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Valve body vacuum heat treatment

The valve body is a main component in the valve; there are different mechanical manufacturing methods according to the pressure level. The valve body has a wide variety of materials and is suitable for a variety of different working conditions.

1.The commonly used materials for the valve body are as follows.

①Grey cast iron, suitable for low pressure valves with working temperature between -15~200%C and nominal pressure PN≤1.6MPa.

②Black-heart malleable cast iron, suitable for medium and low pressure valves with working temperature between -15 and 250, and nominal pressure PN≤2.5MPa.

③ Ductile iron, suitable for low pressure valves with working temperature between -30~350C and nominal pressure PN≤4.0MPa.

④Carbon steel (WCA, WCB, WCC), suitable for medium and high pressure valves with working temperature between -129~425C, of which 16Mn, 30Mn working temperature is between -40~450C, often used to replace ASTM A105.

⑤ Low temperature carbon steel (LCB), suitable for low temperature valves with working temperature between -46~345C.

⑥Alloy steel (WC6, WC9), suitable for high temperature and high pressure valves of non-corrosive medium with working temperature between -29~595C; C5, C12 suitable for high temperature of corrosive medium with working temperature between -29~650C High pressure valve.

⑦Austenitic stainless steel, suitable for valves with corrosive media with a working temperature of -196~6009.

⑧Monel alloy, mainly suitable for valves containing hydrogen fluoride medium.

⑨Cast copper alloy, mainly used in valves for oxygen pipelines with working temperature between -273 ~ 200C.

The above examples are the main categories of commonly used materials for valve bodies. There are many different grades in each type of material, and various grades are suitable for various pressure levels. Therefore, when selecting the valve body material of the valve, the valve body material suitable for the working conditions can be determined according to different uses and different pressure levels.

In addition, the valve body materials include titanium alloy (titanium valve); aluminum alloy (aluminum valve); plastic (plastic valve); ceramic (ceramic valve) and so on.

The vacuum heat treatment process of the valve body blank is mainly based on the material selected, the state of the blank used and the technical requirements after vacuum heat treatment. In addition, factors such as valve operating conditions, vacuum heat treatment equipment and process conditions should also be considered.

2.The vacuum heat treatment process of the valve body blank is introduced as follows according to different materials.

①Vacuum heat treatment of gray cast iron. In order to achieve different purposes, gray cast iron can be subjected to different vacuum heat treatments after casting.

In valve production, the vacuum heat treatment processes often used for parts such as gray cast iron valve bodies after casting are: thermal aging to eliminate casting stress and high temperature vacuum annealing to eliminate free cementite. Thermal aging is a necessary process. High temperature vacuum annealing is used to replace thermal aging only when primary cementite exists in the structure after casting due to improper control of chemical composition and casting cooling rate during casting.

②vacuum heat treatment of carbon cast steel. Steel castings have large casting residual stress after casting, and sometimes the structure of steel castings is coarse and even overheated. These all affect the dimensional stability of steel castings, reduce the mechanical properties of steel and are not conducive to cutting. In order to eliminate the casting stress, refine the structure, improve the mechanical properties and improve the machinability, the vacuum annealing or normalizing + vacuum tempering process is often used for the carbon steel valve body and other parts in the valve production after casting.

③ Vacuum heat treatment of austenitic stainless and acid-resistant steel. The main defect of austenitic stainless and acid-resistant steel is that it is prone to intergranular corrosion. To overcome intergranular corrosion, certain preventive measures can be taken to apply certain vacuum heat treatment to the steel. In valve production, the vacuum heat treatment processes often used for austenitic stainless and acid-resistant steel valve bodies and other parts are: solution treatment (also known as vacuum quenching), stabilization treatment and cryogenic treatment.

④ Vacuum heat treatment of martensitic heat-resistant steel. Martensitic heat-resistant steel should be vacuum annealed in time after casting to prevent cracks and the vacuum annealing holding time should be sufficient (usually 4~8h). The purpose of vacuum annealing of martensitic heat-resistant steel is to relieve stress, carry out recrystallization, refine grains, reduce hardness, improve machinability and prepare structure for final heat treatment. The final heat treatment of martensitic heat-resistant steel adopts normalizing (equivalent to vacuum quenching) + vacuum tempering process.

⑤Vacuum heat treatment of high-quality carbon steel. The heat treatment of high-quality carbon steel takes No. 35 forged steel valve body as an example. After forging, the No. 35 steel valve body should be normalized, and its final heat treatment should be carried out according to the regulations of the valve manufacturing technical documents, generally quenching and tempering.

Arrangement of vacuum heat treatment process.

1.Vacuum annealing and normalizing: It belongs to the preparatory heat treatment of the blank and should be arranged before machining.

2.Aging: In order to eliminate residual stress, for castings with large size and complex structure, it is necessary to arrange a time before and after rough machining.

Aging treatment: for general castings, arrange an aging treatment after casting or rough machining; for castings with high precision requirements, arrange an aging treatment before and after semi-finishing; for parts with high precision and poor rigidity, in rough turning , Rough grinding and semi-fine grinding, each need to arrange an aging treatment.

3.Vacuum quenching: After vacuum quenching, the hardness of the workpiece increases and it is easy to deform. It should be arranged before the grinding process in the finishing stage.

4.Vacuum carburizing: vacuum carburizing is easy to produce deformation, should be arranged before finishing. In order to control the thickness of the carburized layer, finishing needs to be arranged before vacuum carburizing.

5.Nitriding: It is generally arranged at the back of the process, before the final processing of the surface, and should be quenched and tempered before nitriding.