Manufacturing process of seamless elbows

Seamless elbows are a type of pipe fittings used at pipe bends. Among all the pipe fittings used in pipeline systems, they account for the largest proportion, about 80%. Usually, different forming processes are selected for elbows of different materials or wall thicknesses. Common seamless elbow forming processes used by manufacturers include hot pushing, stamping, extrusion, etc.

1. Hot pushing forming

The hot pushing elbow forming process is a process used in the production and manufacturing of seamless elbows. A special elbow pushing machine, core mold and heating device are used to make the blank on the mold move forward under the push of the pushing machine, and the process is heated, expanded and bent during the movement.

Deformation characteristics of hot pushing elbows: The diameter of the tube blank is determined according to the law that the volume of the metal material remains unchanged before and after plastic deformation. The diameter of the tube blank used is smaller than the diameter of the elbow. The deformation process of the blank is controlled by the core mold, so that the compressed metal at the inner arc flows and compensates to other parts that are thinned due to the expansion of the diameter, thereby obtaining an elbow with uniform wall thickness.

The hot push elbow forming process has the characteristics of beautiful appearance, uniform wall thickness and continuous operation, which is suitable for mass production. Therefore, it has become the main forming method for carbon steel elbows and alloy steel elbows, and is also used in the forming of stainless steel elbows of certain specifications.

The heating methods of the forming process include medium frequency or high frequency induction heating (the heating coil can be multiple coils or single coil), flame heating and reverberatory furnace heating. The heating method used depends on the requirements of the formed product and the energy situation.

2. Stamping forming

Stamping elbows are the earliest forming process used in mass production of seamless elbows. In the production of elbows of common specifications, they have been replaced by hot pushing or other forming processes, but in some specifications of elbows, the production quantity is small and the wall thickness is too thick or too thin.

It is still used when there are special requirements for the product. The stamping forming of the elbow uses a tube blank with the same outer diameter as the elbow, and a press is used to directly press the forming in the mold.

Before stamping, the tube blank is placed on the lower die, the inner core and the end die are loaded into the tube blank, and the upper die moves downward to start pressing. The elbow is formed by the constraint of the outer die and the support of the inner die.

Compared with the hot push process, the appearance quality of stamping is not as good as the former; the outer arc of the stamped elbow is in a stretched state during forming, and there is no excess metal in other parts to compensate, so the wall thickness at the outer arc is thinned by about 10%. However, due to its suitability for single-piece production and low cost, the stamping elbow process is mostly used for the manufacture of small batches and thick-walled elbows.

Stamping elbows are divided into cold stamping and hot stamping. Cold stamping or hot stamping is usually selected according to the material properties and equipment capacity. The forming process of the cold extrusion elbow is to use a special elbow forming machine to put the tube blank into the outer die. After the upper and lower dies are closed, the tube blank moves along the gap reserved by the inner and outer dies under the push of the push rod to complete the forming process.

The elbows made by cold extrusion process of inner and outer dies have beautiful appearance, uniform wall thickness and small size deviation. Therefore, this process is often used for stainless steel elbows, especially thin-walled stainless steel elbows. The inner and outer dies used in this process have high precision requirements; the wall thickness deviation requirements of the tube blank are also relatively stringent.

3. Middle plate welding
Use the middle plate to press to make half of the elbow section, and then weld the two sections together. This process is generally used to make elbows above DN700.

Other methods:
In addition to the above three commonly used forming processes, seamless elbow forming also uses a forming process in which the tube blank is extruded into the outer die and then shaped by passing the ball through the tube blank. However, this process is relatively complex, cumbersome to operate, and the forming quality is not as good as the aforementioned process, so it is rarely used.

Extruded elbows use a special elbow forming machine to put the blank into the mold. After the upper and lower molds are closed, the tube blank moves along the gap reserved by the inner and outer molds under the push of the push rod to complete the forming process. The elbows manufactured by the inner and outer die cold extrusion process have beautiful appearance, uniform wall thickness and small dimensional deviation. Therefore, the cold extrusion production process is suitable for the production of thin-walled stainless steel elbows. The inner and outer dies used in this process have high precision requirements and strict requirements on the wall thickness deviation of the tube blank.

Read more: Carbon steel seamless elbow standards or Material selection and application of pipe elbows