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Silver or Stainless?

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Nicholas Farr18/10/2011 20:55:21
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Posted by Wolfie on 18/10/2011 13:17:52:
As a matter of interest, why is stainless non-magnetic surely it has plenty of iron in it??
Hi Wolfie, to understand this fully, you would have to learn a bit of metallurgy. Below is a graph of carbon steels, the temperature on the left and the carbon content at the bottom. The graph shows the metallurgical state that cabon steels are in against temperature.

The next bit is a reply I made in the "Chronos Silver Steel, water or oil quench" thread.
 
Sam, I believe you are describing the allotropy of Iron.
I had to dig out my college notes from 1977 for this one.
Iron can exist in two forms, Ferrite and Austenite. On cooling, it is in its Austenite form at temperatures above the upper transformation line (695 to 920 degrees C)
As cooling continues through the transformation zone (between upper and lower lines) the Austenite changes to Ferrite. During this change Carbon is precipitated from the Austenite, because Carbon is insoluble in (Iron) Ferrite. The precipitated Carbon is in the form of Cementite which goes to form Pearlite, or in the case of steels with over 0.83% Carbon the excess cementite remains on the grain boundries.
Time is needed for these changes to take place. A slow cool will enable the changes to take place and will result in a large grain structure. A faster cool will result in a smaller grain structure.
However, if the cooling rate through the transition range is increased sufficently (i.e. quench) time will not be allowed for the changes from Austenite to Ferrite to take place. In this case there is insufficient time for the Carbon to be precipitated out of solution and this result in the Carbon atoms being forcibly locked in the Ferrite structure. This may be described as a super saturated solid solution of Carbon in Ferrite Iron.
Due to the distortion caused by the Carbon, the grains are acicular (needle like) and are very hard and brittle. The name of this structure is Martensite.
 
As I said earlyer, only Austenitic stainless steels are non magnetic, and this is because there is sufficient Nickel, Nickel and Chromeium or Manganese, to maintain the structure of Austenite.
 
Hope this helps.
 
Regards Nick.
Stub Mandrel19/10/2011 21:19:09
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I think the question then becomes, why is austentite not magnetic...
 
I think (less certainly) that the answer is something to do with the way the atoms are arranged as this affects whether or not their magnetic spins add up or cancel out . if tehy add up, the material is magnetic.
 
I expect wikipedia has the full details...
 
Neil
Andrew Johnston19/10/2011 22:09:13
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Just to confuse the issue; austentite is magnetic. It is paramagnetic, ie, it has a positive susceptibility to an external magnetic field. But the effect is so small that it cannot be detected with the sort of equipment found at home.
 
What most people call a magnetic material exhibits ferromagnetism where the forces involved are easily observed.
 
Just about every material interacts with a magnetic field; it's just a question of the strength, characteristics and observability of the interaction.
 
Regards,
 
Andrew
Gone Away20/10/2011 00:59:22
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Posted by Ian S C on 14/10/2011 08:46:19:
I have been fooled by a bit of chrome plated brass, non magnetic thought it was stainless. Ian S C
 
 
 
I bet a few people have also been fooled by some of the imported "brass wire brushes".
 
Many of these will handily stick to a magnet.
Steve Withnell20/10/2011 17:30:45
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Posted by Sid Herbage on 20/10/2011 00:59:22:
 
I bet a few people have also been fooled by some of the imported "brass wire brushes".
 
Many of these will handily stick to a magnet.
 
J&L sell real brass wire brushes, which are also useful!

Nicholas Farr21/10/2011 20:11:22
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Posted by Robin Woodward on 20/10/2011 10:20:17:
Hi All,
 
Nick just took me back to 1970........................................................
 
 
I vaguely remember the terms "Face Centre Cubic" and "Body Centre Cubic" as regards where the atoms are and this may have something to do with the strength of the Magnetic forces, I am sure there are some who have a better grasp of the theory than I do after 40 years who can answer this.
 
Rob
 
Hi Rob, you are thinking of the Allotropy of Iron in which;
at room temperature ferrite has a "body centered cubic" structure and is magnetic, and will only take 0.006% carbon into solution.
 
If the Iron is heated above 720 to 920 degrees C (according to carbon content) it's structure changes to "face centered cubic" and it becomes non magnetic and will take carbon into solution. The name given to iron in this changed state is Austenite.
 
Regards Nick.
Andrew Johnston22/10/2011 12:08:49
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The temperature at which an iron-carbon 'mix' changes from ferromagnetic to paramagnetic is known as the Curie point. For pure iron it is about 770°C, and varies as the carbon content increases. The Curie point is not related to the phase change from BCC to FCC.
 
For low carbon steels the change in magnetism with increasing temperature is a useful indicator of the phase change temperature, but is only an indicator. The two effects are unrelated; they just happen to occur at similar temperatures.
 
Regards,
 
Andrew
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