Every now and again I have the distinct privilege of having a guest-blogger on the site. Elliot Williamson is a good friend of mine from back in the Track and Field days, and has become quite the craftsman in knife making and steel work. Enjoy this post about the process of this age-old craft. Afterwards, visit his site to admire his handiwork and maybe contact him if you want to purchase one of his works, or commission a custom design of your own!
What makes your steel get hard
Just imagine this scene: Two smiths are working long into the night about 4,000, maybe even 5,000 years ago at any given civilization center of the ancient world. They are doing what they normally do, heating iron and pounding it into shape to make a sword or and axe. But one guy needs to go home, and the iron is still in the fire, so he takes it out and dunks it into some water so they can put it away and go home. The next morning they take the blade and are going work harden the edges just like they did with bronze blades, but when the hammer hits the blade it bounces off and leaves a dent in the hammer. “What has happened, this iron is harder than any other iron I’ve ever seen?” Well boys, you just discovered the one of the greatest advancements in technology, the solid state reaction between carbon and iron at high temperatures and the resulting structural change when rapidly cooled. It took until the late 19th century for scientists to be able to describe what was happening chemically during the heat treatment of steel, even though it was being done for millennia. Books have been written about the subject, whole classes in engineering are devoted to it, but it’s not that complicated, in theory.
Let’s go back to our ancient smiths and start from where they were starting from: Iron ore. It was probably a potter who first discovered that some rocks melt at high-enough temperature, but once the word got out every one with a firing oven was sticking rocks in it. Iron does not exist naturally on this planet in metallic form, but it does in meteors and some of the oldest iron implements are meteoric iron, we know because they contain nickel in higher percentages than found in the Earth’s crust. So once iron ore was discovered, and there is lively debate on when that was, a process was developed to extract the iron, called bloom forging. Large amounts of ore were dumped into a big fire pit with fuel for the fire, i.e. wood, or charcoal, and air was forced through the fire, via an apprentice with a lot of lung capacity, or later with bellows of some sort. This heated the ore enough to melt some of the iron out of the rocks (about 2800 degrees F.), but this left lots of other material in the iron. Once the mass of iron and other material, or the bloom as it is known, was cool it was taken to smaller more reliable heating chamber which came to be called a forge. It was heated to a glowing orange color and then hammered and hammered and hammered to get as much of the impurities out as possible. This could take days. Once the impurities were drawn out enough so that a smooth metallic surface could be raised with a heat in the forge and some hammering, then it was ready to be drawn out to shape. Now here is were things get chemical.
The fuel used to smelt the iron ore contained carbon; some of that carbon was trapped in the iron as it melted and solidified. Turns out the more carbon present in iron directly effects how hard the alloy of iron and carbon (steel) can be. Now one major problem with bloom forging is that working the impurities out of the bloom also allows carbon to be pulled out of the steel by the very oxygen being pumped into the fire to make it hotter. So for thousands of years smiths were making educated guesses as to how hard a piece of steel would become after the magical quenching. You can feel the difference when forging the steel, high carbon-containing steel is much harder to draw and shape than low-carbon steel, but there is a huge range in hardness in steel and it must have taken years to be able to know which steel would make a good sword and which was better suited for a farming tool. Today we know that steel containing .2% carbon or higher will heat-harden into the 40s on the Rockwell C hardness scale, as you get to .4% you can reach the 50s and once you cross the .5% threshold the steel will harden up to 60 HRC. The Rockwell C scale is just one of many scales used to determine the hardness of metal. It is done by a machine which strikes the metal in question with a set force and the indentation is then measured. But what does it all mean? 50 HRC is considered to be a spring temper, meaning the steel with flex, but not bend, returning to its original shape once the load is removed. The leaf springs in your car are the best example of this. But carbon content is not the real key to hardened steel, it is just the door.
The true fundamental answer to why steel hardens when heated and cooled rapidly is all about the relationship between carbon and iron on a chemical level. Post bloom forging the carbon in the steel is lumped around in the iron, creating pockets of hard-carbon within the softer iron. This distribution of carbon within the iron is call ferrite, yes it has name as do all the different distribution states of carbon in iron and there are quite a few, but we will stay with the big three. Ferrite is sort of the base state of steel, where all the alloying elements are doing their own thing. Carbon likes carbon so it hangs out together, just as iron does. This affinity is natural, but it can change and heat produces this change. At high heat, 1900 degrees F. and higher, both carbon and iron become agitated with all that extra energy around and they shed that energy by glowing and moving around. This is one of the quirky things about steel, this movement is not a phase change, the steel is still a solid piece of steel, but on a molecular level the carbon is being repelled from itself and collecting iron atoms in covalent bonds that arrange around the carbon into crystalline structures called carbides. Don’t let the name fool you not all carbines center around carbon, it was just first observed with carbon and the crystalline structure has been called a carbide ever since. So at this almost 2000 degree temperature a lattice work of carbide develops and this state is austenite which is an almost even carbon distribution. Now if it were possible to instantly remove the heat and freeze this structure we would have and almost perfect crystalline structure in our steel and it would be similar in hardness to a diamond, but we can not. We have to settle for a slightly less organized structure can martensite. The distribution of carbon is still fairly uniform, but the crystalline structure is imperfect with some fully formed carbine and some not so fully formed. We can freeze this structure and that was what our ancient smiths did when they took their steel from fire to water within about a second. The rapid dissipation of heat-energy doesn’t allow the carbon to re-attract to itself, thus the carbides stay locked in the iron matrix. The more carbon, the more carbides, the greater the chance of any impact hitting a carbide, and bingo, a harder steel. But there are some other things at work here as well.
The quenching process is not perfect and some areas in the steel do in fact retain the austenitic structure, great right? Wrong, diamonds are hard, and so are perfect carbides, be they are also brittle because the iron surrounding the carbon forms weak covalent bonds with the other iron atoms. Here is lays the great balancing act of steel: Too hard means breaking, to soft means bending and warping. Luckily martensite has the best of both world; carbides for hardness, but imperfect structure for malleability. To get rid of the left over austenite and take the brittleness out of steel you must temper it. If you hold the steel at given temperature for period of time, the austenite reverts back to martensite. But hold it at too high a temperature or for too long and the martensite reverts back ferrite as you give the carbon energy to break its bonds with the iron in its carbides. This must have been a real pain in the ass for ancient metal workers. In modern metallurgy we have tons of data and every type of steel produced is required to have a graph depicting its “S” curve, which is basically the temperature by time and yield structure in line graph form, and a table of tempering temperatures by time and yield hardness. So today there is a “recipe” for any given steel and any given hardness you hope to achieve.
This is all much more complex and really more of a study in quantum mechanics than I have made it out to be and I have taken some liberties with terminology to help demystify the whole process. But for all the complexity in explaining the hardening of steel, it is fairly simple in practice. There are many different kinds of steel with widely varying alloy contents and each steel has its own temperature were the solid state change occurs and this call the critical temperature. But one thing does remain constant the color at which the steel will glow when the austenite temperature has been reached, and the lost of ferromagnetism. There is certain bright orange color and a lost magnetism which is easily recognizable, especially when the magnet doesn’t stick anymore. But the color is how smiths have been judging the critical temperature for millennia, and it works. The Japanese sword smithing tradition has a saying: “When blade glows with the color of the raising sun, then blade has gained its soul.” Tempering is little bit trickier and involves a lot of trial and error. Most of the time 400 degrees F. for an hour or two will take the brittleness out of the steel, but with some of the high alloy steel with much more than just carbon in them the tempering temperature can be up to 1000 degrees F. for two rounds of two hours each. So thank goodness of tempering tables.
Again this just a basic overview of the heat treatment of steel. There is so much more I could discuss, but won’t as I have read several books on heat treatment and they tend to be rather dull. I hope that you never look at a leaf spring, knife, or I-beam the same way again.
Happy hardening,
Elliot Williamson
Ferrum Forge Knife Works
Vincent Tedjasaputra, Ph.D. 