Recommendation? Carbon Steels at High Temper?

M

MinusFortyForge

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Jan 26, 2025
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Hi Blade Forum, long time lurker decided to post here.
I'm wondering if anyone has done a very high temper on (relatively) simple forging steels and what their experience was with that. I mean, past the Tempered Martensite Embrittlement (TME) phase of tempering so probably at least 700 degrees F to 900 degrees.

Here is my background and set up which will provide context to why I ask.
Hobby forge, i'm not selling or doing anything amazing. my goal is "good enough." I use a hole-in-the-ground forge and bags of lump charcoal for fuel. I enjoy doing it this way and I don't forsee any change in my set-up in the future.
Large blades are a blast to make but as you know, my only way to quench harden them is to choose one section of the blade - because if I do multiple sections I believe the heat from the adjacent sections would over-temper. That's what makes me wonder, could I thus intentionally over-temper a steel like 1084, 1095, or 26c3 (i wanna keep my steel choices simple as I ain't doing soak-times in this setup) ... something that's essentially iron+carbon, into the 50-55 HRC Range? or would it plummet into the low 40's? Has anyone done anything like this?
If I could achieve high impact resistance-toughness on a long blade while still maintaining, say, high 40's lower 50's HRC, that would be... good enough!

Thanks by the way for all the great content and reading over the years here. I was able to produce an 80crv2 kitchen knife for my kitchen that was good enough that my wife uses it daily, and I have made several gift-knives for friends from simple steels that they greatly enjoy, thanks in part to what i got out of reading this forum.
 
so i think this is a silly question ....many swords and knives have been forged /heat treated/tempered by these methods by man kind since the dawn of time
a fire pit/forge powered by wood/charcoal.... from the Vikings to Samurai.....
not trying to be a bad guy here !!! but after forging myself for many years using a propane forge this is a silly question to ask....
it has little to do with the source of the heat ...and everything in how you use the heat source t accomplish what you want/need
 
Pinoy Knife said:
so i think this is a silly question ....many swords and knives have been forged /heat treated/tempered by these methods by man kind since the dawn of time
a fire pit/forge powered by wood/charcoal.... from the Vikings to Samurai.....
not trying to be a bad guy here !!! but after forging myself for many years using a propane forge this is a silly question to ask....
it has little to do with the source of the heat ...and everything in how you use the heat source t accomplish what you want/need
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It may be a silly question, but from your response I gather you don't understand what i'm asking as your response is not related to my silly question. Perhaps you might try to read what i am asking again and let me know if you have experience with what i am asking?
Thank you!
 
I'm confused by your description of your process. You say you quench harden only one section of blade. that means rest of blade isn't hardened. Secondly, what temperature are you using for tempering.,that'll govern your the hardness. And only the area of the quenched section.
 
It's simpler to austenitize at lower heat and temper under TME. Target the austenit temp per wanted hardness. So if you want a tough blade and you recon 55HRC is in the ballpark, get the temp/hardness tables and pick the temp you need. Quench and temper normal. This way is less fuss and faster.
 
velegski said:
I'm confused by your description of your process. You say you quench harden only one section of blade. that means rest of blade isn't hardened. Secondly, what temperature are you using for tempering.,that'll govern your the hardness. And only the area of the quenched section.
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I am pondering individually quenching a long blade, one section at a time, which would mean at least 4-5 quenches along the blade. in doing it this way, the adjacent already quenched sections would be exposed to i'm guessing at least 750 F temperatures, thus, making the temper far beyond the range we see prescribed in heat treating guides (for Carbon Steel, not Stainless or complex alloys) which hardly ever exceed 450F in order to avoid TME. That's an explanation as to why i am asking this. What I am effectively asking is:

Has someone out there has done a 750-900 F temper on simple steels/carbon steels. If so, how did that work out? was the blade functional, or did it become too soft? I know that lower carbon steels like 6150 achieve 52-54 HRC with tempering around 450F, but could you take a steel quenched at say 66 HRC and temper it down to ~50? Or would that be unfeasible due to the (i am assuming) formation of plate rather than lath martensite? Or would the plate martensite perhaps react well to a very high temper?

Thanks!
 
I don't think it is a "silly question" as much as it is a question that is hard to understand. I'm guessing your real question is how to HT a large blade in a hole in the ground forge. The tempering is done after the hardening. There is no such thing as hardening one area and then the next. I don't understand your question about a 900°F temper to solve a HT issue? I'll try and help with two answers.

1) A hole in the ground forge is what many people use for forging that don't have a dedicated forge. Nothing wrong with that. I have done a katana in one.
Swords can be done in them just fine. Temperature control is by having the air blast controlled by a gate valve. Pumping the blade back and forth through the hot spots is how you get the steel evenly heated. When the whole blade is evenly red and non-magnetic ... quench in whatever quenchant you are using. The steels you listed are suitable for HT in a simple forge setup and can be quenched in canola oil ... or even brine. The main thing is to have a large enough quench tank for the blade size.

2) Tempering requires controlled heat and time. You can temper part of a blade and then another part and the result will be OK. This is often done on a long sword that won't fit in the HT or kitchen oven. Temper one half, then the other, repeat, repeat. That will give the blade a more-or-less even temper. It isn't perfect, hut will work. Other methods of tempering a long blade are "walking the temper", where you use a torch or hot block of iron and apply it along the spine. Slowly go down the blade watching the temper colors walk down towards the edge. Keep moving to prevent over-tempering any area. It takes a little practice, but this is a historic method of tempering. You are shooting for an even straw color near the edge.

As you see in both answers, heat control and even heating is the key. However, your described method of sectional hardening (if I understand it right) won't work.

Note - Tempering a blade to 900°F would be useless as far as a 1095 knife is concerned. the result would be around Rc42. At 700°F it would be in the mid-40's.

Show us a photo of your forge, your tempering method (forge, oven, torch?) and some details on blade size and such. Try and give us a more detailed idea how you do the HT.
 
Stacy E. Apelt - Bladesmith said:
I don't think it is a "silly question" as much as it is a question that is hard to understand. I'm guessing your real question is how to HT a large blade in a hole in the ground forge. The tempering is done after the hardening. There is no such thing as hardening one area and then the next. I don't understand your question about a 900°F temper to solve a HT issue? I'll try and help with two answers.

1) A hole in the ground forge is what many people use for forging that don't have a dedicated forge. Nothing wrong with that. I have done a katana in one.
Swords can be done in them just fine. Temperature control is by having the air blast controlled by a gate valve. Pumping the blade back and forth through the hot spots is how you get the steel evenly heated. When the whole blade is evenly red and non-magnetic ... quench in whatever quenchant you are using. The steels you listed are suitable for HT in a simple forge setup and can be quenched in canola oil ... or even brine. The main thing is to have a large enough quench tank for the blade size.

2) Tempering requires controlled heat and time. You can temper part of a blade and then another part and the result will be OK. This is often done on a long sword that won't fit in the HT or kitchen oven. Temper one half, then the other, repeat, repeat. That will give the blade a more-or-less even temper. It isn't perfect, hut will work. Other methods of tempering a long blade are "walking the temper", where you use a torch or hot block of iron and apply it along the spine. Slowly go down the blade watching the temper colors walk down towards the edge. Keep moving to prevent over-tempering any area. It takes a little practice, but this is a historic method of tempering. You are shooting for an even straw color near the edge.

As you see in both answers, heat control and even heating is the key. However, your described method of sectional hardening (if I understand it right) won't work.

Note - Tempering a blade to 900°F would be useless as far as a 1095 knife is concerned. the result would be around Rc42. At 700°F it would be in the mid-40's.

Show us a photo of your forge, your tempering method (forge, oven, torch?) and some details on blade size and such. Try and give us a more detailed idea how you do the HT.
Click to expand...
Thank you most kindly for your informative and interesting response! So I don't think i have a picture on hand but i have a circular pit with a 2" steel pipe going diagonally down and reaching about the bottom-centre of the pit. It produces one hotspot in the forge near the middle. I use various electric blowers or even a chargable usb stroller fan if i'm looking to keep my temperatures lower but still forge. my primary electric blower is a modified shop vac which has an aluminum plate over the air intake. However, even closed, the air blast can still reach steel melting hot and I have melted many projects!
So what I would love to know is, how would one pump the blade through the hot spots? Just leave the blade on top of the coals, or place it on the bed of coals, cover it, heat it, then move to the next section, covering and heating?
So 900F is in the "i might as well make this pearlite" level of hardness, got it! thank you for answering that.
 
The ones best suited for knifemaking are trench shaped. Some folks put a horse tub or box over the forge to keep it usable for a long time.

Roughly, this is the build:
Dig a trench about 24" long and 8-12" wide. It is best to raise the edges of the trench above ground a bit by using the dirt you dig out. You can even make a rectangular wooden box to fill with dirt and place it on the ground as a frame and shape the trench inside the box.
Make the trench sides four to six inches higher than the ends.
For a big sword, you can make it as long as needed.
If you have clay soil available nearby, get a wheelbarrow of it and use it to line and reinforce the forge.
Once the trench is done, build a wood fire in the trench to "cure" the soil. Clean out the ashes and proceed with that air supply.

Get a piece of 1.5" or 2" steel pipe. You want it about 3-4 feet longer that the trench, so 6 feet is great. It doesn't matter if it is black iron or galvanized.
Bend the pipe at the trench bottom length so the open end raises up from the ground when it sits in the trench.
Drill 1/4" holes spaced about 3" apart along one side where the pipe will sit in the trench. Cap off one end or pound it shut and weld the seam.
Place the pipe in the trench bottom and build up dirt around where it sticks out to support the pipe. Add more dirt to cover the pipe for about a foot past the trench.

For the air blast, many folks use a shop vac or leaf blower for the air supply. Use a gate valve (PVC works - just make sure the pipe is long enough to stay cool) to regulate the airflow.

From there it is just a matter of filling the trench with charcoal, wood chunks, coal, etc. Once it has burned down to even coals, slowly adjust the air flow to get the coals just right and start forging or doing HT.

If the trench forge is raised a bit, you should be able to slide the sword through the top of the coals. If it is long enough, the blade can be held in the coals.

Google "Japanese Box Forge" or "making a katana in a trench forge" (or things like that) and you will see how the blade is pumped through the trench of coals.

I do the same pumping" with my 16" gas forge. I pump the blade in and out through the ports until it is all ready for the quench.
 
Stacy is right that you'll need a different design of forge that can heat the blade at once

There's no way to get part of a blade to ~800c and not overheat the surrounding bits. This will result in bands around your quenched section that will be way too soft, won't hold an edge, and will bend. You need to get the entire piece up to temperature and quench it
 
Alex Topfer said:
Stacy is right that you'll need a different design of forge that can heat the blade at once

There's no way to get part of a blade to ~800c and not overheat the surrounding bits. This will result in bands around your quenched section that will be way too soft, won't hold an edge, and will bend. You need to get the entire piece up to temperature and quench it
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This is quite useful to me thank you,

I'm very happy I posted this question here and I'm thankful to all the folks who have posted.
At the moment i cannot change my forge into a trench due to space and wife limitations, however, what I take away from this for the moment is that I need to move my thinking from separate sections into getting thinking about how i can use what I have to heat the largest single section possible instead. Fortunately I still have months until Spring so I have plenty of time to think about this.

Back to your reply, Alex Topfer, I just want to be sure I understand you. By doing sectional quenches I would create a significant difference in the structure of the steel between quenches, and those lines of difference would become weak points due to the lack of consistency?

Thank you most kindly!
 
MinusFortyForge said:
Back to your reply, Alex Topfer, I just want to be sure I understand you. By doing sectional quenches I would create a significant difference in the structure of the steel between quenches, and those lines of difference would become weak points due to the lack of consistency?
Click to expand...
Yeah. You heat and quench one section so it's hard. Then when you do the next section up you're heating the border of the first section, and it'll get up into sub-critical anneal ranges. So after the quench the two hardened sections will be separated by a heat affected zone with poor properties. I don't know how thick it would be, but it won't hold an edge there and it will bend more easily

See if you can find a local person who would help, or a local heat treating business
 
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You shouldn’t be controlling your toughness solely on HRC alone. You can run a knife hard and control the toughness with geometry and steel selection.

You can run 8670 or 5160 to 60 hrc and still have plenty of toughness.
 
MinusFortyForge said:
Thank you most kindly for your informative and interesting response! So I don't think i have a picture on hand but i have a circular pit with a 2" steel pipe going diagonally down and reaching about the bottom-centre of the pit. It produces one hotspot in the forge near the middle. I use various electric blowers or even a chargable usb stroller fan if i'm looking to keep my temperatures lower but still forge. my primary electric blower is a modified shop vac which has an aluminum plate over the air intake. However, even closed, the air blast can still reach steel melting hot and I have melted many projects!
So what I would love to know is, how would one pump the blade through the hot spots? Just leave the blade on top of the coals, or place it on the bed of coals, cover it, heat it, then move to the next section, covering and heating?
So 900F is in the "i might as well make this pearlite" level of hardness, got it! thank you for answering that.
Click to expand...
see if this helps you understand the movement

pump

MinusFortyForge said:
Has someone out there has done a 750-900 F temper on simple steels/carbon steels?

and the answer to your question is no, no one here would do such a thing
 
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Stacy, that video where he heats that blade by "pumping" was incredible to watch. The evenness of the colour he achieved prior to the quench was just wow. I'll be saving that video. I certainly cannot hold myself to that standard but I'll definitely hold that as my goal.
I noticed the tang and the section of the blade that meets the tang wasn't red when it went in the quench but he did ensure he heated it. would these sections be pearlite in microstructure then?
In Larrin's incredible Knife Steel Nerds articles i've read about pearlite and it being undesirable frequently, due to low hardness and brittleness, but I cannot find anywhere on the internet if pearlite is more brittle / less tough / less impact resistant than tempered martensite, or if the brittleness of pearlite is in relation to ferrite or other softer phases of steel. If I understand correctly, on the clayed section of hamon'd blades, would that be pearlite?
 
Yes, pearlite.
It is common to leave the main part of the tang below the austenitization point so it does not harden. Usually, the hardened area goes about 1" up from the shoulders of the ricasso. This aids in drilling tang holes. It also keeps the tongs from getting too hot.
Pumping isn't as hard as it looks ... just even strokes and watching for where to be slower and where to be faster in order to get an even heating end to end. Most folks pump too fast, which just wears you out.

TIP:
1) When HTing a blade with a forge, the edge is what you are watching. It doesn't matter if the spine ends up pearlite as long as the edge is martensite. In many types of blades a pearlite spine is desired, as in a katana, wakizashi, bowie, or large camp knife. That is what yaki-ire is all about in Japanese blades.
2) Even with a smaller blade, move and turn the blade as it heats up, so the edge heats evenly. Watch out for the thinner tip which will heat really fast. One trick is to hold the blade by the point and heat the main part until red, then reverse the tongs to the tang, and finish heating. Another is to stick the tip out of the rear forge port of a gas forge, or out of the hot spot in a coal forge.
 
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