Push Elbow

The push elbow is the most widely used manufacturing process for elbows in industrial piping today, especially for medium‑ and low‑pressure, large‑diameter seamless elbows. It features high production efficiency, uniform wall thickness, and relatively low cost. Below is the detailed manufacturing process of push elbows, described in process order:

I. Raw Material Preparation & Cutting

• Material selection: Seamless steel pipes meeting relevant standards (such as 20# carbon steel, 304 stainless steel, etc.) are selected as tube blanks according to order requirements.
• Cutting: A long steel pipe is cut into short sections of specified length using a band saw or flame cutting machine. The length depends on the elbow specifications (diameter and angle) and the elongation rate during pushing.

II. Tube Blank Heating (Critical Step)

• Medium‑frequency induction heating: The cut tube blank is placed into a medium‑frequency induction heating furnace.
• Principle: Eddy currents generated by electromagnetic induction heat the metal of the tube blank directly.
• Features: Fast heating speed, uniform temperature, and precise control over the heated zone (usually only one end of the blank is heated).
• Temperature control: The blank is heated to forging temperature, typically between 900°C and 1200°C, depending on the material. At this temperature, the metal has optimum plasticity and is easy to deform.

III. Push Forming (Core Step)

This is the key stage of elbow forming, usually performed on a dedicated push elbow machine.

• Mold installation: A mandrel and forming die are installed on the guide rails of the pushing machine.
  • Mandrel: Located inside the tube blank, it determines the inner bending radius (R) and wall thickness of the elbow.
  • Die: Located outside the tube blank, it controls the outer diameter of the elbow.
• Pushing process:
  • The heated tube blank is fitted over the mandrel.
  • The pusher advances the blank forward at a constant speed.
  • As the blank passes through the die, it bends under combined thrust and die constraint.
  • Due to high‑temperature metal flow, the tube elongates and thins slightly while bending.
• Angle control: By setting the stroke, elbows of 90°, 45°, or other angles can be formed.

IV. Sizing & Shaping

• Angle correction: Freshly pushed elbows may have angle deviations (e.g., 92° needing correction to 90°) or imperfect roundness.
• Mechanical shaping: The hot or cold elbow is placed in a sizing die and pressed by a hydraulic press to meet standard angles and dimensions.

V. Heat Treatment

• Purpose: Internal stresses and microstructural changes occur during pushing. Heat treatment is required to relieve residual stress, restore toughness, and improve corrosion resistance.
• Process: The elbow is heated in a heat treatment furnace for normalizing or annealing, followed by furnace cooling or air cooling.

VI. Beveling & Machining

• Beveling: A beveling machine cuts standard welding bevels (V‑type or U‑type) on both ends for on‑site welding installation.
• End machining: The end surfaces are turned to ensure flatness and perpendicularity to the centerline.
• Dimensional inspection: Outer diameter, wall thickness, and bending radius (R) are strictly controlled during machining.

VII. Surface Treatment & Inspection

• Surface cleaning: Scale is removed, usually by shot blasting, to reveal a clean metallic surface.
• Non‑Destructive Testing (NDT):
  • Ultrasonic Testing (UT): Detects internal cracks, laminations, and defects.
  • Radiographic Testing (RT): For critical piping elbows, inspects welds (for welded blanks) or heat‑affected zones.
  • Magnetic Particle Testing (MT): Detects surface cracks.
• Hydrostatic test: The elbow undergoes a hydrostatic pressure test to ensure no leakage under specified pressure, verifying strength and seal integrity.

VIII. Marking & Packaging

• Stamping: Specifications, material, standard, heat number, and other identifiers are stamped on the elbow surface.
• Anti‑corrosion treatment: Carbon steel elbows are usually coated with anti‑rust paint; stainless steel elbows generally are not painted but protected against scratches.
• Packing & delivery: Bundled or crated according to transportation requirements.

Summary

The push elbow manufacturing process can be summarized as:cutting → heating → pushing (mandrel + die) → shaping → heat treatment → machining → inspection → delivery This process offers continuous, high‑efficiency production and produces long‑radius elbows with relatively uniform wall thickness.