When doesn't a magnet stick to steel

[GoldSkula]

We all know the trick of testing a piece of steele with a magnet. I know the temperature is something like 800 degrees celsius but does anyone know the (semi)exact temperature when this happens.

 

[Stacy E. Apelt - Bladesmith]

It is called the Curie Point - 1414° to 1418°F for steel. ( 768°-770°C).

The atoms rearrange as they prepare to form austenite and lose their ferro-magnetic property. Interesting enough, it will return to being magnetic around 1300°C until it becomes liquid at 1500°C. This phase is called Delta-iron. It looses its ferro-magnetism again as a liquid.

Here is a short article I wrote a few years back about this subject:
Steel is ferromagnetic, which means the iron atoms react by trying to align with the magnetic field of the earth. All carbon steel is magnetic at normal temperature. Some types of stainless steel is not magnetic. More on that later.

Normally steel has its atoms arranged in a body centric cubic structure (BCC). This is called alpha iron and is magnetic. The normal structures knifemakers deal with at room temperature are called pearlite and martensite.

When the steel is heated above the curie temperature, which is about 1414F, the structure changes to gamma iron and becomes face centric cubic (FCC). At this point it is not magnetic. It has become austenite.
When the steel cools down past the curie point again, it normally changes back to alpha iron and becomes magnetic again. Cooled slowly, it becomes pearlite at 900F. If cooled fast enough, it stays austenite until it reaches 400F, where it begins to converts into martensite.

If the steel is alloyed in such a way that it remains austenite at room temperature, it stays non-magnetic. This is why austenitic stainless steel is non-magnetic or barely magnetic. Austenitic stainless will not harden, and is good for guards and bolsters, but not for blades. Martensitic stainless steel is what we use for knives because it will harden.

In HT, we heat the steel up, and as it crosses 1414F it stops sticking to the magnet because it becomes austenite. We heat about 50-75F higher to get to 1475-1500F to allow the alloy ingredients to get properly distributed, and then quench in a quenchant fast enough to get the steel cooled down past 900F without the structure changing to pearlite. The rate of cooling varies from 1 second to drop past 900F for simple carbon steel like 1084/1095, to many seconds for alloy steels like O-1 and 52100, to many minutes for high alloy stainless steel. The quenchant type should match this need. Once safely past 900F it can, and should, cool slower. It will remain austenite and be non-magnetic until it reaches 400F, where the conversion to martensite starts. It will continue to convert to martensite until most all the steel is that structure. For carbon steels, room temperature is good. For high alloy and stainless steels, it needs to drop to -100F before all the conversion is finished.

 

[Larrin]

The curie point is not the temperature it transforms to austenite, it is the temperature that ferrite is no longer magnetic. Usually knife steels transform to non-magnetic austenite before they reach the curie point.

 

[GoldSkula]

Thank you for the replies!

 

[mete]

We have three temperatures, Ms, Md, Mf. The start , finish, transformation temperatures and that where deformation [cold work ] will cause transformation. Cable made of 304 SS will often be partially transformed . Ask your magnet to help you !

 

[Stacy E. Apelt - Bladesmith]

Thanks Larrin. I edited the wording to make it more clear. That was an old article I put together to answer a similar question about what is happening as we heat and cool steel, and intended ( at the time) to be a basic description.

 

[Larrin]

We all know the trick of testing a piece of steele with a magnet. I know the temperature is something like 800 degrees celsius but does anyone know the (semi)exact temperature when this happens.

It varies by composition (manganese lowers the temperature, for example) and kinetics (speed of transformation). Using this diagram it is 738°C for steel with >0.67% C but that is for a steel with only carbon added to it and assumes an infinite hold time at 738°C:

I have some micrographs at the end of this article for 1080 steel: https://knifesteelnerds.com/2018/02/28/austenitizing-part-1-what-it-is/
If you hold 1080 at 1346°F for 5 minutes it is fully transformed to austenite and therefore non-magnetic. At higher temperatures it would become nonmagnetic more quickly.