Is 304 Stainless Steel Magnetic? 5 Reasons Your Parts Attract a Magnet
- Key Takeaways
- Why Is 304 Stainless Steel Usually Non-Magnetic?
- Cold Work Can Make 304 Magnetic
- Cutting and Shearing Can Create Local Magnetic Response
- Weld Zones May Contain Delta Ferrite
- Nickel Content Can Affect Magnetic Response
- The Part May Be a Mislabeled Stainless Steel Grade
- Is 316 Stainless Steel Magnetic?
- Is Magnetic 304 Stainless Steel a Problem?
- Does 304 Stainless Steel Rust?
- FAQ
If a magnet sticks to your 304 stainless steel part, that does not automatically mean something is wrong with the material. Properly annealed 304 is generally non-magnetic, but it can develop a magnetic response after cold working, such as bending, rolling, or drawing. Weld metal can also attract a magnet if it contains delta ferrite. As a rule of thumb, magnetism around a bend, formed area, cut edge, or weld usually points to the fabrication process. Strong, uniform attraction across flat, undeformed areas is a different story and may be a reason to check the material grade. Here are five common reasons a 304 stainless steel part can become magnetic, and what that means for the finished part.
Key Takeaways
Annealed 304 stainless steel is generally non-magnetic, but manufacturing processes can change its magnetic response.
Cold working, including bending, rolling, drawing, and forming, can cause 304 to become noticeably magnetic.
Shearing and other local deformation can create a small magnetic response near cut edges, while weld metal can be magnetic because of delta ferrite.
Strong, uniform attraction on flat, undeformed areas is a good reason to verify the material grade and certification.
For applications that require very low magnetic permeability, the material grade, processing history, and welding procedure should be controlled and verified.
Table of Contents
- Key Takeaways
- Why Is 304 Stainless Steel Usually Non-Magnetic?
- Cold Work Can Make 304 Magnetic
- Cutting and Shearing Can Create Local Magnetic Response
- Weld Zones May Contain Delta Ferrite
- Nickel Content Can Affect Magnetic Response
- The Part May Be a Mislabeled Stainless Steel Grade
- Is 316 Stainless Steel Magnetic?
- Is Magnetic 304 Stainless Steel a Problem?
- Does 304 Stainless Steel Rust?
- Frequently Asked Questions
Why Is 304 Stainless Steel Usually Non-Magnetic?
Annealed 304 stainless steel is an austenitic stainless steel, meaning that in its annealed condition it should have a very low magnetic permeability. The nickel content is what stabilizes that austenitic structure; chromium does the corrosion-resistance work and actually favors ferrite. Generally speaking, the more stable the austenitic structure, the lesser its response to a magnet. Stable austenite will show little to no response to a magnet.
1. Cold Work Can Make 304 Magnetic
304 can be cold worked by bending, rolling, stamping, or other processes that cause plastic deformation. As 304 is cold worked, some of its austenitic (non-magnetic) structure transforms to a structure known as martensite. Martensite is magnetic, so the greater its presence, the greater the part’s response to a magnet.
For example, imagine two identically sized sheets of 304. One is flat, and the other is formed into a tube. If you wave a magnet over the two pieces of sheet, the formed piece is likely to have a stronger response to the magnet than the flat piece because forming introduces martensite. The greater the severity of the deformation, the greater the amount of martensite will be introduced, and thus, the greater its response to a magnet will be. You can sometimes observe this by waving a magnet along a formed part, observing stronger responses at corners and sharp bends.
In many applications, the introduction of martensite and its accompanying increased permeability is not a problem. However, for applications with strict permeability requirements, annealing can convert martensite back to austenite. This process is known as solution annealing. Whether or not a part can be solution annealed may be contingent upon part size and shape as well as surface finish requirements.
2. Cutting and Shearing Can Create Local Magnetic Response
When sheet, bar, tube, or other products are cut via shearing or other processes that cause plastic deformation, a thin layer of martensite is introduced. This can cause the cut edge to be slightly magnetic while the rest of the material may show little to no response to a magnet.
For example, you may find that a piece of 304 sheet will be non-magnetic in the center, but its edges are slightly magnetic from shearing, blanking, slitting, etc. Grinding, polishing, machining, and other processes that deform the surface can cause magnetic response as well.
3. Weld Zones May Contain Delta Ferrite
Parts can also be more magnetic around welds. This wouldn’t indicate a material error, as the bulk of the material can still be 304 stainless steel, but it’s important to understand the reason for this effect.
When the material in the weld zone melts and resolidifies, delta ferrite can form. This has a different crystal structure than austenite, but it is magnetic. Stainless steel filler metals are actually designed to produce a small amount of ferrite (5–15% for 300-series stainless steels) in welds because it helps avoid certain types of hot cracking during solidification, explains TWI.
In addition to being more magnetic where the weld itself is, the area immediately around the weld can show increased magnetism because the material has been locally changed by the welding process. In applications where magnetic permeability is a concern, it’s important to specify acceptable levels of ferrite and test with a proper instrument instead of holding up a magnet.
4. Nickel Content Can Affect Magnetic Response
The amount of nickel in stainless steel stabilizes the austenitic structure, making it less likely to turn to martensite (and thus become more magnetic) during cold working. This means that two separate lots of 304 may exhibit varying amounts of magnetism after being formed or bent.
Magnetic permeability data taken from ATI’s technical data sheet for 302, 304, 304L, and 305 stainless steels shows how nickel content can affect magnetic response to cold working:
Magnetic permeability at 200 H (oersteds)
| Cold work (%) | Type 302 | Type 304 | Type 304L | Type 305 |
|---|---|---|---|---|
| 0 | 1.004 | 1.005 | 1.015 | 1.002 |
| 10 | 1.039 | 1.009 | 1.064 | 1.003 |
| 30 | 1.414 | 1.163 | 3.235 | 1.004 |
| 50 | 3.214 | 2.291 | 8.480 | 1.008 |
Source: ATI 302/304/304L/305 technical data sheet.
As shown in the table, 305 stainless steel, which contains more nickel, can withstand significant cold working while maintaining low magnetic permeability. In contrast, the lower-nickel alloys demonstrate significant increases in magnetic permeability, particularly after more than 10% cold work.
The relevant chemistry differences between these four grades are small but they matter:
Composition in weight percent per ASTM A240
| Grade | Carbon (max) | Chromium | Nickel |
|---|---|---|---|
| 302 | 0.15 | 17.0–19.0 | 8.0–10.0 |
| 304 | 0.07 | 17.5–19.5 | 8.0–10.5 |
| 304L | 0.030 | 17.5–19.5 | 8.0–12.0 |
| 305 | 0.12 | 17.0–19.0 | 10.5–13.0 |
Source: ATI 302/304/304L/305 technical data sheet.
Note that 304L’s “L” indicates that it’s a lower-carbon grade of stainless steel to avoid sensitization in welded applications. Carbon also stabilizes austenite, which is why, at high amounts of cold work, 304L can be more magnetic than 304 — 8.480 against 2.291 at 50% reduction. Don’t specify 304L expecting a less magnetic part.
When selecting stainless steel grades that will be exposed to significant cold working in service, it’s important to consider how well each grade retains its austenitic structure. Materials like 305 can be an advantage where low magnetic permeability is important.
5. The Part May Be a Mislabeled Stainless Steel Grade
In contrast to cold-worked and welded areas being more magnetic while other sections of a part are not, strong and consistent attraction to a magnet throughout a stainless steel part likely means it isn’t the expected stainless steel grade. This is particularly likely to occur in sections of a part that are relatively flat and haven’t been significantly cold-worked, bent, or welded.
Certain stainless steels, such as 430, are ferritic and thus naturally magnetic. If the wrong stainless steel was ordered or delivered to the shop, it’s possible that 430 could be used when 304 was specified.
If there’s question of a part’s stainless steel grade, check the material’s test certificate to make sure it matches the heat and lot numbers on the material. For critical or higher-risk applications, chemical analysis using positive material identification (PMI) is a good idea.
Is 316 Stainless Steel Magnetic?
In general, annealed 316 sheet or plate will not hold a magnet. ATI publishes magnetic permeability data showing permeability to be less than 1.02 at 200 H for annealed austenitic stainless steels like 316. However, just like other austenitic stainless steels, some areas may be magnetic depending on the application. Areas around edges that have been cut can attract a magnet, as can areas of a part that have been cold worked. Parts made from 316 stainless steel may be welded together, and welds attract magnets.
ASTM A240 calls out nickel content of 316 stainless steel to be 10.0–14.0% as compared to 304’s 8.0–10.5%. The added molybdenum content of 2.00–3.00% improves pitting and crevice corrosion resistance, which is why 316 is chosen for chemical and paper and pulp service. Note that molybdenum is a ferrite former, and ATI does not recommend 316 for seawater itself — it rates 316 for water up to roughly 2,000 ppm chloride, against about 100 ppm for 304. Higher nickel contents stabilize austenite, so 316 stainless steel is less likely than 304 to be attracted to a magnet after the same amount of cold working.
Welding with proper 316 weld filler metal results in welds that contain a small amount of ferrite, which helps avoid hot cracking. This small amount of ferrite is why welds in 316 stainless steel attract a magnet while the base metal does not.
ATI publishes magnetic permeability data for cold worked austenitic stainless steels (302, 304, 304L, 305) but doesn’t publish equivalent cold-work data for 316; its data sheet says typical values are available on request. However, as with 304, the amount of nickel and the amount of cold work are the important variables in determining how strongly 316 will be attracted to a magnet. Simply changing specifications from 304 to 316 will not automatically result in a lower permeability part. Process controls and testing are necessary for finished products if lower permeability is required.
Is Magnetic 304 Stainless Steel a Problem?
In general, no. A slight attraction to a magnet will not affect strength, corrosion resistance, or other properties. Areas that have been formed may be attracted to a magnet while flat, unformed areas will not. Edges may also attract a magnet, as may welds. This does not indicate that the part is not stainless steel or is not 304 grade stainless steel. These are just the effects of cold forming and welding.
In general, if a magnet sticks to areas that have been formed or bent, it is not cause for concern. If the part is supposed to be 304 and a strong attraction is felt on flat, unformed areas, then you may want to verify the material is 304 by checking the material’s certification.
If your application involves a very low permeability (MRI machines being an example), you will want to verify that the permeability is low enough for your application. These cases are difficult to evaluate by magnet. Generally speaking though, if you receive a part that is supposed to be 304 stainless steel and it is slightly attracted to a magnet, it would not generally be cause for rejecting it.
Does 304 Stainless Steel Rust?
Although it is difficult to believe, stainless steels can and do rust. However, they have a thin oxide (chromium based) coating on the surface that resists corrosion. This coating can be scratched or damaged off, but given sufficient oxygen it will reform. Exposure to salt and chlorides increases the chance of seeing surface corrosion on 304 stainless steel. Tight crevices that collect moisture and salt can also show signs of corrosion.
When cutting, grinding, machining, or otherwise using tools on stainless steel, it is possible to transfer small amounts of free iron onto the stainless steel that may rust. This may give the impression that the stainless steel itself has rusted even though the stainless itself has not. Free iron can be cleaned up and stainless steels can be passivated after fabrication to help re-form the protective oxide layer. Passivation is especially important if the application of your 304 stainless steel involves exposure to water, marine, or sanitary conditions.
Whether or not stainless steels attract magnets after cold working and whether they resist corrosion are two unrelated properties.
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Frequently Asked Questions
Can 304 be magnetic?
Generally speaking, 304 stainless steel in its annealed condition is non-magnetic. However, some parts can be made to be magnetic during cold-working, such as rolling, bending, or drawing. During welding, a magnetic response may be detected when testing near the weld because delta ferrite is present in the weld metal.
Can 316 be magnetic?
316 stainless steel in its annealed condition is an austenitic, non-magnetic stainless steel, with magnetic permeability generally less than 1.02 at 200 H — the same figure ATI quotes for annealed 304. Its higher nickel content gives the austenite greater stability, so it tends to pick up less magnetism than 304 after the same amount of forming. It can still be made magnetic after welding or cold forming, and low permeability can’t be guaranteed in all conditions.
If my finished part is slightly attracted to a magnet, does this mean it isn’t 304 stainless steel?
It may contain some martensite, but it does not mean it is bad stainless steel. Heavy forming and deformation will change some of the non-magnetic austenite phase to martensite, which is magnetic. If the magnetism is localized, for example on a cut or bent edge or next to a weld, then the magnetism is a result of cold-working. If the material is not deformed and has strong and consistent magnetism, we would recommend verifying the material’s stainless steel grade.
Can you determine the type of stainless steel using a magnet?
Due to other types of stainless steel being magnetic, and austenitic stainless steels becoming magnetic upon working, the attraction to a magnet is not always an accurate way to determine the type of stainless. Checking the material’s test report or performing a PMI test will verify the type of stainless steel.
Is there a way to decrease the amount of magnetism on a part after receiving it?
A full solution anneal would reduce the amount of deformation-induced martensite; however, this is not always practical, depending on the part. It is best to discuss permeability requirements at the start of a project so proper measures can be taken to ensure requirements are being met. These measures can include using an even more stable austenitic stainless steel, controlling welding and working practices, and performing permeability testing of the finished part to ensure acceptance and rejection limits are being met.