Magnetism and Steel

Stacy E. Apelt - Bladesmith

Stacy E. Apelt - Bladesmith

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Knifemaker / Craftsman / Service Provider
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I answered a question off forum about magnetism, and though it might help others:

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). This is called austenite, and is not magnetic.
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 converts to 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.

Stacy
 
Bringing back memories from my materials class I had in college. At that point in time, I had no real interest in the properties of materials but, now that I'm into knives, I wish I would have paid more attention.
 
One of the new scrap yard guys at my local scrap yard may have learned this lesson the hard way, I was given a commercial stand up freezer that had a non-magnetic stainless steel door and an aluminum case, I cut two sheets of the aluminum for my HT oven, when I took it in I told him the back was aluminum and he promptly told me no it wasn't, it was non-magnetic stainless, he took a magnet out of his pocket and proved it to me, so I played dumb and said ok, aluminum was .30 per lb and dirty stainless was .94 per lb the freezer weighed without the copper and compressor 270lbs

I cut the sheets out of the back using a skill saw with a regular 18 tooth wood blade something you would never do to stainless, when I cut steel roofing it takes a 40 tooth blade otherwise all the teeth break instantly

This was also when I learned there was such a thing as non-magnetic ferrous metal, you could have knocked me over with a feather
 
If a lot of makers would just simply grasp those few paragraphs, and properly define the words therein, they could answer most of their own questions and solve many of their own problems.
 
I've never done the magnet test to test temperature, but I've always wondered if the people that do ruin their magnets. Generally most magnets are ruined by high heat. Does the quick test cause no damage or do they weaken over time?
 
I'm still struggling with some of these concepts. Can anyone explain why austentite is non-magnetic?
 
oic0 said:
I've never done the magnet test to test temperature, but I've always wondered if the people that do ruin their magnets. Generally most magnets are ruined by high heat. Does the quick test cause no damage or do they weaken over time?
Click to expand...
I've used one of those HF pick up magnets a lot. You do not leave the magnet in close proximity for any longer than it takes to determine whether the steel is above critical. Some of my most anxious moments occurred when the magnet stuck to a glowing subcritical piece of steel.:)
 
lastbrunneng said:
I'm still struggling with some of these concepts. Can anyone explain why austentite is non-magnetic?
Click to expand...

An easy way to think about it, when steel is magnetic all the iron atoms are lined up and facing the same direction so they can work together, when you add nickel all the iron atoms are forced to face in different directions and can no longer work as a group

when you heat magnetic steel the heat causes the iron atoms to release the bond they have on each other and allows them to face in different directions
 
Thanks for posting this Stacy. I have had folks explain it to me and read it about it too, but this is the best explanation I have seen to date.
 
I wonder if that guy Stacy could explain Delta ferrite that appears at high temperature and is magnetic ! LOL
Actually magnetism is a physics problem not a metallurgical one ! They are doing some interesting research on supermagnets to develop alloys that work at higher temperatures.
 
Stacy, what about H1 steel which is purported to be austenistic and magnetic?
 
Magnetic austenite isn't that unusual. Plain carbon and low alloy steels can be partially or even fully austenitic before becoming non-magnetic. The austenitic transformation starts below 1400 deg. F. For certain steels it will be finished below 1400 deg. F. Rarely is this information used in hardening steel blades. The lower temperature means much longer soak times and/or multiple quenches are required for full and consistent hardening, and very accurate temperature control for a relatively long time is required.
 
mete - I actually had delta ferrite in the text and deleted it as it just muddies the waters and has no bearing on knifemaking.

Others, there is always a way to push some type of steel to do something it does not want to do normally....but the stable form of steel structures at room temp is austenite - non-mag, martensite/pearlite - mag.\
many stainless steels that have partial magnetic property is due to a mix of alloys and structures.


When learning a technical field like metallurgy with the intent of applying it to a skill like knifemaking, it is far better to learn the rules and not the exceptions. Once the rules are fully understood, the exceptions will often be clear on their own.
 
Just giving you a hard time Stacy !
When we cold work an austenitic steel we will partially transform the structure to martensite so the piece can become slightly magnetic . This becomes confusing to some if they don't know the process.
H-1 , special cases may be hardened by other tha Fe-C martensite transformation. Normally they are hardened by precipitation etc.
 
I have often wondered... if during the adventure to non-magnetic with a welding torch, if you were to have the super strong but small magnets on the handle of the knife blade blank, would the magnets influence the steel in a positive manner as it came back to magnetic steel?
 
MT Borg said:
I have often wondered... if during the adventure to non-magnetic with a welding torch, if you were to have the super strong but small magnets on the handle of the knife blade blank, would the magnets influence the steel in a positive manner as it came back to magnetic steel?
Click to expand...

I read a paper that indicted an influence. But the magnetic field was far greater than that provided by small magnets. Also, the magnets would be ruined by the prolonged exposure to the high temperature.
 
Using STRONG magnetic fields to influence such things as alignment of atoms ,etc., matter in technical items like turbines, but don't matter at all in things like a knife blade.
 
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