When purchasing stainless steel castings, have you ever noticed that austenitic stainless steel castings, which are supposed to be non-magnetic, exhibit magnetic properties? You might suspect that the castings are substandard and reject the entire batch, resulting in a sharp increase in costs. In fact, this is a normal phenomenon, and these slightly magnetic castings can be demagnetized and restored to a non-magnetic state through a secondary solution heat treatment. This article has everything you want to know about magnetism in stainless steel castings, helping you save significantly on costs.
Causes of Magnetism in Stainless Steel Castings
Stainless steel castings, particularly austenitic stainless steel castings, are non-magnetic. However, changes in their internal microstructure and improper investment casting processes can impart magnetism to the castings. Understanding the causes of magnetism will deepen your knowledge of stainless steel magnetism. It’s beneficial for you to quickly identify which steps went wrong when your stainless steel castings exhibit magnetism, significantly reducing the time required to troubleshoot the issue.
Microstructure
The crystal structure within stainless steel plays an important role in the magnetic properties of stainless steel castings.
If the internal structure is face-centered cubic (FCC), as in austenitic stainless steel, the extremely precise and symmetrical arrangement of atoms within the material causes the internal magnetic forces to cancel, resulting in non-magnetic properties.
If the internal structure is body-centered cubic (BCC) or body-centered tetragonal (BCT), as in martensitic and ferritic stainless steels, the asymmetrical arrangement of atoms results in strong magnetic properties.
Improper Cooling
During the investment casting process, excessive wall thickness can slow cooling, resulting in an uneven distribution of metallic elements within the austenitic stainless steel casting. Elements such as molybdenum and chromium, which promote ferrite formation, tend to segregate in slow-cooling localized regions, which leads to retained delta ferrite, preventing the ferrite in those regions from fully transforming into austenite during cooling. Since ferrite itself possesses magnetic properties, the casting retains residual magnetism.
Work Hardening
Some austenitic stainless steel castings are non-magnetic after casting but become magnetic during post-processing. This is because the casting surface is subjected to intense mechanical stress such as machining, grinding, or shot blasting, which induces magnetism in 304/316 stainless steel castings. These immense stresses cause work hardening; that is, the internal crystal structure transforms from austenite to strain-induced martensite, resulting in a significant increase in magnetism in the machined areas of the casting’s surface.
Magnetic Properties of Common Casting Grades
There are many common types of stainless steel, but because of differences in their internal crystal structures, the magnetic properties also differ. Understanding the magnetic properties of different stainless steels will help you select the appropriate material for your working environment and prevent issues caused by choosing unsuitable materials such as those that are non-magnetic or excessively magnetic, which could affect the operation of your machinery.
| Type | Common Grades | Crystal Structure | Magnetic Properties |
| Austenitic Stainless Steel | 304(CF8) 316（CF8M） 316L（CF3M） | Face-Centered Cubic (FCC) | Non-magnetic in the fully annealed state |
| Ferritic Stainless Steel | 409 430 444 | Body-Centered Cubic (BCC) | Strongly magnetic |
| Martensitic Stainless Steel | 410 420 440C | Body-Centered Tetragonal (BCT) | Strongly magnetic |
| Duplex Stainless Steel | 2205 2507 | Austenitic + Ferritic | Magnetic |
| Precipitation-Hardening Stainless Steel | 17-4PH 15-5PH | Body-Centered Tetragonal (BCT) | Moderately to strongly magnetic |
The Impact of Magnetism
The enhancement of magnetism is essentially an increase in ferritic content, but a moderate increase is not necessarily a bad thing for austenitic stainless steel. While it may affect the operation of instruments, it also enhances the casting’s mechanical strength. Therefore, if the operating environment of your castings does not require them to remain completely non-magnetic, a slight degree of magnetism can actually be beneficial. The most obvious benefit is a significant reduction in your costs associated with repeated testing and rework to eliminate magnetism.
Advantages
If manufacturers can control the ferrite content within austenitic stainless steel castings to around 3%–10%, this slight magnetic property can give the castings strength far exceeding their original levels.
- Increased Tensile Strength
Since ferrite is harder than austenite, an increase in the ferritic phase leads to a simultaneous increase in the casting’s tensile strength, yield strength, and hardness. This allows your stainless steel castings to have a longer service life when subjected to impacts and high pressure during operation, making them less prone to permanent deformation and significantly reducing maintenance costs of frequent replacements. Furthermore, due to the hardness provided by the ferritic phase, you do not need to switch to more expensive alloys to achieve the strength, resulting in substantial savings on material costs.
- Improved Resistance to Stress Corrosion Cracking
For 304 and 316 stainless steel castings, if stress-relief annealing is not performed, residual stresses from machining will remain within the casting. When the casting is exposed to a marine environment rich in chloride ions, these residual stresses interact with chloride-induced corrosion, leading to stress corrosion cracking. If the casting possesses an appropriate ferritic structure, it can effectively arrest crack propagation, significantly extending the casting’s crack-resistant lifespan in such chloride-ion pitting environments, and effectively preventing cracks in pump and valve castings that could lead to fluid leaks and system shutdowns.
- Improving Fluidity
Austenitic stainless steel already possesses good fluidity when poured into the mold. The addition of ferrite further reduces the viscosity of molten steel and improves its fluidity, allowing it to fill complex, thin-walled areas of the casting more effectively than before. This significantly reduces the risk of casting defects, such as misruns and cold shuts, caused by slow molten steel flow, addressing these issues at their source. It saves time and the costs of remaking defective parts, ensuring on-time delivery.
Disadvantages
However, if the casting contains too much ferrite, or if the operating environment requires complete non-magnetism, the magnetic properties of the surface of austenitic stainless steel castings can cause significant problems for you.
- Electromagnetic Interference
Due to their non-magnetic properties, austenitic stainless steels are commonly used in medical MRI equipment and precision sensors, which rely heavily on accurate electromagnetic signals. If the magnetic permeability of the stainless steel is significantly greater than 1, the magnetism it carries will distort the surrounding electromagnetic field, causing equipment malfunctions and data errors. This can directly lead to the shutdown of your project and result in significant losses.
- Physical Adhesion
Pump casings, valve bodies, and impellers made of austenitic stainless steel play a crucial role in fluid control. If these castings are magnetic, they will continuously attract iron filings and metal particles from the piping or the fluid. When the system is operating at high speeds, these metal particles will rub against the pump and valve castings, causing wear and fluid leaks that require costly repairs.
- Reduced Corrosion Resistance
Stainless steel castings rely on a dense, uniform passivation film on their surface to resist corrosion. However, if the internal ferrite content is too high, it will interact with the original austenitic structure, creating a galvanic cell effect. The passivation film on the casting’s surface will continuously break down when exposed to corrosive media such as strong acids or strong alkalis, leading to widespread corrosion and surface flaking. This will significantly reduce the casting’s service life in corrosive environments, resulting in substantial maintenance costs.
How to Eliminate or Control Magnetism
If you discover that a stainless steel casting exhibits magnetism during casting, there’s no need to recast the entire batch; excessive magnetism can be eliminated or controlled with specific methods. Understanding the methods will help you manage internal magnetism in castings at the source and promptly find solutions when the casting’s magnetic strength far exceeds your expectations.
Solution Annealing
In the investment casting process, solution annealing is the most effective method to demagnetize stainless steel investment castings. Holding the casting at a high temperature of 1050°C–1100°C dissolves ferrite and martensite formed internally during machining, leaving a pure austenitic microstructure. Moreover, after holding at this temperature, the casting must be rapidly water-quenched to ensure carbides and ferrite are eliminated.
If the casting surface becomes magnetic again during subsequent grinding or machining, a secondary solution annealing must be performed to eliminate the magnetism.
Chemical Composition Balancing
Before the stainless steel is melted, magnetism can be addressed at the source by controlling ferrite content. The manufacturer can use a spectrometer to appropriately increase the proportions of elements that minimize ferrite formation, including nickel, manganese, and nitrogen. Subsequently, the Schaeffler diagram can be used to predict the internal microstructure of the stainless steel, ensuring that the ferrite content within the casting remains within the range from the beginning of the melting process.
Machining Optimization
In the investment casting process, machining steps such as shot blasting, grinding, and machining are major causes of increased magnetic properties on stainless steel surfaces. However, this increase in magnetism can be controlled; foundries can avoid using high-intensity shot blasting, which could prevent the work-hardening martensite and surface magnetism. Additionally, by optimizing cutting parameters, using sharp cutting tools, and ensuring adequate coolant, manufacturers can reduce the risk of transforming the original austenitic structure into martensite under excessive compressive stress, which increases magnetism.
Applications Requiring Non-Magnetic Castings
Although a slight magnetic property can be beneficial for general structural components, in industries that require strict non-magnetic conditions, even the weakest magnetic field can affect system operation and even cause danger. Understanding which specific industries have strict non-magnetic requirements will help you determine whether your castings need to be completely non-magnetic. This can help you avoid paying for unnecessary “complete non-magnetic” and reduce additional testing costs.
Medical Industry
In the medical industry, stainless steel is often used for MRI scanner structural components, surgical instruments, and other medical castings because of its high hygienic standards. If a stainless steel casting exhibits high magnetic properties, the structural components made from it will interfere with the static magnetic field inside the MRI scanner, causing artifacts in the images and leading doctors to misdiagnose tumors, resulting in serious medical accidents.
If surgical instruments such as hemostats made from these are magnetic, they will automatically attract tiny metal particles from the air during surgery. They may even exert a magnetic pull near electronic monitoring equipment, significantly impeding the surgeon’s precise manipulation.
Fluid Control Industry
In the fluid control industry, micro-flow pumps such as metering and medical infusion pumps require extremely high flow accuracy. If a stainless steel pump housing casting is magnetic, it will not only affect the drive motor’s magnetic field efficiency, leading to inaccurate fluid delivery, but also attract magnetic particles in the fluid, which can scratch the internal seals and cause fluid leakage.
In the precision fluid control industry, solenoid valves are key components for precise fluid regulation. They rely on the electromagnetic force generated by an energized coil to drive the internal plunger. However, if the stainless steel valve body is magnetic, it will hold the plunger in place after power is cut off, causing the plunger to stick and preventing it from resetting completely. In precision fluid control, even millisecond-level delays or incomplete closure can directly lead to loss of flow control, media leakage, or equipment errors.
Magnetic Inspection Standards
There is no universally accepted international standard for the magnetic properties of austenitic stainless steel, which depends entirely on the operating environment of your castings. For different stainless steel castings, there are different magnetic property standards. Understanding these standards will give you a technical advantage when discussing the magnetic content of your castings with manufacturers.
For Non-Magnetic Applications
For medical devices and sensors, the factory should strictly adhere to ASTM A342, perform 100% high-temperature solution heat treatment and rapid water quenching, and use the Low-Mu Indicator to verify that magnetic permeability does not exceed 1.02.
For General Industrial Hardware
For components such as valves and pipe fittings, the factory should comply with the ASTM A351 standard and use a calibrated Ferritescope to measure the ferrite number and control the ferrite content between 3% and 10%, thereby improving tensile strength and resistance to stress corrosion cracking.
Final Thought
Understanding the causes and standards for magnetism in stainless steel casting empowers you to optimize material specifications without incurring unnecessary re-casting costs. If you’re looking for a reliable stainless steel investment casting manufacturer, please contact us. Pingheng Machinery, with excellent quality control, rigorous inspections, standard testing equipment, and one-stop service, can provide stainless steel castings that perfectly meet your requirements.
FAQ
Q1. How do you control the magnetic properties of 304/316 castings?
We strictly control every step of the process in accordance with ASTM A342 and ASTM A351 standards. Before casting, we use a spectrometer to strictly control the chromium and nickel content in the 304/316. We also control the temperature and water-quenching rate during solution heat treatment and manage surface stress during grinding to ensure that the magnetic properties meet your requirements.
Q2. Does a change in magnetic properties affect the corrosion resistance of stainless steel castings?
If the magnetic properties are due to work hardening during machining, and some of the austenitic structure is transformed into martensite, it will not significantly affect corrosion resistance. However, if the magnetic properties are caused by improper solution heat treatment, which leads to the precipitation of ferrite or carbides and the breaking of the passivation film, this may slightly reduce the casting’s resistance to pitting corrosion.
Q3. Are 304 and 316 castings used in non-magnetic environments scrapped if they exhibit magnetic properties?
Not at all. 304 and 316 castings that have just been poured or machined may exhibit slight magnetic properties, which is a perfectly normal phenomenon. They can be restored to their non-magnetic state simply by undergoing a secondary solution heat treatment.
Q4. Is there a difference in magnetic properties between 304 stainless steel castings and 316 stainless steel castings?
Yes. 316 contains approximately 2%–3% molybdenum and a higher nickel content, resulting in a more stable austenitic structure than 304. Therefore, under the same heat-treatment or machining conditions, the internal structure of 316 does not easily transform into strain-induced martensite. So 316 typically exhibits lower magnetic properties than 304 and is less prone to developing magnetism due to minor deformation during machining. If you want to learn more about selecting the right alloy for your application, please click ”CF8 vs CF8M”.
Q5. How do you ensure that the magnetic properties of your stainless steel castings meet the required standards?
In accordance with ASTM A342 and ASTM A351, we are equipped with professional ferrite detectors. Before stainless steel castings leave our factory, we will analyze the ferrite content and provide material analysis reports for you. We also conduct either 100% inspection or sampling inspection based on your requirements. This ensures that the internal magnetic properties of the castings fully meet your specifications.
