Most of the stainless steel we encounter in our daily lives is austenitic stainless steel (300 series, such as 304, 316). This includes the knives, forks, and thermos cups we use, as well as the railings and handrails of buildings, which are typically made of 304 or 316 stainless steel. Because these items are non-magnetic and we frequently handle them, we tend to assume that all stainless steel is non-magnetic.
However, this assumption is not accurate. Stainless steel isn’t always non-magnetic. Its magnetic properties depend on its microstructure and specific alloy type.
Stainless steel is classified into four main types: austenitic, ferritic, martensitic, and duplex steel. If you wish to learn more about their differences, you can click here.
We will discuss the magnetic properties of these different types of stainless steel.
Most tableware is austenitic 304 stainless steel
Austenitic Stainless Steel (300 Series): Magnetic Properties and Applications
Austenitic stainless steel, commonly found in grades 304 and 316, is typically non-magnetic. This is due to its face-centered cubic (FCC) crystal structure which prevents the formation of magnetic areas. These steels have higher levels of nickel and chromium, which help stabilize this structure and prevent magnetism from developing.
Applications: They are used in a wide range of applications including kitchen utensils, 304 stainless steel kitchen sink, medical equipment, stainless steel push button switch, and building structures.
Ferritic Stainless Steels (400 Series, such as 430): Magnetic Properties and Applications
Ferritic stainless steels have a body-centered cubic (BCC) crystal structure, similar to pure iron, which makes them magnetic. Due to the low nickel content, the BCC structure is not destroyed, so the magnetism is maintained.
Applications: They are commonly used in automotive parts, appliances, and industrial equipment.
The most direct difference between FCC and BCC crystals is in the atomic arrangements. The face-centered cubic structure has an atom at all 8 corner positions, and at the center of all 6 faces. The body-centered cubic structure has an atom at all 8 corner positions, and another one at the center of the cube. More information: What Is the Difference Between FCC and BCC?
Martensitic stainless steels (e.g., 410, 420) & Duplex stainless steels
Martensitic stainless steels (e.g., 410, 420)
Similar to ferritic steels, martensitic stainless steels have a BCC structure and are, therefore, magnetic. In addition, they can be hardened by heat treatment.
Applications: They are used for knives, surgical instruments, and tools.
Duplex stainless steels
Duplex stainless steel’s microstructure is a mixture of austenite and ferrite, so it is partially magnetic. Thus, it offers a balance of strength, corrosion resistance, and magnetic properties.
Applications: Used in chemical plants, marine environments, and the oil and gas industry.
Effects of Cold Working on the Magnetic Properties of Stainless Steels
It’s important to note that even usually non-magnetic austenitic stainless steels can become slightly magnetic if they undergo significant cold working, such as bending, stretching, or forming. This can cause a portion of the austenite structure to transform into a magnetic martensite structure.
Advantages and Disadvantages of Stainless Steel Magnetism
The magnetic properties of stainless steel have advantages and disadvantages in different applications, depending on the application requirements and the type of stainless steel used. Here are some advantages and disadvantages of magnetic stainless steel:
Advantages:
1. Identifiability of Magnetic Materials
Magnetic stainless steels, such as ferritic and martensitic stainless steels, can be easily identified by a simple magnet test, which is helpful for sorting and classification.
2. Equipment for Applications with Magnetic Requirements
Magnetic stainless steels are suitable for applications that require magnetism, such as certain types of sensors, magnetic separators, and appliances that must be fixed or suspended.
3. Lower Cost
Some magnetic stainless steels, such as 430-grade ferritic stainless steels, are generally less expensive than non-magnetic austenitic stainless steels, such as 304-grade, making them more cost-effective in certain budget-constrained applications.
4. Mechanical Properties
Martensitic stainless steels have high strength and hardness and are often used in tools and equipment that require wear and impact resistance, such as knives and surgical instruments.
Disadvantages:
1. Low corrosion resistance
Magnetic stainless steels (ferritic and martensitic) are generally less corrosion resistant than austenitic stainless steels (grades 304 or 316) and therefore perform poorly in corrosive environments.
2. Poor welding performance
Magnetic stainless steels generally have poor welding performance and are prone to welding defects, which may require unique welding processes or materials to overcome these problems.
3. Not suitable for specific high-cleanliness applications
Magnetic stainless steels absorb magnetic particles, which may introduce contamination risks in environments with high cleanliness requirements (such as pharmaceuticals and food processing).
4. Magnetic interference
In some applications (such as electronic equipment or precision instruments), magnetism may cause electromagnetic interference that affects the equipment’s normal operation.
Summary
The magnetic properties of stainless steel are primarily determined by its microstructure. Austenitic (300 series) stainless steels are generally non-magnetic due to their crystal structure and high nickel content. On the other hand, ferritic and martensitic stainless steels are magnetic due to their different crystal structures. It’s essential to understand these differences when selecting stainless steel for magnetic applications.
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FAQ
Not all stainless steels are attracted by magnets. Austenitic stainless steels (such as grades 304 and 316) are generally non-magnetic or only exhibit very weak magnetism. Ferritic stainless steels (such as grade 430) and martensitic stainless steels (such as grade 410) are magnetic and can be attracted by magnets.
The magnetism of stainless steel is related to its crystal structure. Ferritic and martensitic stainless steels have a body-centered cubic (BCC) crystal structure, which allows the formation of magnetic regions, so they are magnetic. Austenitic stainless steels have a face-centered cubic (FCC) crystal structure, which hinders the formation of magnetic regions, so they are generally non-magnetic.
304 stainless steel is an austenitic stainless steel with a face-centered cubic (FCC) crystal structure. This structure makes the atoms inside the material tightly arranged, and it is not easy to form magnetic areas, so it usually does not show magnetism.
Yes, austenitic stainless steel may partially transform into a martensitic structure after cold working (stretching, bending, or forging), introducing some magnetism. However, this magnetism is usually weak and less intense than ferritic or martensitic stainless steel.
You can use a simple magnet test to distinguish. If the stainless steel is strongly attracted to the magnet, it may be ferritic or martensitic stainless steel. It may be austenitic stainless steel if there is almost no attraction or it is fragile.
Not exactly. Although austenitic stainless steel (usually non-magnetic) generally performs better in corrosion resistance, some magnetic stainless steels, such as 430-grade ferritic stainless steel, also have good corrosion resistance in specific environments. However, martensitic stainless steel generally has poor corrosion resistance.
Magnetic stainless steels are widely used in applications that require magnetic properties, such as household appliance parts, automotive parts, knives, and some construction applications. Their hardness and strength make them suitable for these applications.
The main disadvantages of magnetic stainless steel are its poor corrosion resistance in some environments and its welding performance, which could be better than austenitic stainless steel. In addition, magnetism may cause electromagnetic interference in some applications.
In some applications, magnetism is necessary. For example, magnetic stainless steel is used in motors and sensors because they need to interact with magnetic fields. In addition, magnetic stainless steel is easy to recycle and sort through magnetic separation processes.
For austenitic stainless steel, if cold working introduces magnetism, it can be reduced or eliminated by annealing (heating to an appropriate temperature and then slowly cooling). This heat treatment can convert part of the martensite back to austenite, thereby reducing the magnetism.
