Temper Question (You Know What I'm Going to Ask...)

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Sharperthansticks

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I was sharpening my Benchmade Anthem on a Ken Onion WorkSharp machine using a leather stropping belt. (You can buy this belt separately; it doesn't come with the machine.) And then my finger touched the blade, and I recoiled in pain from the heat. "S--t," I thought. "That temper may be in trouble."

Within a few seconds, I touched the blade again, and it was not hot at all, which means that (a) it was never that hot to begin with, (b) 20CV steel cools quickly, or (c) some combination of (a) and (b).

Next started the compulsive Googing to find out when steels begin to lose their temper. Then I found a graph that showed the temperature range for tempering 20CV, and though I'm nothing near a metallurgist, the graph seemed to indicate that the lower end of the temperature scale was well below 200C. Well, hey, if we're going to be obsessive about this, let's not stop now; I actually did more research and attempted to call one of the manufacturing facilities so that I could ask an engineer how likely it was that I ruined the temper. The facility is closed until Tuesday. (A security guy answered the phone.)

So, I can't provide you with exact data on how hot the knife got nor how long it was at its maximum temperature, but do you guys think there's a significant chance I ruined the temper?
 
I'm no expert, but usually when something like this happens a blade will have turned a straw or light brown right on the edge normally at the tip where it's thinnest.
 
Hickory n steel said:
I'm no expert, but usually when something like this happens a blade will have turned a straw or light brown right on the edge normally at the tip where it's thinnest.
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Yeah, I've read that bit about the discoloration. I didn't notice any, but it's worth mentioning that the 20CV in the Anthem is somewhat brownish to start with.
 
I don’t think I’d be comfortable hitting my Anthem blade on that machine.
If it isn’t really dull, just strop by hand.
I really doubt you over heated it, unless you were using a lot of pressure at high speed for a long time.
Joe
 
This is why I will likely never use one of these.

While it is unlikely that you compromised the entire blade, I find it highly plausible that the apex is compromised, and the heat treatment is pretty much wasted, since this is where the rubber meets the road...

If you've ever run a grinder against rusty, scaly steel, it's amazing how quickly the thin pieces of scale heat up to red hot... They can't shed the heat fast enough, and warm up much faster than whatever mass of base metal they are attached to...

This is what I envision happening at the apex during powered sharpening... (It's well known that the best edge retention on a given blade is often not revealed until after several manual sessions, after the factory-compromised edge is removed)
 
If you are going to use powered belts to strop you should be pulling the knife off the belt regularly and feeling it for heat. Keep a cup of water handy and some paper towel or drying rag. If the blade is getting warm to the touch (warm not hot) just dunk it, dry it and go back to the belt.

If the knife has gotten hot enough that it can burn you then I don’t know if the rapid cooling from dunking will only damage the steel more. The trick is to not let the steel get that hot.

Or better yet, strop by hand.
 
I use the WSKO with the blade grinding attachment to sharpen, and I've never had a blade get beyond moderately warm. I don't use a leather belt, though, so maybe they generate more heat? I always use the minimum speed setting on the WS too, just to be safe. In any case, I'd be surprised if you got your blade hot enough to mess up the heat treat.
 
Thanks for the food for thought, people. Guess I won't know for sure until I cut with the blade for a while (though I don't have a benchmark to compare it to).
 
The tempering temperature for those types of steels can be pretty high. Even if you did damage it, a couple of hand sharpening sessions will fix it. Cut lightly into the stone to remove the damaged edge, sharpen to a burr, repeat, and then finish and it should be fine.
 
me2 said:
The tempering temperature for those types of steels can be pretty high. Even if you did damage it, a couple of hand sharpening sessions will fix it. Cut lightly into the stone to remove the damaged edge, sharpen to a burr, repeat, and then finish and it should be fine.
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I wish I was good at hand sharpening. I'm not, which is why I bought the WorkSharp. Yeah, I goofed by going too hot on the blade, but I'll be checking more frequently, now. (The procedure for using the leather strop on the machine is different than the general procedure for sharpening, which is what led to this problem.) I'm not a pro with the machine, but it usually does a good job for me.

As for cutting into the stone, wouldn't a future sharpening naturally take off the edge, anyway (whether sharpening by stone or via machine/sandpaper)? (If this was really necessary, I'd cut lightly into 1500-grit sandpaper.)
 
From Bohler's data sheet.
M390 at 63 HRc goes down to 61 after THREE temers ONE HOUR each at 400 F (200 C).

I guess the austenizing process takes much longer time than that for sharpening.
It could still occur (I read somewhere that Elmax is very prone to heat damage and need a very slow feed for grinding).
 
In my early days I ruined many edges grinding. if it is burning hot to the touch, then you likely burned the edge. Good news is you can remove the burned metal and reprofile.
 
There was a thread on this site about this. The creator of said thread used Markal markers (or similar in function) to show edge temperature.

I tried to locate the thread, but I was unsuccessful.
 
Tempering is both time and temperature . Especially at the lower end of the temperature range, the time required to change the temper is an hour or more. A few seconds at 400F isn't going to change the temper.

Also, "hot enough to burn your hand" is typically anything over 140F. So there is no telling how hot it actually got.

When the knife was initially given its final sharpening in the factory, it wasn't done by hand, and the machines I have seen used for final sharpening in factories are not water cooled.

So, unless you spent half an hour with the blade at that "hand burning" temp, I think you can just take it as an eye opening experience.
 
Different people are saying vastly different things here. Since the question is not a matter of taste but of fact, at least some people must be speaking not based on facts but on conjecture.
 
Fact is yes you can ruin an edge and it doesnt take 400 degrees for 1 hour. Many have done it. I have. Roman Landes has as well as many more. The apex of an edge being very thin can peak at temps well above 400 degrees and 140 degrees isn't going to be a sharp pain when you touch it. As fast as you heated the edge it will cool down. But the damage is done. If you take it slow you should be fine.
 
knarfeng said:
Tempering is both time and temperature . Especially at the lower end of the temperature range, the time required to change the temper is an hour or more. A few seconds at 400F isn't going to change the temper.

Also, "hot enough to burn your hand" is typically anything over 140F. So there is no telling how hot it actually got.

When the knife was initially given its final sharpening in the factory, it wasn't done by hand, and the machines I have seen used for final sharpening in factories are not water cooled.

So, unless you spent half an hour with the blade at that "hand burning" temp, I think you can just take it as an eye opening experience.
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While I am not disagreeing with your post, I am curious about a few asides. Please bear in mind that I am not privy to all of the intricacies involved with factory knife making.

As far as I know, There is still a bit of grinding that is done post HT is done "non-cooled", be it on a dry belt for grinding bevels post HT, roughing bevels or final sharpening. And the higher the temperature attained, the duration required for damage is lessened.

With this in mind, how much heat do you think is sunk into a factory edge?

With a softer belt building heat faster than a hard backed belt of similar grit due to more surface area contact, would it be safe to presumethe same about a leather strop? Should the sueded side build heat faster due softer surface, or less due to a rougher surface?

How much edge damage would one expect from a home powered system, if used slightly incorrectly? (If one could consider the OP as using his in such a manner.)

I know that heat imparted into the edge is proportional to the grit used, a higher grit built heat faster due to more surface contact and less material removed.
Theoretically speaking, Does this have a limit?

Might a 1um belt (if one were available) have less heat buildup due to having such a small abrasive size, or would it depend more on the substrate density (soft/spongy = more heat, hard backing = less heat).
 
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