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Another Fun Heat Treating Topic This Week.

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Kevin R. Cashen
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Here is another one, that I thought was worth sharing. It is fun, because it is easy when you understand the process but may seem mysterious for many.

One of the weekly phone calls, or e-mail, that I received this week had a good question. I will paraphrase it something like this-

“Kevin, I can see the bright glow created by pearlite formation very clearly when cooling steels like 1075 or 1095, but I have a very hard time seeing the same effects in 5160 or L6. Is it just me or is it actually different with these steels?”

I thought this was a good question because it deals with real concepts that are actually much deeper than they first appear. First, if you have the same concerns as the questioner, it is not just you, there is an actual reason for this, and it is important to know about when normalizing.

On heating, a simpler carbon steel, say 1075, things will begin to glow dimly at around 1100°F to 1200°F and then get continually brighter until around 1325°F when there will be a band of shadow that will move through the steel from the edges to the center. This is decalescence, the endothermic reaction that occurs as the crystalline iron structure is reformed into austenite solution. This reorganization of the atoms requires more energy and thus creates a dimming, or shadow, in the steel as the heating is arrested. This is what the first “critical temperature” of Ac1 is – the first arrest on heating.

I include the preceding paragraph to give perspective on the true topic of this discussion, which is what happens when that austenite solution is cooled during normalizing. When the same steel is cooling it will actually go much cooler than the arrest point of decalescence and that is why “recalescence” has a different designation of Ar1, as it occurs at a much lower temperature than decalescence. But recalescence is much more dramatic. As the blade is going from dull red to black there will suddenly be a bright wave of energy that will sweep across the steel as the exothermic reaction of pearlite formation occurs. It is very impressive in something like 1075 because it is right in that heat range where a few degrees will really glow in the darkening steel. It is very easy to normalize 10XX series steels by eye because of this. With the only appreciable alloying being manganese, there it very little to suppress this reaction in this range.

But the very same thing that make alloy steel, like 5160 or L-6, easier to harden in oil, is the exact same thing that will make recalescence a very anticlimactic, if not all out boring, affair when normalizing these steels. The alloying, mainly chromium, suppresses the pearlite reaction, pushing it much lower into the ranges where there is little glow, if any, left in the steel. With 5160 you will still get pearlite, but it will be a very fine pearlite, created at much lower temperatures than 1075. This finer pearlite, mixed with traces of upper bainite, has given some people the impression that 5160 can air harden, but it really cannot form martensite this way. The issue is with the traces of upper bainite. Upper bainite is the worst of all possible outcomes with the brittle tendencies mixed with low strength and this is why 5160 may seem to break like it has been hardened after an air cool, but it really wouldn’t have any of the other qualities of hardening that you would want.

L6 on the other hand is very resistant to forming pearlite at all and will make mostly an ugly mix of bainites as it air cools. It can reach 61HRC from an air cool but will be surprisingly brittle while still lacking overall strength. It won’t “recalesce” until around 800°F, so you can wait all day for the glow but your will not be seeing it. If you have any doubts about this, the other indicator of the iron recrystallization is the ever-popular magnet. Check a heated bar of 1075 with a magnet and as that glowing band appears, the magnet will also begin to grab the steel. Then try the same with L6 and you will find the steel is completely black before the magnet begins to pull on the slowly forming upper bainite.

It is the suppression of pearlite formation via alloying that moves us from fast oils and waters for one steel (1075), to medium speed oils which will fully harden a steel like 5160 or L6. When pearlite is not formed it is much easier to cool fast enough to avoid the bainites and the next stop is martensite at around 400°F for many steels we work with.

For this reason, while I am not a fan of relaying on a magnet for decalescence, a magnet is quite handy for spotting these differences on the way down during normalizing when, there may be no incandescence left for our eyes to work with.

"One test is worth 1000 'expert' opinions" Riehle Testing Machines Co.

 
Posted : 12/07/2020 3:53 pm
Karl B. Andersen
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Nice. Thank you.

Karl B. Andersen

Journeyman Smith

 
Posted : 12/07/2020 6:28 pm
Dan Hertzson
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"medium speed oils which will fully harden a steel like 5160 or L6"

Kevin, Just a quick side question: Other than concerns about cracking developing from overly accelerated hardening, is there any practical reason why 5160 can't, or shouldn't, be hardened in a fast oil like Parks 50?

Thanks.

 
Posted : 12/07/2020 10:18 pm
Dean Pavia
Posts: 197
Estimable Member Apprentice Bladesmith
 

Thank you Kevin, very interesting and helpful.

 
Posted : 13/07/2020 1:56 pm
Kevin R. Cashen
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|quoted:

"medium speed oils which will fully harden a steel like 5160 or L6"

Kevin, Just a quick side question: Other than concerns about cracking developing from overly accelerated hardening, is there any practical reason why 5160 can't, or shouldn't, be hardened in a fast oil like Parks 50?

Thanks.

Let's put it this way- just read the same question, but with "Parks#50" replaced with "water", which is what that oil was actually designed to replace.

"One test is worth 1000 'expert' opinions" Riehle Testing Machines Co.

 
Posted : 13/07/2020 9:00 pm
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