Tempering Simple Steels Accurately

Two items of note before going into detail about tempering: 1) you can temper anything pretty easily if the tempering temperature is below 450F. That means that if you have an overhard Japanese tool or anything else that’s just junk to you, it’s likely that it could be improved tempering. 2) I am not a knifemaker, and i don’t use high speed or very high carbide content steels that have a very high temper or two tempering ranges.

If you’re going to temper complex steels, you’ll need to get something designed to temper high speed steels.

Otherwise, the focus here is tempering recently quenched tools, but for most things used in woodworking hand tools, a good quality oven or toaster oven will be all that you need.

What is tempering?

Quenching relatively simple steels is a matter of converting nonmagnetic microstructures of steel – austenite – to martensite, and doing so as completely as possible. You can read knifesteelnerds.com if you want to know more about those states, but for our purposes, we assume a quenched tool starting with brittle martensite and peak hardness. For almost all uses other than a blunt scraper, you’ll find untempered steel doesn’t hold up to anything – not even sharpening.

Tempering is a process of heating the untempered microstructure to the same microstructure with somewhat less hardness and more toughness. For hand tools, I’ve found with O1, XHP (similar to or same as V11), 1095, 26c3, etc. to prefer 375F to 425F. As a rule of thumb, you’ll trade about a point of hardness for each 25 degrees. that means big temperature variations are unacceptable and we’d really like to achieve something more like a 10 degree deviation at most.

How much tempering, when can it be done?

A single temper with really simple steels will take care of 90% of the result and leave you with something usable to very good. Some retained austenite can be converted to martensite during the first temper, so official schedules generally recommend tempering twice.

Your job if tempering steel is to get steel through its thickness to be an even tempering temperature. Whether you do that in hot oil, in a kitchen oven, whatever it may be doesn’t matter. However, other than very simple small applications, I’d recommend staying away from tempering over a flame.

Most schedules suggest tempering right away. If you arrived here after reading from the quenching page that it’s nice to park steel in a freezer to really chase as high of initial hardness as possible – at least on a budget where liquid nitrogen is out of the question – then that really means it doesn’t matter that much. Tempering sooner will reduce warpage, but good technique quenching should limit that and we’re chasing hardness above perfection in warp avoidance. Long story short, temper sometime before you use a tool

I’d recommend tempering twice. And if you make something you really like, have a stable setup.

Tight Budget Equipment

The range that we’re discussing here – 375F-425F is well covered by a toaster oven. Toaster ovens probably vary a lot in quality, but I bought the cheapest toaster oven I could find on eBay – something like $24 shipped. the toaster oven cycles on and off and surface temperatures below the elements deviate wildly, so an easy way to mitigate that is to use the trays that come with the oven and block direct heat from whatever you’re tempering, and put the tool you’re tempering in a sandwich of metal to increase mass, have something else take the direct heat from the heating elements and have the overall tool temperature variation be smaller.

400F tempering generally results in a color change on the surface of bright steel to about light straw color. if you see purples and blues, the edges of your tools are getting too hot, and the remedy is you start over and re-quench. However, a little blue on the bevel of an iron may be misleading if the entire subject item isn’t blue – try the tool first.

You’ll also want to invest in one of the cheap hanging thermometers (analog) that are designed for ovens. If you use a kitchen oven instead of a toaster oven, you’ll find in some cases that your oven is not close to what it says the temperature is, and that while the oven cycles, the temperature varies like a sine wave pattern. In all cases in my experience, even convection ovens have drastically different temperatures in different spots. So, use the same spot in the same oven with the same setup. Consistency is key. Right next to the glass on a toaster oven is also, of course, not going to be as hot as the center of the toaster oven.

If you have a non-contact thermometer, you can also put something non-reflective in the oven with the piece you’re tempering and get a reading. More sources of information are better. In my case, as long as there is some shielding to the steel being tempered, I’ve found the indicated temperature on the toaster oven to be within about 10 degrees of what it says. It’s easier to get the same temper than it is with a larger oven.

Exactly what I use and Do

I use the aforementioned inexpensive toaster oven. This rarely heats lunch – it’s in my shop and quench oil makes it smell like a smoky candle. If you can only temper in a regular oven, O1 steel and vegetable oil is a good idea or your house will stink like petroleum or smoking candles.

You can see in this picture that there is a lower tray. This toaster oven came with a second flat tray that can go under or above, and I will sometimes put the steel between both trays. It takes a very long time for the “metal sandwich” to get to temperature doing that, though, so use of the top tray to shield away from heat is usually a only if the metal sandwich outsides are thinner.

In the picture are two heavy aluminum plates, and thinner may mean unwanted older plane irons, or 1/4″ mild steel bar stock.

I temper twice, an hour per cycle. The steel needs to cool between tempering cycles, but not long, so I pull it from the sandwich, and put it on the floor or the bench for a minute or two and then cool in water and then start the second cycle. There’s no need for a big rest between cycles. That term, an hour, means an hour after the thermometer shown reaches a stable temperature.

For ease, if quenching isn’t done right before bed, I will throw the aluminum plates in the toaster oven as soon as I drop the steel off after quench in the freezer, and then after an hour or so, start the tempering process.

For simple steels, that’s it. needling away at ensuring temperature stability and accurate measuring is important, and the toaster oven, plus the thermometer, plus a third check if available is nice.

There are other alternatives for temperature stability, too. If you have a small metal can and can put chisels, knives or irons within the can and not touching the sides, that’ll work well. You can also use a container with sand in it – I use the metal sandwich because it’s available and works. Grade yourself on outcome, not style. If you quench and temper accurately, even with a cheap setup, you’ll see almost no hardness variation with oil hardening steels and variation of less than 1 at the extremes with water hardening steel.

One last point – the Japanese tools

I find 375-425F to be a great range for everything listed above, and I’m partial to the sweetness/hardness combination of 400F. However, Japanese tools that specify 65/66 hardness are at the lower end of hitachi’s (white/blue steel) range. You’ll see comments that this or that blacksmith is tempering at 100 Celsius, or 212F. There’s nothing to gain tempering tiny amounts, but it does provide bragging rights for people with no nerve endings.

If you find yourself with an older japanese tool that’s extremely extremely hard, it may be 25 or 50 degrees of temper away from being a real treat. You’ll need to remove any wooden parts, abide by the accuracy discussion mentioned above and start tempering at 325F and then increase by 10-25 degrees depending on your patience until you find something you like.

White steel and steels like 26c3 will be 2 points or more harder than O1 or A2 at the same temper, so sweetness in the result can come with high hardness. 26c3 can hit 64 hardness after a hand quench and temper at 400F. The samples that I sent for testing averaged 63.8, and O1 averaged closer to 61.5 on the c scale. 26c3 is similar to japanese white 1B. Had I tempered it around 325F, it would’ve been less tough than I like, but would’ve hit 66 hardness or so.

Unless you’re working pallet loads of paulownia, you’ll probably find 325-350F will get you a nice japanese chisel from one that started overhard, and you may still find yourself with a very sweet 64F chisel at a 375F temper.

Higher and Higher Temper Can Yield Subpar Results

Larrin Thomas (knifesteelnerds.com) writes about tempering embrittlement or something similarly named. Apologies for poor handling of proper nouns if I mangle names or proper noun, but what this means is more important than exactly what it’s called.

The useful fact in this case is that simple steels often reach a point where adding temper leaves them both softer and less tough. Or, put differently, the result is worse no matter how you look at it as the tradeoff to get better toughness by reducing hardness is lost. We don’t want to delve into this. From what I gather, this range for simple steels can vary, but is above 425F. If you want a softer steel than O1 at 425F (about 60/61 hardness), you’re better off switching to something with less carbon and tempering it in its sweet spot than you are trying to temper a harder steel softer and softer.