Just Curious

[george tichbourne]

What makes 300 series stainless virtually non magnetic and 400 series stainless magnetic? Has it anything to do with the nickle or the carbon?

I am asking because lately I have been recieving stainless in my shop labled 304 and 316 with a definite magnetic signature, usually from Asia.

 

[mete]

The nickel in the 300 series makes austenite stable at room temperature .The 400 series , without the nickel ,can be heat treated. The 300 series has an Ms below room temperature but there is the Md temperature [martensite transformation by deformation] which is at or slightly above room temperature. The Md temperature is the point where it is possible to transform at least partially to martensite by working the steel .If you check cable made of 300 the working [cold drawing] will do this so the cable will weakly attract a magnet.The 300 series , if annealed, should not attract a magnet but might if it's hard [1/2 hard etc] .If it's annealed and attracts a magnet then it's been mislabeled or it's some of the 'funny' steel that come on the market recently.

 

[george tichbourne]

The unusual magnetic 304 and 316 is usually hot rolled and pickled product.

I put it down to poor quality control at the point of manufacture. The magnet will be attracted almost as much as on mild steel.

 

[Matt Shade]

I was gonna try to answer this but I'm glad Mete found it first

I'll toss out my overly simplified answer, to see if maybe I would have got it right.

Whether something is magnetic or not has to do with electron spin and how they are aligned. That comes down to atomic structure.
We know that the steel we work with has 2 main structures that we are interested in, Martensite and Austenite. There are others as well, pearlite, bainite, cementite, ferrite etc. but the austenite and martensite are the 2 main ones to talk about right now.
Austenite is defined as a solid solution of carbon in FCC (gamma) iron. Gamma iron is not magnetic.
Ferrite is a solid solution of carbon in BCC (alpha) iron,(alpha iron IS magnetic). Martensite comes about by trapping a carbon atom in ferrite, so you are still dealing with alpha iron, which is magnetic.

In the tool steels we use, austenite is only formed at high temperatures. That is why they turn non magnetic when we heat them to critical temp. When we drop them to room temp we either form ferrite or martensite, both of which involve alpha iron, so then it is magnetic.

300 series stainless steel is an austenitic stainless. It is in austenitic form at room temperature (apparently because of the nickel, didn't know that until now ). So it is in its non magnetic form at room temperature.

Right

 

[Dan Gray]

dang and I was just going to say yes carbon will make it magnetic..

one of these days reading this stuff over and over it's going to sink in to my brain.