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Magnetic Permeability

You probably have noticed that many insert installers use a magnet to remove broken off insert tangs from the bottom of blind holes. Perhaps you have wondered how this is possible, as stainless steel in general is considered to be non-magnetic.

Type 304 (UNS S30400) corrosion resistant steel specified in MIL-I-8846 and AS7245 is the material used to produce KATO Tanged and Tangless® Inserts. To understand magnetic permeability, you must first define a reference point. The magnetic permeability of air is defined as 1.0 Gauss per Oersted (G/O). Messrs. Gauss and Oersted were famous mathematicians/scientists who like many of their peers had engineering terms named after them in recognition of their work. Our Mr. Al Qaqish is hoping to someday reach this level of achievement, however we do anticipate difficulty in pronouncing “kakeeshes”. Perhaps the short form “Qaqs” (pronounced quacks) would be simpler.

OK, back to the subject. The Type 304 stainless steel wire that we use to manufacture inserts is procured from the mill with a round cross section, and it has a maximum allowable magnetic permeability of 1.02 G/O in the annealed condition

The wire is cold rolled to the precise diamond shaped cross section, and the result of this process is a work hardening of the wire from a tensile strength of approximately 150,000 PSI to over 200,000 PSI. This process also produces a very smooth (8 to 16 micro-inches) finish on the wire. The work hardening process causes the stainless steel material to become slightly magnetic, thus allowing the broken tang of an insert to be removed using a magnet. The degree of work hardening is also dependent on the nickel content of the material. The lower the nickel content, the higher the degree of work hardening.

The magnetic permeability of a standard free-running insert usually falls between 2.0 and 10.0 G/O. The variation is due to the different lengths and wire sizes.

Usually locking type inserts have a slightly higher G/O value due to the additional work hardening that results from the formation of the locking coil. This is exciting stuff – right?

If you have any questions contact the KATO Tech-Group. To view more technical articles please visit KATOpedia.