What a great topic. And a Huge one... Something that comes to mind are the various alloys and what exactly they're responsible for? I believe sulfur for instance helps in machinability? Perhaps someone that knows would be able to help me understand "the most important" ones (taking carbon for granted, here) and the knife specific "thing" they do? Characteristics like wear resistance, toughness, etc. I hear/read frequently about folks saying how one still has superior edge holding vs another steel. (I'm also taking for granted that the proper HT has been done for any given steel.) So, all things being equal, what are those alloys that are making these different characteristics show up?

And what about stainless-is there a certain something that makes a steel be considered in that category or is there a percentage of something that must be reached...?

I hope that makes some level of sense. I guess when I see the make up of a given steel in percentages, I can name most of them, but honestly don't understand how one would provide varying characteristics by looking at them side by side. I just know stuff like low manganese steel can make for a great hamon based on what I've heard or read.

Thanks for any insights, hope these were the kinds of questions you were looking for. If not, please feel free to ignore <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//biggrin.gi f' class='bbc_emoticon' alt=':D' /> .

Jeremy

Must be Steve wanted me to post a little more, and decided to get my attention <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//biggrin.gi f' class='bbc_emoticon' alt=':D' /> By the way Steve, I think I have found the answer to your question of analyzing the chemistry of individual pattern welding layers, it actually hit me last night while I was researching some upgrades to equipment that I have.

The question is cubic unit cells actually ties into the second question of allotropes. A material that is allotropic is one that can exhibit various phases in the same form. Iron, due to its ability to alter its unit cell, can take on different states and exhibit markedly different properties.

Now back to FCC (face centered cubic)and BCC (centered cubic). These are unit cells of the atomic stacking of iron. Iron is crystalline so its atoms are arranged in a very orderly and repeating pattern. That pattern is cubic in nature, think of a cube with an iron atom at each corner. At room temperature there is a space in the center of the cube that is also occupied by an iron atom, think of a cardboard box with a ping pong ball suspended by a string so that it hangs in the center of the box, but there are also ping pong balls glued to each outside corner. This is body centered cubic- BCC. It is not a very efficient stacking so it takes up more area but it has less space between the iron atoms for anything to fit. Carbon atoms are small and can only rest between the iron atoms. So BCC has two notable characteristics that are relevant to this discussion- it has little room for carbon in solution so carbon tends to be separated out at room temperature and being a less efficient stacking it tends not to deform easily so iron is hard to hammer or bend at room temp.

When iron is heated to incandescent, at a certain temperature the atomic stacking will shift to face centered cubic (FCC). To visualize this think of the “5” face on a pair of dice, but imagine each die only has “5” faces and no others. So you have a cube that on each face there is an atom in the center of the face but the center of the body is not occupied, i.e. there is not ping pong ball in the box, but there are ones glued to the outside in the center of each side. This is a much more efficient stacking for metal atoms and so it takes up less space, but more importantly it has lots of room for carbon atoms to occupy between the iron atoms. So in this atomic arrangement iron is easy to deform and bend (forging time!) and it can hold vast amounts of carbon in solution. So now you can see why the allotropic nature of iron is of very keen interest to what bladesmiths do, forge and heat treat.

Heavier alloys differ from carbon in how they fit into the iron matrix. Carbon is also an alloying element but, as mentioned above, carbon is a small atom that fits in the spaces between the iron atoms and thus it is known as “interstitial” alloying element. What we normally call “alloys” (chrome, nickel, etc…) are elements with atoms large enough that they cannot fit between iron atoms and so must actually replace an iron atom in the stacking, these are known as “substitutional” alloying atoms. Interstitial atoms have plenty of paths to move in between the iron atoms, especially at higher temperatures, so they are very mobile, but substitutional atoms can only move by swapping positions with adjacent iron atoms so they move very slowly and only with very high temperatures. This is why two damascus layers can share all their carbon and equalize but still retain their own separate alloying.

Jeremy, sulfur only helps in machinability, but is a negative in just about all other aspects. It is a nasty element that does not play well with iron atoms and so it tends to literally sit on the sidelines and sabotage the game. By loading up in the grain boundaries it causes brittleness at low temperatures (they believe that is what sank the Titanic) and at high temperatures it gets gooey and causes the steel to turn into cottage cheese when you hit it, something us smiths really dislike; we call it “red short”. This is the main reason for manganese in simple steels, to chemically combine with the sulfur and render it harmless to the steel.

Carbon is indeed the most important alloying element in steel, but the others vary in importance based on the properties you value most. As we already observed, for the most part sulfur is a negative, unless you value machinability, then this contaminant becomes an asset. If you are looking for hardenability chrome is a very useful addition. If you are looking for fine grain vanadium is pretty handy. If you value abrasion resistance vanadium or tungsten (particularly at higher temperatures) are good. If you want a steel that can take a beating from sudden loading nickel or silicon are nice to have. All the steels we have to choose from today were designed, chemistry wise, to fit a particular application by enhancing certain properties.

Certain metals will form an oxide layer that will sort of seal off its surface to further oxidation, causing the finish to be very stable over time, iron is not one of them <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//sad.gi f' class='bbc_emoticon' alt=':(' />. But chromium is one of them. And when we add enough chrome to steel (around the 14% mark) it begins to lend its oxide layer to the outer surface of the steel and we get stainless.

Thanks very much for the explanation, Kevin. I know I'm not the only one who's glad you're around to answer HT and metallurgy questions <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//cool.gi f' class='bbc_emoticon' alt='B)' /> .

Jeremy

Thank you Kevin! Good stuff and things to think about when considering steel.

Brion

Great explanation Kevin. Thank you!

thanks Kevin

Every time I think I have a handle on this metallurgy thing you open your mouth and remind me I am not as smart as I think I am.. that is a good thing by any measure. (just ask my wife she will tell you <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' /> )

here is my question

when cooling from austinite to form martinsite (assuming you cool quick enough get full conversion)it is my understanding that the change is set up once the steel drops bellow 900degf or so but will not turn over and collapse in to the new structure until 400degf or so. this is an effect I use on just about every blade I heat treat but I don't understand why it is happening, so why?

MP

|quoted:

thanks Kevin

Every time I think I have a handle on this metallurgy thing you open your mouth and remind me I am not as smart as I think I am.. that is a good thing by any measure. (just ask my wife she will tell you <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' /> )

here is my question

when cooling from austinite to form martinsite (assuming you cool quick enough get full conversion)it is my understanding that the change is set up once the steel drops bellow 900degf or so but will not turn over and collapse in to the new structure until 400degf or so. this is an effect I use on just about every blade I heat treat but I don't understand why it is happening, so why?

MP

The martensitic transformation is one of the more unique occurrences in steel and it is all really based upon denying the steel the diffusional processes that would naturally occur. We have already discussed the ability of FCC to hold much more carbon in solution than BCC. This solution, what we call austenite, is the result of carbon “diffusion” where the carbon atoms move into and fill the spaces created between the iron atoms by heating. The opposite is true on cooling when diffusion will allow the carbon atoms to move out of those spaces and gather up in groups to form carbide.

When steel is cooled naturally, i.e. slower like in air, the carbon atoms have time to move out of the iron matrix as it shifts from FCC to BCC. This normally happens in the range from 1000F to 1200F and the phase that the carbon in carbide form separated from the iron is pearlite. But since diffusion also requires time for the carbon to move we can mess with it by rates of cooling and if we cool fast enough to trap the carbon within the iron matrix it will force the iron to remain FCC below the pearlite transformation range. You now have metastable austenite that has the squeeze being put on it as it cools from 900F to 500F. To demonstrate give this a try- quench to 600F and then pull the steel out of the liquid, the magnet will not stick to it as it is still austenitic.

Now eventually as they give off energy the need for the iron atoms to assume body centered stacking will become greater and greater until a critical point will be reached. This critical point where the need for the FCC stacking to move to body centered is enough to overcome the effect of the trapped carbon is dependent on the amount of carbon trapped. So if you only have .5% carbon trapped in solution the transformation can get underway at 500F or 550F but if you have .9% carbon in solution holding up the FCC you will need to force more cooling on it to reach the tipping point and it may be 350F or lower.

This point at which the shift to body centered occurs is called M_s (martensite start). The martensitic transformation is unique to the point of bizarre in a few ways. First is that it is not diffusional, the carbon is trapped and doesn’t move at all, so time is no longer a factor, in fact all martensitic transformation occurs almost instantaneously (around the speed of sound). In the blink of an eye entire planes of atoms suddenly tilt and in a shearing motion shift into a new body centered stacking. Note that in the last couple of paragraphs I have said “body centered” but not BCC (body centered cubic). With the carbon atoms trapped in the interstices BCC is no longer possible so instead the FCC shifts into a new distorted stacking called body centered tetragonal (BCT), and this is what we call martensite. Because it is a highly distorted lattice martensite has more volume (the steel grows) and it has very little plastic abilities, the iron atoms cannot move to accommodate any deformation.

Since there is an M_s point, there is also an M_f point, but that designation is not often used anymore and has been replaced with M% on most I-T curves. The M_f point designates where you have forced as much FCC into BCT as you can with your level of cooling, which obviously stops at room temperature under normal conditions. But if the FCC structure is too saturated with carbon M_f is pushed below room temp and you need to resort to freezing the steel to accomplish the rest of the transformation. This is why control of our soak temperature is so important with steels that have more than .8% carbon. If a bladesmith notices a hardness difference by freezing the steels we normally work with he needs to correct his soak temperature as he is overheating and putting too much carbon into solution.

In respect to the alloying elements of steel... I've been playing around with 1084 powder steel these past couple of weeks and I've been having a blast when it comes to creating mosaic billets. I had the idea of packing a can with a few boxes of MAZE hardwood trim nails (AISI 1095) to build up a speckled pattern. I added a little nickel to ensure contrast, but in doing so, it got me thinking.

When it comes to powdered 1084, a simple addition of nickel goes a long way in regards to aesthetics, but what about the addition of other alloys that are in powdered mesh form? chromium, molybdenum, cobalt, etc... What are my limitations when it comes to experimenting with the alloying elements of PM on a small scale? exp: by weight; adding an amount of vanadium to the 1084 in order to gain the benefits of preventing grain growth at high temps along with toughness, etc...

In order to rope in such a doozy of a question I'd like to preface it with the following; I know that the question is a bit naive, because the answers are dependent on so many variables.... and... I'm not inclined to believe nor am I suggesting that there is a parallel between powdered metallurgy and Jamba Juice. This is just what happens when a generation X'r watches a youtube video about Ulfberht and finds himself within arms reach of some powdered 1084.

thanks for the explanation Kevin, seems like I should have know that now that it is explained.

Zack I would think it would depend of a few factors 1 how fine the powder is, and 2 if the introduced alloy will move with in the structure, ie carbon and nickel move chrome and I believe vanadium do not, if not I would think they would tend to form super large carbides.

Here is another on that has been a bone on contention in our shop.

Currie point- it has been my understanding that this point is more or less fixed across alloys and is not really linked to austinite formation, magnetism having to do with electron spin rather than the structure of the matrix. One of my business partners argues that the two have to be linked and offers as evidence that all austinenitc alloys (300 series stainless for example) are non magnetic and the fact that magnetism does not return unto Ms point.

MP

Zach,

Just a comment on mixing carbon steel and most alloys. If you are suggesting that you can mix powdered 1084 and chromium or tungsten or vanadium powder, then that is not something that occurs without melting the solids. Simply welding the alloy powder with steel powder will not produce an equivalent to commerically produced steel with the same alloys. Most of the alloys in the steels we know smelted into the steel. Some of these alloys do not melt until 3500 F or higher. Depending on the alloy and the size (mesh) of the alloy and quantity you may just be putting solids into the steel that weaken the blade's structure. Also, some alloys in the pure form are toxic so you have to choose carefully.

Some manufacturers have placed diamonds on the surface of the steel but if the diamonds are contained in the blade steel, hard as they are, the blade's integrity is compromised.

To get niobium (comparable to vanadium) into the steel I needed to generate substantial heat. To do this I used a thermite process generating over 4000 degrees F. I had a couple of failures but also a couple of successes. It is not that easy to mix an alloy like chrome with a simple steel to create an alloy of steel using that element.

Dan