The Basics of How to Heat Treat 1084 Carbon Steel

There are many articles, videos, and books on how to treat carbon steel for knife making and many of them are very informative and extremely detailed. This article is a basic overview of how I treat 1084 steel, it is based on a combination of research and my own experiences.

Background
In the beginning I was doing knife making purely as a hobby and the first knife I made was very much an experiment. I used an old car spring and heated it in a firepit with coal. I thought the temperature should be hotter, so I added air from a portable air pump. I didn’t know the heat of the fire and I didn’t know the type of metal I was heating… so understandably it was a disaster. The fire was way too hot and when I quenched it some of the blade melted completely. That was the first practical lesson of many.

Quenching
Nowadays after studying articles, videos, and books, I have a gas forge, I know it’s operating temperature and I know the properties of the metal that I am heating. The forge is a simple chamber made from Hebel block with a gas burner inserted into the top of the forge at a 45-degree angle.

Blank being heated in the forge

I invested in a high temperature thermometer and measured the average inside temperature of the forge and the temperature of the metal being heated – along the length of the metal knife blank. 1084 needs to be heated to 850 degrees Celsius so I adjusted the gas flow regulator to achieve that temperature. I took note of the colour of the metal at that temperature.

Next step was to move the blank back and forth to achieve a constant temperature across the full length of the blade, and to ensure that gas flame wasn’t concentrating on one area for too long. I constantly tested the blade with a magnet on the end of a metal rod – to see when the blade had gone from magnetic to non-magnetic. The result was a nice even dull orange colour that I now recognise instantly. If it goes bright orange it is too hot.

Checking the colour of the blade

When I get the full length of the blade to that colour I immediately plunge it into a large container of canola oil that has been heated to around 40 degrees Celsius. To get the oil to that temperature I put a reinforcing rod in the forge for a few minutes as soon as I turn the forge on and plunge it into the oil to heat to the correct temperature – prior to the first heat treatment. A cooking thermometer probe helps achieve this. It takes some trial and error. Make sure you have a decent amount of oil (at least 10 litres) because each heat treatment increases the temperature, and you don’t want it increasing beyond 60 degrees Celsius.

Quenching in canola oil

Once the blades have been quenched they are very hard but also very brittle. They need to be tempered to release the stress and achieve some flexibility. Cool the blank with water and clean it. Then use sandpaper on the handles to scratch off some of the black heat treatment residue.

Tempering
Put the blanks in an oven for 2 hours at 200 degrees Celsius. Wait till the oven is up to temperature before you start the 2-hour timer. Tip: buy a small oven with built-in 2-hour timer. I use a temperature thermometer to measure the temperature of the blanks inside the oven. Rest one of the blanks on the probe.

Blanks in the oven with temperature probe

Digital thermometer with high temperature probe

After 2 hours cool down the blanks (you can use water – it won’t affect the heat treatment) and make sure they are straight. I use a straight edge or the granite kitchen island bench to check this. If it is not straight, use a jig or clamps with spacers to straighten the blank – before the second tempering. There are many ways to do this, and I suggest searching online to find a method that suits you best. I use a homemade jig to straighten any wayward blanks.

Homemade straightening jig

Inspection
After another 2 hours heat treatment, check that the blanks are straight and the sanded area on the side of the handle is a straw colour. If they are straight and straw coloured you are good to go! If it is purple, the steel has been over heated and you will need to start the heat treatment process again. If they are still not straight repeat the straightening process and put them in the oven again at 200 degrees Celsius for another hour. Repeat until straight.

Checking the colour of the handle where it was sanded