Seamless Steel Elbows: Steel elbows are pipe fittings used at pipe bends. They account for approximately 80% of all pipe fittings used in piping systems. Different forming processes are typically selected for steel elbows made of different materials or with different wall thicknesses. Common forming processes for seamless steel elbows include hot pushing, stamping, and extrusion. The hot pushing steel elbow forming process uses a specialized elbow pushing machine, mandrel, and heating device. The blank, placed on the mandrel, is pushed forward by the pushing machine, where it is heated, expanded in diameter, and bent into shape. The deformation characteristics of hot pushing steel elbows are determined based on the principle that the volume of the blank remains unchanged before and after plastic deformation of the metal material. The diameter of the blank used is smaller than the diameter of the steel elbow. The deformation process of the blank is controlled by the mandrel, allowing the compressed metal at the inner arc to flow and compensate for the thinning of other parts due to diameter expansion, thus obtaining a steel elbow with uniform wall thickness.
Hot-push forming of steel elbows offers advantages such as aesthetically pleasing appearance, uniform wall thickness, and continuous operation, making it suitable for mass production. Therefore, it has become the primary forming method for carbon steel and alloy steel elbows, and is also used in the forming of certain specifications of stainless steel elbows.
The heating methods during the forming process include medium-frequency or high-frequency induction heating (with multiple or single heating coils), flame heating, and reverberatory furnace heating. The choice of heating method depends on the requirements of the product being formed and the available energy. Stamping forming of steel elbows was the earliest forming process used for the mass production of seamless steel elbows. While it has been largely replaced by hot-push or other forming processes in the production of commonly used specifications of steel elbows, it is still used for certain specifications due to low production quantities, excessively thick or thin walls.
It is still used when products have special requirements. Stamping forming of steel elbows uses a tube blank with the same outer diameter as the steel elbow, which is directly pressed into shape using a press in a die.
Before stamping, the tube blank is placed on the lower die, and the inner core and end dies are inserted into the tube blank. The upper die moves downward to begin pressing. The steel elbow is formed by the constraint of the outer die and the support of the inner die.
Compared to hot-push forming, the appearance quality of stamped steel elbows is not as good. During forming, the outer arc of the stamped steel elbow is in a stretched state, without excess metal from other parts for compensation, so the wall thickness at the outer arc is reduced by about 10%. However, due to their suitability for single-piece production and low cost, stamped steel elbows are mostly used for the manufacture of small batches of thick-walled steel elbows.
Stamped steel elbows are divided into cold stamping and hot stamping, usually selected based on material properties and equipment capabilities.
The forming process of cold-extruded steel elbows uses a dedicated elbow forming machine. The tube blank is placed in the outer die. After the upper and lower dies close, the tube blank moves along the gap between the inner and outer dies under the push of the push rod to complete the forming process.
Steel elbows manufactured using the internal and external die cold extrusion process have an aesthetically pleasing appearance, uniform wall thickness, and small dimensional deviations. Therefore, this process is often used for forming stainless steel elbows, especially thin-walled stainless steel elbows. This process requires high precision in the internal and external dies and has stringent requirements regarding the wall thickness deviation of the tube blank.
Medium plate welded steel elbows: A medium plate is pressed into half of the steel elbow profile using a press, and then the two profiles are welded together. This process is generally used for steel elbows with a diameter of DN700 and above. Besides the three commonly used forming processes mentioned above, seamless steel elbows can also be formed by extruding the tube blank into an external die and then shaping it by passing a 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 processes, so it is less commonly used.
