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The Newbies Arena Are you new to knife making? Here is all the help you will need.

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  #1  
Old 08-29-2006, 12:37 PM
AlanR AlanR is offline
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To temper or not

I've successfully (I think) finished heat treating my second blade (<4"), this time using an oxy/acet torch. Really simple and quick, I liked it. I've read where it's best to temper very soon after the blade is quenched but I waited so I could test the hardness first to see where I was at in the process. I tested 3 times near the tang (so I wouldn't mark up the blade too bad) and got readings of 61, 65, 67. I know it's a bit softer, but not much, toward the tip of the knife because a file bites more in that area.

A couple of questions:

Is tempering necessary if hardness is in a decent range for the purpose of the knife? This won't be a prybar or heavy action knife.

My second question is just as a curiosity and falls inline with my first - Does tempering do anything other than lower the hardness, lower brittleness, raise durability (I'm assuming these all go hand-in-hand)?

Thanks,
AlanR
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  #2  
Old 08-29-2006, 01:20 PM
AcridSaint AcridSaint is offline
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Tempering is going to even out the hard spots to some degree and normalize the knife a bit. If parts are softer than the temper will make the steel, it's not going to do much for them, but the hard spots will all even out making your edge cleaner and more consistent when sharpening and cutting. Even if the stress in the steel doesn't crack the blade you'll probably end up with chipping at the edge later on if some parts are say 67RC. Since you don't have a good way to test the hardness of the whole knife, tempering is some very cheap insurance.

I think on of the main benefits of tempering is normalizing the steel to a point where it won't rupture or crack under its own stress at some later point. I also like to stick mine in the freezer between cycles, I don't know how much it helps but thus far it hasn't hurt. I read Fitzo and Kevin Cashen talking about it once and decided that I might as well do it too.

Just my amateur thoughts, some folks out here will likely have better answers.


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  #3  
Old 08-29-2006, 01:30 PM
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Ray Rogers Ray Rogers is offline
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I know that you are testing using hardness test files and so your accuracy could be questionable but a range of 6 points is HUGE! So, if that range is really true then you must temper for all the reasons AcridSaint just listed or the internal stesses will cause a failure sooner or later (most likely sooner).

You didn't say what steel it is so I can only guess at what a proper working hardness would be but for most steels that can be properly hardened with a torch, even 61 would be too high for most applications, even it the knife is for light duty. You don't have to use it as a pry bar to break it. A knife that hard can easily crack or break if you drop it on a cement floor, especially if the internal stresses haven't been relieved ...


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  #4  
Old 08-29-2006, 01:41 PM
AlanR AlanR is offline
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Thanks for the replies. It encourages me to temper the blade tonight.

Just to clarify: The metal is damascus. Testing is done with a Rockwell tester not files, the file comment came about just because I was running a fine file along the edge to see what would happen. I'm guessing the variation comes from the different metals/layers in the damascus.
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Old 08-29-2006, 07:17 PM
Drunkenduck Drunkenduck is offline
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Not knowing anything more than the points you tested were near the tang, I'd say that that is awfully hard for the tang or the back of the blade. Actually I'd say that that would be too way too hard for the edge. I'm glad to hear that you are going to temper the blade because I'd be afraid of catastrophic failure if you didn't (breaking the blade and having a ruined knife).

Doug Lester
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  #6  
Old 08-29-2006, 07:49 PM
Coutel Coutel is offline
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Hi Alan...I am curious to know what steel was in the damascus mix?....Rc 67 seems high, and I was wondering what steel would give that reading.

After hardening, did you clean the steel before ou took readings?... I was thinking that maybe that high reading was on some scale?

Definately temper.


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  #7  
Old 08-29-2006, 08:22 PM
AlanR AlanR is offline
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Hey all,
The damascus is leftover from my first project using "ThunderForged" damascus from TKS. I tested in a few other spots near the tang and got similar numbers. I've now messed with damascus steel enough to know that I'll not mess with it again until I've completed quite a few more knives and learned quite a bit more about material science

So I tempered at 350 for 1 hour to begin with which didn't yield much change in hardness so I moved up to 400 for an hour and that did the trick it seems. Testing now shows anywhere from 58-61 and now I'm going to try to sand the marks down a bit and finish assembly of the knife.

Now that you mention it the scale hadn't been completely cleaned off yet. Also, I haven't been trained specifically in how to use the hardness tester - only watched a few times while guys on the floor checked the hardness on some tooling. So I can see where, very easily, the readings could be 2-3 pts off especially knowing who was doing the testing *cough*. I'm also not sure when the tester was last calibrated or if it even needs to be.

-AlanR
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Old 08-29-2006, 09:09 PM
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Ray Rogers Ray Rogers is offline
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Having built a hardness tester myself once I feel compelled to comment. First of all, for a regular Rockwell tester to have any hope of reading correctly the steel must be clean, flat with two parallel sides, and with no warp. I am assuming that the material being tested is a blade and that the tester is not fitted with and special jigs or holders to allow for odd shapes, and of course, that the machine has been calibrated to read hardness in the normal blade range. If the machine wasn't calibrated close to Rc 60 it probably will not read the average blade's hardness correctly..

If any of those conditions have not been met the readings will vary greatly just as yours did. And, there is one further condition and that is that the blade in question must be of homogenous steel, .i.e., not damascus. Because of the way Rockwell testers work they cannot be relied upon to read damascus accurately. Think about it, there are at least twio steels there and they are intended to be at different hardnesses so that they will etch a nice pattern. And no, before you ask, the number you get will not necessarily be an average of the various hardnesses in the steels. Bottom line: Rockwell testers aren't of much use on most pattern welded steels....


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Old 08-30-2006, 06:09 AM
AcridSaint AcridSaint is offline
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I wanted to add that the tang still sounds quite hard, how does it react to the file now? Are you quenching the tang when you quench the blade?


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  #10  
Old 08-30-2006, 08:40 AM
AlanR AlanR is offline
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Quote:
Originally Posted by AcridSaint
I wanted to add that the tang still sounds quite hard, how does it react to the file now? Are you quenching the tang when you quench the blade?
The tang itself isn't hard. The very back of the blade near the tang is where I'm referring to. [Sorry if my terminology is adding confusion] I can see if that area is brittle all the stress from use goes directly thru there.

Should I heat the area adjoining the tang up and let it cool slowly to bring it back down? Thanks for everyone's patience. I'm always impressed with the information and methods to relay it on to newbies.

-AlanR
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Old 08-30-2006, 09:04 AM
AcridSaint AcridSaint is offline
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It's not a bad idea to have the back of the blade softer than the edge, just be careful that you don't get the edge too hot. If you draw it back with a torch you might want to stick the edge in some water. Just getting it hot should soften it up enough, I wouldn't worry about controlling the cool down like an anneal, just let it air cool after you heat it up.


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  #12  
Old 08-31-2006, 09:41 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Quote:
Originally Posted by AlanR
...Is tempering necessary if hardness is in a decent range for the purpose of the knife? This won't be a prybar or heavy action knife.

My second question is just as a curiosity and falls inline with my first - Does tempering do anything other than lower the hardness, lower brittleness, raise durability (I'm assuming these all go hand-in-hand)?....
AlanR
Alan, let me try to help with your questions with one of my boring metallurgical descriptions soft steel at room temperature will have its atoms arranged in a BCC (Body Centered Cubic) configuration, quenched steel will have its atoms forced into an unnatural BCT (Body Centered Tetragonal) configuration by the carbon atoms that were trapped between the iron in the quench. This is a highly stressed state, and if you have actually exceeded HRC 65, you are definitely there. This is the condition that often results in blades self destructing without tempering, and is above and beyond the mechanisms we need for high wear resistance and strength.

Tempering is the use of heat to affect those changes in the quench. With heat carbon atoms can move, the speed at which they move is dependent on the amount of heat. At the quenching heat things move very fast since spaces are wide open for carbon to zip about. In tempering things move much slower so we have much tighter control over those carbon atoms trying to wiggle and squeeze their way about. When heating to temper, virtually nothing happens until you reach the neighborhood of 250F. then the first change will begin. From 250F-350F you should see no significant drop in overall Rockwell hardness, but instead what will happen is that the BCT state will relax into a more stable BCC, this will stabilize the steel and greatly reduce the threat of self destruction. This is the basis for the so called "Snap Temper" that so many folks use if they cannot get to a proper temper right away or are going to further shock the blade with something like cold treatments.

Above 350F other changes occur. Superfine, submicroscopic tempering carbides will begin to precipitate out; it is this removal of the trapped carbon that is responsible for the slow and gradual loss of overall Rockwell hardness, and is pretty much the extent of it in the tempering ranges used by knifemakers (with some affects on retained austenite occuring above 400F). Continued heating above 600F or more will result in coarser iron carbide globs forming until one heats high enough that they are annealing the steel.

A single temper is beneficial, but two or three tempering cycles will have a very nice equalizing effect on the final Rockwell hardness and you could see the spread of your HRC readings narrow significantly, but only in a full temper, I have my doubts that snap tempering would do much in this regard.

One last note, as has been pointed out, depending upon the materials used and the quality of the welds, Damascus can be a challenge to get an even HRC spread on. However if steels very similar in heat treatment are used the readings can be spot on. The O1/L6 steel that I work with continues to amaze me in the consistency of the HRC numbers. Properly heat treated, I can dial things in to less than ½ a point Rockwell taken anywhere on the sample, some of the homogeneous steels I use can’t get that level. However I have tried testing some very dissimilar mixes that couldn’t ever get within 5 points HRC- very frustrating. If Thunderforge steel is what they say it is, chemistry wise, you should be able to get very close, not great but better than the spread you are describing.
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  #13  
Old 08-31-2006, 12:38 PM
AlanR AlanR is offline
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Kevin, that's awesome.

Thanks for the detailed reply. The "snap temper" you described is probably what I saw the first time I tempered (at 350) and didn't see much change in hardness. The second time I went up to a bit over 400 and saw quite a bit of change and I wasn't seeing quite the variance of the numbers. Given, I may be testing wrong completely anyway but the numbers I was getting after the 400 temper are in the range of 59-61 so I think I'm happy with that.


Thanks for the information - I hope it sticks in my brain

-AlanR
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