As a supplier of stainless steel closed die forgings, I get a lot of questions from clients about magnetic properties. Most folks come to us with a basic question: “If it’s stainless steel, why is this forging sticking to my workshop magnet?” It’s a fair question—stainless steel is often grouped with non-magnetic metals like aluminum or copper for general use, but the reality is way more nuanced, especially when we’re talking about closed die forgings, which are the custom, precision-shaped parts we make in our facility. I’ve spent years working with these parts, troubleshooting, testing, and answering this exact question, so let’s break it down clearly, without too much jargon that makes your head spin. Stainless Steel Closed Die Forgings
First, let’s start with what stainless steel actually is. It’s an alloy, right? Made mostly of iron, with at least 10.5% chromium to resist corrosion. That chromium is the key to its rust-proof superpower, but other elements mixed in change almost everything else—including magnetism. The main categories of stainless steel are austenitic, ferritic, martensitic, duplex, and precipitation hardening. Each has a different atomic structure, and that structure determines how magnetic the part will be. Closed die forgings are made by heating metal and pressing it into a custom die with massive force, which shapes the grain structure of the metal in a way that other manufacturing methods (like casting or machining) don’t. That grain structure is huge for magnetic properties—something we see every day at our shop.
Let’s go through each type, because not all stainless steel forgings act the same. The most common austenitic stainless steel—like 304 and 316—is often called non-magnetic. These are used for everything from food processing equipment to pharmaceutical parts, where you don’t want magnetism interfering with products. But here’s the catch: austenitic stainless steel isn’t perfectly non-magnetic. It’s what we call paramagnetic, meaning it has very weak magnetic attraction. You won’t pick up a 304 forging with a fridge magnet, but a strong industrial magnet will barely pull on it. And here’s where closed die forging makes a difference. When we shape austenitic stainless steel in our dies, the high pressure and heat can cause a tiny part of the austenite structure to transform into martensite—a harder, more magnetic phase. It’s not a lot, just a thin layer on the surface sometimes, or along the grain lines. We see this all the time when we test parts after forging: a brand-new 316 forging might have a faint magnetic pull that disappears after a little annealing (heating it up slowly to soften the grain structure and reverse that transformation). If you machine the surface, you’ll often remove that magnetic layer too, leaving the true weak paramagnetism of bulk 316.
Then there’s ferritic stainless steel, like 430. This one is magnetic, full stop. It has a body-centered cubic (BCC) atomic structure, which aligns easily with magnetic fields. Ferritic stainless steel is used for exhaust parts, kitchen appliances, and trim—places where corrosion resistance is needed but high strength isn’t as critical. When we forge ferritic stainless steel, the closed die process actually reinforces that magnetic structure a little, because the forging aligns the iron and chromium atoms in a way that makes magnetism even more consistent across the part. You can pick up a 430 forging with a basic fridge magnet, no tricks here. That’s usually a relief for clients who need magnetic parts for things like sensor housings or clamping components.
Martensitic stainless steel, like 410 and 420, is another magnetic type. It’s hard, strong, and used for things like valve parts, turbine blades, and tool components. Its atomic structure is also BCC, similar to ferritic, but it gets extra strength from heat treatment (quenching, which is cooling it quickly after forging). That quenching locks the structure in a way that makes it extremely magnetic. We have to be careful with martensitic forgings because their magnetism is so strong that it can interfere with nearby equipment if the part isn’t isolated. That’s a point we always make to aerospace and oil and gas clients who are using these parts in sensitive systems.
Duplex stainless steel, a mix of austenitic and ferritic, is a middle ground. It has roughly equal parts of each structure, so its magnetism is moderate—stronger than austenitic, weaker than ferritic or martensitic. Duplex is used for offshore pipes, chemical processing equipment, and heat exchangers. Closed die forging of duplex stainless steel has to be done at very precise temperatures to keep that balanced structure; if we get the temp wrong, it can make the part either more or less magnetic than specified. That’s why our quality control team does magnetic permeability tests on every duplex forging that leaves our shop—we don’t want to deliver a part that’s off-spec for a client’s project.
Now, let’s talk about why closed die forgings are different from other stainless steel parts. A lot of people ask us, “I bought a 304 stainless steel bolt from a hardware store, it’s not magnetic, why is your 304 forging sticking a little to my magnet?” The answer is all about the manufacturing process. Closed die forging subjects the metal to extreme pressure as it forms. That pressure creates what’s called “work hardening” and can cause phase transformations, like the austenite to martensite shift I mentioned earlier. When you cast a stainless steel part, it cools slowly, so the grain structure is more uniform, with less martensite. Machining a part from a solid bar can also leave surface stresses, but forging applies that stress in a way that affects the entire part’s grain structure, not just the surface. Another thing: forging aligns the metal’s grain flow with the shape of the part. For magnetic materials, grain alignment makes magnetism more consistent in the direction of the flow—so a forged valve shaft might be slightly more magnetic along its length than across its width, something we note in our material test reports for every order.
We also deal with post-forging treatments that change magnetic properties. Annealing is the most common: heating a forging to 1000-1100 degrees Celsius, holding it there, and cooling it very slowly. That reverses the martensite transformation in austenitic forgings, making them almost completely non-magnetic again. We always anneal parts for clients who need strict non-magnetic properties, like medical device parts or parts for MRI machines—wait, yes, MRI machines are a big one! I once had a client reach out panicking because their stainless steel forging for an MRI lab component was magnetic, and they thought they’d ordered the wrong material. It turned out we’d skipped the final annealing step because they needed it in a hurry. We ran a test anneal overnight, and the magnetism was gone, no issues. That’s a lesson we learned early: always confirm if a client needs a specific magnetic level before shipping, because annealing adds lead time but is non-negotiable for sensitive applications.
Another factor: cold working. Even after forging, if a part is machined, bent, or shaped after it leaves our shop, that cold work can create more martensite, making it magnetic. We get calls from clients who say “your part is magnetic, but I didn’t do anything to it!” Usually, it’s just a strong magnet they’re using testing it—fridge magnets are weak, so they might pick up a faint magnetic layer that’s not functional. For accurate testing, we use a gaussmeter, which measures magnetic field strength, not just pull. That’s part of our standard testing protocol now: every part gets a gaussmeter reading, so we can give clients a precise number, not just a “it’s magnetic/non-magnetic” answer.
Let’s cover common myths I hear all the time. First myth: “All stainless steel is non-magnetic.” Nope, as we talked about—ferritic, martensitic, duplex are all magnetic to some degree. Second myth: “If it’s magnetic, it’s not stainless steel.” No way—many high-grade stainless steels used in critical applications are magnetic. A valve for an oil well might be martensitic stainless steel, magnetic, and last 20 years in corrosive saltwater, longer than a non-magnetic alternative. Third myth: “Magnetism means the part is defective.” Absolutely not. In fact, for parts that need magnetic properties, like those used in electrical motors or sensors, magnetism is a desired characteristic. It’s all about matching the material and forging process to the application.
As a supplier, how do we support clients who care about magnetic properties? First, we start by asking detailed questions: What’s the part used for? What standards does it need to meet? Do you require a specific magnetic permeability or a gaussmeter reading below a certain number? We have a library of material data for every stainless steel grade we forge, so we can tell you exactly what to expect. For example, 304 austenitic closed die forgings will have a permeability of around 1.02 (very close to non-magnetic) if annealed, while a 410 martensitic forging will have a permeability of 100 or higher. We also keep detailed process records, so if a client has a question about a part they ordered years ago, we can pull up the forging parameters, heat treatment, and test results to confirm its magnetic properties.
I’ll give you a real example from last year. A medical equipment manufacturer came to us needing 50 custom clamps for use in an MRI suite. MRI rooms require parts that are strictly non-magnetic—any ferromagnetic material could be pulled into the machine at 1.5 or 3 Tesla, which is deadly. They’d had bad experiences with another supplier before, who sent them “304 stainless steel” clamps that were slightly magnetic, even after claiming they were annealed. We tested our 304 billet before forging, did a test forging, annealed it, and ran gaussmeter tests—we got a reading of less than 2 gauss, way below the 5 gauss limit the MRI required. The clamps worked perfectly, and they’ve been a repeat client since. That’s why attention to magnetic properties isn’t just a technical detail for us—it’s about keeping our clients’ projects safe and on track.
Another example: a construction company needed closed die forged anchor bolts for a skyscraper project in a coastal area. They needed bolts that could resist salt corrosion (so stainless steel) but also that could be aligned with a magnetic drilling tool on the job site. We recommended duplex stainless steel—its moderate magnetism was perfect for the drilling tool, and its corrosion resistance was enough for the salt air. We forged the parts, tested their magnetic permeability at 1.8, exactly what they needed, and they’ve used those bolts on three more projects since.
So, to wrap this up, the magnetic properties of stainless steel closed die forgings aren’t a one-size-fits-all answer. They depend on the alloy composition, the forging process (how we shape the metal, the temperature and pressure we use), and any post-forging treatments like annealing. As a supplier, our job is to understand those variables, test every part to make sure it meets your needs, and communicate clearly so you don’t have to guess if your part will work for your application. If you’re working on a project where magnetic properties matter—whether you need a non-magnetic part for medical or MRI use, a moderately magnetic part for industrial tools, or a highly magnetic part for aerospace components—we can walk you through the best grade, forging process, and testing protocol to get exactly what you need. Feel free to reach out to our team to discuss your closed die forging requirements; we’re here to answer any questions, run tests, and help you spec the right part for your job.
Iron Green Sand Castings References:
- ASM International. (2020). Properties of Stainless Steels. ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials.
- Brandes, E. A. (1998). Smithells Metals Reference Book, 7th Edition. Butterworth-Heinemann.
- Lindberg, R. A. (2010). Forging: Materials, Processes, and Tools. CRC Press.
- International Organization for Standardization. (2018). ISO 9445: Stainless Steel Forgings. Requirements and Test Methods.
Hebei Shata Machinery Co., Ltd.
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