How to determine the quality of 304 stainless steel pipes and 316L stainless steel pipes

To determine the quality of 304 and 316L stainless steel pipes, one needs to consider six aspects: material composition, appearance and surface condition, dimensional accuracy, mechanical properties, corrosion resistance, and process verification. The details are as follows: 

1. Material composition verification

1. Spectral analysis: Use a portable spectrometer to detect the element content and compare with the national standard (GB/T 24511):

304: C ≤ 0.08%, Cr = 18 - 20%, Ni = 8 - 10.5%, and there should be no excessive S and P impurities (S ≤ 0.03%, P ≤ 0.045%). 

316L: C ≤ 0.03%, Cr = 16 - 18%, Ni = 10 - 14%, Mo = 2 - 3%. The key point is to confirm whether the Mo content meets the standard (lack of Mo will significantly reduce the corrosion resistance). 

2. Spark identification: 316L contains Mo, so the tail end of its spark is shorter and has fewer branches. 304 has a slightly longer tail end and a slightly lower brightness (comparison with standard samples is required). 

II. Appearance and Surface Quality

1. Visual Inspection:

The surface of the high-quality pipe should be smooth without cracks, folds, scars, or bubbles. The welds (if any) should be uniform and full, without undercut (depth ≤ 0.5mm), incomplete fusion, or other defects. 

The inner wall of the 316L pipe may show a slight grayish-black color due to the oxidation of the Mo element, but there should be no oxide layer or severe rust (the surface of 304 should have a silvery-white metallic luster). 

2. Acid washing test: Wipe the surface with a 5% hydrochloric acid solution. For 304, there should be no obvious corrosion marks; for 316L, which contains Mo, the corrosion resistance is stronger and the corrosion rate should be more than 30% lower than that of 304. 

III. Dimension Accuracy Inspection

1. Outer Diameter and Wall Thickness: Measured using a vernier caliper. The deviation must comply with the GB/T 17395 standard:

For outer diameters ≤ 50mm, the allowable deviation is ±0.3mm; for outer diameters > 50mm, ±0.8% of the outer diameter. 

When the wall thickness is ≤ 4mm, the deviation is ±0.3mm; when the wall thickness is > 4mm, it is ±7.5% of the wall thickness (for 316L, due to the high difficulty in smelting, the tolerance for wall thickness may be slightly stricter than that of 304). 

2. Straightness: Use a ruler to align with the pipe wall. The curvature per meter should be ≤ 1.5mm, and the total curvature throughout the entire length should be ≤ 0.15% of the total length. 

IV. Mechanical Performance Testing

1. Tensile Test:

304: Yield strength ≥ 205 MPa, tensile strength ≥ 520 MPa, elongation ≥ 40%.

316L: Yield strength ≥ 170 MPa (due to low carbon content, the strength is slightly lower than that of 304), tensile strength ≥ 480 MPa, elongation ≥ 40%. 

2. Hardness test: The test is conducted using a Rockwell hardness tester (HRB). The hardness values of 304 and 316L should be ≤ 90 HRB. If the hardness of the annealed tubes is > 100 HRB, there may be processing hardening or improper heat treatment. 

V. Corrosion Resistance Verification

1. Intergranular Corrosion Test (GB/T 4334):

After the 304 tube is immersed in oxalic acid, the microstructure should have no intergranular corrosion grooves; for 316L, due to its ultra-low carbon design, the intergranular corrosion tendency is significantly lower than that of 304. The weight loss rate after the test should be less than 0.5g/m². 

2. Pitting corrosion test: The sample is immersed in a 6% FeCl₃ solution for 24 hours. The pitting depth of 304 should be ≤ 0.1mm, while for 316L (which contains Mo), the pitting depth should be ≤ 0.05mm (a difference of approximately 50%). 

VI. Process Performance Inspection

1. Compression Test (GB/T 241): The specimen is compressed to 1/3 of its outer diameter. No cracks shall occur at the weld seam (The ductility of 316L is slightly better, and the incidence of cracks after compression is 10% to 15% lower than that of 304). 

2. Expansion test: Use a 45° punch to expand the opening to an outer diameter of 1.2 times the original size. Both 304 and 316L should show no cracks during the expansion process. Due to the presence of Mo in 316L, it has a stronger plastic deformation ability during the expansion. 

Quick identification techniques

Magnetic test: Both are austenitic stainless steel. They have no magnetism under normal conditions, but after cold processing, a weak magnetism may be generated (the magnetic property of 304 cold-drawn pipe is slightly stronger than that of 316L). If there is strong magnetic attraction, it may indicate inferior material. 

Solution testing: Add "304/316L testing solution". 304 appears colorless or light red, while 316L (due to the presence of Mo) shows a deep blue color (the color change should be observed within 10 seconds). 

Summary

High-quality stainless steel pipes must meet the component standards (with a particular focus on Cr, Ni, Mo and C contents), have a smooth surface, strict dimensional tolerances, and comply with the standards for mechanical and corrosion resistance properties. For 316L pipes, it is necessary to specifically confirm whether the Mo element is in sufficient quantity (spectroscopic analysis shows that Mo should be ≥ 2%), while for 304 pipes, attention should be paid to whether the carbon content exceeds the limit (C ≤ 0.08%). During procurement, suppliers should be requested to provide material certificates (including spectroscopic reports), mechanical performance test reports, and intergranular corrosion test reports. In necessary cases, a third-party testing institution can be entrusted for retesting.