molten metals will lead to stains. Low grade casting may also cause other defects.
The improvement of castings to meet the requirements of X - ray quality depends on the grinding, repair, heat treatment and repeated testing and testing of the defective parts. Even in this case, the valve seat and washer face or bump welding end may display fine line cracks that require heavy welding and machining.
The casting process built a fine particle structure and improved the physical properties of the metal. In the practical use of parts, a correct design can make the particle flow in the direction of the main pressure.
Forgings require that each piece is consistent, without any porosity, excess space, inclusions, or other defects. The component produced by this method has a higher ratio of strength to weight than that of weight. These elements are usually used in aircraft structure.
The advantages of forging are: the advantages of forgings include stretchable length, shrinkable cross section, shrinkable length, extensible cross section, variable length and variable cross section. The types of forgings include free forging / hand forging, hot die forging / precision forging, top forging, roll forging, and die forging.
Why do you choose the forgings? Compared with castings, the valve body of forging valve has more uniform structure, better density, better strength integrity, better dimensional characteristics and smaller size error. The directional structure (pipeline) has a higher performance than the casting in the whole strength and stress.
(1) high strength
Hot forging promotes crystallization and grain refinement, allowing the material to achieve maximum possible strength and consistency, and the minimum variation between parts and parts. The flow of the particle flow is precisely along the body profile, and these continuous streamlines help to reduce the incidence of fatigue or common failures.
Forgings, which meet the requirements of the design structure, have always been one of the most important advantages of the forgings, to some extent at the top of the above characteristics.
(3) structural integrity
Forging eliminates internal defects and produces a coherent and consistent metallographic organization that ensures excellent performance. In places where stress and internal corrosion are serious, forgings can guarantee long service life and failure free service.
The dimensional consistency of closed die forging results in the complete control of the key wall thickness, avoiding the defects caused by the shifting of the core in the casting process. Through the impact of high quality, no separation steel ingot and 10 thousand to 30 thousand ton presses, it ensures the consistent metallographic structure without internal defects.
The quality assurance of the forgings, through the use of forgings, as well as its consistency and high quality, disappears the X - ray demand for the 1 types of casting parts. The United States Navy also holds the same attitude when it uses forgings as parts on nuclear submarines and aircraft carriers. For the finished product, the full requirements of the ASME code are ultrasonic inspection (U.T.), magnetic powder (M.T.) or liquid penetration test (P.T.). Discarded forgings found by U.T., M.T., or P.T. methods are very rare. The delivery of the components can be controlled, so the delivery of the valves is more reliable. The directional structure of the valve body of the castings and forgings compares the creep fatigue strength of the forgings more than 3 times higher than that of the casting under the condition that the forging has a very high temperature change.