Why does stainless steel sometimes exhibit magnetic properties? Let’s hear what a heat‑treatment specialist has to say.

23 Aug,2019

Why does stainless steel sometimes exhibit magnetic properties? Let’s hear what a heat‑treatment specialist has to say.
　　People often assume that if a magnet sticks to stainless steel, it proves the material is genuine and of good quality—no magnetism means it’s authentic and trustworthy; if it does stick, they conclude it must be counterfeit. In reality, this is an extremely one-sided, impractical, and flawed method of assessment.
　　Stainless steels come in a wide variety; at room temperature, they can be classified into several types based on their microstructure:
　　1. Austenitic grades: such as 304, 321, 316, 310, etc.;
　　2. Martensitic or ferritic types: such as 430, 420, 410, etc.;
　　Austenitic steels are non-magnetic or weakly magnetic, while martensitic and ferritic steels are magnetic.
　　Stainless steel commonly used for decorative tube sheets is typically the austenitic 304 grade, which is generally non‑magnetic or weakly magnetic. However, fluctuations in chemical composition due to smelting or variations in the processing condition can sometimes result in magnetism. This does not indicate that the material is counterfeit or substandard—so what accounts for this?
　　As mentioned above, austenite is non-magnetic or weakly magnetic, whereas martensite and ferrite are magnetic. Due to compositional segregation during smelting or improper heat treatment, small amounts of martensitic or ferritic phases may form in austenitic 304 stainless steel. Consequently, the 304 stainless steel will exhibit a faint magnetic response.
　　In addition, when 304 stainless steel undergoes cold working, its microstructure can transform into martensite. The greater the degree of cold deformation, the more extensive the martensitic transformation, and the stronger the steel’s magnetic properties. For example, with steel strips of the same grade, when used to produce Φ76 tubes, no significant magnetism is observed; however, when used to produce Φ9.5 tubes, the magnetism becomes noticeably stronger due to the larger cold‑bending deformation. Similarly, when manufacturing square or rectangular tubes, the deformation is greater than that of round tubes—particularly at the corners—resulting in even more pronounced magnetism.
　　To completely eliminate the magnetism of 304 stainless steel arising from the aforementioned causes, a high-temperature solution treatment can be employed to restore a stable austenitic microstructure, thereby removing the magnetic properties.
　　It is particularly worth noting that the magnetism of 304 stainless steel, arising from the aforementioned reasons, is entirely different in magnitude from that of other stainless steels, such as 430, and from carbon steel; in other words, 304 steel consistently exhibits only weak magnetic properties.
　　This tells us that if the stainless steel strip exhibits weak magnetism or no magnetism at all, it should be classified as 304 or 316 grade; if, like carbon steel, it displays strong magnetism, it can be determined not to be 304 grade.
　　Heat treatment cannot account for the non-magnetic nature.
　　The non‑magnetic nature of stainless steel cannot be explained from a heat‑treatment perspective, since the outcome remains unchanged regardless of whether heat treatment is applied. Magnetic properties can only be accounted for in physical terms. In the world, materials are classified into three categories based on their magnetic behavior: ferromagnetic, paramagnetic, and diamagnetic substances.
　　The statement that stainless steel is non‑magnetic is incorrect; rather, stainless steel is paramagnetic, not ferromagnetic. As a ferritic steel, its magnetic transition occurs at 770°C, while cementite undergoes this transition at 230°C. In the iron–carbon phase diagram, these temperatures correspond to the A2 line (or point) and the A0 line (or point), respectively. The temperature at which a material loses its ferromagnetism and becomes paramagnetic is known as the Curie point, a property intrinsic to both pure iron and its carbon‑containing compound, cementite. This physical characteristic cannot be altered by heat treatment. Thus, the Curie point marks the temperature at which ferromagnetism gives way to paramagnetism. Above 770°C, iron transforms into gamma‑iron, commonly referred to as austenite; consequently, austenitic stainless steels are often described as “non‑magnetic” (though they are in fact paramagnetic). Therefore, any steel with an entirely austenitic microstructure is considered “non‑magnetic.” Among stainless steels, only austenitic grades exhibit this “non‑magnetic” behavior. Similarly, wear‑resistant steel ZGMn13, though not a stainless steel, is also “non‑magnetic” because it exists in an austenitic microstructure at room temperature.

Previous:

A Brief Analysis of Ultrasonic Testing Methods for Stainless Steel Pipes

Next:

Applications of Stainless Steel Pipes in the United States