Any tips for heat treating mild steel?

[AndrewJohnKarow]

I just finished 2 plasma cut blades. How should I go about heat treating the blades after sharpening? I only have an oxy/acetylene torch to heat the blades so how should I go about it. The blades are made of mild steel about 0.77 in wide and about four in long. I would upload a photo but I forgot my USB cord.

[Jerrod Miller]

By "mild steel" I'm assuming you got it from a hardware store, meaning it is A36. In that case don't bother, it doesn't significantly respond to heat treat. You can search the forum for several discussions on the topic; just don't use the forum search tool, use Google and a site search. site:bladesmithsforum.com <search term>

 

Example: site:bladesmithsforum.com A36 heat treat

[AndrewJohnKarow]

Ok do you think if I quenched it in used motor oil that it could absorb some of the carbon or did I just spend 2 hours for nothing on the two blades.

[Gabriel James]

To expound on what Jerrod mentioned a little: If you want a functional knife then you must start with hardenable steel-- and even better -- knife quality steel. Each steel has its own heat treat regiments required to obtain a hard sharpenable utensil. Thats why a lot of makers do not use mystery steel from the scrap yard (not knowing the steel means you dont know IF its hardenable or how to HT it specifically) but prefer to purchase known steel. Look up knife steel types and have your go at it for real this time. It's not a complicated process but you have a few holes in your approach so far. Try a bit more reading and youll get there. Heating with your oxy torch should be fine, but you also need to have a quench media and a tempering plan for after hardening.

 

Case hardening is a long time 12hours + at high temps to absorb into the "skin" of the steel some carbon to be case hardened. Primarily only done in industrial settings due to the operating costs. You just spent 2 hours practicing grinding for the real thing. those are throwing knives or letter openers at best.

[Riggertrey]

Got to agreement with Gabriel you did learn something that you will use later on. Don't bother trying to case harden a36. Shoot me a pm. I have some cut pieces of bandsaw blade 2" wide foot or so long appx 1/8" thick. I use for Damascus and for fillet knives

[AndrewJohnKarow]

I wasn't grinding I was programming a CNC plasma cutter. I was replicating a blade that I had broken the tip off of. It was a lot of small complicated and precise measuring.

[Brian Dougherty]

The grinding part should come between the profile shaping and the heat treat. You'll want to get your plasma cut blanks "Ground" down to more of a knife blade shape prior to heat treating. You don't have to use a grinder, you can use files, or whatever, but the slang term is "Grind".

 

Don't take it to a thin edge before you heat treat it. leave the edge with some meat to it so that you don't have cracking or warping problems in the quench. then finish grind after you quench and then temper it. You could leave it full thickness before heat treating, bit then you have to remove a lot of hardened steel.

 

Given your other post advising someone to always orient their quench tank north, I think some more reading is in order. There is a lot of bad info on the internet. This place is a great exception to that. You'll find lots of great people willing to share hard learned knowledge. make the most of it, and good luck. This stuff is addictive.

Edited May 20, 2016 by Brian Dougherty

[Alan Longmire]

Not to mention on good steel the plasma cutter will leave a zone of decarb that must be ground off before final shaping. Since this was mild steel it doesn't matter, it's just practice.

[Hoy's Forge]

If this is the only mild steel knife you ever made your way ahead of me! I forged blades out of mine roof bolts(a36) for 6 months back when I first started. I always wondered why my sword bent so easily! 1986 no such thing as internet forums. If it wasn't Jim Hrosilous' book I might still be trying it LOL

[AndrewJohnKarow]

Ok thanks for the info guys. (the quench tank orientation came from an old friend from Arizona long time blacksmith)

[AndrewJohnKarow]

These are the blades I've been working on.

[Wayne Coe]

Seems that no one remembers about SuperQuench.

5# rock salt

24 oz of Dawn blue dish detergent

8 oz of Shackalee II (you can use Jet Dry from the dish washer section at the grocery store or any other surfactant).

Place in a 5 gal container and fill with water and stir until all of the salt is dissolved.

Robb Gunter invented this for hardening mild steel. In a demonstration he cut a section off of a mild steel bar and made a chisel from it and hardened it in SuperQuench, then used the chisel to cut the chisel to cut the original bar. Robb has told me that you should get a 53-54 RC and should then temper it at about 450 degrees. The tempering does not take any of the hardness out of the blade but will refine the grain structure.

While it will not make as good a blade as a good steel alloy, it will harden. I have had my EDCs made of Rail Road Spikes and half a horse shoe and had to sharpen them less often than my wife's Case Equine Special.

[Jerrod Miller]

Seems that no one remembers about SuperQuench.

5# rock salt

24 oz of Dawn blue dish detergent

8 oz of Shackalee II (you can use Jet Dry from the dish washer section at the grocery store or any other surfactant).

Place in a 5 gal container and fill with water and stir until all of the salt is dissolved.

Robb Gunter invented this for hardening mild steel. In a demonstration he cut a section off of a mild steel bar and made a chisel from it and hardened it in SuperQuench, then used the chisel to cut the chisel to cut the original bar. Robb has told me that you should get a 53-54 RC and should then temper it at about 450 degrees. The tempering does not take any of the hardness out of the blade but will refine the grain structure.

Not really metallurgically sound info there. If you look at a TTT for mild (let's give the benefit of the doubt and say some nice 1025) you'll notice it is impossible to get much martensite. Tempering at 450F will drop it a couple points HRC if you do have some martensite, and not really do much refining of the grain. Grain refinement requires a bit of heat, deformation, and/or phase change; none of which can be said for a 450 F temper. If you don't lose any hardness at 450 F in a plain carbon steel, then you don't have martensite and thus have a letter opener. As for the chisel cutting the original bar, that isn't overly impressive. A 40 HRC chisel will definitely go through 20 HRC steel. That doesn't make it good for a knife. It's all good practice though, and not-so-great blades can still be useful. They can even be better than store bought, many of which are junk and yet people find them useful.

[AndrewJohnKarow]

I kinda agree with Jerrod you need a phase change to refine the grain.

[Joshua States]

Not really metallurgically sound info there. If you look at a TTT for mild (let's give the benefit of the doubt and say some nice 1025) you'll notice it is impossible to get much martensite. Tempering at 450F will drop it a couple points HRC if you do have some martensite, and not really do much refining of the grain. Grain refinement requires a bit of heat, deformation, and/or phase change; none of which can be said for a 450 F temper. If you don't lose any hardness at 450 F in a plain carbon steel, then you don't have martensite and thus have a letter opener. As for the chisel cutting the original bar, that isn't overly impressive. A 40 HRC chisel will definitely go through 20 HRC steel. That doesn't make it good for a knife. It's all good practice though, and not-so-great blades can still be useful. They can even be better than store bought, many of which are junk and yet people find them useful.

I'm glad someone (other than me) said this. I would not have said it quite so well........

Edited May 24, 2016 by Joshua States