Magnetic Properties of Stainless Steel
We often receive questions regarding the magnetic properties of stainless steel. Hopefully, the brief discussion that follows will provide answers to some of these.
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1. Can Stainless Steel be Magnetic?
The simple answer is yes, however this is not a measure of quality or performance.
There is a common misconception that the best way to check if a fastener, or any item for that matter, is made out of stainless steel is to place a magnet next to it. Many people believe that if the magnet sticks, then the fastener is not stainless steel. This is not the case.
There are several different types of stainless steel that are used to manufacture commonly available fasteners, most notably “410” and “300” series.
Type 410 stainless will be strongly magnetic. Types 305 and 316 (marine grade), seldom exhibit magnetic properties.
Fasteners made of the other 300 series stainless that are classified as “18-8” types, e.g. 302, 303 & 304 may or may not exhibit magnetic properties. Therefore using the “refrigerator magnet test” is not a reliable or desirable test to determine the quality of the stainless.
2. What May Cause Stainless Steel Fasteners to be Magnetic?
There are two significant factors which may contribute to the magnetic properties of stainless steel.
- The process by which the fasteners are manufactured.
Most common stainless steel fasteners are “cold formed”. During the cold working stage (forming the head, rolling the threads, etc) the fastener is subjected to high mechanical stresses. These stresses may cause the part to become magnetic.
- The nickel content of the stainless.
Nickel strongly influences the potential magnetic properties of stainless steel. To be classified as an “18-8” stainless steel it needs to meet certain specifications. Nickel is specified to be between 8% and 13% of the stainless alloy. The “18” number refers to the chromium content. Chromium is the component that provides most of the corrosion resistance properties to common stainless steels and nickel helps to reinforce that property of the chromium.
Nickel is a very expensive element. When nickel prices are high, stainless alloys are often manufactured to the lower side of the specification range to keep the cost down. As a result the 18-8 type stainless steels will tend to exhibit stronger magnetic properties. However it is the chromium content, not the nickel content which is the major contributing factor to the corrosion resistance of stainless steel.
To view a chart of composition specifications for the common types of stainless steel used for fastener production, click here: Composition Specifications
For more in-depth and technical information there are a number of easily searchable web links available on the internet.
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