304 stainless steel is inherently austenitic and non-magnetic. The presence of magnetism is primarily caused by the transformation of austenite into magnetic phases due to processing, or by compositional deviations. The specific reasons are detailed below:
- Cold Working During Pipe Manufacturing: Processes such as cold drawing, cold rolling, and bending after welding introduce stress. This stress causes the austenite to transform into martensite. Consequently, the magnetism increases with the intensity of the processing and is most noticeable at the pipe ends and bent sections.
- Welding Effects: Uneven cooling after the high temperatures of welding can cause local formations of ferrite or martensite, which are magnetic. Therefore, the weld seam and its surrounding areas typically exhibit stronger magnetism.
- Compositional Deviations: If the nickel and chromium content falls below the 304 standard, the austenite structure cannot be stabilized. This allows ferrite to precipitate naturally during production, resulting in magnetism throughout the entire pipe.
- Subsequent Processing: Secondary operations such as cutting and polishing introduce stress, which can also induce localized phase transformations and magnetism.
Magnetism should not be used as the sole criterion for judging 304 stainless steel pipes. Qualified 304 pipes are originally non-magnetic or only weakly magnetic. However, they are highly likely to become magnetic after cold working or welding, yet they remain genuine products, and magnetism does not affect their performance. Conversely, some counterfeit pipes(such as 201) are strongly magnetic, while others may mimic 304 with low magnetism. Relying solely on a magnet for testing can easily lead to misjudgment. The core criterion for identifying 304 stainless steel is its chemical composition ratio (Chromium ≥18%, Nickel ≥8%), not its magnetism.
