Testing for Overheated Edges

[EOU]

We've been thinking - and that's a dangerous thing. There has been much discussion on the Exchange this past year about overheating edges during the grinding process. Some of our members have conducted edge retention tests that might indicate that an edge had been detempered or these test results might simply be indicative of any number of other variables, singly or in concert, that might affect edge retention. If one were to lay poor edge retention firmly on the doorstep of overheating it would be nice to have some direct evidence that, indeed, the edge temperature had been raised to a temperature level during the grinding process, at or near, where detempering might have occured.

Of course the rub here is that measuring temperatures at or very near the edge apex is, to say the least, problematic. Furthermore, measuring maximum temperatures in real time, while the edge is being ground, seems near impossible. But perhaps not. In our former lives, we did something similar to this with discrete electronic components. We used temperature sensitive tapes that turned color once a preset temperature had been achieved. For our purposes, back then, if the high temperature limit was never triggered then the component was unlikely to suffer damage from overheating. It occurs to us that something similar to this approach might provide, if not the answer, at least a better idea of what kinds of temperatures are being achieved under what kinds of grinding conditions.

For starters, here's a possibility; a description of temperature sensitive lacquer followed by a list of available temperature ranges that fit our area of interest. 

Description
OMEGALAQ™ Liquid LAQ Series uses the same material as used in OMEGAMARKER™ in lacquer suspension. Lacquer serves a transport function only. OMEGALAQ™ may be thinned to ensure workable consistency. Thinning OMEGALAQ™ will not affect its accuracy. Order GREEN LABEL THINNER. 

To use, simply shake or stir to a uniform consistency and apply a thin smear to the working surface before heating begins. It dries almost instantly to a mark. When the stated temperature is reached on subsequent heating, this mark liquifies sharply. A melted OMEGALAQ™ coating does not revert to its original dull-opaque appearance but remains glossy transparent which makes subsequent interpretation a simple matter.
 
0175 for 175F (79C)
0200 for 200F (93C)
0225 for 225F (107C)
0250 for 250F (121C)
0275 for 275F (135C)
0300 for 300F (149C)
0313 for 313F (156C)
0325 for 325F (163C)
0350 for 350F (177C)
0363 for 363F (184C)
0375 for 375F (191C)
0400 for 400F (204C)
0425 for 425F (218C)
0450 for 450F (232C)
0475 for 475F (246C)
0488 for 488F (253C)
0500 for 500F (260C

Much higher temps are available as well. Here's how it might be done; Paint the edge apex on both sides and then grind on one side for a determinant period of time/pressure. Of course the lacquer where the grinding has been conducted will be gone but should remain immediately adjacent to the ground area and, hopefully, in its entirety on the unground side. Then examine (probably microscopically) to see if the preset temperature level of the lacquer has been triggered. Lot's of possible permutations to this test. Different temperature levels of the lacquer, different grinding times and levels, and different grinding techniques.

We're throwing this idea out for comment. We have some very sharp and experienced cookies hanging around the BESS Exchange these days and we'd like to open this idea up for discussion and suggestion. As always with the BESS Exchange, we work as a team.

[SteveG]

Absolutely a great idea! I don’t sharpen with power equipment, so probably won’t test it, but I’d sure like to hear what someone like Grepper would find.

[Larrin]

Sounds like an interesting idea. Probably easier than attaching thermocouples.

[EOU]

Thanks Larrin and Steve. We're all about finding out where the fences are for starters. Might be a good idea to begin with a belt grinder. Begin with a short period and light pressure and see if we trip the sensor paste. If not, a considerably longer period with more pressure, then check again etc. Depending on what we see and at what level we see it we may need to call on a good "non-power" man like you Steve.

By the bye. Just had a conversation with someone who maintains that the term we used "detempered" is not really an accurate term. Over heating would simply cause the edge to be tempered again, albeit at a potentially different temperature than the rest of the blade. It would not be as if the edge had lost all of its original temper and hardness simply because, during the grinding process, it was heated to a temperature that approximated its original tempering temperature.

[Larrin]

I’ve never seen the term detempered before. I think you made it up. Overheated is an appropriately generic term, since it could range all the way from overtempered to reaustenitized.

[Bud]

Now I'm mad I didn't think of this. We use thermal paste on bearing races that are liable to overheat at work. If it turns color it time to replace. Good thinking and hope that it works out. Lets see if belt grinders get blades anywhere close to tempering temperatures or not. My money says no but I've made bad bets before.

[Mark Reich]

Mike, tremendous respect for your determination to find as much truth on this subject as anyone, Ever, to my knowledge.

You had me at, "We used temperature sensitive tapes that turned color...". The light came on immediately. I wanted to weep and rend my garments.

Coming up with such a perfect answer is very cool. Ultimate simplicity makes it sheer friggin Genius. Tempilaq has been around forever. Many people already use it on knives. No one has ever thought to put some on the edge.

I'm anxious to read the rest of this, but the opening volley sounds absolutely brilliant.

[Geno]

This is an interesting topic which also often came up in a german board messerforum.net where Dr. Herbert Weisshaupt in 2007 where he looked at this scientifically but sadly it is only in german but you can propably get some info out of the graphs in the attached PDF. https://www.messerforum.net/showthread.p...quot-Thema

There is much more info than that over there but nothing in english sadly. I can try to translate some points albeit I am not verry good ad it if you are interested in soemthing specific.

[KnifeGrinders]

I’ve never seen the term detempered before. I think you made it up. Overheated is an appropriately generic term, since it could range all the way from overtempered to reaustenitized.

The term "detemper" is used in the New Zealand study
The Measurement of Knife Sharpness and the Impact of Sharpening Technique on Edge Durability

"...it is thought that powered, mechanical type sharpening overheats and thus detempers an edge.
The softer, detempered structure of the metal causes the edge to fold over more quickly and more easily, resulting in a dull blade."

***
We are talking of a change in the very narrow band along the edge apex, within 1-2 micron from the apex. Methods based on visual detection may not be suitable for the micro-scale, but we won't know till we try, and I don't think this should be tried without a metallurgical microscope...

[EOU]

Thanks to all! Please Larrin, "I think you made it up " don't be so brutally honest. A little literary license is acceptable from time to time. We considered "distemper" but found that it can be vaccinated against. In addition to KG's reference we also found ourselves in good company with everyone's favorite electric knife sharpener and we quote "Chef’s Choice recommends using a multi-stage sharpener-whether electric or manual-with guides (to ensure angle control) and diamond abrasives (which will sharpen any metal alloy and never overheat/detemper the blade)." Having put forth our defenses, we agree and prefer "overheat" as well.

Mark, thanks much for your support and we have yet to research "Tempilac" but that is exactly the kind of help we are looking for here. We just ran across the Omega product and it looked as if it might work so advanced it as a possibility. If something else might work better we'd love to hear about it.

Which brings us to Geno. Thank you very much for the link to the Dr.'s paper. Very useful and informative indeed. We have a lot of "lookers" on many of the Exchange's studies but not so many contributors so your input was much appreciated. I had a couple of takeaways from the Dr.'s paper and please correct us if we've missed or gotten something wrong here. It would seem that this is primarily a study of how much heat (as it relates to grinding steel), at how long a time, various steels begin to lose some of their hardness. We did find it interesting that he didn't seem to think that color changes in the steel were a good indicator of this loss of hardness but my translation wasn't good to enough to understand why he feels that way. Here's a couple of highlights for everyone here;

Applying his general formula and at at 300F (150 C) of a knife beginning harder than HRC62 it would might look something like this for a particular steel, Hardness reduction vs. time @ 300F

62 HRC     23 minutes
61 HRC     21 hours

60 HRC     3  days

59 HRC     6 months

58 HRC     16 years

He references 600C (1100F) several times as some sort milestone temperature and we're not certain why other than we suppose that most of the hardness might be removed from the steel at that temperature if it were held at that temperature for a long enough time. He gives an example where a particular steel, when heated to 600C, was reduced in hardness from HRC56 to HRC48 in 36 seconds. Someone with better German translation skills than our own might be able to glean more and more accurate information from this study than we did.

KG. What is the reason for your suggested use of a metallurgical microscope? We would be only trying to discern color changes in the lacquer ,as we understand the process, and it would seem that an optical scope would suffice.