Introduction: Heat Treating Knives
The purpose of heat treating is to bring steel to a hardened state. The correct hardness depends on the application of the steel being treated. Knives need to be hard enough to hold an sharp edge through continuous mechanical abrasion, yet be soft (flexible) enough to stand up to forceful use without breaking.
Equipment and tools:
- Visegrip pliers or tongs
- small forge or charcoal fire of sufficient heat (search for forge or smelting instructables)
- fire proof quench container with lid (I used an old cookie tin)
- quenching medium such as used motor oil
- BC fire extinguisher (the kind that puts out grease and oil fires)
- Heat resistant gloves and face shield.
- kitchen oven
- fireproof material for regulator block (I used aluminum tube)
- one mild or high carbon steel knife blank (forged or stock removal)
The quenching method I decided to use for this knife was the “edge quench”. I learned this method from $50 Knife Shop by Wayne Goddard. Instead of dropping the heated knife into the quenching medium tip first, submerging the entire knife, the edge quench involves submerging one third to one half of the blade's width (cutting edge first) into the quenching medium. A regulator block is used to hold the blade at the correct depth. The quenching medium I used was old motor oil. After the blade has been quenched, its hardness is still not suitable for usage. In its hard and brittle state, the quenched blade will shatter like glass if dropped, it must be tempered before it is put to use. Tempering involves heating the blade to a non-critical temperature (350 – 450 F) to slightly soften the steel (I used a kitchen oven). A tempered blade will hold a sharp edge and still retain strength and flexibility.
Step 1: Surface and Edge Preparation
The heat treating method is the same for knives made from forging, or stock removal. Whatever method you used,the final bevel should be on the blade and the surface should be brought to the desired finish. This was one of my first forged blades and I found the hammer markings appealing, so I did not polish the blade to a mirror shine, but I used a file to cut the bevel and a rough stone (100 – 200 grit) to get it fairly smooth, and as even as possible. Remember, once it is hardened, it will be much more difficult to remove material via filing and sanding. The edge is the thinnest part of the blade, and therefore more prone to cracking during the quench. Use a strip of 150 grit sand paper to dull the edge and reduce the chance of cracks or warping in the edge.
Step 2: Normalizing
Normalizing involves bringing the unhardened blade to critical temperature a few times to stabilize the structure of the steel. A normalized blade is more resistant to warping during the quench, although some degree of warping is unavoidable.
Heat the blade evenly and touch it to a magnet to test its temperature. Steel becomes non-magnetic at critical temperatures, so torch it, test it against the magnet, and let it cool to room temperature three times to normalize it.
Step 3: Quench Setup
I used this aluminum tube for the regulator block. You won't be able to see past the surface of the oil, so I used nails to indicate the position and direction of the block below the oil.
The quenching container MUST BE FIREPROOF! Do not use plastic Tupperware! It is also a good idea to have an airtight lid for the container to smother flames. I filled the container with water and marked the water level with a red marker (see photo) where the the blade rested one third to one half its depth under the water's surface on the regulator block.
In order for the oil to provide the right qualities for quenching, it needs to be warmed. Wayne Goddard says that cold oil “is not wet enough”. $50 Knife Shop suggests the oil should be 90 – 140F , but I just used my finger to judge when the oil became bath-water warm. I used a hotplate set on LOW HEAT. Heat the quench container filled with water first to get an idea of what heat to use for the oil.
Before you even start up the forge for the quench, prepare your work area by laying out all the tools and equipment you will need for the process. Make sure you have your container lid and a few sets of pliers. Make sure you have a BC fire extinguisher (the kind that puts out grease/oil fires) nearby. Mentally rehearse all of the steps you need to make.
The most important detail is that the knife enters the oil at or above critical temperature. The sequence I use is to heat the blade until it becomes non magnetic, then stick it back into the forge and push/pull the blade's full length through the heat once more, making sure the piece is evenly colored. When the blade comes out of the forge, you should move it into the oil slowly enough not to splash oil, and quickly enough not to lose significant heat. Be prompt about it.
Dunk the hot blade into the oil, onto the regulator block for approximately 15 seconds.
After the 15 seconds, submerge the entire blade into the oil beside the block.
Note: The reason the regulator block is used instead of simply filling the container with enough oil that the blade rests 1/3 to ½ its width against the bottom of the container is so that the blade can be fully submerged after the edge hardening. Laying the blade on its side without a regulator block to fully submerge it risks warping due to one side of the blade being cooled faster than the other.
When the bubbling and hissing of the oil subsides (around 45 seconds, time is not critical here) pull the blade out and allow it to cool in open air. Put the lid back on your quenching container to smother any flames.
Step 5: The File Test
Take the quenched blade to your bench when it is cooled. Rest a file on the bevel of the blade and move it back and forth, up and down the length. If all went well, the file should feel glassy as it slides across the bevel. Compare the file's “bite” on the bevel to the unhardened area on the tang. There should be a noticeable difference in the file's exchange with the steel.
Step 6: Tempering
A Note on Tempering: Every article I have read on tempering gives a notice to knife makers who choose to use their kitchen's conventional oven for tempering. Every piece of literature related to backyard knife making I could find gives a foreboding note about angry women coming after you for smoking up their kitchens by leaving motor oil covered steel scraps in the oven.
Some suggest buying a toaster oven for the sole purpose of knife tempering. I just scrub the hardened blade in water with dish soap and then hit it with a wire brush attachment in a power drill and then another soapy water scrub. This seems to get off the majority of the oil, and I have never noticed a smokey smell. Be sure to dry the blade before it goes in the oven. Recommendations for tempering temperatures vary, If you know precisely what type of steel you are using, search online for the correct temperature. The knives I have made are what I assume to be mild steel, coming from sources like hedge clippers and lawnmower blades. With my lack of metallurgy expertise admitted, I temper at 400F.
Cook your blade for one hour, allow it to cool to room temp, and return it to the oven for another hour.
After 3 one hour cycles, the tempering is complete. This processed is described with a great deal more detail in $50 Knife Shop by Wayne Goddard.
Step 7: Finishing the Blade
During the heat treating process, the surface of the steel was likely marred by scale buildup and oil residue, so it will be necessary to re-finish the blade to the desired level of quality. I used the roughest stone I have (100 – 200 grit stone from the hardware store) to put the edge back on the blade. From here, work your way up through your stones to achieve the desired edge.
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6 years ago
I just heat treated and tempered my blade to brown/straw as you showed here, but the edge was still too fragile and the tip broke. So my question is: Is possible to heat it too hot before quenching? Blade was almost bright orange in the daylight when I quenched it. And I´m 100% sure I did tempering correctly. Please help me so I can avoid this in the future.
Reply 1 year ago
I am going to add to this, even though this is quite aged. When heating hardenable steel, you've got to get to that salmon color. It's slightly past where it loses magnetic properties and just beyond where table salt melts. Far too many newer smiths quench too cold, which leads to cracks, which then lead to breakage. While tempering is an art form in itself, you've got to get that steel just over 1500° F to harden correctly. Anything below that will give you detrimental results.
Reply 6 years ago
It's really hard to see the color in daylight - our forge is pretty dim so we can see it pretty well. But even then, I don't usually trust my eyes for the quench. To make sure it's ready, you can get a magnet and hold the blade near it. If it's not attracted to the magnet, you've reached the right temperature. Just be careful - if any part of you gets between the blade and magnet, you can get a red hot knife stuck to your finger!
Reply 6 years ago
Quenching should be done when the steel reaches a bright red colour, 800-900 fahrenheit. Forging and shaping the steal is done at a bright yellow/orange colour, 1700-2100 fahrenheit. quenching at temperatures over 800-900 degrees can result in a brittle blade and/or your blade could warp or crack.
Reply 6 years ago
Quenching and Tempering refer to two specific heat treating processes. It seems like you may not have tempered 100% correctly. Quenching is much easier, and only requires that the metal reach Austenizing temperatures and then is rapidly cooled. Tempering on the other hand, is much more specific. It requires a quenched material, quenching being as described above, be taken to temperatures below the first transformation temperature of that specific alloy (normally 1100-1300) and held for 1 hour per inch of material. It is then cooled in the furnace, dropping temperatures roughly 400 degrees per hour (again, depending on the alloy) down to 700 or 800 degrees (again, depending on the alloy) and then air cooled. Higher alloys require controlled cooling to much cooler temperatures, as they commonly are more suseptible to diffusion (oxidation basically) at lower temperatures. Hope this helped. I fear they use their heat treating process labels loosely around here.
Question 2 years ago
So my Son is 19 and has been watching forged in fire since it began. In fact he's obsessed with it and wants to start giving knife forging a go.
I've just priced up only half of what he would need and that's going to cost me over £500 which I don't have.
Plus any extras that I don't know about. Any help as to where I should start would be helpful. Thanks.
Answer 1 year ago
This ain't as cheap in the UK as it's made out to be. Steel £20, firebricks £30-40, regulator and hoses £30, burner £30+. A better option is to buy a devil's forge on ebay
It will last longer and you can probably resell it a lot easier if it's not for you...
Answer 2 years ago
I came across this video on youtube, i am also interested in starting blacksmithing/forging... and i also love Forged in Fire. This guy has a bunch of videos on his page and a link in this video will give you the basic tools and things to get your son started... good luck and hope he enjoys knife making :)
11 years ago on Introduction
you say that in its quenched state it is still brittle. but with the blades i have made after i quenched them they were still very strong.
Reply 2 years ago
After the initial quench the knife is brittle and over time will continue to harden and be more brittle. By tempering a few times at 400 degrees you are slowing the hardening process and allowing flexibility to the blade while still maintaining an hard edge. Tempering the blade relieves stress in the blade itself and is a very important process.
Reply 10 years ago on Introduction
You might need to use a faster oil or a brine solution for your type of steel. What kind of steel are you using? I often have the same result with steels that only harden on the surface, the core remains tough and supports the brittle 'sheath', but this only happens with some steels that I use. Remember, the quicker you quench without cracking, the finer the grain structure and the tougher the blade will be at a given hardness. Good luck with your blades.
Reply 10 years ago on Introduction
my first blades were made from low-mid steel. (dresser drawer sliding bracket, and a shelving bracket.) the first one (my only instructable) does not bend easily and does not hold a great edge. (like i said low-mid steel) however it works like a child sized machete. the second one from a shelving bracket bends a little easy, but hold its edge and can be bent some distance before diffuclty is hit. and can be bent back without weakining.
only first blades cause im still working on my 3rd one (sword). the knives i could finish in 4-5 hours in my coal/wood fire pit. brick, wood or rock was used as anvil (all broke hehe) but i need anvil or anvil like thing for sword. last time i use a block of wood as one and i split it in half.(freshly cut too, very hard.)
Reply 10 years ago on Introduction
If you can get a hold of a piece of old rail road track or one of those antique irons you should be able to use that as an anvil without it breaking, I've used a railroad track for mine, but since it's covered in divots and dents from people doing the same to it before I had it, I use a smaller iron as a "finishing" anvil to get the final shape. Those will be easier to get if you live in an area with ranches or farms though, they may be harder to find in cities for free. As for the steel, I'd have to say that you'd have better luck making durable knives from a higher carbon steel, I've found a bunch of old files from my family farm and I've figured out a pretty good heat treat on Tool Shop brand files (really cheap where I get them) to where I can chop through several oak boards and still have them razor sharp. I'm trying to get some higher quality steel online though. NJ Steel Baron has some good prices and pretty good quality too, I'll hopefully be ordering some 52100 steel from them, I've seen that steel chop through hardened bolts so that's my new goal as a knifemaker. That's a pretty cool knife you make in your instructable, good luck with any other knives or swords you make. If you have any questions don't hesitate to ask.
Tip 2 years ago on Step 6
Hedge clippertand lawnmower blades are not mild steel and are great for making knives as well as all kinds of other hand tools. Chisels, pry bars, files etc.
Tip 3 years ago
please do not use motor oil it is toxic food grade oils usually work just as well here is a thread on oils for heatreating
3 years ago
I am currently tempering my first blade in my kitchen and after I put it in I ran to the corner store and when I came back I got yelled at for putting metal that was quenched in motor oil in the oven. Then I sat back down at my computer and this was open right to that paragraph warning of the angry women of the house. Couldnt help but laught at myself and next time will wash the blade instead of just wiping it off.
5 years ago
Mild steel does not harden. You should use steel with at least 0.5% carbon, preferably 0.8% to 0.95%.
Reply 5 years ago
It can be hardened, but in practical terms only for the first few mms. You will need to employ an old technique called case hardening. It is impractical because it is expensive. The case hardening material is actually very easy to make. You can but it but it stupidly expensive. It gets expensive because you need to weld a case, put the part in it with the hardening powder, then heat it extensively in a kiln which is expensive on the electricity supply. Anyway once you have soaked it in the kiln at temp for many hours, leave it overnight, crack it open and you will have a case hardened Piece of steel.
5 years ago
okay, after reading this, there are some guides I can add. I am from the UK, so 40°c is the temperature I was taught as a journeyman Smith. For oil quenching steels. When quenching in water cool but never cold was always the rule of thumb. And as for warping of the blades. I know it does happen, it has happened to me on occasion but it is VERY easy to fix, sometimes it is unavoidable. I have found it happens more with laminated steels and the twisted damascus types.
Perhaps this is due to one side always being under more tension than the other due to the twist? Just a theory.
Anyway, I digress....but if there is one piece of advice I can offer, if you want to make lots of blades, make yourself a belt grinder/sander. I did, and it saved me many many hours/days/weeks of endless filing. Just go ahead, make a couple of decent knives, and don't hold back on the finish and you will be begging for a belt sander.....trust me.
7 years ago
I hear treated some blades last night and they came out COVERED in all kinds of crud, would you have any useful advice for me? Lol, I'm desperate lol