How come...........

[Jerry Shorter]

it takes 4 to 6 hours at 400 to 450 deg's to properly temper a blade. (depending on who you are talking to) and it only takes a micro-second at 600+ deg's (blue spot when grinding) to render a blade useless :confused: just something that has puzzled me for a long time so figured it was time to find out. So would someone please explain.

Jerry

[Quenchcrack]

Tempering is a chemical reaction where carbon and iron combine to form carbides that precipitate in the martensite through out the blade. For the carbides to form, the carbon must diffuse toward the point where the carbide is precipitating. This takes more time at 400F than at 600F. Time and temperature are inversely related. To get a specific hardness you can use a low temperature for a long time or a high temperature for a short time. The problem is being able to exactly hit the desired high temperature and hold it for the precise short time. Better to use the lower temperature/longer time. As for the blue spot, it is, of course just an oxidation color that shows the metal hit 600F+. It does not in anyway permanently hurt the blade if you have not heat treated it. If it is already heat treated, the blue spot will be a bit softer, and possibly somewhat more brittle, than the rest of the surrrounding steel. It is often refered to as burning the steel but this is not entirely accurate. To actually burn the steel, you need to get it to white hot and sparkling. At this temperature, you really have ruined the steel. A blue spot formed at 600-1000F is far less damaging, especially if you have not heat treated it. If it is already heat treated, depending on where the blue spot is, I suppose you could just polish it out. I would not do this if the spot was within 1/2" of the edge,though.

[Jerry Shorter]

Thanks for the quick response Quenchcrack, it gets me a little closer to understanding. When you say that Time and Temp are inversely related, are they also none linear? If so it must be a really steep curve for that blue spot to be that much softer. On the Rockwell scale, what would you guess as the difference. I would think that there is more to it that just hardness but that is a good starting point. Are there charts available or better yet a mathematical formula to illustrate the difference.

When I get as confused as I am about this I like to start at the very bottom.

Jerry

[RJ Martin]

Jerry: Two comments for you. First, tempering times are generally established for thicker sections and generally anticipate that it will take some time to get the entire workpiece up to temperature. Because the heat transfer is not too fast at lower temps (400-600F), it is always best to err on the conservative side for tempering time. However, knife blades are very thin, and so they heat through evenly and relatively quickly. Also, the diffusion that occurs does not take long once the temperature is reached. So, 2 hours is plenty of time

Grind burn on a knife edge involves contact temperatures far higher than you might expect at the point of contact between the abrasive and the blade. I would never assume that the blade had only been tempered to the level indicated by the color of the oxide layer-I'd assume it got much higher (400 or 500 degrees, easily). And, as quenchcrack pointed out, you now have a local area that is markedly different from the surrounding areas and is therefore prone to failure because it is too soft, or too brittle.

Remember, when regrinding a blade, just because you can hold the blade in bare hands and handle the heat, it doesn't mean that the actual temperatures at the point of contact with the abrasive aren't much hotter. If you want to see for yourself, put your thumb right behind the area you're grinding and see how fast you get burned!

Going slow and using sharp belts and a good lubricant or coolant on the belt is essential.

[Jerry Shorter]

RJ: Thanks for the response. I see what you mean about the thin cross section on the edge of a knife. (the thinner she is the quicker she gets hot) and I have Ki-yiped a few times doing just what you suggested with my thumb behind the grind spot. As far as the temp of the "burn" spot goes, if it reaches say 1000 Degress F. re-profiling past the blue to purple range up into the bronze range would'nt really help because it really got much hotter than that. Am I getting it? So maybe it is just best to hang that one up and start all over because it is probably too thin to re Ht.

Sorry about being so dense about this. When I don't understand something I can be like a porkypine and stick you from all different directions.

Jerry

[AwP]

So maybe it is just best to hang that one up and start all over because it is probably too thin to re Ht.

If you just hang it up, it won't be finished into a knife... If you try to reHT it, worst that can happen is that it breaks and it can't be finished into a knife. Since you would be out a knife either way you might as well try the HT again and hope it works.

[RJ Martin]

Jerry: Hard to say.....If the knife is for you, I'd consider sharpening it up and doing some cutting on some hard wood so you can assess the quality of the overheated part of the blade.

You'll find out pretty quick if there's a problem.

I wouldn't recommend selling it.

[Quenchcrack]

OK, I guess I have to concede that the temperature of the steel might go much hotter for a few thousandths of an inch deep. However, if the steel is NOT HEAT TREATED is still won't hurt the steel. IF it IS heat treated, then you might have to do some grinding to get it out. If you tempered the blade at, say, 450F for two hours, then created a blue spot that got to 1000F for a fraction of a second, it will have a marginal effect on the hardness, maybe a point or so Rockwell C. Generally, as long as you do not exceed the tempering temperature during grinding, it will not hurt the steel even if you do burn yourself. However, it really depends on the type of steel. Steel with high chromium, vanadium or molybdenum will resist softening more than plain carbon steel. I think the real issue here is one of professional integrity. Would you buy a blade if you knew it had a blue spot in it and it was polished out?

Regarding the linearity of the inverse relationship, yes, it is fairly linear within a given alloy group. It may change from alloy to alloy.

[Jerry Shorter]

"Would you buy a blade if you knew it had a blue spot in it and it was polished out?"....Nope! and I wouldn't consider sending one out either, unless I could prove without a shadow of a doubt that no harm was done to the blade.(after temper blues). And I highly doubt that I'll ever be able to prove that, so I guess when that happens I'll just have to start over with the HT.

Sooo. When re-heatreating would it be best to anneal or would normalizing be a better way to go prior to the quench cycle heat.

Jerry

[AwP]

Normalizing is better, if you anneal it can take alot longer to get it back into solution when you actually HT, and longer soak means more decarb and scale with the risk of still not getting it all into solution.

p.s. I'm assuming a carbon or low alloy steel, if you're using stainless or something then ignore me because I have no clue.

[Jerry Shorter]

Ok, thanks Cough'n

Jerry

[Quenchcrack]

This is an interesting question and it actually can be determined how deep into a piece of steel the structure is affected. I can forge out a victim blade, grind it enough to get a nice surface on one side, heat treat it, then put it on the grinding belt and burn a blue spot in it. I work in a plant that has a very complete metallurgical lab. I can cut the victim blade through the blue spot using a coolant flood to make sure I don't burn it worse. I can then mount the specimen, polish and etch it and view it under high magnification. Our microscope takes very nice digital color photographs and I can post the photos here. As soon as I learn how........

[Jerry Shorter]

Originally Posted by Quenchcrack

This is an interesting question and it actually can be determined how deep into a piece of steel the structure is affected. I can forge out a victim blade, grind it enough to get a nice surface on one side, heat treat it, then put it on the grinding belt and burn a blue spot in it. I work in a plant that has a very complete metallurgical lab. I can cut the victim blade through the blue spot using a coolant flood to make sure I don't burn it worse. I can then mount the specimen, polish and etch it and view it under high magnification. Our microscope takes very nice digital color photographs and I can post the photos here. As soon as I learn how........

Quenchcrack: That would be great. Would you do that please? That would go a long way in seperating the fact from fiction. As for posting pics, when you get ready let us know and we will walk you through it. I thought that there was a sticky or tutorial about posting pics but danged if I can find it. The main thing is you need to have find a photo hosting site. Jamey Saunders (member) can help you out on that, just email him.

Thanks a lot for you help.
Jerry