Differential Tempering

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grizzled gizzard

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Aug 31, 2010
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I can't find anything on drawing the spine back or differential tempering, and I'm not coming up with anything very effective and controllable.

I make small neck knives 0.1" to .125" thick, and 3/4" deep to knives as large as 5/16" thick by 1 3/4 deep, all out of 52100.

I'd like to fully harden them, then just temper about half the depth back to a rockwell of (I'm guessing) 45 or so, to be tough and spring-ish.

Does that sound feasible? I can't seem to get the spine hot enough without overheating the edge.

What is the best method?
 
I'm no knifemaker, but the best way likely is to just edge quench to desired hardness and let the rest air cool.
 
One way is to watch the oxidizing colors. Quench the blade then brush off the scale on one side. Use a glowing hot piece of steel from the forge or a propane torch. Heat the blade from the spine and watch the colors. The tang, ricasso, and the back of the blade should be blue. The edge should be a light yellow. It is OK for the colors to progress from light yellow to blue. As soon as you get the colors you want, cool in water. It is OK to brush the knife again and re-temper portions of it if you didn't get the blue colors where you wanted them. Remember, you can make the steel tougher and softer with repeat temperings but you cannot make it harder except by quenching it again.
 
A fully quenched blade with a soft-draw back will be stiffer and springier than an edge-quenched blade, which I like.
The most failsafe way to do it is to quench the whole blade, then temper the whole blade in the oven twice for two hours each to the temperature you want your edge tempered at. Then get a shallow water bath set up (metal cake pan, etc) and torch heat the spine of your blade while keeping the edge in the water. Rock the tip now and then, and watch your colors to make sure you're not softening any parts you dont want to.
I like to do this very thoroughly, to a blue-gray from both sides- it's easy enough to not draw the spine temper fully and leave some hard spots where you won't expect them.
 
What Salem said. I'd just add that I use a sand/water mixture for physically stabilizing the blade and preventing waves/splashing.
 
What Salem said and what Brett said as well. The wet sand in a flat tray gives you a hands free way of doing this type of draw.

Fully hardening a blade followed by tempering and drawing the spine produces a better blade than edge quenching.

Fred
 
That's what I do, too. Though... the metallurgy tells me it probably isn't necessary and there is always a chance of tempered martensite embrittlement.

Here is a diagram for reference. Notice the decrease in toughness after 400F? After 425F or so, you are only loosing HRC points with an actual loss in impact toughness. At 800F the toughness is back up with an HRC in the low 40's. I don't know how this translates to "real world" performance. But it is something to think about.

I read it as... Get your spine up past 800(almost to dull red) and quench in oil... or don't bother at all. I might be wrong.

SCIENCE!

Hardness-and-toughness-of-steel-after-tempering.jpg
 
Rick,

The temper embrittlement seems to have less of an effect as the carbon increases in a steel. At 50 points of carbon the dip has flattened out considerably (Verhoeven). I haven't seen any information, but extrapolating, 1095 should show a minimal effect.
 
Bo T said:
Rick,

The temper embrittlement seems to have less of an effect as the carbon increases in a steel. At 50 points of carbon the dip has flattened out considerably (Verhoeven). I haven't seen any information, but extrapolating, 1095 should show a minimal effect.
Click to expand...

I couldn't find any firm data on TME or TE with carbon content above .50% either. It would be nice to confirm but I suspect you are right. Thanks.
 
I'm trying to do exactly what Salem described. To the letter.

Edge quenched blades do not... well, they do cut the mustard and everything else, but they aren't strong enough.

Not knowing what else to do, I tried the pan of shallow water, but I WAS making waves, and not getting what I wanted.

Adding sand is a GREAT tip!

Having the knowledge about the temp/toughness curve is PRICELESS!!

Y'all corroborating stories totally takes the guess work out for me.

THANKS A TON GUYS!!!

Seriously, you guys are friggin Outstanding! ;)
 
When your tempering the spine can you use a propane torch or does a welding torch work better?
When you get the spine heated do you quench it when you get the heat you want?
Thanks this has been a great thread!
 
Greetings,
Someone stated that edge quenched blades are not tough! Can you explain this further? Very interested in this discussion!

Regards,

Dennis Paish
 
If someone stated that, it wasn't me. I think an edge quenched blade can be super tough, I just personally prefer a stiff, spring-tempered spine.
I think it all boils down to personal pref/philosophy of use on this one.
Ok, I'll admit that I just posted so I can see my new sig line in action!
 
Dennis Paish said:
Greetings,
Someone stated that edge quenched blades are not tough! Can you explain this further? Very interested in this discussion!

Regards,

Dennis Paish
Click to expand...
Grizz said that edge quenched blades aren't as strong and he is correct. Martensite is inherently stronger than mixed pearlite.

ETA...
Perhaps I should clarify any potential semantic misunderstandings. Strength and toughness are two very different things

Toughness - The ability to deform(rather than break) once the elastic limit has been reached.
Strength - The ability to resist plastic deformation past the elastic limit. (Steel has the same elastic limit, regardless of heat treatment)
 
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Rick, I appreciate your participation on this thread.

I have a couple questions though. I agree and understand the parts in your quote I haven't colored, but I don't understand "elastic limit" in the blue highlighted parts. Also, I really don't understand the red part. How can steel have the same elastic limit regardless of how it's heat treated?

Basically, I guess I haven't ever heard of an "elastic limit".

*Quote, Rick*
Toughness - The ability to deform(rather than break) once the elastic limit has been reached.
Strength - The ability to resist plastic deformation past the elastic limit . (Steel has the same elastic limit, regardless of heat treatment) * Unquote Rick

Thanking you in advance for your time and clarification.

Really appreciate it man.
 
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Elastic limit, the point of stress at which the material will no longer return fully upon release. I.e. it will either bend, (plastic deformation) or snap.
 
Grizz... look up "Modulus of Elasticity" or "Youngs Modulus".

Basically hardness has nothing to do with elasticity as Salem defined it above. Geometry(cross sections) will dictate how far steel will flex and return to true. Hardness will dictate whether it will resist plastic deformation(and eventually break) or deform(bend) and stay in one piece. It may sound like a no brainer... "I want my knife it bend, not break!" but there is more to it. It depends on whether or not you want your knife to bend and be tough or resist and be strong. I downright abuse my knives and found that strength is an attribute I desire. So I make my blade strong with a full quench and temper back to increase the toughness.
 
One way of thinking about it is the amount of torque that the blade will take before is bends or breaks. A strong knife might take 50 lb-ft of torque before it breaks at a 30 degree bend. A tougher knife might take 35 lb-ft of torque before it starts to take a set at a 30 degree bend. It will be easier to bend an edge quenched blade (mostly pearlite) than a tempered martensite blade. But the tempered martensite will break sooner after hitting its elastic limit. The pearlite will take a set and bend further before breaking.
 
So, if I understand correctly, toughness is how soft the metal is (as supposed to hard and brittle) and strength is how springy the metal is (so it returns to its shape), and the elastic limit is the same regardless we just perceive it differently because hard steels break and soft steels bend.
Am I way off? I really want to understand
 
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