Magnetism in welding: how to deal with magnetic blow

Why Magnetic Blow Happens

Magnetic blow is caused by residual magnetism in the steel. High-strength steels, with their fine grain structure, tend to retain magnetism more easily than regular mild steels. Magnetism can be introduced into the material in several ways — for example:

• Magnetic lifting cranes commonly used in production halls

• Previous welding operations, especially when using high DC current

Once the material becomes magnetized, the magnetic field can distort the electric arc and push it away from the weld joint.

How It Affects Welding

Magnetic blow can disturb all types of arc welding processes.

• Pulse MAG welding is particularly sensitive to it.

• TIG and MMA (stick welding) are also heavily affected.

• Short arc welding is the least sensitive, but still not immune.

In practice, magnetic blow causes arc instability, spatter, and arc breaks. In severe cases, welding becomes impossible because the arc refuses to stay in its intended position.

How to Measure Magnetism

Magnetic fields in steel can be measured using a gaussmeter or magnetometer, typically in millitesla (mT). While there is no exact threshold, both tests and experience show that:

• Around 5–20 mT can already affect arc stability

• Over 20 mT often causes major welding issues

If a measuring device isn’t available, a quick practical test can help. A thin steel sheet can reveal magnetism if it sticks to the end of a pipe (see Picture 1).

A thin steel plate stays attached to the pipe due to magnetism.

How to Remove Magnetism

Magnetism can be removed from steel parts using AC current. Traditionally, this requires wrapping a cable around the workpiece and carefully reducing the alternating current to zero—a method that can be quite tricky without specialized equipment.

Now, there’s an easier way.

Kemppi’s new Master T TIG machine family includes an automatic demagnetization function in all AC/DC models. The process is simple: Just follow the on-screen instructions, press the START button, and in about 30 seconds, the magnetism is gone. No extra tools needed.

The demagnetizing function in the Kemppi Master T 355 AC/DC.

The demagnetizing function in the Kemppi Master T 355 AC/DC.

Case Example – SSAB Raex 500 Steel

High-strength and wear-resistant steels, such as SSAB Raex 500, are especially prone to magnetism issues. The following video demonstrates how the Kemppi Master T 355 AC/DC can effectively demagnetize such material, restoring stability and smoothness to the welding process.