Core Manufacturing Process of Carbon Steel Elbow

The core manufacturing processes of carbon steel elbows are mainly the following, each process is suitable for different specifications, wall thickness, angle and batch requirements:

1. Hot Push Bending / Induction Hot Forming

Core process: This is the most commonly used process for producing small and medium caliber (usually DN15-DN600) and standard wall thickness elbows (90°, 45°, etc.).

Cutting: Cut the carbon steel pipe into the required length.

Heating: Use the medium frequency induction coil to quickly heat the area to be bent of the tube blank to the plastic deformation temperature (about 1050°C - 1150°C).

Push forming: Under the action of hydraulic thrust, the heated tube blank is pushed into a special mold (core mold and outer mold). The tube blank undergoes plastic deformation under the constraint of the mold, bends and expands along the mold cavity, and finally forms the shape of the elbow.

Cooling: The formed elbow is cooled (usually air-cooled).

Subsequent processing: cutting ends (removing irregular ends), beveling, shaping (if necessary), heat treatment (depending on requirements), surface treatment (sandblasting, painting, etc.), inspection.

Advantages:

High production efficiency, suitable for mass production.

Good wall thickness uniformity (especially good control of thinning of outer wall thickness).

Smooth inner wall and low flow resistance.

Dense material structure and good mechanical properties.

Relatively thin elbows can be manufactured.

Disadvantages:

High initial mold cost.

Mainly suitable for standard angles and radii (mainly R=1.5D).

2. Stamping / Press Forming

Core process: Mainly used to produce elbows with standard wall thickness (SCH40/SCH80, etc.), especially common in smaller specifications (DN15-DN50).

Cutting: Cut steel plates or steel strips into fan-shaped blanks (or tube blanks) of specific shapes.

Heating (optional): For thick-walled or large-angle elbows, the blank may need to be heated (hot stamping). Thin-walled and small-angle elbows can be cold stamped.

Stamping: The blank is placed in the mold. Under the action of a large press, the upper and lower molds are closed to make the blank plastically deformed and pressed into two halves (or a single piece) of the elbow.

Welding (for two-half welding): The two pressed halves are welded together along the longitudinal seam to form a complete elbow. There is also a single-stamping integral forming process (less common).

Subsequent processing: Beveling, shaping, heat treatment (depending on requirements), weld treatment (grinding, flaw detection), surface treatment, inspection.

Advantages:

High dimensional accuracy and regular appearance.

High production efficiency (especially for small specifications).

Mature molds and relatively controllable costs.

Disadvantages:

For large-diameter elbows, very large presses are required.

There are welds in welded elbows, which are potential weak points and require strict quality control.

Cold stamping requires high plasticity of materials and limited deformation; hot stamping requires heating equipment.

The control of wall thickness uniformity (especially inner thickening) is not as good as hot push.

3. Fabricated Welding / Segmented Bend

Core process: Mainly used to manufacture elbows with large diameter (DN600 or above), non-standard angles, large radius or special wall thickness, as well as "shrimp waist" elbows.

Cutting: Cut the steel plate into multiple trapezoidal or fan-shaped tiles (tube sections).

Rolling/pressing: Roll/press each tile into a specified arc shape on a plate rolling machine or press.

Group welding: Group multiple arc-shaped tiles in sequence, weld the inner and outer longitudinal seams, and form a complete elbow. For the "shrimp waist" elbow, multiple standard short sections are beveled and welded.

Subsequent processing: shaping, heat treatment (especially for thick walls or important occasions to eliminate welding stress), non-destructive testing of welds (RT/UT is very important), surface treatment, water pressure test, inspection.

Advantages:

Not limited by caliber, angle, radius, and maximum flexibility.

High material utilization (especially for small pieces).

No large-scale special molding equipment is required.

Disadvantages:

Low production efficiency and high labor costs.

There are many and long welds, which are the main stress concentration points and potential failure points, and the quality control requirements are extremely high (non-destructive testing is essential).

The inner wall is not smooth enough (there is weld excess height), and the flow resistance is large.

The dimensional accuracy and roundness are relatively low.

Common and key subsequent processes:

Heat treatment:

Purpose: To eliminate residual stress caused by molding or welding, improve metallographic structure and mechanical properties (especially toughness and plasticity), and meet standard requirements (such as ASTM A234).

Common methods: Normalizing, normalizing + tempering, annealing. The specific process depends on the material grade (such as WPB, WPC) and standard regulations.

Surface treatment:

Purpose: Remove scale, rust, oil, etc., improve appearance and corrosion resistance (especially for subsequent painting).

Common methods: Sandblasting (shot blasting) for rust removal.

Nondestructive testing:

Purpose: Detect internal and surface defects (cracks, inclusions, pores, incomplete penetration, etc.) to ensure product safety and reliability.

Common methods:

Magnetic particle testing: Detect surface and near-surface defects (ferromagnetic materials).

Penetrant testing: Detect surface opening defects (all materials).

Ultrasonic testing: Detect internal defects and wall thickness (especially weld areas).

Radiographic testing: Detect internal defects (especially welds, commonly used X-rays or gamma rays).

Dimensional inspection: Check whether the key dimensions of the elbow, such as angle, radius, end outer diameter, wall thickness, center-to-end distance, etc., meet the standards.

Hydraulic pressure test: Verify the pressure bearing capacity and sealing of the elbow (usually carried out at the pressure specified in the standard).

Groove processing: Prepare standard grooves (such as V-shaped and U-shaped grooves) for on-site welding.

Marking and packaging: Mark according to standard requirements (material, specification, pressure level, standard number, manufacturer, etc.), and package appropriately to prevent transportation damage.

The choice of core process depends on:

Diameter of elbow

Wall thickness (Wall Thickness / Schedule)

Bending radius (Center-to-End Dimension)

Bending angle (Degrees - 90°, 45°, 180°, etc.)

Production batch (Quantity)

Cost requirements (Cost)

Application standards and quality requirements (such as ASTM, ASME, EN, etc.)

In short, hot push is the mainstream and efficient process for small and medium-sized standard elbows; stamping is often used for small-diameter standard elbows; and welding forming is the key process to solve the manufacturing problems of large-diameter, non-standard or special-requirement elbows. Regardless of the core forming process used, subsequent heat treatment, non-destructive testing and strict dimensional/performance inspection are essential links to ensure the quality and safe operation of carbon steel elbows.