Metallurgical Terms Made Simple
Understanding Those Intimidating "Ites"

By Kevin Cashen, ABS Master Bladesmith

Many people, when they begin delving into bladesmithing, encounter many metallurgical terms that seem bigger than they actually are. I have often said that my grandmother didn't know anything about the "aerobic/anaerobic chemical process of various Saccha romyces cerevisiae fungi strains upon pulverized Triticum aestivum in the presence of H2O." No she didn't, she just made good bread! Anything can be made to sound really intimidating and impressive if you use enough $5 words to describe it .

In the simplest possible terms, steel is an alloy of iron and carbon. In metallurgical terminology, the iron portion is known as Ferrite and the carbon part is known as Cementite. Both terms describe an iron/carbon mixture, just one has much more iron than carbon and the other has much more carbon than iron. As you change the equilibrium between the two, you get the various states of steel which are known by other ites.

O.K., you have a piece of steel, let's not worry about any "ites" for the time being -it could be pearlite, gooberite or even cashenite for all we care (cashenite! I like the sound of that!). All we really need to know right now is what steel it is. For the purpose of our discussion, let's say it is AISI 1084. So we wish to forge a blade from our steel so we place it in the forge and slowly begin to heat it. From room temperature up to 1350^oF (Ac1) nothing, which is pertinent to this discussion, occurs. At 1350^oF something begins to happen. Internally the atoms are very excited and begin to mingle. Some properties of iron change a little bit and cementite begins to mix with ferrite to form solid solution known as austenite. Heating to a selected temperature above critical (Ac1) for the purpose of forming austenite is called austenitizing the steel (or "trashing the steel", depending upon how high you go). The specs for this steel say that a good austenitizing temperature could be 1500^oF and at this point the steels internal structure will have completed the transformation to austenite. Austenite has some interesting qualities. It is not attracted to magnets (more accurately--gamma iron is non-magnetic) like other forms of iron, and it moves very readily when manipulated. This is your neutral ground, your starting point.

From here, depending upon how you bring the steel down from this temperature, you will make any of our other "ites". Each of them have their own unique physical properties which, when utilized in the proper ways, result in different qualities for various applications. In the following paragraphs we will explore these applications and qualities and how they can result in a well made blade. But for now we have made the most significant step in forming austenite. It seemed easy enough, but to a beginner the "geek is speaking Greek" as in the following:

Steel geeks: The slightly hyper-eutectoid alloy, upon being heated to Ac1 makes the allotropic shift from the body centered cubic atomic lattice structure of alpha iron to the face centered cubic atomic lattice structure of gamma iron and begins to dissolve free cementite to form austenite. At 1500^oF complete austenization has occurred (it's so geeky it sounds dumb!).

Normal people: You place a burner under the aquarium and, needless to say, the fish become a little excited! Iron fish swim a little looser and carbon fish go berserk and swim within the iron fish schools in their frenzy. If you tried to move any of your aquarium decorations around at normal temperatures, it would be difficult because of all the sluggish schools of fish in the way. But now, with them bouncing off the walls, it is pretty easy to move things without much interference. An ever-present problem with heating the tank is the fact that carbon fish are so jumpy that they will jump right out of the tank and be lost! So be careful how high, how long, and how often you heat.

Next we will forge our steel to the desired shape. To do things right, we will get the steel a little warmer, since most steels have a recommended forging temperature that is a bit higher (several hundred degrees) than the critical temperature. It is very important, however, never to over heat the steel. High carbon or tool steels should never see a white heat in forging. If you get white sparks, cut off that area and start all over. Austenite is very pliable and moves easily under the hammer and any imperfections that we could cause are cancelled by the fact that the crystals will just reform anyhow. Now you begin forging your masterpiece and soon you have it just the way you want it to look.

Now, after heating one section more than another, cooling other parts quicker than others, beating the snot out of it, and generally raising cane with the internal structure you have a very interesting hodgepodge of grain structures, stresses, and internal misalignments. What do you do? There is no such thing as a chiropractor for steel so the next best step is to normalize. To normalize you heat the piece back above the critical temperature and then pull it out and admire its beauty for a few minutes. You do nothing else. No pounding, quenching, or insulating, you just leave it alone and let it cool in the air. If done right, the entire piece evenly turned to austenite and then sat back and relaxed for a nice easy ride back to room temperature forming whatever crystalline structure it pleased throughout. Warning: in richer alloys that have a tendency to air harden, normalizing is better known as "crappy quenching" and isn't always beneficial.

Steel geeks: The slightly hyper-eutectoid alloy is heated to above Ac1-2-3 for complete austenization then allowed to cool to below Ar1in still air for the purposes of correcting undesirable banding and non uniformity in grain size caused by the plastic deformation and random temperatures, in preparation for subsequent heat treatments. (Sounds good! I guess!?!)

Normal people: While you were rearranging your aquarium you heated some areas more than others. You couldn't help slamming some fish against the walls. You now have partially scattered schools with lost carbon fish here and there. All the fish are a nervous wreck! You put the heat evenly to the bottom of the tank and get them all excited one more time and then leave them alone to relax a bit.

Now that you have let the steel have its way for a minute or two, it is time for you to take charge once again and force it into a structure that you want. In order to grind or machine the steel easily, it will have to be as soft as it can get. It is important to do this before any material removal for many reasons, two of which are: 1. Un-annealed steel with hard spots can be a real bugger to work with (your grinding belts will hate you for it!). 2. A thin "skin" of the steel will be decarburised (carbon fish jumping right out of the tank) by the long time at temperature and must be ground off to get at the good, hardenable steel underneath. You will also want a nice, even disbursement of carbon and iron for the subsequent treatments. So now our goal with the anneal operation is another ite called pearlite. To achieve this you will be heating the steel to 1500^oF again. Once it is evenly heated, it will be annealed. This is accomplished by very slow cooling from the austenatizing temperature. The steel can be buried in an insulating material or the anneal can be done in a temperature controlled oven or kiln. With simple carbon steels, insulation may be sufficient. With alloy or tool steels, which harden much easier, an oven is needed. As we take our time cooling, carbon has plenty of time to escape from amidst the iron in austenite and we start making pearlite.

So, as the steel cools slowly, the iron and carbon are permitted to separate and form their own groups. If this is done slowly enough at the right temperatures, the carbon will precipitate into globby little spheres, hence the term spheroidise or spheroidal anneal. Spheroidizing requires rather close temperature control so it is less common than the most popular lammelar anneal. 1084 steel, that is slow cooled from above critical temperature, will be lamellar annealed. The word lamellar comes from the fact that the carbon and iron separates into relatively flat stacked up structures that make the pearlite. Pearlite is so named because, under the microscope, the layers of flat iron and carbon can look like mother of pearl. So imagine a big pile of old, used, pearly looking roofing shingles made of carbon and iron, and you get the idea. Slower cooling allows the cementite and ferrite to separate farther from each other so it makes a coarse pearlite. Stuff the hot blade in an insulating material and call it good. As the steel takes a very long time to properly anneal, you are done for today.

Steel geeks: For a lamellar anneal, heat through Ac1-2-3 range to thoroughly austenitize, then slowly cool to Ar1 to allow marked precipitation of cementite from the gamma iron matrix as it returns to the allotropic phase of alpha iron, forming a coarse pearlitic structure. (And a partridge in a dendrite steel tree!)

Normal people: You reheat the tank again but slowly reduce the heat so that the fish can stay warm and energetic without getting sluggish. This allows the iron fish to form their originals schools with the ordered configuration and the carbon fish find each other and regroup. All are much at ease and stress free from being with their own kind again.

The next day you retrieve your work, which should now be room temperature and made up primarily of pearlite, and begin to do your final shaping. The piece will be all gray and scaly and you will notice that, if forced, it will bend easily. The steel can also be cut by a file, mill, or drill bit, fairly easily, and it grinds like butter. Well not just like butter, since butter would plug up your belts and get slung all over the work area, making a slippery greasy mess. After a good day at the grinder you now have a shining example of your craft at its best, but it is useless as a blade. You sharpen it and try it out and it gets dull cutting your bread and butter over lunch. You are now ready for the most crucial step. It is time to make the steel hard.

This is accomplished by... yep, you got it! We are on our way back to austenite. This time we will be hardening the steel so our temperatures when making austenite will be much more critical. Whatever size the grains of the steel are when we quench the steel, that is the size they will be from then on (unless, of coarse, we re-austenitize the steel and thus ruin the heat treat). At or above our critical temperature grain growth begins, like puddles of quicksilver being pushed together to form larger and larger pools. This increases with time and temperature so the higher we go above critical temp, the more rapid the growth. How long do we keep it at temperature? Here is where the term "soak time" becomes important. A number of people make a tragic, yet understandable, error at this point. For many simple steels, the all-important specs recommend a soak time of 5 minutes per inch of thickness. You did catch the operative word there, didn't you? Per INCH of thickness! Most heat treating specs are for industrial applications where they are heating large blocks and pieces, which take some time to heat throughout. We are working with fractions of an inch for our blades so adjustments must be made. It is worth mentioning that richer alloys and some stainless steels do require a much greater soak time, however. But here the idea is to get as close to critical temperature as possible and then, as soon as transformation to austenite is complete, get it back down as efficiently as possible.

Whether you have the luxury of a finely controlled salt bath, oven, or just the old forge, you begin heating the blade yet again, but this time you must be very careful to heat evenly to just the desired temperature and no more, and certainly no less. And then, when the carbon is all inter-mingled with the iron, we will trap it there by lowering the temperature so quickly that the two cannot precipitate out of the solution quick enough to form pearlite. When the steel is evenly heated to a full austenitic state, then you must waste no time in getting it nice and evenly cooled as quickly as possible without destroying the piece. This is accomplished by quenching.

The medium that steel can be quenched into can range, depending upon the steel, from water based solutions to air. For our steel we will use very quick oil. An entire book could be written on the different phases of the quench and quenching mediums alone. The reasons oil works well is low vapor and the gentler cooling that it is capable of in the third phase of the quench when the real work has been done but the steel is most prone to stress. Warpage is the ever-present threat when quenching swords. But there are some ways to minimize the threat. Even heating to the critical temperature is very important, as well as matching your quench medium to your steel. There is an interesting balancing act here. If the quench is not fast enough to completely miss the time frame in which cementite can precipitate out, at around 1000^oF a fine pearlite could form in small pockets throughout the piece, causing soft spots and other problems. At one time, this unwanted stuff was called primary troosite (like we needed another "ite"!) but thankfully we have now simplified things by identifying it as the fine pearlite. On the other hand, if you over compensate for this with a quench that is too fast for the steel, you will shock the steel causing stresses that can manifest themselves in ways ranging from warpage to cracking and breaking.

So once it is heated, the steel is plunged beneath the surface and super cooled at a rate sufficient to trap the carbon atoms within the groups of iron before they can precipitate out of the solution. If we have successfully bypassed pearlite, then we are on our way to success! But we are not there yet. From 1000^oF down to perhaps 420^oF our blade is still made up of austenite. Wait a minute! Isn't that the stuff you get at 1350^oF+? Yes, but austenite is pretty peculiar stuff. It can hang around at relatively lower temperatures under the right circumstances. Besides we have trapped the carbon in the iron, it can't form pearlite, so what choice does it have but to remain as austenite. Without any intervention on our part, austenite will hang around until the Ms point, when it will transform into a hard, needle-like crystalline structure called martensite (the M in Ms is for martensite).

Martensite is steel in its hardest form. Very hard, brittle and full of stress, it is seldom useable in the as-quenched condition. Since it forms at relatively low temperatures, if you have proper temperature control you can interrupt the cooling at around 400^oF and have a look for any warps, twist or kinks. If you find some, put on some good leather gloves and correct it! You are holding onto a blade that is made up of400^o+ austenite which, as we have already found, is more movable than other forms. Austenite is just not very stable and will try to break down into other forms when it can. If the austenite was held above the Ms range long enough, it would say "to heck with you then!" and move on to form bainite instead. Crazier yet, if the alloy is much richer than we are discussing here, or the quench is extremely quick (impossibly quick for the steels, and thickness we are discussing) it would be as if the austenite was riding a runaway elevator. Passing pearlite with no problem, the austenite would plummet in temperature fast enough to watch its exit floor of martensite zoom past. Then where does it go? It just hangs around in limbo at room temperature wondering the heck happened. This is called retained austenite and can be a problem in richer steels that have plenty of time to avoid pearlite. It will eventually just take the stairs up out of the basement and become martensite, which will make your blade that was supposed to be Rc58 jump to Rc60 when you are not looking!

Steel geeks: Harden the piece by heating to 1500^oF and then super cool in accordance with the critical rate of the TTT curve for the slightly hypereutectoid alloy below Ar1 to avoid the pearlite nose. At Ms the acicular structure martensite will begin to form. Highly rigid structure and internal stresses will result in higher tensile strengths, lower impact toughness and ductility, and higher wear resistance with a hardness of Rc65. (you get all that?)

Normal people: Reheat the aquarium and carefully watch for a nice even dispersment of carbon fish within the schools of iron fish. Carbon fish will find a place to hangout in the place in the center of the cubic iron fish groups which the center iron fish have vacated. You now have several schools made of both excited iron and carbon fish. If you heat too much or too long, in their panic they will start merging into fewer and larger schools which will be bad for everybody involved. When all is right, you quickly freeze the aquarium, trapping all the carbon fish within the iron schools. Nobody is moving anywhere! All the fish are rigidly in their forced places and incredibly stressed out! You couldn't move any decorations in the tank with if you had to. But there is so much stress that the slightest push could shatter the aquarium!

After the piece is safely at room temperature and fully hard, the next step is to make it useable by tempering. The word tempering is often incorrectly applied to the hardening operation and has thus caused much confusion over the years. Tempering is not hardening, its purpose is, in fact, to reverse some of the effects of hardening! How is tempering done? That's right! The steel is heated again! But his time it is heated to a much lower temperature. As-quenched martensite when heated, to 200^oF and above, begins to change. Some of the trapped carbon is allowed to slip out and group together in boundaries around the grains. This eliminates some of the stress, making things a bit more stable. The goal of tempering, however, is not exactly the opposite of hardening. The desired effects are to stabilize structure, increase impact strength and overall toughness with much of the internal stresses relieved while sacrificing the least amount of abrasion resistance or overall hardness. Hardness will, however, be lost to a small extent.

At this time it may be necessary to briefly explain the measuring of the property known as hardness. In our business, everybody loves to throw the ominous and enigmatic Rockwell numbers at folks. Often used to prove one's savvy in the ways of steelology, I often wonder how many people know, or really care, what these numbers mean. How often at a show, when asked by some guy wanting to impress the girl on his arm "what does this Rockwell at," I have felt the overwhelming temptation to answer "235.5 z scale" just to see if he catches on or not. Rockwell hardness (written as Rc or HRC) is a measurement of the materials hardness or resistance to penetration of a diamond needle-like penetrator. The test is performed by placing a test piece under the penetrator and then a minor load is placed upon the penetrator to plant it firmly on the steel. Every description I have ever heard of the process seems to place some mysterious importance on this first or "minor" load, when it actually plays a very small role in the measurement. It does seat everything firmly in place and lifts the measuring mechanism into a position so that it can be excluded from the measurement (like resetting digital scales to zero after setting on the cup that will hold the liquid to be weighed). Then a major load (150kg) is placed upon the penetrator with maximum possible precision. The number that results is a precise measurement of the depth of penetration from the minor load set point to the depth of penetration of the major load. Our steel would see its maximum hardness at Rc65 and then losing all edge holding abilities below Rc50 (probably higher, depending upon your preferences).

To temper the steel, the fully hardened piece is placed in a kiln or oven and heated evenly to above 200^oF. If we just go to 350^oF we relieve some of the stresses but do nothing for reducing brittleness. To temper we will go higher. At 400^oF we will have a blade with a hardness of Rc58, this would be great for a knife blade with a fine balance of edge holding versus strength, but swords blades putt a greater emphasis upon toughness than a knife. With continued heating, according to the steels specs, we will get Rc55 at 500^oF and Rc50 at 600^oF. So now we determine what levels of toughness or shock resistance as opposed to edge holding that we want for our application and then make the required compromises to achieve those ends.

Steel geeks: Upon heating to 200^oF the alpha martensite, formed by super cooling the slightly hyper- eutectoid alloy, transforms to the body centered cubic structure of beta martensite. Further heating results in the precipitation of cementite from the beta martensite in sub-microscopic particles (what the... !?! Really small thingy's!)

Normal people: Very gently warm the tank so that some of the carbon fish can slip out and regroup around the stressed schools and give a little more elbow room for everybody.

There you have it! A blade made from steel that you have put through all the most common ites. And if all was done correctly it is better off from all those ites. I hope you have enjoyed and had as much fun reading this article, as I had in writing it. My intention was sincerely not to poke fun at or insult anybody's intelligence with talk of geeks or fish tanks, but to clearly demonstrate how some things that can be made to sound ridiculously complicated or technical are actually quite simple. Does one need to know all the five syllable words to make blades? Not really--if they follow the recipe, there isn't any reason at all why they cannot make really good bread! But, if one wants to get fussy and really fine tune things by getting predictably consistent results in order to push the limits of performance, then knowledge is power!

Definitions

Ac1: The temperature at which iron, in its normal (room temperature) state of body centered cubic structure (alpha iron), makes the shift to face centered cubic structure (gamma iron) capable of dissolving cementite to form the solid solution austenite.

Annealing: Heating the steel to the critical temperature (austenitic condition), and then cooling very slowly at a controlled rate to produce a structure of coarse pearlite. Steel is annealed to reduce the hardness, improve machineability, and produce a desired microstructure. "Spheroidal annealed" steel consists of a ferrite matrix, with all the carbon as spheroidal carbides of relatively large size scattered evenly throughout the ferrite matrix. Failure to anneal before subsequent heat treatments can result in stress and uneven grain growth.

Austenite: The homogeneous phase of steel consisting of a solid solution of carbon in the gamma form of iron (face centered cubic lattice) . Named for English Metallurgist Sir W.C. Roberts Austen.

Austenitizing: The most critical of all heating operations. It requires the careful heating of the steel to critical temperature (above Ac1) and then holding at temperature long enough for all the carbide and ferrite to form the solid solution known as austenite. High austenitizing temperatures or excessive soak times result in weaker steel due to enlarged grain structure.

Bainite: The microstructure formed when quenching of austenitized steel is interrupted and held at temperatures above the Ms point for extended periods of time and then allowed to cool to room temperature.

Cementite: The constituent of steel that primarily represents carbon. A hard brittle iron carbide (Fe3C) consisting of 6.67 percent carbon and 93.33 percent iron. Doesn't seem like much carbon but, within the alloy of steel, it is a lot.

Eutectoid: Carbon iron alloy consisting of .80% carbon, which will form an entirely pearlitic structure upon slow cooling to ar1. 80% is a compromise since I have found much confusion/contradiction in various references from .77% to .85%. Even the ASM Heat Treater's Guide shows .80% on pg.4, Fig.5 and then states .77% on pg. 6. In the article I refer to 1084 as slightly hypereutectoid alloy because it supposed to have .84% carbon , but close enough!)

Ferrite: The constituent of steel that primarily represents iron. Absolute forms of pure iron do not exist in steel as the iron particles always have other elements mixed in, hence ferrite as opposed to iron. It is in essence a solid solution in which alpha iron is the solvent.

Hardening: see quenching

Martensite: The most common form of hardened steel that results from super-cooled austenite in which carbon has been trapped between the iron acting like a keystone and holding the structure very rigidly in place. It is highly stressed and brittle and is named for German metallurgist Adolph Martens. Under the microscope Martensite looks jagged and needle-like.

Normalizing: Heating the steel above the critical temperature then allowing to slow cool in still air. Refines internal makeup by breaking up non-uniform structures, removing residual stresses and creating greater uniformity in grain sizes.

Pearlite: A structure of precipitated ferrite and cementite formed upon slow cooling of austenite. It is the most commonly known structure within annealed steel. Lamellar in makeup, it is made of alternating layers of ferrite and cementite. Pearlite is the soft and stress free state of steel.

Quenching: Having heated steel to a temperature at which austenite is formed, the rapid cooling in order to trap cementite within ferrite and form the structure martensite, resulting in hardened steel. Quenching can be done in a number of mediums (air, oil, water, brine, sodium hydroxide or heated salts) depending upon the alloy. It is a poor practice, however, to use a quenching medium which exceeds requirements.

Spheroidized cementite: The spheroidal cementite precipitate which results from spheroidize annealing.

Tempering: The heating to a relatively low temperature and holding there for a time for the purpose of slightly altering the structure of an as-quenched steel consisting of martensite, precipitating some of the cementite back out and imparting toughness while decreasing brittleness.

Other ites, about which we just don't give a darn: Along with the aforementioned troosite, I have also found references to such curious and extinct creatures as troostite with an extra "t", and sorbite. The best I can figure is that these were once terms for bainite, or smaller cousins of T. Rex which preyed upon iron molecules in the late cretaceous period (fossil records are quite incomplete). Ledeburite is a high carbon bearing structure in cast irons, which we all know makes such excellent swords that it is a good thing to remember! I also think there was mention of a ledeburite in a couple of the Dr. Suess books, as it could be a constituent in the cat in the hat's fur balls. And of course graphite, which is the straight cylindrical shaped crystal structure which gives alpha and gamma pencils their strength but in the annealed form makes the very erasable #2!

Kevin R. Cashen is certified by the American Bladesmith Society as a Master Bladesmith. A winner of various awards including "Best Custom Straight Knife" (for a short sword!), Kevin is well known for his high performance art swords rarging from rapiers to Viking swords. He is also the maker of our January cover sword (click thumbnail to expand), which is a Viking sword sporting pattern-welded steel and Celtic gold designs. Kevin resides in Hubbardston, Michigan. e-mail: krcashen@mvcc.com

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