Re: [MW:16627] MAGNETIC CHECK

Mr. David,

It is commonly stated that "stainless steel is non-magnetic". This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the micro structure of the steel. A totally non-magnetic material has a magnetic permeability of 1

Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1.

The best austenitic stainless steel types for low permeability applications are those with high austenite stability as these have low permeability in both annealed or cold worked conditions. These include the nitrogen bearing types, 304LN (1.4311) and 316LN (1.4406) or the high nickel types such as 310 (1.4845).

It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work (or) welding are liable to increase the amount of martensite and ferrite respectively in the steel and beyond a limit, it may lead to SCC during service.

In welding it could be due to heat input i.e., non-equilibrium cooling (always use a lower heat in put for SS) rate, selection of welding process, shielding gas, Nitrogen (a strong austenite former) pick up which has influence on the ferrite level.

Your weld may have an increased ferrite content (compared to base metal) and attracted a magnet. Hence check the ferrite level in the weld & base metal and you may get an answer for the problem,

Sridhar

From: David.Chan@Exterran.com
To: materials-welding@googlegroups.com
Date: Mon, 28 Jan 2013 13:27:40 +0800
Subject: [MW:16610] MAGNETIC CHECK

Hi Experts,

We have an issue where the Customer is going around on the a stainless steel (S316L) using a potable to check on the materials and welds

He has checked on some areas near the weldment where there is a force of attachment and the magnet is still sticking to the area

The magnet drops off when the magnet is moving away from the weldment

I request for a technical understanding to clarify with the customer

Appreciate your kind assistance

Thank you

Best Regards

David Chan

Republic of Singapore

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