How to choose a forged ball body for ball valves? Explanation of roundness, hardness, and material

How to choose a forged ball body for ball valves? First, give a direct judgment: as long as the ball valve adopts any one of metal hard seal, particle containing medium, high temperature and high pressure, or frequent opening and closing, the ball blank should be forged; Low pressure and room temperature soft sealed ball valves are usually sufficient with cast balls. Next, let's talk about what the sphere undertakes in the valve, the differences between the three types of raw materials, and why roundness and surface roughness are key.

1、 The sphere is the only moving seal in the ball valve

The sealing of a ball valve relies on one thing: the surface of the ball and the valve seat are pressed together when closed, forming a continuous sealing band. When the valve is opened, the ball rotates 90 degrees to align the channel with the pipeline; When closing, turn it back and use the pressure of the medium or the pre tightening force of the valve seat to press the sealing tape tightly.

This structure determines that the sphere is sensitive to three things. The first is geometric accuracy, whether the sphere is a true sphere or "looks like a sphere" directly determines whether the sealing tape can be continuous. The second is the surface condition. Metal hard sealing belongs to the adhesion between metal and metal. If there are scratches, pits, or microscopic high points on the surface, the medium will seep through from there. The third is the quality of the matrix. If there are shrinkage porosity, pores, or inclusions inside the sphere, they are prone to develop into crack sources in subsequent processes and during use.

Soft sealed ball valves can compensate for some geometric errors to some extent through the elasticity of the valve seat material, while hard sealed ball valves do not have this margin - so the requirements for the ball body in hard seals are one level higher than those in soft seals.

2、 Three types of raw materials, the difference lies here

The first type is casting balls. Casting can produce rough blanks that are close to the final shape in one go, with high material utilization, cost advantages, and the ability to produce large diameters. However, the internal structure of the casting is relatively coarse, and there may be shrinkage porosity, porosity, and inclusions. If there are hidden dangers in the substrate before surface hardening treatment, problems are likely to occur later. It is suitable for applications with low pressure, room temperature, soft sealing, and low requirements for lifespan.

The second method is direct turning of bar material. Take a round steel and get on the bed to get the ball cart out. The process is simple and it is very convenient to make small batches of samples. There are two problems: first, the material utilization rate is low, and the cost of large-diameter spheres will significantly increase; The second is that the metal streamline of the bar material itself is a straight line along the rolling direction. After being rolled into a ball, the streamline is cut off on the spherical surface, and the stress state is not as good as that of the formed part.

The third type is forged shaped balls. The billet is forged into a spherical shape using a mold or free forging, and the metal undergoes plastic deformation under pressure. The internal structure is compacted and refined, and the streamline is continuously distributed along the shape of the sphere. For spheres that require surface hardening treatments such as welding, spray welding, or nitriding in the future, a dense and uniform substrate is crucial - the hardened layer is hard and brittle. If the substrate is uneven in hardness and hardness, local peeling may occur during use.

One sentence summary: Low pressure room temperature soft seal, all three types can be used; The advantages of forging spheres are only reflected in situations where they are hard sealed, contain particles, under high pressure, high temperature, and frequently move. The extra money is exchanged for seal reliability and maintenance cycles.

3、 Why are roundness and surface roughness key

The sealing band width of a hard sealed ball valve is very narrow, usually in the circle where the spherical surface contacts the valve seat. Once the roundness exceeds the tolerance, the contact pressure of the sphere will be unevenly distributed along the circumference when it rotates to the closed position. Areas with high pressure will wear out quickly, while areas with low pressure will be leakage channels.

The most common manifestation on site is that the new valve has passed the pressure test, but after several months of use, it begins to leak internally. Upon disassembly, there is an uneven wear mark on the spherical surface. This situation is often not a problem with the valve seat, but rather the geometric accuracy or assembly concentricity of the sphere.

The same applies to surface roughness. Hard sealing requires a surface roughness of mirror level on the spherical surface. The larger the roughness value, the more difficult it is for metal to metal bonding to form a continuous sealing band, resulting in greater initial leakage. One of the benefits of forging a sphere is that the matrix structure is uniform and fine, making it less prone to "uneven hardness and roughness" during grinding and polishing, and resulting in a more stable surface quality.

Here is a reminder: the final roundness and roughness of the sphere are achieved through precision machining and grinding processes, and forging factories deliver blanks or rough machined parts. When placing an order, it is necessary to clearly state the machining allowance, which party guarantees the final accuracy, and what standards are used for acceptance, in order to avoid unclear responsibilities in the later stage when the blank is fine but the finished product is not qualified.

4、 How to match the material

Carbon steel is the most common choice for water, steam, and oil pipelines that are used at room temperature to medium temperature and have no strong corrosion in the medium.

Stainless steel is used in chemical and food pharmaceutical pipelines with corrosive media and cleanliness requirements. The common austenitic stainless steel series is particularly suitable for low-temperature working conditions, with good impact toughness performance at low temperatures.

The strength of duplex stainless steel is one level higher than that of ordinary austenitic stainless steel, and its ability to resist pitting and crevice corrosion is also better. It is suitable for environments with seawater and chloride ions.

Heat resistant steel and nickel based alloys are used in extreme conditions of high temperature or strong corrosion, such as the high-temperature section of power plants and sulfur-containing oil and gas environments. This type of material has high forging difficulty and a narrow heating temperature window, requiring manufacturers to have corresponding material experience.

Fujian Mingda can process finished and semi-finished products of nickel based, iron-based, copper based, titanium based and other alloy materials. The long-term material categories include nickel based high-temperature alloys, nickel based corrosion-resistant alloys, duplex stainless steel, super austenitic stainless steel, and heat-resistant steel, which can be customized according to drawings or grades.

5、 Specification range and supply status

The conventional coverage range of spherical products is NPS half inch to 40 inch (DN15 to DN1000), pressure rating 150 to 2500LB (PN20 to 420), materials include carbon steel, alloy steel, stainless steel, special alloys, etc. The standard system supports ASTM, DIN, JIS, GB, and production can be arranged according to customer design and size.

There are three levels of supply status, choose according to your own processing capacity:

Forging billet state. Only for forging and forming, all subsequent processing will be done by the buyer themselves, suitable for valve factories with complete machining capabilities.

Rough machining status. The forging is rough machined, leaving a margin for precision machining. The buyer only performs precision machining and grinding, which can save one process.

Precision machining status. Complete machining according to the drawings, and the buyer can directly assemble or only perform surface hardening treatment, which is suitable for purchasers with limited machining capabilities who wish to compress the process.

6、 These situations need to be explained clearly in advance

Large spherical objects with diameters exceeding the conventional coverage range should be evaluated separately based on equipment capacity, as the capacity of the rolling ring and forging equipment determines the upper limit of the weight of a single piece.

The medium environment, material, and heat treatment status that are sensitive to intergranular corrosion should be determined together. It is not enough to only write the grade without specifying the heat treatment requirements.

In situations where low-temperature impact performance is required, the temperature and qualified indicators of the impact test should be written into the technical agreement. The qualified impact value at room temperature does not mean that it is also qualified at low temperatures.

The party responsible for surface hardening treatment should be defined in the contract, as the quality of the hardened layer depends not only on the substrate but also on the treatment process itself.

7、 Please provide a clear quotation for these seven items in the inquiry

Ball diameter and pressure rating, material grade and execution standard, supply status (forging blank, rough machining or precision machining), individual weight or external dimensions, quantity, heat treatment requirements, non-destructive testing and acceptance requirements. These seven items are complete, and the manufacturer can provide accurate forging processes, mold schemes, and delivery times at the same time, reducing the number of back and forth confirmations.

Fujian Mingda Valve Manufacturing Co., Ltd. is located at No. 11 Ninghe Road, Wendu Industrial Zone, Fuding Industrial Park, Ningde City, Fujian Province. The factory covers a total area of 24 acres and is equipped with 8T and 5T electro-hydraulic hammers, 15T and 5T operating machines, 12T multi station discharge machines, as well as fully computer-controlled 60T and 40T natural gas heating furnaces and vertical ring rolling production lines. The annual production capacity can reach more than 20000 tons, and the products cover valve bodies, spheres, valve covers, valve seats, valve stems, blocks, pressure plates, fixed shafts, support plates and other valve forgings. If there is a need for spherical or other valve forgings, please send the seven items above for technical personnel to verify the process plan and provide a quotation.

Contact consultation: Fujian Mingda Valve Manufacturing Co., Ltd., phone 13958857568, fax 0593-7261298, email 450993478@qq.com .