Stainless steel is a metal alloy with various desirable properties, and one of these is magnetism – the ability to attract and repel magnetic materials.
All grades of this material have varying magnetism, which can limit or define their applications. In this article, we’ll discuss how magnetic are stainless steel categories and what processes affect this behavior.
Let’s discuss magnetism first.
The attraction or repulsion between certain materials caused by the motion of electric charges within them creates a magnetic field.
This phenomenon is called magnetism, and here are a few terms and concepts associated with it:
These are the regions around magnets or moving charges where they exert an external magnetic force.
Types of Magnetic Behaviour
There are four main types of magnetic behavior such as
This is the most commonly exhibited form of magnetism. You’ll find it in materials, like nickel, iron, and cobalt.
When you apply a magnetic field to these elements, the magnetic moments of their individual atoms start pointing in one direction. This creates a net magnetic moment detected as a magnetic field.
Almost all magnetic stainless steels exhibit ferromagnetism.
Paramagnetism can be found in materials like aluminum, platinum, and titanium.
Here, the magnetic moments of atoms are aligned in random directions. However, they can create a weak net magnetic moment under the influence of another field.
This is relatively weaker than the other types of magnetism. You can find it in all types of materials, including stainless steel. However, due to its weak nature, it is harder to observe since other types of magnetism often overpower it.
Here, atomic magnetic moments point opposite to the external magnetic field. This results in a net magnetic moment opposite to the applied field.
Materials like manganese oxide and chromium exhibit antiferromagnetism. Here, the magnetic moments of nearby atoms aligned in opposite directions, canceling the overall magnetic effect entirely.
The Magnetic Stainless Steel Categories.
The magnetism of stainless steel categories and grades depends on their components, microstructure, and processing conditions. Generally, austenitic stainless steels are non-magnetic. And in contrast, ferritic and martensitic ones are magnetic.
However, this is not always the case, and some variations of these types may exhibit different magnetic properties.
1. Ferritic Stainless Steels.
These stainless steels have a high chromium content and low levels of carbon. They have a body-centered cubic (BCC) structure and are generally magnetic. Their magnetic properties depend on:
- alloy composition
- processing conditions, and
- impurities or defects.
An increase in chromium content can increase their corrosion resistance while decreasing their magnetic properties.
These properties make ferritic steels useful where magnetism is desired, like in the automotive, aerospace, and architectural industries.
2. Martensitic Stainless Steels.
This group is characterized by its high carbon content and low levels of chromium. It is also magnetic and has a body-centered tetragonal (BCT) structure. Its magnetic properties can be influenced by
- carbon content,
- alloy composition, and
- processing conditions.
The magnetic response of these stainless steels increases if the carbon content is increased or other elements, such as nitrogen or cobalt, are added.
Factors Affecting Magnetism in Stainless Steels.
Apart from the composition, magnetic properties can also be affected by external factors such as temperature, stress, and external magnetic fields.
- At high temperatures, the thermal energy disrupts the magnetic moments of the atoms. This can weaken or even eliminate the magnetic response of stainless steel.
- Applying stress to the material can alter the crystal structure of the material, disrupting its magnetic behavior.
- The presence of external magnetic fields can influence the alignment of atoms inside the material, disrupting its natural magnetic behavior.
Non-Magnetic Stainless Steels.
Austenitic stainless steels are non-magnetic. Let’s discuss them in detail.
1. Crystal Structure.
Stainless steels with a face-centered cubic (FCC) crystal structure are non-magnetic. In an FFC structure, the magnetic moments of the atoms are oriented in random directions. Due to this random orientation, the magnetic moments cancel each other out completely.
This structure is why austenitic steels are non-magnetic. However, you can introduce weak magnetic properties in them by adding an excess amount of elements like nickel to their composition.
These are primarily used in:
- Medical implants.
- Food processing.
- Chemical processing industries.
How Welding Affects the Magnetism of Stainless Steel.
During welding, the stainless steel is subjected to a fast heating and cooling cycle, changing its crystal structure. This significantly impacts the material’s magnetic properties.
There are four common types of welding techniques, and they all can affect magnetic properties. Gas tungsten arc welding can significantly reduce the SS’s magnetic properties due to the weld metal’s slow cooling rate.
On the other hand, techniques like gas metal arc welding, shielded metal arc welding, and submerged arc welding can all increase the magnetic properties of the weld material due to their rapid cooling rates.
How Heat Treatments Affect the Magnetism of Stainless Steel.
Temperature significantly affects the magnetic properties of materials. Like welding, the heat treatment processes of stainless steel also involve exposing the material to varying temperatures.
During these, you heat your material to a specific temperature and then let it cool at a controlled rate. The process changes the crystal structure of the material, which can affect its magnetic properties.
Here are three common heat treatments that can have such effects.
- Annealing: This process involves heating the material to a specific temperature and then cooling it slowly, forming an austenitic crystal structure. This tends to reduce the material’s magnetic properties.
- Quenching: Unlike annealing, quenching involves rapidly cooling after heating the material to a specific temperature. This causes a martensitic crystal structure to develop, increasing the material’s magnetic properties.
- Tempering: In tempering, the target material is slowly cooled after heating it to a particular temperature. This process can slightly reduce the magnetic properties of the material.
Stainless steels can exhibit varying degrees of magnetism. While magnetic properties are favorable in some industrial applications, such as motors, they may not be desirable in others, like medical equipment.
Austenitic steels do not exhibit magnetism, whereas ferritic and martensitic steels show it. And although the magnetic properties of the types of steel are embedded in their crystal structure, they can be affected by external sources such as temperature and magnetic fields. So, processes like welding and heat treatment of stainless steel can affect the magnetic properties of the material.
If you’re looking for stainless steel grades with specific properties, feel free to contact us.
1. Can magnetic stainless steel be recycled?
Stainless steel can be recycled irrespective of its magnetic properties. It is one of the most recycled materials around the globe. The alloying elements of stainless steel, such as iron, nickel, and chromium, are all recyclable materials that can be molten in a furnace and reprocessed as stainless steel.
2. Is magnetic stainless steel more expensive than non-magnetic stainless steel?
The cost of stainless steel can vary depending upon many factors like availability, the cost of raw materials, and the seller’s policies. However, magnetic stainless steels require extra alloying elements, making them pricier than their non-magnetic counterparts.
3. How do you clean magnetic stainless steel?
To clean magnetic stainless steels, it is essential to use a gentle solution that is non-abrasive and non-corrosive. The process involves using a mild detergent solution with warm water and a sponge or cleaning cloth to wipe the material’s surface.