Quenching Oil Question

So if nearly half the 1095 blades that are brine quenched end up cracked or whatever, wouldn't the other half be on the verge of cracking or have micro-fractures already? I am just thinking, wouldn't a less-than-perfect oil quench where not all the pearlite has transformed be better than a blade that was quenched in the brine and may contain fractures? This may be a stupid question, but that is a mark of intelligence in the making. If pearlite is basically un-hardened steel, then wouldn't having a small amount of it in the steel matrix of a hardened blade be a potentially good thing? Wouldn't it make for a more flexible, tough blade? Kinda like having hard and soft steel folded together to make a stronger steel? Go ahead, I'm ready for whatever sarcasm you have to deliver......Let me just get out those dixie cups......
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Tougher, yes, more flexible, no. Ductility and flexibility are not the same thing. The only way to make a blade more flexible is to grind it thinner. This is because when you flex a blade you are actually stretching the steel on the outside radius of the curve. Thus, you rely on the elasticity of the steel to return that stretch true. The ability of steel to stretch and return does not vary by composition or heat treat.

However, your point is on the right track as far as it goes. Given that most of us hobbyist scale makers don't have fully optimal equipment, if you have a choice between having some fine pearlite or retained austenite mixed into the steel, or having a network of fine microcracks, I can't think of a single maker I know who would pick the cracks.

The whole point of this thread, however, seems to me a refutal of the idea that this is a choice even us sub optimal hobbyists have to make. If our austenizing temperature and soak time are appropriate for the steel we are treating, and the quenching rate is similarly appropriate, the point is to convert to something approaching 100% martensite, without the stress cracks.

Take away the appropriate austenizing, soak, or quench rate, and substitute it for something less than appropriate, and you are back to making sacrifices. This is where Kevin gets on his soapbox, as the claim is so often made that you can improvise, make do, and use various methods that may or may not provide that measurably appropriate heat treat and still not make any sacrifices. When people deliberately stick their heads in the sand, and then continue to spout off about the superiority of their voodoo magic, it tends to get under his skin, which is almost always educational for the rest of us to watch.
 
1095 is a water quench steel, 1084, 1080 etc are listed as brine quench steels. In our size and application brine should be fine for 1095, it isn't going to be so fast that you get micro-fractures in the steel.

It's my belief that the large number of blades that get lost in brine are ground too thin or get an uneven vapor barrier causing distortion and cracking. I understand that it is -very- fast for the steel and that can cause stress and rip it apart, but I don't think we see this happening too often, especially if we're talking about straight quenched blades.

It's my opinion (just my opinion) that people should just stop using 1095 all together if they aren't going to use a "proper" quench. If a person doesn't want to buy the "right" oil and they aren't willing to take a loss in brine then why use 1095? 1080 and 1084 will make a knife quite on-par and they are so much more forgiving in their heat treatment. Why would you risk large amounts of fine perlite on 1095 when you can get near complete martinsite in 1084? Oh, and 1080/1084 is cheaper ;)
 
I just might try the brine quench with 1095. I make convex ground blades, so they're not overly thin to start out with. I might have a hard time getting the proper quenching oils up here in Alberta anyways. Just a couple questions, how much salt do I need to add for it to be considered a brine and what temperature should it be at before I quench. Thanks.
Also, would a 1/8" thick convex ground blade taken down to 1/16" at the edge be too thin for this type of quench?
 
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" weasel pee and bear grease quench" :eek::D

That's funny; I don't care who ya are! :thumbup::D
 
i just quenched a knife with an 8" long blade yesterday in room temperature used canola oil and it turned out great. no cracks and the blade is hard. (i did get hungry after the quench since the oil was used to fry chicken in :D)
 
Heat treating isn't everything!

There,... I said it!

It's almost like some folks think heat treating is a magical transmutation process which will always result in a superior knife regardless of any other considerations... (geometry etc.)... like heat treating is the only thing they know and the only truly important consideration to make. It gets me when some of the bladesmiths who lack in other areas keep trying to make heat treating some kind of catch all or holy grail in terms of quality and performance. Then, heat treating starts to look like just a bunch of marketing hype and BS designed to mislead the public and sell knives for an elite few.

I'll say this though, If everything else is equal then the blade with the superior heat treating should perform better,.... but when is everything else equal?

In terms of marketing and commercial success, in just about any business including custom knives, "Design" is going to be what sells the piece,... not heat treating.

Overall performance is all about #1. Design, #2. Design, #3. Design...

No matter how good the heat treating is, it won't make a bad design any better.
 
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Many of the issues now being touched on in this thread get quite complicated. My inside joke to Fitzo regards a recent conversation with him in which I simply lost my motivation to continue posting the same facts, not because new faces wanted to hear them but I became quite discouraged by the number of old faces who seemed to have gotten none of it over the years and I asked Fitzo if what we were doing was tantamount to bailing out the Titanic with a Dixie cup. As I told him, we can build towers of factual data only to have them toppled with a single pen stroke of the “experts” referred to in earlier posts. However today I feel more energized, folks like Mr. Reichert are responsible for that with their patience with our cynicism and desire to hear more facts than fancy.

I have some input on a few topics raised so I will break each down into separate posts to avoid confusing myself. I will simply try to ignore Tai's input for now so that I can get useful information posted without the entire thread descending to the depths he would prefer. Perhaps later I will have time to play:), but for now where to begin…
 
The concept that industry has given us of water, oil and air hardening steels is based upon their standards for depth of hardening, thus I often refer to them as “deep” or “shallow” hardening. The deep or shallow part comes from the very old industry standard of the Jominy quench test. In this test a 1” round bar of the steel in question is austenitized and then subjected to a water spray. The sample it then sectioned and hardness tests and examination is done to determine the depth of hardening. In this test something like O1 would be a deep hardening steel due to the fact that the cross sectional hardness may go right to the core. W1 W2, 1080, 1084, 1095 etc… would all be shallow hardening steels since they would form a well defined hardened skin of varying thickness over a soft pearlitic core.

With oil hardening steels, an oil bath is all that is necessary to get through hardness on such a test sample. On water hardening steel a quench as severe as water or brine would be necessary to get through hardness. Much of industry uses cross sections at or larger than the standard Jominy sample.

Quenching occurs in perhaps 4 distinct phases, there is the vapor jacket forming phase where the hot steel first contacts the liquid and instantly converts it to gas. There is the full vapor jacket phase where that gas forms a complete liquid barrier which can totally insulate the steel from the cooling liquid during the most critical time for cooling (from 1200F to 900F). Then there is the vapor discharge phase where the jacket would normally break down that the rumbling and violent boiling occurs. Finally there is the direct liquid cooling phase where the liquid has unlimited ability to use conduction and convection to extract the heat from the steel, it is in this final phase that actual hardening occurs.
 
In the simplest terms hardening steel is a matter of putting carbon into solution and trapping it there to form a supersaturated solution and room temp where I would rather be a more stable condition. The more stable condition normally forms at around 1000F when the carbon and iron separate out into pearlite. With quenching our goal is to cool things too fast for this separation to occur, so with simple steels the secret to proper hardness is not making pearlite. Thus the more rapid the cooling to 900F the better. If we succeed the steel still is not hard until it approaches temperatures below 500F. at this point the new highly stressed phase of martensite (hardened steel) will form. When it forms the steels expands markedly and if this expansion is way out of balance the steel will pull itself apart, thus the more gentle the cooling during martensite formation the better.

So the ideal quench cools a s quickly as possible from 1200F to 900F and then cools much more slowly from 500F to room temperature. Heat treating oils are designed to do this, only God knows what he designed canola or olive oil to do, but I doubt it was quenching steel. Water, however, does the exact opposite of what is best, it rapidly forms a thick vapor jacket that doesn’t want to quit until things are below 212F and then cools very rapidly to room temperature once the jacket collapses. It gives us the worst possible effects for the operation. Water does not warp blades from cooling too quickly through the pearlite stage, it messes things up because the violent bubbles cool things very unevenly. Adding brine causes precipitates to form on the steel which aids in breaking the vapor jacket.

The vapor jacket is why agitation is extremely important with both oil and water. Now when the martensite transformation begins, a dramatic shearing action occurs at the speed of sound at a sub microscopic level within the steel, push this along any faster than is necessary and you can see how you are not doing the steel any favors. This is where water excels at making that dreaded “ping” sound with our blades.
 
Tai, first let me say that I love your designs and the flair with which you shap your work.

That being said, I think that you can have a poorly designed knife with bad geometry, or a knife with superb design and geometry, and as long as you nail the heat treat and get the thing sharp, it's going to cut and last. It will still be a knife. Now a poorly designed knife won't win any cutting/chopping competitions, but it will handle mundane daily tasks and hold an edge well as long as the heat treat is correct, design aside.

On the other hand, you can have a superbly designed knife with wonderful geometry, and if you muff the heat treatment, it will be a beautiful piece of art that doesn't hold up to daily use. So in light of that, it would seem that the correct heat treatment is paramount over design in the creation of a hard use knife.

Tai, I completely agree with you that design and heat treatment must go together to get the most out of a knife. Design is critical in performance. I'm not trying to disagree with you, just throwing my perspective in there.

--nathan
 
Every blade we can work with is considerably different in cross section than the standard Jominy sample. We often work with steel that is ¼” or less thus heat extraction to the core needs to be much, much less drastic to avoid pearlite, and many spec sheets for the “water hardening” steels suggest oil in sections under ¼”. Quenching speeds beyond that which is needed to avoid pearlite are not only unnecessary, they can be detrimental. You want maximum hardness due to carbon in solution, not extra strain induced hardness that will result in unnecessary stress.

Now here is where things get more complicated. And here is where the differences from 1084 and 1095 start to show. The way the steel hardens will be profoundly affected by the amount of carbon put into solution in your heating (you can very effectively adjust hardenability up or down simply by how high or how long you heat it, but that would require 5 more threads to fully explain). With carbon steels the magic number is .8% carbon. Obtainable hardness climbs steadily with every added point of carbon until .6% is reached then it begins to level off until not more gains are made at .8% . Beyond this range is overkill. When you put over .6% carbon into solution you begin to make a large brittle plate like martensite, that is very prone to micro-fracturing. If you don’t carefully watch your austenitizing temps with any steel over the eutectoid range (.77%- .80%) your blade will indeed be riddled with fractures that you will need 1000X magnification to see, but they are there.


Here is a micrograph taken from a Japanese sword that shows fine pearlite (the little black cancerous looking splotches) peeking out amidst the martensite. A file will not detect this stuff:
finepearl.jpg



Here is a micrograph of plate martensite in 1095 riddled with micro fractures, these are not detectable under normal examination but result in a severe loss of overall toughness and can eventually contribute to fractures on the intergranular level, in plain English that would be- “snap!”:

plate1.jpg


Here is a micrograph of water quenched 1095 where the cracking made it to the intergranular level and it is only a matter of time that the steel comes apart (this eventually happens in all of my water quenched samples if they are not immediately tempered):

5.jpg
 
Kevin, That sounds like loser talk to me...

It sounds like once again you are sacrificing looks and performance for metallurgy...
 
No matter how good the heat treating is,... it won't make a 3 inch ball bearing cut.
 
Tai Goo said:
No matter how good the heat treating is,... it won't make a 3 inch ball bearing cut.
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Nuts. I was anticipating strong demand for my new Ball Bearing Straight Razor design. Think of it--no more honing, no more nicks when shaving... It's all out the window now. :)

Josh
 
Tai Goo said:
No matter how good the heat treating is,... it won't make a 3 inch ball bearing cut.
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We're talking about knives (and knife-shaped-objects), not ball bearings. :) It's all perspecitve anyhow. With any two geometrically identical knives, heat treatment is going to determine if one performs better than the other.

With any two identically heat treated knives, geometry is going to determine if one performs better than the other.

Apples and oranges. (But heat treatment still seperates a knife from a knife-shaped object).

--nathan
 
Tai Goo said:
No matter how good the heat treating is,... it won't make a 3 inch ball bearing cut.
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Tai, I think an important point is that heat treating is one of the more misunderstood parts of blade making.

I completely agree with you about the importance of design, but there aren't many new guys logging on to this forum and asking why their ball-bearing shaped knife won't cut anything.

Design sense is something that has to develop over time. Heat treating ought to have a much shorter learning curve. The problem is that there's so much misinformation that there that heat treating becomes shrouded in mystery. If there wasn't so much misinformation, we would all follow a basic set of principles and get pretty good results.

Josh
 
If it looks like a knife, feels like a knife and cuts like a knife,… Hey! It must be a knife!

If there was one point I could make that would help people understand bladesmithing, sell knives and just be happy,… it would be that,… We aren’t making knives to satisfy microscopes and Rockwell hardness testers. We are making knives to satisfy people!

With this in mind there really isn’t any metallurgically correct way or best way to make a knife,… only and infinite number of ways to interpret and apply it.

What’s better or best is subjective and just a matter of personal preferences, tastes and opinions. No two people are exactly the same.

It’s all good if kept in perspective.
 
silver_pilate said:
We're talking about knives (and knife-shaped-objects), not ball bearings. :) It's all perspecitve anyhow. With any two geometrically identical knives, heat treatment is going to determine if one performs better than the other.

With any two identically heat treated knives, geometry is going to determine if one performs better than the other.

Apples and oranges. (But heat treatment still seperates a knife from a knife-shaped object).

--nathan
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Geometry comes first and heat treating comes second. The geometry is what defines the knife as a knife or cutting instrument, and heat treating is the means by which we support the geometry.
 
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