The production processes of flanges are mainly divided into four types: forging, casting, cutting and rolling.

Cast Flanges and Forged Flanges

Cast flanges feature accurate blank shape and dimensions, small machining allowance and low cost, but they have casting defects such as air holes, cracks and inclusions. Besides, the internal streamline structure of castings is poor, which becomes even worse for machined parts.

Generally, forged flanges have lower carbon content than cast flanges, making them less prone to rusting. Forgings boast fine streamline structure, dense texture and superior mechanical properties compared with cast flanges.

Improper forging processes may lead to coarse or uneven grain size and hardening cracks, and the production cost of forging is higher than that of casting.

Forgings can withstand higher shear force and tensile force than castings.

The advantages of castings lie in their capability to form complex shapes at a relatively low cost.

Forgings are characterized by uniform internal structure and the absence of harmful defects like air holes and inclusions found in castings.

The differences between cast flanges and forged flanges can be distinguished by production processes. For instance, centrifugal flanges are a type of cast flange.

Centrifugal flanges are manufactured through precision casting. Compared with ordinary sand casting, this casting method delivers a much finer internal structure, greatly improved quality, and fewer problems such as loose structure, air holes and sand holes.

First, it is necessary to understand the production and manufacturing method of centrifugal flanges. The process for producing slip-on welding flanges by centrifugal casting consists of the following steps:
① Melt the selected raw steel in an intermediate frequency electric furnace to raise the molten steel temperature to 1600－1700℃;
② Preheat the metal mold to 800－900℃ and maintain a constant temperature;
③ Start the centrifuge and pour the molten steel obtained in Step ① into the preheated metal mold in Step ②;
④ Allow the casting to cool naturally to 800－900℃ and keep the temperature for 1 to 10 minutes;
⑤ Cool the casting with water to nearly room temperature, then demold and take out the finished casting.

Next is the production process of forged flanges:
The forging process generally includes material cutting of high-quality steel billets, heating, forming and post-forging cooling. Common forging methods comprise open die forging, closed die forging and loose tooling forging. Manufacturers select appropriate forging methods according to the weight and production batch size of forgings.

Open die forging has low production efficiency and large machining allowance, yet it requires simple tools and offers high versatility. Thus, it is widely used for single-piece and small-batch production of forgings with simple shapes. Open die forging equipment includes air hammers, steam-air hammers and hydraulic presses, suitable for manufacturing small, medium and large forgings respectively. Closed die forging features high productivity, simple operation and easy realization of mechanization and automation. Closed die forgings have high dimensional accuracy and small machining allowance, with a more reasonably distributed fiber structure, which can further extend the service life of parts.

1. Basic Processes of Open Die Forging

In open die forging, the shape of forgings is gradually formed by processing billets through basic deformation procedures. The core basic processes include upsetting, drawing out, punching, bending and cutting.

1. Upsetting: It refers to hammering the billet along the axial direction to reduce its height and expand its cross-sectional area. This process is commonly used for forging gear blanks and other disc-shaped forgings, and is divided into full upsetting and local upsetting.
2. Drawing out: This forging process increases the length of billets and reduces their cross-section, which is widely applied to produce blanks for shaft parts such as lathe spindles and connecting rods.
3. Punching: The process of punching through or blind holes on billets with a punch.
4. Bending: A forging procedure to bend billets into specific angles or shapes.
5. Torsion: The process of rotating one part of a billet at a certain angle relative to another part.
6. Cutting: Used to split billets or remove excess material ends.

2. Closed Die Forging

Short for die forging, closed die forging shapes heated billets in forging dies fixed on dedicated forging equipment.

1. Basic Processes of Closed Die Forging: The complete workflow includes material cutting, heating, pre-forging, finish forging, skin punching, trimming, quenching and tempering, and shot blasting. Common forming operations involve upsetting, drawing out, bending, punching and shaping.
2. Common Closed Die Forging Equipment: Main equipment includes die hammers, hot die forging presses, horizontal forging machines and friction presses.

In simple terms, forged flanges have better overall quality, mostly produced by closed die forging, featuring fine crystal structure and high strength, with a correspondingly higher price.

Both cast flanges and forged flanges are mainstream manufacturing methods for flanges. The selection depends on the strength requirements of applied components. For scenarios with low strength demands, lathe-processed flanges are also available as an alternative.

III. Cut Flanges

Cut flanges are produced by directly cutting discs with reserved machining allowance for inner and outer diameters and thickness from medium steel plates, followed by processing bolt holes and water lines. The maximum diameter of such flanges is limited by the width of the medium steel plate.

IV. Rolled Flanges

Rolling is a process that cuts strips from medium steel plates and bends them into circular shapes, mainly adopted for the production of large-sized flanges. After rolling forming, the workpiece is welded, flattened, and then processed with water lines and bolt holes.