In part one of a two-part series, guest editor Nic Westermann covers the basics of heat treating steel.
I have wanted to talk about heat treating tools for a long time as it’s something I do on a daily basis as a toolmaker. It is a difficult subject to write about though. There is a lot of theory, which you will find this article largely concerning itself with. It is, perhaps, similar to the English language: lots of people can speak it, but there are so many exceptions and rules it’s hard to get it perfect, and people around the world will hold strong opinions about what is actually right.
I do think, though, that this is a going to be a useful series. Heat treating allows you to modify and hopefully improve existing tools, but if you take it even further you can make tools from scratch. As with my sharpening articles, the emphasis is on understanding what is going on – when you understand the process, you can apply it to your individual situation, and it will vary from tool to tool. There isn’t a single heat treat recipe that can be applied across the board and yield good results.
However, I’m not going to discuss any complicated chemistry here, I’ll be concentrating on the theory in simple terms, with a quick visual representation of the process. This article is not a guide or ‘how to’, I will be covering the actual methods and relevant health and safety issues in the next article.
Health & safety disclaimer
The processes in this article should only be attempted by someone capable of performing them safely. GMCPublications takes no responsibility for any damage or injuries caused to anyone attempting to follow the procedures in this article. All safety precautions must be adhered to and all necessary safety equipment must be worn. If you do not think you can safely perform the procedures described in this article, we recommend that you do not undertake them.
Overview
The basics of heat treating are very simple, if you have a hardenable steel and heat it to around a cherry red colour, the iron and carbon combine in solution. If cooled very slowly they will separate out again and the steel will become very soft – this is annealing. If you were to let it cool a bit faster the steel would be not quite as soft, but still not hard – this is normalising. If you cool it very quickly, the iron and carbon will not be able to separate out and will be effectively frozen together. This fast cooling is quenching and will result in a steel that has been hardened.
The steel, though, would be hard in the same way as glass is, it would result in an edge that, although hard, would chip when highly stressed and flexed. If you made an edge from normalised or annealed steel the edge would roll when flexed. We are looking for a balance between these two states, an edge that will flex but spring back into shape. This is done by tempering a tool that has been hardened. The hardness is reduced slightly but the toughness increases massively, the hotter you get an edge the softer it will get until it becomes useless. Tempering is simply a question of reheating the steel in the range of 180-250°C.
As a final simplification of the process, you get it hot, then get it cold quickly, then finally warm it up a bit. In essence it is that simple. To break up all the text I have shown this process completed in one heat, using the residual temperature in the shank of the steel to provide the heat for tempering. However, although quick and simple it is not a process that I would use commercially, nor recommend you do as it is not a very controlled process.
Steels
You do need to start with a useable steel, this comprises iron and carbon, the percentage of carbon having the largest effect on the hardenability of the steel. You need a minimum of 0.4% C for a steel to harden. Most tool steels used in carving tools have between 0.6% and 1% carbon in. Other alloys may be added to modify the properties of the steel. As the percentage of carbon is low, it is often described in terms of points – so a 0.80% steel would have been described as having 80 points of carbon.
There are many different ways steel is classified, but some can be easily read, for example 80CrV2 means a 0.80% carbon steel with added chrome and vanadium.
As a general rule, the more carbon a steel has the harder it will get, but is also more prone to breaking. This is one of the reasons files are frowned upon – old ones were made of very high carbon steel, over 1% as they had to be hard enough to cut steel, but the trade-off was fragility. Not too bad on a tool that had no shock loading when used as intended, but if used as a turning tool a catch could cause problems, plus a file is formed by cutting hundreds of tiny teeth into it, each one a potential point for a fracture to propagate from unless they are all ground out. This amount of work to get a safe, usable billet of steel can make files looks like pretty poor starting points.
People often use drill bits thinking that as they are made from high speed steel, they will hold their edge for a long time. However, the shanks of drills are left soft so they don’t damage the jaws of the drill chuck, so if you do this, make sure you grind your edge from a hard section of the bit.
Bi-metallic high-speed steel hacksaw blades are usually made with a tiny section of HSS for the teeth and a plain carbon steel for the rest of the blade, so once you have ground the teeth it is likely you no longer have any HSS left. But the best starting point is a carving tool. It’s a pretty good bet that you will be able to heat treat that without any major issues.
It’s important to gain an idea of how hard the resultant steel has become. In a professional workshop this is usually measured on the Rockwell C scale, determined by the depth a specially cut diamond will sink into the steel under a certain load. Most carving tools are in the range of 58-60 HRC, although tools that have a tougher life, such as adzes, can be as low as 55.
However, a hardness tester is expensive. You can buy a set of hardness testing files relatively inexpensively and these can give good results with practice. There are two other methods to estimate hardness in a more empirical method that are reasonably easy to use and repeatable. First, a tool after hardening should scratch glass, but not after it has then been tempering as it will have softened just enough by this process.
In a similar way, a sharp, fresh file should skate over hardened steel but just start to bite on steel that has been tempered – be aware you can destroy a file quite quickly doing these tests. Although steel needs a rapid quench to harden properly, it actually hardens at quite a slow rate. If it doesn’t seem to have hardened, leave it an hour and retest. If you use a calibrated hardness tester, you will notice that the steel will often increase by one unit 24 hours after quenching.
Heating
You need a way to heat the steel evenly and a way to check or estimate the temperature. For heat, a gas ring on a domestic oven will do for small tools. A blowtorch aimed at a fire brick can work, or even a lump of charcoal. If you can make a little cave to keep the heat in, this will be more efficient. Work in a well-ventilated space, outside if possible. I’ll be talking about specific safety precautions in the next article.
To estimate temperature a magnet is a great tool, but get a metal one, not a plastic fridge one. When steel becomes non- magnetic this is a sign that you are in the right temperature range for hardening (quenching). It takes a time to take effect though, so let the steel come up to temperature slowly and keep checking. Ideally you want to let the steel sit for a few minutes (often 5-10 mins is better, but this seems like an agonisingly long time).
You can use a thermocouple to more accurately replicate heat treat temperatures, but be prepared to experiment. A thermocouple may give a repeatable reading but at 800°C the few I have will diverge by up to 20°. Depending on what you are using as a heat source, the outside layer of the steel may lose some carbon, usually due to the oxidising effects of the flame, and not harden properly. So be prepared for a bit of grinding after heat treatment to get back to unaltered steel.
Quench media
Steels are sometimes classified by their quench media, thus the well-known O1 is hardened in oil and a steel used for files such as W2 is hardened in water. There are air hardening steels – in an air blast – but I have yet to see a carving tool made from one. Water gives a more rapid quench than oil, and has the potential to make a harder tool, but too rapid quenching can cause warping and even cracking, which will destroy a tool.
You can modify how rapidly the water quenches. Adding salt to make a brine will speed it up, as will adding detergent, but if a tool needs this to harden the steel must have very little carbon in and will be pretty marginal at best. Warm or even hot water will, not surprisingly, cool the steel more slowly. But if you decide you need a slower quench then oil is a better bet than water.
There are dedicated quenching oils that have different speeds, but you can get good results with vegetable oils. It sounds unlikely, but if you warm the oil you will get a quicker quench, the oil will be more mobile and transfer the heat away from the steel quicker. It wants to be about as hot as a drinkable cup of tea, around 50-60°C. As with heat sources, this is theory only at this stage. As already mentioned, I’ll be talking about safety issues in the next article.
Tempering
This isn’t like quenching or hardening – it doesn’t matter what the steel is in when it is tempered. You can temper it in water, but obviously could only get up to 100°C. You can temper in oil, but it tends to smoke quite dramatically over 200°C. The simplest is the domestic oven. A top tip here – if you have quenched in oil the tool will smoke in the oven, so wash it in soapy water before putting in the oven. I put them in a baking tray with a loose wrap of silver foil round them, which tends to stop delicate tips of tools overheating as the oven cycles and the heat varies. Leave it for an hour for the temperature to even out.
I’ll mention a bit about temper colours here. When clean steel is heated in the tempering ranges here a very thin oxide layer builds up, and light is refracted through it and changes colour. The steel will go from straw, to brown, purple, blue and finally, when the oxide layer is too thick for the light to pass through, grey. However, despite what you may have heard, these colours are not directly related to the temperature of the steel, higher carbon steels will take on colour (i.e. thicker oxide layers) quicker. An oily surface will also build up the layer quicker.
It’s a way to temper, but not my preferred one. Also, when tempering it’s better to allow the steel to sit at your desired temperature for an hour, and this is difficult when judging temper colours as you can’t tell if the steel has got colder. If it has there is no visible change in colour as the oxide layer can only get thicker, not thinner. Grain size has a bearing on how well a steel will cut. It’s similar to cutting a thread in boxwood compared to some quick-grown pine. Fine grain will hold detail better and be stronger.
Steel that is kept at high heat for a long time will exhibit a coarse grain structure. This is similar to crystals growing in metamorphic rocks that are subject to very high heat over a long period of time. It can be caused by the steel being forged at too high a heat when made, or rarely by being overheated on its quenching cycle. If you have a tool that chips, it’s worth looking at the grain size of the steel in the chip. Sometimes chipping is caused by the tempering cycle being too cool and leaving the steel to hard, however, sometimes chipping is caused by large grain size and this can also be addressed.
One way to break up these large grains is by thermo cycling – getting the steel hot and letting it cool. There is no need to let it cool really slowly (annealing), cooling in still air breaks the grains down more efficiently. This process relieves stress in tools and makes blades less likely to warp on quenching, so it is recommended to do at least one normalising cycle. If you are having difficulties with the steel, three normalising cycles and a quench is often used.
In the next article I will put all this together and show how to approach and troubleshoot heat treatment safely in the home workshop, even if you don’t know the exactly what steel you are using.
The sharpening clinic is open
As the name suggests, I would like to help carvers with sharpening problems – this will allow me to focus my articles on tools that are relevant to you, the readership. I am looking for readers to send a brief email with a description of the tool, the sharpening equipment they are using and problems they are having. Please do not send images at this stage as it clogs up my email system far too quickly.
I will try to answer all emails but will only be selecting one tool per article. You would then send the tool to me at your expense, I will sharpen it and make it the subject of the article and send it back to you at my expense. Turnaround will be up to a month as I will need to get the tool well before the deadline to be certain I can fulfil my obligation to WC of turning in a quality article each issue. If not selected, please do not send me your tools. I don’t have time to sharpen them in my day-to-day business, and I don’t have the budget to return them to you if you do. Also, due to the time scales involved with overseas post, currently this is only open to carvers in the UK.
If you are interested, and I hope you are, then please email me at nic.westermann@btconnect.com
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