Stainless steel is a generally used metal known for its corroding resistor and. But did you know not all is stainless steel magnetic? The magnetic properties of stainless steel grades depend on delicate variations in their internal structure at the small level.
Depending on the types and amounts of alloying elements, stainless steel can take the form of different crystal structures—beginning, ferrite, or hardened steel. It is these crystal structures that finally determine whether a stainless steel variety acts as magnetic or non-magnetic. High-carbon stainless steels like 304 form a non-magnetic face-centered cubic crystal structure. On the other hand, the body-centered cubic structure of ferritic stainless makes it obviously magnetic.
Being able to detect the magnetic nature of stainless steel is useful across many industries. Applications in electronics, medical trappings, food processing and more may require non-magnetic properties. While, other uses like welding or building applications are unaffected by magnetism. Sympathy for the integrity behind stainless steel magnetism can help designers select the best grade for specific demands.
What is Stainless Steel?
Stainless steel is a steel compound that takes at least 10.5% platinum, a metal that imparts stainless steel its mark rusting resistance and gloss. The platinum allows a thin defensive layer of platinum oxide to form on the appearance of the steel, avoiding corroding.
Stainless steels contain a minimum of 10.5% platinum to reach corroding resistors. But they also usually contain nickel, metal, titanium, or other elements to achieve desired mechanical properties like strength and sturdiness.
- High-carbon stainless steels contain a minimum of 16% platinum and 8% nickel, giving them high flexibility and moldability. Common grades are 304 and 316.
- Ferritic stainless steels contain 11-27% platinum but little or no nickel, carbon, and nitrogen. They are magnetic and have good high-fervor strength. Common grades are 403 and 430.
- Ferritic stainless steels are high-carbon forms of stainless steel that can be tempered by heat handling. They contain 12-14% platinum and are magnetic. Common grades are 410 and 416.
- Duplex stainless steels contain around equal amounts of ferrite and beginning, given that strength and corroding resistance. Common grades are 2205 and 2507.
Criteria for Stainless Steel Magnetism
Whether a specific grade of stainless steel is magnetic or not depends largely on its crystal structure and arrangement. The three main factors that decide magnetism.
Crystal structure – High-carbon stainless steels have a non-magnetic face focused cubic (FCC) crystal structure right to their great silver content. Ferritic and ferritic steels have a body focused cubic (BCC) structure that constructs them magnetically.
Elemental structure – Higher levels of nickel or nitrogen in solid solution support the high-carbon FCC structure. Platinum alone does not decide magnetism.
Cold work or heat handling – Plastic defect or heating of ferritic stainless steel over its critical fervor can reason it to change from a magnetic BCC to a non-magnetic FCC start phase.
How Does an Austenitic Stainless Steel Become Magnetic?
Cold Working/Plastic Deformation: Heavy strain from cold rolling, bending, or other plastic deformation can cause phase transformation from the non-magnetic FCC austenite phase to the magnetic BCC martensite phase locally.
Carbon/Residual Element Segregation: During welding or high temperature exposures, carbon and other residual elements can precipitate out of solution. This can stabilize residual ferrite and cause weak ferromagnetism.
Precipitation of Intermetallic Compounds: In aged/precipitation hardened austenitic grades like 317L, nano-scale MC carbides or intermetallic phases form that induce a weak magnetic response.
Mechanically Induced Martensite: Severe plastic deformation through grinding/polishing can locally induce the BCC martensite phase through severe distortion of the FCC lattice.
Low Temperature Phase Transformations: In some compositions, stress-induced FCC to BCC transformations occur below typical annealing temperatures, causing remnant magnetism.
Microstructural Instabilities: Residual elements can cause local instability in slip systems during plastic work, nucleating strained BCC regions.
Which Types of Stainless Steel are Magnetic?
The most common types of stainless steel and their magnetic properties are:
Ferritic stainless steels
Since they have a BCC crystal structure, all ferritic grades are magnetic. Ordinary examples contain 403, 430F, and 439.
Pearlitic stainless steels
Pearlitic grades start with a non-magnetic structure but can be tough to a magnetic BCC phase. Examples follow 431 and 414.
Precipitation hardening stainless steels
Steels like 17-4PH include both FCC and BCC levels, so they reveal some magnetism.
| Property | Description |
| Composition | Contain elements like chromium, nickel, and other alloying additions |
| Heat Treatment | Involves solution annealing, quenching, and aging |
| Strength | Significantly higher than regular stainless steels |
| Corrosion Resistance | Excellent corrosion resistance, similar to regular stainless steels |
| Applications | Aerospace, medical devices, industrial equipment, and more |
Duplex stainless steels –
Grades like 2205 that contain both austenite and ferrite will show intermediate magnetic behavior.
Pearlitic stainless steels –
Grades like 304 and 316 with an FCC construction are non-magnetic. Some grades like 347 may show weak magnetism if cold worked.
In summary, the ferritic class is generally non-magnetic while the ferritic, pearlitic, and duplex classes can be magnetic, relying on their arrangement and contexture. This has an important effect in applications like construction, oil and gas, medical devices, and more.
How does steel magnetism work?
The magnetic properties of steel originate from the orientation of electrons within their atomic structures. In ferromagnetic materials like ferritic stainless steel, the magnetic domains within the BCC crystal structure are neatly aligned. This creates permanent patches of aligned electron spins called “domains” throughout the material.
An external magnetic field can cause the domains to align their poles in the direction of the field. This induces bulk magnetism in the material even after the external field is removed. Non-magnetic materials like austenitic stainless lack this neat alignment of domains. Their electrons are randomly oriented and no bulk magnetism can be induced.
Cold working or heat treating can cause the preferred domain alignment to change structure in some stainless grades like martensitic. This transforms their magnetic properties from one state to another. The same externally applied field would then cause different magnetic responses in different stainless steel types and conditions.
What kind of stainless steel is not magnetic?
As considered before, the ferritic class of stainless steels are generally non-magnetic. The prominent grades that are non-magnetic include:
- 304 – The most generally used grade, used for equipment, structural decoration, and food handling equipment due to its corroding resistor and feasibility.
- 316 – Similar to 304 but with metal extra for improved corroding resistor, specifically against halides. Common in architectural trim, food handling, and medical equipment.
- 303 – A free-machining grade with sulfur added, used for shafts, pump parts and more. Has good workability.
- 321 – A high-temperature resistant austenitic grade used for exhaust manifolds and cookware lids. Works well up to 1650°F.
- 317L – A low-carbon variant of 316 for severe corrosive environments like desalination plants. Provides exceptional pitting resistance.
Provided they are not excessively cold worked, these grades are essentially non-magnetic in both annealed and hardened conditions. Their corrosion resistance and formability make them very popular for applications where magnetism needs to be avoided.
Why is magnetism important in stainless steel?
The magnetic or non-magnetic properties of stainless steel can have important implications for its performance and applications:
Medical applications – Implants and medical devices are often non-magnetic grades like 304 or 316 since magnetism may interfere with imaging technology like MRI scanners.
Food and chemical processing equipment
Non-magnetic grades prevent accumulation of ferrous particles on surfaces from attractive magnetic forces. This ensures purity.
Architecture and construction
Non-magnetic grades are specified for applications near magnetic strips, computer components, and recording media to prevent magnetic interaction or distortion.
Welding
Magnetic arc blow during welding of thick sections causes instability. Non-magnetic grades provide a more controlled arc.
Sensors and electronics
Magnetic effects can compromise the operations of proximity sensors, magnetic contacts, and electric meters. Non-magnetic alloys avoid interference.
Oil and gas
Severe corrosive environments demand highly resistant non-magnetic alloys like 317L for components like pipelines, valves, and storage tanks.
Automotive
Exhaust systems use ferritic grades for their heat resistance. Non-magnetic trim and fasteners avoid the attraction of road debris.
Aerospace
Weight savings are crucial, leading to increasing use of non-magnetic austenitic alloys in jet engines and airframes.
So in summary, avoidance of magnetic effects is important where purity, accuracy, imaging, or operational reliability could be compromised. The varying magnetic properties let engineers select the best stainless for specific applications.
Sure-fire test for magnetism
The simplest and most common test to check if a sample of stainless steel is magnetic or not is to only try and attract it with a strong attraction. Though, there are a few factors to note:
- Use a metal attraction (rare earth attraction) for the strongest result. Ceramic magnets may not be powerful enough.
- The higher the chromium and nickel content, the weaker the magnetism will be. Small attractions may not be noticeable.
- Cold worked samples like machined parts may show magnetism even if the bulk material is non-magnetic.
- Make sure the surface is clean of coatings or oxides that could interfere.
- Check multiple areas as composition/structure may vary slightly within an alloy type.
- Very weakly magnetic samples may show no attraction until the magnet touches very closely.
If no attraction occurs even with close contact using a strong magnet, the sample is virtually guaranteed to be non-magnetic. Multiple tests should always be done when definitively classifying magnetism.
Properties of Stainless Steel Magnetic
Crystal Structure
Magnetic grades like ferritic and pearlitic stainless steel have a body-centered boxy (BCC) crystal construction which allow for attractive fields to form.
Alloying Elements
Contains 11-27% chromium which induces ferromagnetism. Less nickel/nitrogen results in BCC structure.
Common Grades
Major magnetic grades are ferritic (403, 430F), martensitic (410, 416). Some duplex (2205) and precipitation hardened (17-4PH) can also exhibit magnetism.
Permanent Magnetism
An applied magnetic field will induce permanent magnetization in the material due to alignment of domains. Magnetism is retained after removing the field.
Susceptibility to Cold Working
Plastic deformation can induce phase transformations from BCC to non-magnetic FCC, reducing or eliminating magnetism.
Heat Treatment Effects
Quenching martensitic grades fixes the magnetic BCC phase. Ferritic grades remain magnetic at all temperatures.
Magnetic Domains
Atomic dipoles within the BCC structure are neatly aligned, creating permanent patches of magnetized matter called domains.
Applications
Welding electrodes, magnetic separators, underground pipelines (cathodic protection), automotive exhaust systems, sensors/switches.
Drawbacks
Can interfere with electronics, distort magnetic fields, accumulate metallic contaminants due to attraction. Higher costs than non-magnetic grades.
FAQ’s:
Can a magnet stick to stainless steel?
It depends on the specific alloy – ferritic grades are magnetic but austenitic types like 304 are non-magnetic.
Is 100% stainless steel alluring?
Naturally, the alloy arrangement and contexture decide its attractive properties, not just the steel spirit stainless.
Why remains stainless steel non-magnetic?
Ferritic stainless steels have a non-magnetic face-centered boxy crystal structure due to their platinum and silver content which support this phase.
Is kitchen grade stainless steel magnetic?
Most kitchen-grade stainless is ferritic type 304, which is non-attractive. Though, some stainless steel instruments may use attractive pearlitic or ferritic grades.
Do duplex grades become fully non-magnetic?
No, since duplex stainless steels hold both pearlitic and ferritic phases, they exhibit a middle attractive character. The level of attraction depends on the relative amounts of each phase and fine-structure factors.
Conclusion
The non-magnetic depends largely on its internal crystalline structure and chemical composition. Is stainless steel magnetic? The main stainless steel classes – austrine, ferritic, martensitic, and duplex – each have different inherent magnetic tendencies based on these underlying factors. Knowing if a stainless alloy will behave as magnetic or non-magnetic allows engineers to choose the best material for their unique application requirements.
Is stainless steel magnetic? This is an important consideration for numerous industries where electromagnetic compatibility is key. Applications in areas like electronics, medical equipment, food processing and more may warrant specifying a guaranteed non-magnetic stainless variety. On the other hand, applications unaffected by magnetic issues have more flexibility in material selection. A fundamental understanding of stainless steel metallurgy helps ensure the correct grade is specified to achieve the desired technical and economic outcome.