How I re-harden file?

[pieinthesky]

I understand what you are saying Ray but there is some fun and reward from using old files/rasps. Overcoming the problems and figuring out which files are suitable and how to heat treat them is all part of the challenge.

I havent tested my rasp knife to destruction and cannot say for sure that the HT is the best it could be. I have however subjected the knife to some serious chopping and battoning (across the grain), it is still in one piece, it has kept sharp well enough and the edge has not rolled or chipped.

I can safely say if the rasp had been case hardened, this layer would have been removed from the edge during grinding and the edge of the knife would not have performed well. Also the file tempered down from 65 Rc to 56/57 Rc as expected so I am pretty sure the steel is OK.

I am getting a few more rasps tomorrow and hope to have a good range of makes. I might cut all the tangs off and anneal and harden them to be sure. If I learn anything useful I will post it here. If anyone is interested?

[pieinthesky]

I got some time to have a play and my results are below. Any assistance in interpreting them would be appreciated.

Firstly I took 4 different makes of 14 inch farriers rasps and measured the Rc at the tip end and the handle end:

Save Edge 66 Tip 61 Handle
Bassoli 65 44
Mercury 55 41
Hellier 44 39

Next I sawed off the tangs. Except for the Mercury on which I used a cutting disk the tangs were all soft enough to be hacksawed easily. I used the tangs as I wont need them to make knives and any case hardening is less likely to be present. I filed groves in each tang for identification and ground the surface of each side to remove any case hardened layer.

First I heated them to 825C, soaked for 10 minutes and quenched in oil. Hardness was as below

Save Edge 46
Bassoli 56
Mercury 23
Hellier 18

I then re-heated them all to 840C and quenched in water. Hardness was:

Save Edge 56 - 63
Bassoli 61 - 63
Mercury 50 - 56
Hellier 42 - 58

They all seemed to have hardened so can I conclude they are all carbon steel and not case hardened?

As you can see I did not get consistent hardness throughout each test piece. I am not sure why this is but I did not take to much time getting a good finish on the test pieces and they were just chucked in a bucket of water all together so quench was probably far from even.

[Ray Rogers]

First of all, I'm highly suspicious of your Rockwell results. Getting accurate readings with a Rockwell tester is as much an art as simply science and it requires careful preparation of the metal surface which, from your description, I don't think was done here. All that aside, your results probably showed lower figures than the truth in most cases. Numbers like 18 are completely inaccurate on the C scale, assuming your machine is calibrated for about 60, but it does indicate that the steel was very soft or perhaps uneven, lumpy, had scale on the surface, or it was improperly supported.

Case hardened steel is carbon steel. Depending on the carbon content it will harden up if you heat it and quench it. It won't harden enough to make a good file though which is why the case hardening process is used on it. Pretty much any steel used in a file will make a knife if you aren't too particular about exactly how well it performs. A piece of completely mild steel can make a blade, after all, and it can be sharpened and it will cut at least for a while.

All of which takes us back to where we started: you can spend a enormous amount of time on old files trying to guess exactly what you have and testing for the best way to heat treat it so that you can produce a blade that will have mediocre characteristics more often than not. Or, you can spend a few dollars on good blade steel thus removing all the doubts and questions and produce a high quality blade in a short period of time. Files and rasps can definitely work but it's up to you to figure out what it will take to make it work this time with these files. Then, be prepared to start the entire process all over again with the next pile o'files ....

[Doug Lester]

First of all hardness testers are only have a precision of =/- 2 HRc so the Bassoli is consistent and the Save Edge and Mercury aren't all that bad. I'm thinking that the quench wasn't all that bad but there are strange things that can happen with the vapor jacket when using plain water so there might be something there that interfered with even cooling. How was the surface prepared for measurements after the quench? Could there have been any scale left on the surface? Was it even? Were any of these measurements taken right at the edge? I'll explain the reasons for that last question below.

From the measurements that you obtained I would venture to guess that all of the files were made with enough carbon in them to make knives from. The Bassoli and the Save Edge might have a little something in the way of an alloy that increases hardenability to a small degree. The Bassoli more than the Save Edge. The Mercury and Hellier are from real shallow hardening steels that need an aggressive quench.

One of the things that you are witnessing here is that steel can act a little weird in quenching if the thickness is around 1/4" or less in thickness. There is relatively little difference between the rate of cooling on the surface of a flat bar and the middle so the quenchant will not form a hardened jacket around a soft center if it's not fast enough to harden the ball all the way through. It will all cool to form the products whether that is pearletic steel, martensetic steel, or a combination due to a slack quench evenly throughout the bar except right along the edge of the bar where the 90 degree angle there forms a little corner where the steel acts like it is thinner. Actually, it is thinner if you make your line of measurement at a 45 degree angle from the flat surface which allows the edges to cool faster. This same effect is responsible for the "natural" quench line/hamon sometimes seen with some blades made from shallow hardening steel. The blade is thick enough at the edge to form pearletic steel and thin enough at the edge to form martensetic steel with a thin band between where the steel slack quenched.

I'm thinking that the above might have an effect on the wider range of hardness measurements, especially with the Hellier. It could be that it was right at the thickness where it wouldn't harden or not harden but tried to do both and slack quenched. Just a guess on my part though and only as a contributing factor for that really wide range you got with the Hellier.

Doug

[pieinthesky]

Thank you both for very informative answers. The one thing I keep learning, over and over is how complex this subject is, how much I dont know and how much I dont know I dont know

My tester is accurate as I checked it with three different test pieces and it was pretty much spot on.

The samples however were very quickly prepared with an 80 grit belt and were not particularly flat or straight. My results were initially much more varied and tightened up when I changed from a wide platen on the tester to a spot platen, I guess this tells a story. I kept away from the edges though you cannot get too far away on a file tang.

I will clean up the surfaces a bit better and re-test my samples.

One question for the experts please. What ball park Rc reading should I be getting after quenching and before tempering. Can this be measured to check for a slack quench? I dont seem to find this figure quoted anywhere.

And I know its easier to buy decent steel, I have a stock of O1, RWL34 and 12Csomething waiting to be used but I still want to play around with files. I am sure I will learn something useful (I already have with your help) and if I make a few half decent knives in the process I will be happy.

Many Thanks

Mike

[Ray Rogers]

QUOTE: What ball park Rc reading should I be getting after quenching and before tempering. Can this be measured to check for a slack quench? I dont seem to find this figure quoted anywhere.


That question can't be answered specifically because we don't know what steel you are using. Generally though, any bladeworthy steel will Rockwell above Rc 60 just after quenching, most will be at or above 63. And, if you can't find those figures quoted anywhere that is simply because you don't know what steel you are using. If you did know, you'd look at the HT specs provided by the manufacturer and you would see that figure as well as all the others that you will need to properly HT that particular steel. That last part is an indication that there can be a significant difference in the properties of a blade that you guess has been HTed about right and one that follows the published specs. This is especially true of alloyed steels like O1 which is why any old heat and quench process used on O1 seems to work pretty good but that's only because that maker never tested an O1 blade that has been properly HTed . All of which is by way of explaining that no matter what you do with those files the blades you make will likely fall short of the true potential of that particular steel whatever it may be ....

[pieinthesky]

OK I give up