Many people think that stainless steel is not magnetic, and often use magnets to identify whether the product is stainless steel. This method of judgment is actually unscientific.
Stainless steel can be divided into two categories according to the structure at room temperature: austenite and martensite or ferrite. The austenitic type is non-magnetic or weakly magnetic, and the martensite or ferritic type is magnetic. At the same time, all austenitic stainless steels can be completely non-magnetic only in a vacuum state, so the authenticity of stainless steel cannot be judged by a magnet alone.
The reason why austenitic steel is magnetic: austenitic stainless steel itself has a face-centered cubic crystal structure, and the surface of the structure is paramagnetic, so the austenitic structure itself is not magnetic. Cold deformation is the external condition that turns part of the austenite into martensite and ferrite. Generally speaking, the deformation amount of martensite increases with the increase of the cold deformation amount and the decrease of the deformation temperature. That is to say, the larger the cold working deformation, the more martensitic transformation and the stronger the magnetic properties. Hot-formed austenitic stainless steels are almost non-magnetic.
Process measures to reduce permeability:
(1) The chemical composition is controlled to obtain a stable austenite structure and adjust the magnetic permeability.
(2) Increase the material preparatory treatment sequence. If necessary, the martensite, δ-ferrite, carbide, etc. in the austenite matrix can be re-dissolved by solid solution treatment to make the structure more uniform and ensure that the magnetic permeability meets the requirements. And leave a certain margin for subsequent processing.
(3) Adjust the process and route, add a solution treatment sequence after molding, and add a pickling sequence to the process route. After pickling, conduct a magnetic permeability test to meet the requirement of μ
(5) Choose suitable processing tools and tool materials, and choose ceramic or carbide tools to prevent the magnetic permeability of the workpiece from being affected by the magnetic properties of the tool. In the machining process, a small cutting amount is used as much as possible to minimize the occurrence of martensitic transformation induced by excessive compressive stress.
(6) Degaussing of finishing parts.
Post time: Sep-26-2022
