There are a lot of misconceptions about stainless steel. Many people think that all stainless steel is magnetic, but this isn’t the case. In fact, there are five different classes of stainless steel, and only some of them are magnetic.
In this article, we will discuss the different types of stainless steel and their respective levels of magnetism. We will also answer some common questions about stainless steel and its magnetic properties.
So if you’re curious about whether your stainless steel appliances are magnetic or not, keep reading!
Stainless Steel – Introduction:
Stainless steel is an alloy of iron, carbon, and chromium. It is a popular material for many applications because it is durable, corrosion-resistant, and has a low Maintenance cost.
Stainless steel is an alloy made up of metals such as iron and chromium.
The main reason why it does not corrode or rust easily is because of the elements present in it: iron, chromium, silicon, carbon, nitrogen and manganese. According to standards, stainless steel must contain 10.5% chromium at the least and 1.2% carbon at most.
Role of Chromium in Stainless Steel:
Chromium is the element that makes stainless steel rust resistant. When exposed to oxygen, chromium forms a thin layer of chromium oxide on the surface of the steel. This invisible layer protects the steel from corrosion and oxidation.
The minimum amount of chromium in stainless steel should be 10.5%. If the chromium content is less than 10.5%, the steel will not be fully protected from rust and corrosion.
Role of Nickel in Stainless Steel:
Nickel is another element present in stainless steel. It increases the corrosion resistance of the alloy by forming a protective layer on its surface. Nickel also makes stainless steel more ductile and less likely to crack under stress.
The minimum amount of nickel in stainless steel should be 8%. If the nickel content is less than 8%, the steel will not be fully protected from rust and corrosion.
Types of Stainless Steels:
There are five different types of stainless steel, and each one has its own set of properties.
1. Austenitic Stainless Steel:
Austenitic stainless steel is the most common type of stainless steel. It contains high levels of chromium and nickel, making it extremely corrosion-resistant. Austenitic stainless steel is non-magnetic.
2. Ferritic Stainless Steel:
Ferritic stainless steel contains high levels of chromium, but low levels of nickel. It is magnetic and less corrosion-resistant than austenitic stainless steel.
3. Duplex Stainless Steel:
Duplex stainless steel is a combination of austenitic and ferritic stainless steel. It contains high levels of chromium and nickel, making it more corrosion-resistant than either austenitic or ferritic stainless steel. Duplex stainless steel is also magnetic in nature, however, its not as magnetically strong as ferritic stainless steel.
4. Martensitic Stainless Steel:
Martensitic stainless steel is a type of stainless steel that BCT structure in its matric which offers magnetic properties. AISI 410 is most well known example.
Martensitic stainless steel contains iron, which gives it a ferromagnetic crystal structure. The magnetism in martensitic steel comes from the high iron content.
Martensitic stainless steel is very strong, but has reduced chemical resistance compared to austenitic stainless steel because carbon is trapped in its crystals.
5. Precipitation Hardened Stainless Steel:
Precipitation hardened stainless steel is a variety of regular stainless steel that has been heat-treated to make it stronger and harder. It’s magnetic and highly resistant to corrosion.
The mechanical properties of precipitation hardened stainless steel are obtained from the presence of a second phase, which can be austenitic (17-10 PH) nonmagnetic, semi-austenitic (17-7 PH) slightly magnetic or martensitic (17=4 PH) Magnetic.
From the looks of the above types, it is visible that Austenitic stainless steel are mostly non-magnetic and found majorly in practical applications.
While magnetic properties are observed in ferritic and martensitic stainless steels.
Steel is Ferrous metal, however some stainless steel types are not attracted to magnet. Why?
The answer is in the element composition of stainless steel. As stated above, nickel is an austenitic stabilizing ingredient while chromium is a ferritic stabilizing ingredient. With higher nickel, microstructure will be austenitic and alloy will be non magnetic.
Austentic structure as discussed above has non-magnetic nature due to FCC Crystal structure. If Stainlesss steel of your choice has austentic stabilized at home temperature, than it will have high probability of non magnetic crystal structure.
If we talk interms of grade of steel, you will also be able to see in image, than 300 grades have most of austentic matric resultantly show poor magnetism. While, on the other hand, 400 series are either BCT or BCC matric structure giving very strong pull towards to magnet.
What makes stainless Steel magnetic or Non-magnetic?
The terms chromium equivalent and nickel equivalent are defined for stainless steels. As you might know, chromium (and hence the Cr-equivalent) is a ferrite stabilizer while Nickel (and hence the Ni-equivalent) is an austenite stabilizer.
The ratio of these two determines the ferrite content of stainless steel. The ferrite content can also be known as the ferrite number (almost equivalent toferrite content in practical range). It can be determined by using Schaeffeler diagram as shown below:
Depending upon that ferrite number, you can control magnetic properties of stainless steel. Ferrite content can be measured either from microstructure or using a ferritoscope.
How does nickel make stainless steel not magnetic?
In general Iron has unpaired electrons in the outermost shell and presence of these unpaired electrons in positive magnetic moment. Nickel is not magnetic in general, and also, if they are present in sufficient quantity in Iron, they will fill those unpaired electron shells, resultantly reduced magnetic moment.
How to identify magnetism characteristic of Stainless steel in Quick Manner?
The different types of stainless steel can be distinguished by their magnetic properties; if a permanent magnet is brought close to the material, ferritic and martensitic steels will stick to it while austenitic steels will not. Precipitation-hardened stainless steel falls between these two extremes. To quickly identify the magnetic properties of stainless steel, bring a permanent magnet close to the material; if it sticks, the steel is ferro-magnetic, if it doesn’t, the steel is non-magnetic.
Is medical grade stainless steel magnetic?
Medical implants are often made from non-magnetic or austenitic grade of stainless steel because it’s important that they be MRI compatible. The addition of nickel changes the structure and properties of stainless steel, making it non-magnetic. However, some medical grade stainless steels are slightly magnetic or martensitic (17-4 PH).
Why are surgical instruments made of stainless steel?
Surgical instruments are often made of ferritic or magnetic stainless steel because it’s important that they be sterilizable. The presence of chromium makes the steel more resistant to rust and corrosion.
Why are cooking utensils often made of stainless steel?
This all depends on the type of stainless steel pan you have. Many are not magnetic and therefore will not work with induction cook-tops, but some come with a magnetic disc in the center layer. You’ll be able to tell if it’s compatible by checking for an induction symbol or looking at the manufacturer’s instructions.
All cast iron works as well as carbon steel pans with induction cook-tops. The only problem comes with woks since the flat bottom heats up but not the sides–woks simply work better over gas or an open fire.
Why aren’t stainless steel countertops, like in restaurant kitchens, magnetic, but stainless steel knives?
This has to do with the different grades of stainless steel. The countertops are likely made from a non-magnetic or austenitic grade while the knives are made from a ferritic or magnetic grade.
The addition of nickel changes the structure and properties of stainless steel, making it non-magnetic. However, some grades of stainless steel are slightly magnetic or martensitic (17-4 PH).
So, if you’re looking for a completely non-magnetic surface, austenitic grades are your best bet. If you don’t mind a little bit of magnetism, then ferritic grades will work just fine.
Why do austenitic stainless steel become magnetic after cold working?
Austenitic stainless steels can become magnetic after cold working because of transformation of austenitic structure in BCT tetragonal structure called martensite. The martensitic phase is not completely magnetic but it is attracted to the magnet. The more cold work, the more magnetic the austenitic stainless steel becomes.
Is magnetic stainless steel safe for cooking?
Yes, magnetic stainless steel is safe for cooking. The only caveat is that you need to make sure that the cookware is made from quality materials and is free of any impurities. Additionally, it’s important to properly clean, dry, and season the cookware before each use.
Though iron is not poisonous, too much of it in the body can be dangerous. This usually comes from rusting containers. But even if those containers are washed and dried before use, they may still contain traces of Appropriate elements which can be harmful.
If you are looking for a healthier option in kitchen materials, choose utensils made with pure iron, without any harmful heavy elements mixed in. Unfortunately, some companies cut corners by using cheap iron that contains lead or other contaminants – so be sure to check the purity of your stainless steel before use.
Is 303 stainless steel magnetic?
303 belongs to austenitic stainless steel. It has high toughness and good corrosion resistance. Austenitic stainless steels are non-magnetic or weakly magnetic. 303 stainless steel contains more than 8% Nickel thereby lowering ferritic number. This will increase the amount of austenite in matric making steel non-magnetic.
