One of the most important considerations that a manufacturer or product designer will need to address is whether or not the materials they use can be magnetic. Unfortunately, this is frequently a question that is postponed until the last minute, or which is ignored altogether. This can often be a mistake, and we recommend anyone working with stainless steel should proactively determine whether their application requires a magnetic or non-magnetic metal.
This is especially true with stainless steel because the answer to the above question is that some stainless steels are magnetic and some or not, which allows designers tremendous flexibility when picking out stainless steel. It also means that it’s necessary to pay close attention to the particular needs of your application and to choose a stainless steel alloy that meets your specifications.
The experienced and professional team at Clinton Aluminum is happy to help you weigh the different benefits of various stainless steel alloys so that you can make the best possible decision in terms of material selection. Contact us today to learn more.
Stainless steel alloys are divided into five main families
In order to know whether a stainless steel alloy is magnetic or not, it’s first necessary to know what class of stainless steel it belongs to. Broadly speaking, there are five main families of stainless steel. Of these, the most popular is known as austenitic stainless steel, and the alloying materials primarily consist of chromium and nickel, which gives this family a strong corrosion resistance. The 200 and 300 series of grades are primarily austenitic.
The next most popular class is known as ferritic stainless steel, and these alloys have molybdenum, aluminum, and/or titanium added in addition to chromium and nickel. The combination of alloying elements gives the stainless steel greater ductility and resistance to stress corrosion cracking. Ferritic alloys mainly fall under the 400 series of stainless steels.
Martensitic is another class of stainless steel, and because they also fall into the 400 series of alloys, there is a number of similarities with ferritic stainless steels. In fact, you have to look at the molecular level to tell them apart, where they can be distinguished by the increased carbon content and reduced chromium of martensitic steels.
A special category of stainless steels, known as precipitation-hardening, has aluminum, copper, and titanium as alloying agents. In order to produce these alloys, the metal must undergo a solution and aging heat treatment that provides it with a higher tensile strength compared to the other classes.
That brings us to duplex stainless steel, which is the newest of the five classes. These alloys combine the properties of ferritic and austenitic alloys, hence the name. They are made up of 24% chromium and 5% nickel, providing for increased yield strength and improved stress corrosion resistance, especially when it comes to chloride environments.
Which stainless steel alloy families are magnetic?
In manufacturing and metalworking, the term ferromagnetic refers to materials that are strongly attracted to a magnet. In general, this is what is meant when people say that something is magnetic. Furthermore, the term magnetic permeability indicates the ease by which a magnetic material can be magnetized, with a numeral value close to 1.0 meaning that the material in question is non-magnetic.
If magnetism is an issue for your application, then it is important to distinguish not just whether a metal is magnetic, but how magnetic it is, and how it will respond to exposure to magnets. For example, you need to know whether a metal is “Hard” or “Soft” in terms of its magnetism. When a material is classified as hard magnetic, it will retain some amount of residual magnetism following exposure to a magnetic field. On the other hand, soft magnetic material will revert to low residual magnetism after exposure.
Since all of the most common carbon steels, low alloy steels, and tool steels are magnetic, it would be easy to assume that all stainless steel grades are magnetic as well. This is not the case. As a general rule, the Austenitic stainless steels, which include the 300-series of Chromium-Nickel alloys, but also the 200-series Chromim-Manganese-Nickel alloys, are non-magnetic.
Many of the wrought austenitic stainless steels will contain some amount of ferrite, but most of the time there is not enough to alter the magnetic performance of the alloy. However, in applications where even a slight magnetic sensitivity could have an effect, then these particular alloys should be avoided.
The remaining stainless steels grades are all magnetic. This includes the ferritic grades, duplex grades, martensitic grades, and precipitation hardening grades. Even though the duplex grades are a mixture of austenite and ferrite stainless steels, they retain the ability to attract a magnet.
Finally, austenitic stainless steel castings have a different chemical composition than wrought stainless steels. In order to prevent hot cracking during casting, a small amount of ferrite will be added. Certain austenitic welds will have between 4% to 8% ferrite for this reason. This will add to the finished metal’s magnetic properties. If magnetism is an important concern and even a small amount is to be avoided, it’s possible to find ferrite-free castings, as they are produced under special circumstances for certain specific corrosive environments.
There is a lot more nuance when it comes to the magnetic properties than can be covered in a single article. That’s why it’s essential that manufacturers and designers take into consideration what magnetic properties are most desirable for their application and choose the right material accordingly.
Summary
Magnetism is a complicated but often overlooked factor when it comes to production. Manufacturers who are concerned about the magnetic properties of the materials they are using should consult with knowledgeable partners who have a deep understanding of the science involved. At Clinton Aluminum, our priority is to help our customers make the best purchasing decisions they possibly can. For this reason, we strive to be true partners that can help you through every step of the production process.
Contact us today to speak with one of our professional and friendly representatives.
