Mixed Structures in Knife Steels

me2

Well-Known Member
I was thinking about this last night and thought it might be a topic others have wondered about as well. In all my reading of various shop talk forums, no one has really addressed this in detail.

Much has been made of hardness testing and many more makers are using it to check on the various heat treating methods. Makers have target hardness they aim for, but they often get there by a multistep process of heating, quenching, and tempering. Much has also been made of getting "full hardness" from the quench then tempering back to the desired hardness. More than one new maker has probably wondered if you can't just quench to the desired hardness. Why go to the extra trouble of getting a blade as hard as possible, then drawing the hardness down in another step?

The answer is you can, through careful selection of steel and quenchant, just heat and cool to the desired hardness. The question then becomes why not do it that way. The answer I came up with, while waiting for my son's stomach to calm down in the middle of the night, is mixed structures that can result and the undesirable issues that come from them.
 
For the sake of new members to the heat treating forum, a quick review of the phases we could get from heat treating steel is in order.

Austenite: this is the structure we're after during the initial heating before the quench. The structures below come from changes in austenite as it cools.

Ferrite: this is a room temperature structure of steel. It's soft and fairly ductile. It is nearly always mixed with other structures when talking about knife steels.

Carbides: these are the hard particles in steel that result from the carbon bonding with various alloying elements. Iron carbide, chromium carbide, and vanadium carbide are common in knife steels. These are typically surrounded by ferrite, austenite, or martinsite, depending on what we've done to the steel.

Martinsite: this is the structure we knife nuts are after. It's the reason for quenching. It is hard and fairly brittle and will hold a thin edge.

Pearlite: this is a mixture of ferrite and carbides, arranged in alternating layers. It is steel's own naturally occuring damascus, with alternating layers of hard carbide and soft ferrite. It's soft, ductile, and generally undesirable in cutting edges.

Bainite: this one is not mentioned much in knives, but it's there and deserves attention. It is a mixture of ferrite and carbides, but not in layers like pearlite. It is stronger than pearlite, weaker than martinsite, but tougher than both.

Retained Austenite: this is the same as above, but it's still present at room temperature or below. It causes various issues.

There are other structures possible, but they are rarely encountered in knives, so I'll leave them out.
 
Every steel has an individual threshold, depending on alloying to get all the individual ingredients into a homogenous solution. That's the target, without reaching it you still have a mish-mash of ingredients that is inherently weak and unsuitable for it's intended task. Most common for us, we're chasing the carbon which reacts differently with what ever other ingredient it needs to interact with...which requires different temperatures. This is somewhat simplistic but easy to understand by my shop visitors.

Rudy
 
Just a caveat to all this. Mixed structures aren't always undesirable, but to start mixing them, we need to know what properties we're after and which structures to mix.

Let's deal first with the undesirable mixtures, as this is what would lead to the ability to heat something like 1080 and quench it directly to say 60 HRc hardness without a tempering step.

Martinsite is the hardest of the structures, except carbides. As carbon content goes up, so does the maximum attainable hardness. Quenching our 1080 into water, brine, or a fast oil will give us as much martinsite as possible. However, a tiny amount of pearlite may still form, and some retained austenite will be there as well. The retained austenite and whatever pearlite are present are much softer than martinsite, and will lower the overall hardness of our piece if too much is there. This may sound like a good idea, as it's generally thought softer is tougher and will prevent chipping.

However, in a case of martinsite with an excess of pearlite or retained austenite, we have sort of a worst cast scenario. Tempering of simple steels like 1080 will take care of the retained austenite, or we can use cold treatment to get rid of it. However, these will not get rid of pearlite. Say we quench our 1080 in something like Wesson oil or motor oil. Some recent hardness testing I've seen shows it will be in the neighborhood of 62-64 HRc, vs water or fast oil which will give us 66 HRc or a little more. In the case of either of the softer phases, pearlite or retained austenite, we have a mixture that's rather like good hard concrete with silly putty blobs mixed in. It's important to remember that we are dealing with very thin sections here, and that Rockwell testing doesn't represent those that well. Essentially, you have a very hard matrix (martinsite) with a very soft (relatively speaking) structure sprinkled in, and when that very thin cross section is stressed, the soft structure will just bend.

On the other side of this, let's say you want to make a good chopper out of 1055 or 1060 steel. As the steel is heated to austenite, it will go through a range where it is a mixture of ferrite and austenite. If for some reason, the temperature doesn't go high enough to get rid of the ferrite, when the blade is quenched, there will be ferrite sprinkled into the martensite. The result for a thin edge would be about the same as above.

This scenario also applies to anyone wondering why knife makers would go to all the trouble of getting rid of as much retained austenite as possible in steels where it can be a problem, like most stainless steels.
 
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Every steel has an individual threshold, depending on alloying to get all the individual ingredients into a homogenous solution. That's the target, without reaching it you still have a mish-mash of ingredients that is inherently weak and unsuitable for it's intended task. Most common for us, we're chasing the carbon which reacts differently with what ever other ingredient it needs to interact with...which requires different temperatures. This is somewhat simplistic but easy to understand by my shop visitors.

Rudy

Exactly. Exception is when we want undisolved carbides for wear resistance, so in that sense, my example of 1095 was probably a bad choice. I'll remedy that. However, as you said, for the sake of keeping it simple, and for the simpler steels like 1084 and lower carbon contents, everything goes into solution.
 
Yes, you can do a combination quench and tempering. It's called austempering or isothermal quenching. A couple of problems with it. First it will depend quite a bit on the alloy that you are using as to whether or not you can achieve the target hardness that you want. More so than with carbon content in regular quench hardening and tempering. Another is that it takes a period of time to achieve it because you are going to have to hold the steel at just above the Ms point or at a temperature between the Ms and the Mf point until the austinite finishes converting to lower bainite or a mixture of martensite and lower bainite, depending on your exact method. This could be from a few hours to over a day, depending on the alloy in use. This also has to be done in an oil bath that is dangerously close to it's flash point or a low temperature molten salt bath which has it's own dangers. A commercial marquenching oil is much better to use than, let's say, peanut oil to get a better rate of cooling to beat the nose of the cooling curve, as Kevin Cashen pointed out to me.

Then you have the end product. If you perform this quench at just above the Ms point then you will end up with a blade that is all lower bainite. It might hold an edge quite well an be quite tough but it won't be as strong as a martensetic steel blade. It will tend to bend relatively easily. I did one test blade from 52100 by quenching at 350° to form about 75% martensite and then upquenched it to 430° for four hours to convert the rest of the austinite to lower bainite to get a stronger blade, more of an isothermal quench. It held an edge fine chopping through a 2X4 and bent to just short of 90° before it snapped where I hot stamped it near the handle, not where it was bending. The results were interesting but it took a lot of work. It's not a method to use to make your life easier; it can be really quite involved and time consuming, not to mention a bit more dangerous.

Doug
 
Right on there Doug. The complexity of controlling bainite is why it only got honorable mention above. From the standpoint of eliminating a seemingly unneeded tempering step, proper bainite formation is way too involved. I have had people ask if one could just quench and get the hardness desired, and I have to say yes. These were not knife makers though. I have also seen posts by new makers that indicate they are leaning toward trying this, but generally, they get talked of it by the other makers, but no one explained why other than to say the steel will be too soft. Also, keep in mind I come up with this thread laying in bed after having my son vomit day and night for 2 days. Don't be shocked if it appears to ramble.
 
I hope that your boy is getting better. I used to work in health care and I know how much of a worry the little ones present.

I was just thinking, which is something that I should avoid before my first dose of caffeine in the morning, another one could also do this by selecting a steel that doesn't have much carbon in it to begin with that would have an as quenched hardness in the upper 50's. Maybe something like 1040 or maybe 4140. Of course even there a single tempering cycle of around 300° would be beneficial. I sort of picked this up from reading a history of iron metallurgy in early Europe up to the Viking Era. It seems that there were a few swords from that time made of a lower carbon steel, or low carbon wrought iron depending on your definition, that had been quench hardened, probably by accident, but not tempered.

Admittedly not the best way to go, or it would still be getting done, but it is a possibility. The only reason that I can see to do it is just to say that you made a usable blade without the tempering step or resorting to austempering. The product would be pretty second rate. It would be a step up from work hardening the edge but probably not by much.

Doug
 
Me2, you do offer some industrial examples of mixed structures that give legitimacy to the concept. One needs to remember what newer knifemakers will take from the conversation. An odd thing I have found about newer knifemakers, when they are trying to find support to their plans and thought patterns, is the ease with which they can take almost any exception to the rule and apply it as if it were a rule, trust me, I know I was there once myself.

Yes there are a few exceptions where industry can use mixed phases to achieve the properties they are looking for, but for most properties the more homogenous the structure, the greater the gains in the target property. Strength is one where this is prevalent. Generally a duplexing of phases is a compromise to try to get some benefit from two properties at once.

If it is not a direct conflict of properties, such as pearlite in martensite, i.e. having hardness countered by a softer phase, it can often be the interface of two distinctly different crystalline orientations where problems arise. Even within a homogenous phase this is the case, for example fractures in totally martensitic steel travel along the grain boundaries, the interfaces between one crystalline orientation and another.

An even better example, and one that most folks don’t get the joy of experiencing, is the rather dynamic interface between martensite and its parent austenite. During the Bain strain of habit plane formation, in the making of martensite, the steel is miserably brittle. If you were to whack a blade or kink it too tightly in a bend, when it is around M50% you would be shocked at how unstable it is (please don’t ask how I know this:eek:).

The most predictable way for the average maker to take advantage of duplex structures is to avoid them in the micro sense and work with them in a macro sense. The example I am thinking of here is a martensite edge with a pearlite spine, this permits maximizing of the martensitic strengths. However even here it is the interface that is an issue. One of the main reasons you will find mentioned for the development of “ashi” is to limit fracturing that wants to run along the martensite/pearlite interface. And I have seen suguha type blades that did have their edges decide to depart the spine in quenching.

So one can see the complications that arise even in the exceptions, but for the new guy this topic is rarely about duplexing acicular bainite with martensite. The new guy is asking a much more basic question- “If I am shooting for 59HRC after I am all done, why do I need to quench it any harder than 59HRC?” To this I have an equally basic answer in the form of a question- if a farmer’s crop will only be 6” tall after harvest, then why does he have to wait for it to grow much taller than 6” to harvest it? While it sounds like a stretch there really isn’t much difference, a farmer wants his crop as high and full as it can get before the harvest it to maximize his yield, the knife makers wants his steel as martensitic as it can get before he tempers it, to maximize its strength.

Pearlite on a cutting edge stinks out loud. It is contrary to almost every property you want for a cutting edge, and every point of HRC you drop from the as-quenched maximum equals pearlite formation in the steels we forgers typically use. Now what is the difference if we are dropping that strength back to 59HRC anyhow?

Let’s look at just one aspect- carbide. The carbide in pearlite is courser and lamellar in nature, not particularly good for a very stable fine edge. Stable edges are made from very evenly distributed fine carbide, the finer the better. Carbides found in tempered martensite are so fine you cannot even see them with an optical microscope, and they are everywhere! They precipitate from every point in the martensitic matrix. This is why martensite rules for ultra-fine cutting edges, its homogeneity and fine structure allows it to get shaper and hold that shape better under pressure. Pearlite doesn’t sharpen as well and can’t hold the shape it does take. A pearlite colony formed in insufficient quenching can be huge, such as here:

bloom.jpg


Some of those blobs are almost 20 microns across, and that was from a water quench (the faster the quench the smaller the size of pearlite colonies as well as the carbide spacing in them). To effectively shave hair, an edge should be under 1 micron. So an edge loaded with pearlite colonies would have sections of inconsistency and weakness as much as 20 times greater than what we often accept as a standard of sharpness. Under the microscope these edges look more like rows of rocks than a cutting edge. This gets by some folks because the ragged edge that it can form will rip through paper and other items very aggressively, but it simply is not as sharp and stable as a fully martensitic edge that was tempered.
 
So when martinsite fractures, do you mean along the boundaries between plates or laths, or along the prior austenite boundaries?

Industrially, you can end up with all kinds of things mixed together. Ask Page about the nickel super alloys. But there is a very good reason for each there, and the processors take great pains to make sure each structure is exactly the way they want it. I tried to be very careful about limiting the discussion here to knife steels and knife applications.

I think a lot of times one just gets the structures mixed together and has to learn to deal with it. There will always be a small amount of pearlite in the simple steels, though it may be as small as 1% or less. Edge quench an L6 blade and you might get the whole spectrum of stuff. I started thinking about this months ago after reading some stuff in an American Welding Society publication. Someone had written in and was having issues with their welds not passing the bend tests. The author answered the question in short order and very simply. As the alloy cooled, there was a tendency to get a pearlite like structure, except it was austenite and ferrite in alternating lamellae. Once the alloy cooled to room temperature, the austenite turned to martinsite, requiring a post weld heat treatment, which was too low. Then, when bent, the martinsite was stronger than the ferrite, and never even passed it's yield point. The ferrite however had to take all the strain, and if there was too little ferrite, or the martinsite was too strong, the strain couldn't be accomodated and it broke. This is the same general concept we are talking about. The martinsite won't even yield, while the ferrite/retained austenite/pearlite/bainite will be deformed.

Basically, this whole rambling mess I posted is just a bit of advice to get as much martinsite as possible, and if you can get a hardness of 66-68, take it and temper it back. Don't think that a 1095 blade that starts out with a 63 HRc from the quench and then gets tempered back to 58 is the same as one that starts out at 66 and is tempered to 58. And, unless one doesn't have a choice, don't think quenching that same 1095 in a shallow pan of light motor oil straight to a 59 is really saving any effort for the same end product.
 
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