Most people don’t know the importance of stainless steel and how many diverse applications it has. If you ask people about its magnetism, you will likely receive different responses. Some believe the stainless steel must be magnetic, while some argue that it is a completely nonmagnetic material.
Well, the answer lies in between. There are different types of stainless steel — some are magnetic while others are not.
But why does it matter?
Magnetic properties of a material have a huge impact on the material’s performance and applications. For example, electric current behaves a bit differently near strong magnetic materials. Also, magnetism can complicate material fabrication and other processes like welding.
Below, we have explained different types of stainless steel and what makes them magnetic. To answer this question better (is stainless steel magnetic or not), we have explained everything in detail along with examples. Let’s start with the basics.
Table of Contents
What Exactly Is Stainless Steel?
Stainless steel is an alloy of iron containing at least 10.5% chromium. It may also contain various metals and nonmetals, such as aluminum, titanium, nickel, copper, carbon, nitrogen, phosphorus, or sulfur. These elements are added to increase oxidation resistance, corrosion resistance to specific environments, and impart exceptional characteristics.
Because of these special characteristics, stainless steel is widely used to create a range of products, from kitchen sinks and auto bodies to surgical tools and medical equipment. It is also used in paper, chemical, petrochemical, jewelry, 3D printing, and food and beverage industries.
Things That Make Stainless Steel Magnetic
For stainless steel to exhibit magnetic behavior, it must fulfill the following criteria —
- The alloy contains iron.
- The crystal structure of the alloy should be arranged in a ferrite or martensite pattern.
Since all types of stainless steel have iron, whether or not they are magnetic comes down to the microstructure of the alloy.
Different Types of Stainless Steel and Their Magnetism
There are over 100 grades of stainless steel. Most of them can be grouped into five categories: Austenitic, Ferritic, Martensitic, Duplex, and Precipitation hardening. Each has unique properties and advantages.
1. Austenitic
Credit: ASM International
Nonmagnetic in nature
Austenitic stainless steel has a face-centered cubic crystalline structure. This structure is obtained by adding austenite-stabilizing elements like nickel, nitrogen, and manganese.
It contains about 11% chromium, 8-12% nickel, carbon, nitrogen, and several other elements. While chromium makes stainless steel highly corrosion resistant, nickels enhance its formability, weldability, and ductility. Sometimes, nitrogen is used to add stiffness.
Since this type of steel possesses the same microstructure at all temperatures, it is neither hardened by heat treatment nor influenced by an external magnet.
It is the largest family of stainless steel, making up about 65% of all stainless steel production.
Examples
They are divided into two subgroups:
- 200 series includes 201, 202, and 205 grade
- 300 series includes 302, 304, and 316 grade
The 200 series austenitic stainless steel contains manganese and nitrogen, while 300 series steel achieves its austenitic structure due to the addition of nickel.
Because they are nonmagnetic, they are widely used in applications where magnetism is undesirable, such as medical centers with MRI scanners.
2. Ferritic
Magnetic in nature
Ferritic stainless steels are known for their corrosion resistance, ductility, and magnetic properties. They have a body-centered cubic crystal structure, containing 11-27% chromium and small amounts of ferrite stabilizers like titanium and niobium.
The alloy can retain its ferromagnetic properties up to a temperature called Curie point. Based on chemical composition, this temperature could range from 650 °C to 750 °C. Above this point, the alloy loses its permanent magnetic characteristics.
Like austenitic stainless steel, it cannot be strengthened through heat treatment. However, it can be cold worked and softened via annealing.
Examples
The majority of the AISI 400 series stainless steels are ferritic, including the Grade 409, 410S, 430, 434, 444, and 446.
Because of their higher thermal conductivity and lower thermal expansion coefficient than austenitic steels, they are more suitable for heat transfer applications such as solar heaters and cooking utensils. A few grades contain higher chromium and molybdenum content, which makes them ideal for corrosive seawater applications.
The 410S is one of the most popular stainless steel grades. It is widely used in automotive exhaust components, quenching racks, and mining equipment.
3. Martensitic
Grade 410 stainless steel
Magnetic in nature
Martensitic stainless steel can be tempered and hardened through numerous aging or heat treatment processes. These processes make steel stronger as well as magnetic in both annealed and hardened conditions.
Martensitic steels may have low- or high-carbon steels built around the composition of iron. The ones containing up to 0.4% carbon are used for their excellent mechanical characteristics (shafts, pumps, valves, etc.), and the ones with more than 0.4% carbon are used for their wear-resistant capabilities (plastic injection molds, cutlery surgical blades, etc.).
Examples
Grade 410 is referred to as general-purpose martensitic steel. Their applications include pump rods, valves, ball bearings, turbine blades, shafts, nuts, pins, springs, and micrometer parts.
Grade 420 stainless steel contain 0.15%-0.45% carbon content. Thus, they have a broad range of hardness scales in both hardened and tempered conditions. They are used to create valve trim, cutlery, fasteners, machine parts, and surgical tools.
Grade 440 steel have higher hardening capability than Grade 410 and 420. However, they have limited formability in the annealed condition. They are widely used to make high-quality knife blades, chisels, and surgical equipment.
4. Duplex
Magnetic in nature
The metallurgical structure of the Duplex stainless steel contains two phases — austenite (face-centered cubic lattice) and ferrite (body-centered cubic lattice) in approximately equal measure. They are built to provide increased strength, higher toughness, and more resistance to various types of corrosion.
Their magnetic properties come from ferrite stainless steel. More specifically, they contain 20-28% chromium, up to 9% nickel, up to 5% molybdenum, and 0.05%-.050% nitrogen. The low nickel content gives substantial cost benefits.
Examples
Grade Duplex 2205, 2304, and 2507
But since duplex stainless steels have poor formability, inefficient machinability, and require complex production techniques, they are used mostly for niche applications. The common industrial applications include oil and gas exploration, chemical transport and storage, and pulp and paper manufacturing.
5. Precipitation hardening
Most are magnetic in nature
The precipitation hardening stainless steels are a family of corrosion-resistant alloys, which can be heat treated to provide exceptional tensile and yield strengths (up to 1700MPa and 1500MPa, respectively). The hardening is achieved by the addition of titanium, aluminum, copper, and/or molybdenum.
Based on their final microstructures after heat treatment, the alloys can be classified into one of three groups: austenitic, semi-austenitic, and martensitic.
All martensitic and the majority of precipitation hardenable stainless steels are ferromagnetic. Because of the stress-induced during the hardening processes, they exhibit permanent magnetic characteristics.
These steels are used in aerospace, petroleum, and nuclear industries where a combination of high corrosion resistance, high strength, and a medium level of toughness is required.
Conclusion
While stainless steels are made of iron, not all of them exhibit magnetic properties. Austenitic stainless steel, for instance, is not influenced by any external magnet.
Ferritic stainless steel can retain its ferromagnetic properties up to a temperature called Curie point (750 °C), whereas martensitic steels are magnetic in both annealed and hardened conditions.
And since duplex stainless steel contains both austenite and ferrite steel in approximately equal proportion, they are magnetic in nature.
Recent Studies
Magnetic properties of stainless steel at room and cryogenic temperatures
A team of researchers at the College of the Holy Cross describes the magnetic characteristics of 10 types of martensitic and ferritic steel at different temperatures. They found that cooling these alloys to 77 Kelvin results in stronger magnetic properties. Plus, the alloys’ permeability was reduced by 8%, and the coercive force increased by 14%.
The research shows that austenitic stainless steel (SS-316), when exposed to liquid sodium, can become partially ferromagnetic. This happens due to open volume defects — Nickel vacancies that occur at the surface.
Magnetic behavior of 316L stainless steel doped with Ti–B–C nanocrystals
In this study, researchers investigated how the magnetic properties of 316L steel change when it is doped with nanocrystalline TiC, TiB2, and B4C powders. The outcomes demonstrate the possibility of developing new steel alloys with desired properties.
Surface crack of 304 stainless steel under natural magnetic field
Researchers at the Nanchang Hangkong University in China used multiclass classification support vector machines to identify quantitative defects on 304 stainless steel under a natural magnetic field. They analyzed samples before and after annealing. The results show an obvious upward convex magnetic anomaly.
More To Know
Can Metal Detector Detect Stainless Steel?
A decent-quality metal detector can identify various types of metals, including ferrous, non-ferrous, and stainless steel. However, stainless steels are the hardest to detect. A few very high-end steel (that are used to make medical instruments) are neither magnetic nor conductive, so your detector might not see it.
Grade 302, 304, and 316 are hardly detectable unless they are in a big size. Ferrous and Martensitic stainless steels, on the other hand, are the easiest to find — simply because they are magnetic.
Common Properties of Stainless Steel
While there are several different types of stainless steel built for a range of applications, they all share some common properties:
- High tensile strength
- Corrosion resistant
- Temperature resistant
- Durable
- Long Lasting (low maintenance)
- Recyclable
Global Stainless Steel Market Size
According to the Grand View Research report, the global stainless steel market size will exceed $223.60 billion by 2030, growing at a CAGR of 8.9% from 2022 to 2030.
The public and private investments in infrastructure, as well as residential housing, are expected to support the market growth during the forecast period. Moreover, the rising innovation in electric vehicles and increasing technological advances in the automotive industry will also fuel the stainless steel market size.
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