Proper Thermal Processing or Heat Treatment (HT) is essential for making a high performance blade. Geometry and steel selection are very important as well, but without correct HT toughness and the ability to hold an edge will be lacking.
Different steels require different HTs – the process outlined here is what I use for 1095 steel and for 15N20/1095 pattern-welded steel. The process is similar for 1080/1084 or CruForgeV steels, although 1080/1084 doesn’t require the annealing step described here.
Forging steel, especially at high temperatures needed for forge-welding, disrupts the structure within the steel, especially the distribution of carbides. This is especially true for 1095 steel, which has more carbon than it knows what to do with. This leads to the need for annealing or spheroidizing the steel after heavy forging to ensure the structure of the steel is ready for hardening.
These are the steps to my HT process:
– Apply anti-scale compound
– Normalize
– Grain reduction cycle #1
– Grain reduction cycle #2
– Anneal
– Rough grinding/filing/drilling
– Austenize
– Quench
– Snap Temper
– Temper #1
– Temper #2
and at the end of this process, the blade is ready for final grinding, hand sanding and etching.
Part 1 covers up through annealing, and Part 2 will cover the rest.
After forging something to shape, be it a blade, pry bar, golf tool etc., and letting it cool down, it goes back in the forge for a quick stab at normalizing. For these steels, that means heating it up to roughly 1575 F and then air cooling. In this step, air cooling is most often done by setting the glowing steel on a bed of coke on my coke forge, but sometimes by putting the end of the handle in a vise. This step is done in still air – if it’s windy outside, the forge doors are closed so there isn’t a breeze cooling the hot steel too fast. In general I run the forges with a reducing environment where there is minimal free oxygen, so scaling and decarburization (decarb) is minimal – with a typical heat treat oven, that is not the case.
Anti-scale Compound: Avoiding Decarb
Once I’ve got enough things forged to justify turning on the HT oven (an Evenheat 22.5 LB w/ TAP controller) the first step is to warm up the tools to be heat treated to 500F (soaking at that temperature for at least 10 minutes) so that the anti-scale compound that I use will adhere to the steel. Currently I’m using Brownell’s Non-Scaling Compound (https://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/heat-treating-accessories/non-scaling-compound-sku083015100-1122-4011.aspx). I’ve tried others, but I’ve found this to be the most effective and easiest to apply, and it will stay on for several cycles in the HT oven. I keep this in an oversized salt shaker, stored in a zip lock back. Once tools are done soaking at 500 F, I apply a generous coating of the compound before tools go back into the HT oven for higher temperature processing.
Without anti-scale compound, an inert gas environment, the reducing environment of a forge, or a vacuum, carbon will constantly be lost from the steel, resulting in decarb. That is, you end up with a thin layer on the surface of iron (steel minus the carbon) which will not harden. This is a very bad thing on the edge of a blade, but not necessarily a bad thing elsewhere… If you’re not ‘etching’ the steel after decarb forms, then it’s unlikely you’ll see it. Etching the steel in ferric chloride will quickly show you if decarb is present as it appears sort of hazy (on the left side of the image below, by the forge marksI call it ‘bling’ as it’s usually got a crystalline look to it)
Normalizing
Now that the steel is protected from oxygen, the next step is normalizing – this helps reset the grain structure and make it more uniform and also relieves some of the built-up stress in the steel. With the HT oven still at around 500 F from the anti-scale application, the blades are put in the HT oven and the temp is ramped up to 1575 F and the steel is allowed to soak for approximately 10 min once the oven reaches that temperature. Then the blades are removed and allowed to air cool, usually clamped in a vise. I usually have two batches or more, so the first batch cools down while the next batch soaks in the oven. Again, still air here – I keep the shop doors closed for this step if there’s any wind outside.
Grain Reduction
After normalizing, where we’ve made the grain size more uniform (more normal), the next steps are to further reduce the grain size. This is basically the same process as normalizing, but done at a lower temperature. Blades go in the HT oven, temp is ramped up to 1450 F and everything is soaked for approximately 10 minutes and then removed to air cool. Cooling speed isn’t as critical here, so instead of clamping the handle in a vise, the blades are set on a small shelf of expanded metal to air cool. Once cool, the same process is performed a second time.
Annealing/Spheroidizing
For a while, I used the temper anneal process, where blades were austenized, oil quenched, and then soaked at 1200F for 30 minutes. But once I learned about Divorced Eutectoid Transformation (DET) annealing I did some testing and quickly switched to that process. Annealing is a step that can be skipped with steel like 1080 – just normalizing is enough to make it easy to drill or saw and it’s not required to get it ready for austenizing/quenching. However, with the extra carbon in 1095, annealing is required to break up the carbide structures that form during forging and extended time at high temperatures and to get the steel ready for austenizing/quenching.
Following normalizing and grain reduction, the DET annealing is done by putting the blades in the HT oven, ramping the temperature up to 1380 F, soaking for 30 minutes and then slowly ramping the temperature down to 1200 F at a rate of 670 F/hour. Once the oven reaches 1200 F, the blades are removed and allowed to air cool.
Finally, I put the blades in a large skillet with some water and boil for a few minutes which remove the anti-scale compound.
Next up will be Austenizing–>Quenching, Tempering, testing hardness and checking grain size in Part 2.
In the mean time, check out one of the best books out there an heat treating steels specific to knifemaking:
https://www.amazon.com/Knife-Engineering-Steel-Treating-Geometry/dp/B08D4P9D95