Stainless steel elbow inspection method

Stainless steel elbows are key connectors in the pipeline system, and their quality directly affects the safety, sealing and corrosion resistance of the system. The following is a systematic inspection method for stainless steel elbows, covering five core dimensions: appearance, material, performance, size and non-destructive testing:

I. Appearance inspection

1. Visual inspection

Surface defects: Check whether there are cracks, folds, sand holes, scratches or oxide scale residues on the inner and outer walls of the elbow (especially the weld area).

Weld quality: The weld should be uniform and continuous, without unfused, undercut, weld nodules or pores (use a 10x magnifying glass to assist observation).

Polishing: In food-grade or high-cleanliness occasions, it is necessary to check whether the surface has achieved a mirror polishing effect of Ra≤0.8μm.

2. Coating/passivation layer detection

Blue spot test: Use potassium ferrocyanide solution to detect the pickling passivation effect. If blue spots appear, it means that the passivation film is incomplete.

Salt spray test (sampling): Perform a 72-hour salt spray test according to ASTM B117 standard. There should be no red rust or corrosion spots on the surface.

2. Material composition verification

1. Spectral analysis (OES)

Use a handheld spectrometer to quickly detect material composition (such as 304 must meet C≤0.08%, Cr 18-20%, Ni 8-10.5%).

Key indicators: Mo content (316L contains 2-3% Mo), low carbon requirements (304L/316L C≤0.03%).

2. Chemical titration method (arbitration in case of disputes)

According to the GB/T 223 series standards, sample and test to accurately determine the content of trace elements such as C, S, and P.

3. Mechanical properties test

1. Hardness test

Rockwell hardness (HRB/HRC): The hardness of austenitic stainless steel elbows should be ≤90 HRB (avoid excessive cold hardening).

Portable Leeb hardness tester: suitable for on-site rapid sampling, comparing the standard value to determine whether it is qualified.

2. Tensile test (destructive sampling)

According to ASTM A370 standard, sample and test tensile strength (≥515 MPa), yield strength (≥205 MPa) and elongation (≥35%).

3. Flattening test (special requirements)

Flatten the thin-walled elbow until the inner wall spacing is 3 times the wall thickness, and observe whether cracks or delamination occur.

IV. Dimension and shape tolerance detection

1. Geometric dimension measurement

Angle tolerance: Use an angle ruler or 3D scanner to verify the elbow angle (90° elbow allows ±0.5° deviation).

Wall thickness uniformity: Ultrasonic thickness gauge multi-point detection, wall thickness deviation should be ≤±10% (high-pressure system requires ≤±5%).

End bevel: Check the bevel angle (usually 30°±5°) and blunt edge thickness (1-2mm).

2. Roundness and ovality

Use a caliper or a three-dimensional coordinate measuring machine to detect the roundness of the port (deviation ≤1% of the nominal diameter) to avoid sealing failure during installation.

V. Nondestructive Testing (NDT)

1. Liquid Penetrant Testing (PT)

Spray the penetrant after cleaning the surface, and observe the surface opening defects (such as cracks and pores) after imaging. It is applicable to all materials.

2. Ultrasonic Testing (UT)

Detect internal defects (slag inclusions, incomplete penetration), focus on scanning the weld and the curvature change area of the elbow, and determine the defect level according to the ASME B16.9 standard.

3. Radiographic Testing (RT) (High-voltage Key Parts)

X-ray or gamma-ray transillumination to detect volume defects (pores, slag inclusions), and the film evaluation complies with the ISO 10675-1 standard.

4. Eddy Current Testing (ET)

Quickly screen surface and near-surface defects, suitable for rapid inspection of large quantities of thin-walled elbows.

VI. Additional tests for special working conditions

1. Intergranular corrosion test (sensitization risk)

Sulfuric acid-copper sulfate method (ASTM A262 Practice E): Observe whether cracks appear on the surface after bending the specimen to determine the material's resistance to intergranular corrosion.

2. Metallographic analysis

Observe the grain size (ASTM E112 standard, austenitic stainless steel usually requires 5-8 levels) and inclusion level (≤1.5 level) under a microscope.

3. Pressure test

Hydraulic pressure test: 1.5 times the design pressure for 10 minutes, no leakage or deformation;

Air pressure test (use with caution): soapy water is required to detect tiny leaks.

VII. Common problems and treatment suggestions

1. Weld cracks

Reason: excessive welding heat input or too fast cooling.

Treatment: repair welding after grinding, and PT/UT testing needs to be performed again after repair welding.

2. Local corrosion

Reason: passivation layer damage or excessive chloride ion concentration.

Treatment: Rework pickling and passivation or replace with higher corrosion-resistant materials (such as duplex steel).

3. Dimensional deviation

Reason: Improper cold bending process control or mold wear.

Treatment: Scrap or negotiate concession acceptance (non-pressure-bearing parts).

Summary: The inspection of stainless steel elbows needs to be combined with "visual + instrument + destructive + non-destructive" multi-means linkage to strictly control the material, process and dimensional accuracy. For high-pressure, corrosive or food-grade application scenarios, it is recommended to add special tests such as intergranular corrosion and polishing roughness to ensure reliability throughout the life cycle.

Cangzhou Aosendic Pipe Fitting Manufacturing Co., Ltd. is a large manufacturer specializing in the production of elbows, tees, reducers, elbows and caps. We can implement ASTM, ANSI, JIS, DIN, GB and other standards, basically covering all types of steel.

If you have any purchasing needs or questions, please contact us.

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