Newbie Heat Treat Questions

M

Mark Redmon

Joined
Nov 7, 2007
Messages
451
I’m hoping to get some advice from all you experts out there about heat treating. Specifically, help in avoiding pitfalls that could damage my blades during the HT process. I’ve been reading that blade symmetry is important… but how important is it?
Are we talking precision, or just really darn close? The first blade I ground I did free-hand (and it shows). The plunges are not quite parallel. I think I might try to correct/minimize that before HT though. The second blade I ground has a slight bias (a fraction of a millimeter) to one side. I didn’t want to remove any more stock prior to HT in fear of having too thin of an edge. My intention was to correct this after HT before doing the edge bevel. Also, as these were my fist blades, you can imagine they are far from perfect… there are a few digs (read “waves”) from the belt that have been sanded smooth, and nothing that is what I would consider “too much” to continue working on. I was careful to remove any deep scratches. That being said, there would be some minor variation of density through the tang… should I be worried about warping or cracking? Also, I hope this is the correct forum for this post…

-Mark
 
Symmetry is important, but the kind of steel you use is, as well. Some steels are very forgiving, others are not so forgiving, others still will be terrible. Blade form factor is also an issue: long, thin, slender blades warp far more easlily than short, chubby, thick blades.
The longer and thinner the blade, the more accurate finish should be.
Still, finish is not everything. Heat should be as uniform as possible, and the blade should be pluged vertically in the appropriate quench medium without hesitation and immediately moved in a gentle 8 shaped path for the first few seconds of quench. The harsher the quench, the more likely the blade will warp.
Inner stresses due to forging, or blade manufacturing process can make a blade warp as well, so it's very important to normalize, look for any warping, correct it if necessary and normalize again or, even better, twice more. Then you quench. Some would double quench (that is: heat, quench, heat, quench) but that would significantly rise the chance of warpage, so in very thin, slender blades avoid it unless you know really well what you're doing. Better get a good, straight blade than spoil it trying to get an excellent blade. Most steels that benefit from a double quench are bitchy ones anyway, for what I know.
Steels like 5160, 1050 or 1075 should be pretty forgiving as it comes to warping during quench. 5160 is a much used steel for sword blades due to its ease of quenching and very uniform hardening and excellent toughness.
Steels like W1, W2, O1 and such can be tricky. W1 and W2 are not well suited to slender blades that should withstand a flex as their very high carbon content makes them brittle anyway, even if it produces extreme edge holding capabilities.

So, to sum it up:
1) try to use a forgiving steel, at least for your first experiments.
2) Always normalize at least twice before quenching.
3) You can try exotic quenching techniques with a chubby short blade, but thin, slender blades should be always quenched vertically, without any hesitation.
4) Try to work thin, slender blades as accurately as possible, and to use easy, forgiving steels for these blades. They are the best anyway, as this kind of blade requires a steel that allows flex, that is tough and that hardens as uniformly as possible.
5) soak time is important to have a uniform crystalline structure.
6) Heat uniformly, avoid hot spots: hot spots cause different heating, distortion and grain growth which in turn produces a brittle blade.
 
PS: also, leave a decent edge thickness to avoid warpage. Remember that the edge takes the most of the thermal shock during quenching.
Leaving an edge too thin can lead to a perfectly straight blade with a wobbly edge.
 
Thanks Alarion... That’s good information and it gives me some food for thought. I’ve read a decent amount about the ductility and finishing of different steels, but haven’t seen much on the quenching characteristics, so thank you for that. I'm starting with A-2 and D-2 because I thought that an air hardening steel would be easier, and a good place to start. These are stock removal blades also… I know that some forum members frown upon that, but there’s no way for me to set up a forge while I’m renting. I managed to keep the edge to about 1/32" or so… I could give them a quick run against the belt if that’s on the thin side though.
 
If you're plate quenching, the steels will be much more forgiving than if you used oil/water hardening steels. The biggest reason to fix all of the "minor" things now, is that the steel comes off slower after you heat treat it. If you're trying to even out a plunge, it might be easier to do after heat treating because it's a little easier to over grind it before heat treat. I freehand grind all of my knives, don't worry about that. If it looks even to the eye, that's just about as good as it gets for most makers.

It's my opinion that flex is more a function of thickness and heat treat, high carbon steels will flex just fine if properly sized and heat treated.
 
Thanks AcridSaint. I’m learning a lot at this point. I don’t know any makers personally, so this is all sort of touch-and-go right now. All I know is that in a few short months, my “interest” in knife-making has turned into an obsession. I just wish I knew more about the “common sense” aspects concerning knife-making metallurgy. Flexibility isn’t an issue with these first attempts. These are both smallish blades (about 3.5-4” tip to plunge) from 1/8” stock. I would like to make some kitchen and fillet knives someday, but for now the stock I have is between 1/8-3/16” thick. You reinforced my initial thinking that finishing the smaller problems would be best left until after HT in order to not overgrind…although that’s almost a non-issue with my wimpy equipment :p
 
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