Stainless steel products are divided into stainless steel pipes, stainless steel plates, stainless steel coils, stainless steel round bars and stainless steel profiles.
Today we will first talk about distinguishing the quality of stainless steel pipes, which requires a comprehensive evaluation from multiple aspects such as material, process, appearance, and performance. The following are specific identification methods:
Material identification
Material label:
High-quality stainless steel pipes usually use labels such as 304 (06Cr19Ni10) and 316 (06Cr17Ni12Mo2), which are highly corrosion-resistant.
Poor-quality pipes may be impersonated with 201 (1Cr17Mn6Ni5N), which is easy to rust. It can be verified by material test reports or spectrometers.
Food grade or medical grade must comply with national standards such as GB/T 12771 (fluid pipes) or GB/T 14976 (structural pipes).
Magnetic test:
Austenitic stainless steel (such as 304, 316) is usually non-magnetic or weakly magnetic (may be slightly magnetic after cold working).
If the magnetism is strong, it may be ferritic stainless steel (such as 430) or inferior iron-doped material.
Appearance inspection
Surface finish:
The surface of high-quality pipes is uniform and smooth, without cracks, burrs, rust spots or obvious scratches.
Poor-quality pipes may have color differences caused by rolling defects, oxide scale or incomplete pickling.
Weld quality:
The weld of the welded pipe needs to be checked to see if it is flat and there is no false welding/leakage. High-end products will be leveled internally to ensure a smooth inner wall.
Process and thickness
Wall thickness uniformity:
Use a caliper to measure the wall thickness. Poor-quality pipes may have uneven wall thickness (such as one end thick and the other end thin), which affects the pressure bearing capacity.
Process mark:
Regular manufacturers will mark the brand, material, specification, implementation standard (such as ASTM A312, GB/T 21833), etc.
Chemical and physical properties
Corrosion resistance test:
Salt spray test: High-quality stainless steel pipes will not rust in a salt spray environment for 48 hours.
Simple home test: Spray the surface with salt water and observe whether it is rusted after 24 hours.
Strength test:
The tensile strength of high-quality pipes is ≥520MPa (taking 304 as an example), and low-quality pipes may be brittle or easily deformed.
Other identification methods
Price comparison:
The price of 304 steel pipes is significantly higher than that of 201. If the price is too low, be vigilant.
Certificate and source:
Ask for MSDS (Material Safety Data Sheet) and third-party test reports (such as SGS) to ensure environmental protection and non-toxicity (especially for food/medical fields).
Sound test:
Tap the steel pipe lightly, the sound of high-quality stainless steel is crisp and long, and the sound of low-quality pipes is dull.
Common characteristics of low-quality pipes
There are sand holes, bubbles or cracks on the surface.
The cut section is black or yellow (the material is impure).
No brand logo or blurred label.
Red rust appears after salt spray test (insufficient chromium content).
Application scenario suggestions
Food/medical: Choose 316 or 304L ultra-low carbon stainless steel to ensure that no heavy metals are precipitated.
Industry/Construction: Focus on thickness and compressive strength, and give priority to using precision-rolled pipes from large factories.
