Tempering question I have often pondered

[Chris Railey]

It may sound like a dumb question but I am curious on this one. Let’s say I am working with X steel and my HT is shooting for an as quenched hardness of 65. Lets say my temper recipe is shooting for 61. Now if, for some reason I only got to 63 on my quench. Will my temper draw 4 points of hardness or is it still good for about 61 even though I started at 63 instead of 65.
I know absent a Good way to test hardness its really a moot point but I was just wondering.

 

[Heikki]

I've actually wondered this too.

 

[Sean Jones]

Interesting question. I'll tag along.

 

[Kevin R. Cashen]

If you could have had 65, and got 63, the iron matrix has only about .5% carbon in solution (rough estimate). The remainder could be pearlite, from an insufficient quench or, more likely, in the form of undissolved carbide. Tempering doesn't know what your initial wishes were, and will simply remove more carbon from the matrix to form tempering carbides. The bigger question is what was the knife intended for? If it was a large chopper, you could enjoy greater toughness from the resulting phases formed in the lower carbon environment, essentially what you did was turn your 1095 into 1075 by not engaging its full carbon potential. But if you wanted a hunting knife or fine chefs knife, the edge will not hold up as well and you HRC will drop disappointingly low if you don't adjust your tempering schedule to match.

 

[Chris Railey]

If you could have had 65, and got 63, the iron matrix has only about .5% carbon in solution (rough estimate). The remainder could be pearlite, from an insufficient quench or, more likely, in the form of undissolved carbide. Tempering doesn't know what you initial wishes were, and will simply remove more carbon from the matrix to form tempering carbides. The bigger question is what was the knife intended for? If it was a large chopper, you could enjoy greater toughness from the resulting phases formed in the lower carbon environment, essentially what you did was turn your 1095 into 1075 by not engaging its full carbon potential. But if you wanted a hunting knife or fine chefs knife, the edge will not hold up as well and you HRC will drop disappointingly low if you don't adjust your tempering schedule to match.

I know there is a lot of stuff I am leaving out here but if I am correct in translating what you said into redneck then the temper formula for a said steel is more akin to reducing hardness by a number of points at a specific temp. In other words, this specific tempering temperature should produce enough changes in the steel to drop its hardness by about 4 points.

Yet another example of why one should choose the correct steel for your heat treating methods (among other things). If you cannot be reasonably sure that you reached the full as quenched hardness then it is hard to be sure a specific tempering temperature got you to your desired hardness.

 

[Kevin R. Cashen]

I know there is a lot of stuff I am leaving out here but if I am correct in translating what you said into redneck then the temper formula for a said steel is more akin to reducing hardness by a number of points at a specific temp. In other words, this specific tempering temperature should produce enough changes in the steel to drop its hardness by about 4 points.

Yet another example of why one should choose the correct steel for your heat treating methods (among other things). If you cannot be reasonably sure that you reached the full as quenched hardness then it is hard to be sure a specific tempering temperature got you to your desired hardness.

Absolutely. One of the most common questions that I get is what tempering temperature will give you a specific Rockwell number. I always have to say- sorry, I can't honestly give you that as it is way to subjective to all the previous heat treatments for that blade, and anybody who says they can, is either mistaken or not being entirely honest. When you see those numbers in books, like ASM texts, they are based upon ideal circumstances, under absolute control conditions. After a lifetime of pretty well controlled heat treatment procedures, I personally NEVER punch in a single tempering temp and go right to it for a desired Rockwell, but will walk it into that number with sequential tempering. It almost always comes out exactly as I predict, but the one time it doesn't could be a pretty expensive blade lost, or hours of time wasted to that point. You can always shave HRC points off but, to put them back on, you need to start all over.

It all starts with steel selection, it is one of three critical aspects of the quality cutting tool. We can either set ourselves up for effortless success, or endless frustration or failure, simply by choosing the right steel for the job, and knowing how it will react to our methods.

 

[Chris Railey]

Absolutely. One of the most common questions that I get is what tempering temperature will give you a specific Rockwell number. I always have to say- sorry, I can't honestly give you that as it is way to subjective to all the previous heat treatments for that blade, and anybody who says they can, is either mistaken or not being entirely honest. When you see those numbers in books, like ASM texts, they are based upon ideal circumstances, under absolute control conditions. After a lifetime of pretty well controlled heat treatment procedures, I personally NEVER punch in a single tempering temp and go right to it for a desired Rockwell, but will walk it into that number with sequential tempering. It almost always comes out exactly as I predict, but the one time it doesn't could be a pretty expensive blade lost, or hours of time wasted to that point. You can always shave HRC points off but, to put them back on, you need to start all over.

It all starts with steel selection, it is one of three critical aspects of the quality cutting tool. We can either set ourselves up for effortless success, or endless frustration or failure, simply by choosing the right steel for the job, and knowing how it will react to our methods.

Man, thanks Kevin it never actually occurred to me (though it should have) to walk the temper number down to a specific hardness. That way if I did something wrong in the hardening or the steel just had a bad day, I am more likely to hit what I want. "Sneaking up" on the number instead of a shotgun approach makes much more sense. A good Rockwell Tester is on my short list of stuff to acquire.