Understanding Residual Magnetism and Residual Circuit in Steel

Magnetism plays a crucial role in many industrial applications, particularly when handling steel materials. Two important concepts in this field are residual magnetism and residual circuits. This article will explore these phenomena, their effects on different types of steel, and methods to address them in practical applications.

When a positive magnetic field is applied, the magnetic ions start to fall into alignment.

The more powerful the magnetic field, the more tightly the ions are aligned. If all of the ions are aligned, the material is said to be magnetically saturated (Point b).

When the external magnetic field is removed, the ions do not return to their random state. This leaves some residual magnetism in the material, an effect known as remanence (Point c in Figure 3:).

Residual Magnetism

Residual magnetism is defined as the amount of magnetization left behind after removing the external magnetic field. In another words, the value of the flux density retained by the magnetic material is called Residual Magnetism and the power of retaining this magnetism is called Retentivity of the material

Gauss is a unit of measurement of magnetic flux density. It was named after German mathematician and physicist Carl Friedrich Gauss in 1936. The International System of Units (SI), which uses the tesla (symbol T) as the unit of magnetic flux density.

1 tesla = 10,000 gauss

Field Strength (gauss)	Effect
>200	Permanent magnet
~20-40	Paperclip sticks
>15	Small metal components stick
>10	Small metal shavings stick
>4	Metal dust sticks
~40-50	Interferes with arc welding
~0,3-0,6	Field strength of the Earth

High and Low Carbon Steel

Low carbon steels have small hysteresis and residual magnetism is low. Their residual magnetism is not stable and will disperse naturally. High carbon “tool” steels that can be found in bearings, gears, knife blades will “hold” more residual magnetism and have a tendency to create residual circuit that may require an air gap on pole shoes in order to efficiently drop the part.

Residual Circuit: A Temporary Phenomenon

A residual circuit is a temporary condition that occurs primarily in high-carbon steels. It creates a self-sustaining magnetic flux circuit even when the magnet is turned off. In this scenario, magnetic flux follows a closed loop from the north pole of the permanent magnet, through the steel material, and back to the south pole.
Unlike residual magnetism, residual circuits dissipate once the part separates from the magnet. However, they can cause issues with part release in magnetic handling applications.

Steel Types and Their Magnetic Properties

Carbon Steel (A36, A529, A572, 1020, 1045, 4130)

• Low Carbon Steel (0.05%-0.25% carbon): Small hysteresis, low and unstable residual magnetism
• Medium Carbon Steel (0.29%-0.54% carbon): Moderate magnetic properties
• High Carbon Steel (0.55%-0.95% carbon): Higher residual magnetism, prone to residual circuits

Alloy Steels (4140, 4150, 4340, 9310, 52100)

• Residual magnetism and residual circuits possible

Tool Steels (D2, H13, M2)

• Residual magnetism and residual circuits possible

Stainless Steel

• 300 series: Non-magnetic
• 400 series: Significant magnetic force reduction

Ultra-High Strength Steels (UHSS)

• Exceptional hardness (50-65+ HRC) and yield strength
• Martensitic and TWIP variants can have up to 0.8% carbon content, leading to residual circuit phenomena
• Can cause wear on magnetic gripper surfaces

Addressing Residual Magnetism and Residual Circuits

Vibration

Vibration and impact shocks due to handling can return the domains to their naturally scattered pattern. A small piece of steel can be vibrated to demagnetize it. Sometimes even movement and handling of the part through an assembly line is enough to remove any residual.

Heat

The heat generated from processes such as Hot Stamping, Hot Forming, Welding, paint ovens, machining operations, high frequency quench, and heat treatments can often remove any residual magnetism left behind from handling with a magnetic gripper.

All ferromagnets have a Curie temperature, that is, the temperature where the ferromagnetic property disappears due to thermal agitation. At this temperature, material atoms vibrate strongly enough to demobilize tiny magnetic zones called "domains" in the material.

Degausser

A degausser is an electric solenoid (coil) powered by current can be used to demagnetize magnetic materials. It comes in many forms to suit all industrial requirements including tool, handheld, pen style and table type. In all cases, the current produces a magnetic field and the strength and polarity alternates in order to re-scramble the ions within the part.

Air Gaps

For residual circuits, introducing an air gap or non-ferrous material between magnet poles and steel can break the circuit.

Specialized Coatings

For high-carbon steels prone to residual circuits, Magswitch offers coatings for pole shoes to improve part release.

Wear-Resistant Surfaces

For UHSS materials, engineered sacrificial wear pads can protect magnetic gripper surfaces.

Conclusion

Understanding residual magnetism and residual circuits is crucial for effectively handling steel materials in industrial applications. Low-carbon steels generally have minimal issues, while high-carbon and tool steels require more consideration. By applying appropriate techniques and technologies, these magnetic phenomena can be managed effectively, ensuring smooth operations in steel handling and processing.

For specific solutions to residual magnetism or residual circuit challenges, consult with our magnetic handling experts who can analyze your entire process and recommend tailored approaches.

Magswitch Application Engineers can help you with a solution for your application by understanding your process from beginning to end. If you are interested in learning more about how Magswitch can help you resolve challenges with residual magnetism or residual circuit, please contact us at sales@magswitch.com. We have a team of experts ready to assist you with any questions or inquiries you may have. You can also browse our catalog how Magswitch magnetic grippers are used in different industries and applications.

FAQs

How do I determine carbon content in my steel?

Magswitch Engineering team has compiled a metal chemistry database for many common types of steel and can help you size the magnet and properly estimate a derate based on exact material grade.

Establishing the carbon content in steel is vital to comprehend its characteristics and appropriateness for different uses. Here are various commonly employed techniques:

• Combustion Analysis (LECO Method): This method involves incinerating a sample of steel in a flow of oxygen to create carbon dioxide (CO2). The CO2 quantity is then ascertained using infrared absorption, providing an accurate measure of the carbon content
• Optical Emission Spectroscopy (OES): A small piece of the steel sample is vaporized with an electric spark or arc, resulting in a plasma. Light emissions from this plasma are examined to assess the carbon content
• Infrared Absorption Spectroscopy: As with combustion analysis, this approach relies on detecting infrared light absorbed by CO2 that's produced from incinerating the steel sample
• X-ray Fluorescence (XRF) Spectroscopy: This non-invasive technique employs X-rays to stimulate atoms in the steel, leading to the emission of secondary X-rays. These secondary X-rays are investigated to ascertain the carbon content
• Spark Testing: This comparatively qualitative approach involves rubbing steel against a grinding wheel to generate sparks. Analysis of the spark patterns can suggest the carbon content when compared against established standards

Each of these methods offers distinct benefits and drawbacks, meaning the selection will be based on the precision needed, equipment at hand, and the intended application.

Is there an impact with Residual Magnetism on Arc Welding?

When applied correctly with proper Magswitch tool selection, the tool saturation matches the magnetic field depth matches the thickness of the material being worked on so this minimizes "Stray Flux" from influencing Arc Blow or Arc Deflection in Arc Welding.

What is Stray Magnetic Flux?

Stray magnetic flux, also known as a stray magnetic field, is the portion of the magnetic field that diverges from its intended path, which can arise in various contexts like electrical devices or around permanent magnets. Key points to consider include:

Origin: It originates from the internal magnetization of magnets or from currents in conductors.

Impact: This flux can disrupt the functioning of adjacent electronic devices and sensors, causing errors or malfunctions.

Mitigation: To reduce stray magnetic flux, one can employ shielding, select high-permeability materials, and design circuits to limit leakage.

What is Magnetic Arc Blow, or Arc Deflection?

Arc deflection in welding, often referred to as arc blow, occurs when the welding arc deviates from its intended path. This can be caused by various factors, including disturbances in the magnetic field around the welding area. Arc blow happens when the magnetic field is disturbed by external sources, leading to deflection of the arc. It can cause issues like lack of root penetration, lack of fusion, undercuts, and pores. Arc blow can result in an irregular weld bead, decreased penetration, and defects such as porosity and cracks.