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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #61  
Old 04-09-2015, 06:24 AM
jmccustomknives jmccustomknives is offline
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Cracking during heat treatment can be tracked back to several variables. The first would be that the blade was too hot going into the quench or you used a quench that was too fast for the steel. If you used water for instance, this will crack a blade. The way to determine over heating, since the blade is already cracked is to break it. Examine the break, if you can see crystals then the steel was over heated or you didn't follow good forge theory.
The next thing that can cause cracking could be stress risers. Most of the time these are angles, however hammer marks left in the steel can cause these.
The last things would be forging temperatures. If you are forging too hot (the steel is sparking) then you are lucky if it just cracks as usually knife steels will break apart. The high forge heat will also contribute to large grain growth. Forging at too low a heat will also cause cracking as you are trying to move the metal and it just isn't plastic enough to move so it cracks.
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  #62  
Old 04-09-2015, 07:40 AM
ivance ivance is offline
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My friend makes knives by stock removal method, he told me that the microfractures can also appear if the steel that you use is naturaly tainted. Is this the same for forging?
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  #63  
Old 04-09-2015, 08:04 AM
jmccustomknives jmccustomknives is offline
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I'm not exactly sure what you mean. But if I understand, yes steel can have microfractures without forging. If you were using an old spring it could have microfractures that will show up later. For new steel those are a sign that something went wrong in the manufacturing process.

Breaking your blade will go along way to answering your questions. If it breaks at a fracture (which is what you want it to do) examine first the area where the crack was. Were there oxides formed (black scale) that would indicate the cracks existed before heat treat.

Inclusions caused by forging scale back into the steel will show up differently. These can be caused by forging on a dirty anvil. Always wipe the scale of your anvil between heats. I can't speak for any of the other guys here, but my SOP (standard operating procedure) is to do a rough grind after I finish forging and before the heat treat. That way I can spot any trouble before the heat treat.

Large grains will indicate too much heat. During the forging process use a finishing heat. A magnet will tell you the hardening tempurature but it will also tell you the forging heat you should be using. If the magnet is sticking to the steel it's too cool for forging. The exception to this is the finishing heat. When you finish forging the blade, heat the blade to non magnetic and no hotter. Use light hammer strikes using a flat faced hammer. You aren't trying to move any appreciable metal, just working out the hammer marks. This also works for refining the grain a little. Be sure to normalize.

You didn't say what your quenchant was or what steel your were working with.
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  #64  
Old 04-09-2015, 09:12 AM
ivance ivance is offline
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I am not making my knives yet because i need a lot informations before i start. What i want to say is can steel that is not manufactured in factory, that is meant only for forging process have somekind of error that will result in microfractures during the forging and heat treatment process? I suppose that only the steels that are manufactured and meant only for stock removal method can have error, right?
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  #65  
Old 05-14-2015, 02:14 PM
ivance ivance is offline
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I've been reading one book about heat treatment of tool steels, the autor wrote that during heat treatment first of all is normalization and then annealing, i thought the annealing should be first because the stressess are starting to appear after you grind the blade, when you use new steel, not the recycled one. Can someone give me an advice what should i do first?
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  #66  
Old 05-14-2015, 02:44 PM
samuraistuart samuraistuart is offline
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I too don't understand what you are asking about micro cracks in steel. CAN steel from the manufacturer (whether it is intended for stock removal OR forging) have cracks in it? I suppose so....if there was something that went wrong when the steel was rolled out. NOT GOOD. If you are talking about mystery steel, as in the leaf spring of an old Chevy truck...yes...it too CAN have micro cracks due to stress over it's lifetime. It shouldn't but it can. Also NOT GOOD.

If I bought a bar of steel from a knife steel supply house, will it have micro cracks in it? IT BETTER NOT.

Concerning what you read about tool steel. Tool steels are, usually, hypereutectoid. That means they have more carbon than can be dissolved in ferrite, and they also usually come spheroidized annealed. What that means is the steel manufacturer has "annealed" the piece over many hours, causing the carbon to ball up into spheres. This makes the tool steel very easy to machine/drill/file. However, the spheroidized structure (coarse/heavy spheroidized....not to be confused with fine spheroidized) will not release it's carbon in solution as easily during heat treatment. In order to get it into solution, it needs to be "normalized" first, usually at a temp around 1650F.

Let's say you have some of Aldo's 52100. It comes coarse spheroidized. VERY VERY soft steel to work, but needs a normalizing before heat treating, or you'll likely not reach max hardness level after quench. So the working sequence would be either two ways....

1. Receive the steel. Cut the bevels and drill/file as needed. To heat treat.....normalize at 1650F. Then again (not technically called normalizing now) at 1550F. Then again at 1450F. That is technically really called "thermal cycling". No need to anneal now....because the higher heat of normalizing has erased any previous stresses in the steel. Go right to hardening and then quench and then temper.

2. Receive the steel. Let's say you want to do normalizing/thermal cycling/anneal FIRST before cutting bevels. So it would be 1650F, then 1550F, then 1450F. Now the steel is soft, but it can be softer. If you want to spheroidize anneal it, you can do 1250F for 20 or 30 minutes, air cool, do this a few times. Now it is fine spheroidized. Cut/file/machine as needed. You can, if you like, once all machining is done, do a "stress relief" to relieve stresses, just once at 1250F or so for 30 minutes is all you need. Then go to harden/quench/temper.

There is another method of spheroidize annealing if you have the control. After a quench at 1425F, bring up to 1375F, hold for one hour, then cool to 900F at a rate of 50F per hour. This would produce a very soft fine shperoidized structure. You would do all machining after this, and then either right to hardening, or a low temp (1250f) stress relief if you like.

Normalizing is different than annealing because of what you are trying to accomplish. Normalizing is indeed what it sounds like.....it gets the steel....normal! Carbon and alloys distributed throughout the matrix evenly. That is normalizing. This high heat also will wipe out any stress that might be in the steel.

Annealing is different than normalizing because we aren't trying to distribute alloys evenly, but rather to soften the steel and make it ready to machine.

Last edited by samuraistuart; 05-14-2015 at 02:48 PM.
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  #67  
Old 05-14-2015, 04:56 PM
Doug Lester Doug Lester is offline
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First of all there are no steels that are made only for forging or only for stock removal. Now some steels, due to their make up, are difficult to forge, but if you stay within their working range still can be. Any steel that can be forged can be used in stock removal. All bladesmiths still have to do stock removal. As far as the steel coming out of the foundry with micro cracking goes, I agree with Stuart. It shouldn't be happening but I guess that it could in some extreme cases. Used steel and all bets are off.

Now you can introduce fractures, or even break, steel if you try working outside of it's work range. This is especially a problem with some of the high alloy steels that are usually not forged just for that reason. Some of these high alloy steels can even crumple if they are worked too hot. The best solution to these problems is to work with simpler alloys. You can still work them too "cool" but they are more forgiving.

I work pretty much the same way Stuart does. I forge the blade, get it straight and even, then normalize. After that is grinding, triple normalization (I don't have the heat control to really do thermal cycling), austinization and quenching, then tempering. Then back to the grinding shop for finish grinding, polishing, and assembling.

Doug


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