Stainless steel seamless elbows offer superior corrosion resistance and strength, ensuring precise dimensions and smooth surfaces. They are ideal for high-pressure and complex fluid transport, enhancing piping system safety and reliability.

I. Seamless processing technology

1. Hot Rolling (Extrusion) Process: A prevalent technique for producing seamless elbows. Initially, the round tube billet is heated to approximately 1200°C and pierced using a pressure piercing machine to form a preliminary tube billet. Subsequently, the billet undergoes further processing via three-roll cross-rolling, continuous rolling, or extrusion to achieve the desired dimensions and shape. This method is ideal for manufacturing large-diameter and thick-walled seamless elbows, offering high production efficiency. However, it demands stringent equipment and process control.

2. Cold Drawing Process: Suitable for crafting small-diameter, thin-walled seamless elbows. Hot-rolled pipes are cold drawn or cold rolled to enhance dimensional accuracy and surface quality. This method reduces the outer diameter without altering the pipe's wall thickness, thereby boosting the tube's strength and pressure resistance. Nonetheless, the cold drawing process requires high plasticity in raw materials and precise equipment, resulting in relatively higher production costs.

3. Bending Forming Process: Encompasses stretching, stamping, and roller methods. The stretching method involves adjusting the distance between fixed and adjustable rollers to induce plastic deformation in the pipe during bending, forming the required elbow. The stamping method utilizes a tapered core on a punch press to expand the pipe end to the desired size and shape. The roller method bends the pipe into shape through the combined action of 3-4 rollers. These methods are applicable for producing various seamless elbow types, such as long and short radius elbows.

II. Methods for testing seamless qualification

1. Appearance inspection

Surface quality: Check the appearance of the elbow piece by piece to ensure that the surface is smooth and free of oxide scale, and there shall be no obvious defects such as scarring, folding, delamination, etc. that are greater than 5% of the nominal wall thickness and the maximum depth is not greater than 0.8mm. You can use the naked eye to observe, or use a 5-20 times magnifying glass for detailed inspection.

2. Dimension detection

Dimension deviation: Use measuring tools such as vernier calipers to check with the corresponding standard tables to ensure that the dimensions of the elbow meet the standard requirements.

(1) Outer diameter measurement: Use a caliper or an outside diameter micrometer to measure the outer diameter of the elbow to ensure that it is within the specified tolerance range.

(2) Wall thickness measurement: Use equipment such as ultrasonic thickness gauges to measure the wall thickness of the elbow to check whether there is uneven wall thickness or less than the standard requirements.

3. Chemical composition analysis

Material verification: Use spectral analysis and other methods to detect the chemical composition of the elbow material to ensure that the content of its main elements such as chromium, nickel, carbon, etc. meets the standard requirements of stainless steel materials.

4. Mechanical property test

(1) Tensile test: Conduct a tensile test on the elbow to measure stress and deformation, and determine the strength (such as yield strength YS, tensile strength TS) and plasticity index (such as elongation after fracture A, cross-sectional shrinkage Z) of the material.

(2) Impact test: Conduct an impact test to detect the impact toughness of the elbow material to ensure that it is not easy to break when subjected to impact load.

(3) Hardness measurement: Use a hardness tester to measure the hardness value of the elbow to evaluate whether its hardness level meets the use requirements.

5. Nondestructive testing

(1) Magnetic particle testing: It is suitable for detecting cracks and defects on the surface and near the surface of the elbow. Defects are identified by magnetic traces formed by magnetic particles gathering at the defect.

(2) Ultrasonic testing: Use the reflected waves generated by ultrasonic waves when they propagate inside the material and encounter defects to detect defects and cracks inside the elbow.

(3) Radiographic testing: Including X-ray testing and gamma-ray testing, the rays penetrate the elbow and form an image on the film to check internal defects.

6. Pressure test

(1) Hydraulic test: Conduct a hydraulic test on the elbow to check its sealing and pressure resistance under a certain pressure to ensure that no leakage occurs during actual use.

(2) Air pressure test: Conduct an air pressure test in a pressure vessel that needs to be sealed, inject gas equivalent to 1.25 to 1.5 times the working pressure, maintain for a period of time, and observe the pressure drop and external leakage

7. Other tests

(1) End bevel test: Since most elbow fittings are connected by welding, it is necessary to check whether the shape and size of the end bevel meet the specified requirements to improve the welding quality.

(2) Low temperature toughness impact test: For elbow fittings with special requirements, low temperature toughness impact test is also required to evaluate their performance under low temperature conditions.

(3) Flattening test: Flatten the seamless elbow to a certain size to check whether it has cracks or cracks;

(4) Expansion test: Expand the seamless elbow to a certain angle to check whether it has cracks or cracks.