Testing for Overheated Edges

[Geno]

To EOU:

This is a somewhat theoretical but based on research piece where he took a optimistic number of only 600°C as maximum temperatur on the cutting edge and calculated the resulting hardness over time with the formula on page 3.

But I forgot to mention that he and Roman landes further in the thread postet other research that shows diagrams of heat vs time while grinding and real world tests on knives with micro hardness measueres I wil link them below.

Micro Hardness of a Busse and Cold steel knive
https://www.messerforum.net/showthread.p...post378002

Older german research by Südemann where he drilled holes through a specimen and attacked a thermocouple directly to the grinding surface or just behind a thin layer Page 14 shows how it was done. And at page 17 is the graph of temperature while grinding at 19m/s while the part was ground for one second on a SIC grinding disc 320 grit. Further down the paper there are different speeds and grits in use plotted temperature against time with and without coolant.
https://www.messerforum.net/showthread.p...post378086

And lastly something linked by Roman again from 1909 where they researched sparks while grinding and found that the sparks get hot enough to melt.
https://www.messerforum.net/showthread.p...post378314

[Jan]

Geno, thanks for drawing our attention to the second part of the thread, especially to the posts by Roman Landes. IMHO, when sparks are generated during grinding, then the 600⁰C (1112⁰F) cannot be considered as a conservative estimation of the maximum temperature on the cutting edge.
 
Jan

[EOU]

Thanks again Geno. We read the entire thread https://www.messerforum.net/showthread.p...post378002 and it explained the Dr.'s arrival at 600C as a benchmark temperature so thank you for that. We also found support, in that same thread, of Jan's contention that temperatures higher than 600C might easily be generated and we quote;

"In some forum, somebody was very amused that I wrote in a book that you can assume the temperatures of uncooled steel up to 2000 ° C induced in the uppermost surface layers. 

This person thought why then the blade does not liquefy when grinding the belt sander ...
Well it does, just in a very small area we know as a sparks rain"

There is little doubt left after reading several of these threads and studies of two things; (1) this group is very concerned about the potential for softening of the edge during the power grinding process and (2) this is a group of very smart, courteous and knowledgeable folks steeped in science and testing.

For our part we are forging ahead having ordered a bottle of 325°F Tempilaq. That's where we'll begin. We'll apply it and see what we see. We don't know, Geno, if you are a member of the German forum you reference or not but it would be very interesting to see what this group might think of our very simple experiment.

[EOU]

Tempilaq is arriving day after tomorrow and we're kicking ourselves in the butt for not realizing that Mcmaster Carr stocks this stuff in the complete temperature line.  We could have already received it last week if we had realized it. Here's the Carr link if anyone wants to do a little testing of their own;  https://www.mcmaster.com/temperature-indicating-liquid

[KnifeGrinders]

EOU, the World stand still in wait for your test results

[grepper]

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.

I find Mr. Mike to be an irritating fellow at times.  I get settled in my ways thinking I actually understand something and here he comes, with great alacrity and totally sans compunctions, perfectly willing to question the status quo and upset the apple cart all in the name of gaining a true understanding rather than peacefully going with the flow and blindly accepting comfortable old wives’ tales and accepted wisdom.  Very annoying.

More than once I have had a conversation with him where he proposed ideas that I initially inwardly brushed off as misguided or goofy only to later, in a forehead slapping moment, grasp the significance and ramifications of his thinking.  Mr. Mike’s ideas are well worth giving credence and due consideration.  

Not trying to be obsequious, but rather simply to say, thanks Mike!

[Mike Brubacher]

"I find Mr. Mike to be an irritating fellow at times."  If my wife had been the author of your post Grepper she would have just finished right there. Thank you for the encouraging words, at least the ones I was able to find in the dictionary. So if you don't mind, I'll try to explain my poor behavior and contrary nature. 

No matter the industry or craft, our activities seem governed by conventional thinking and wisdoms. That's how we learn and the more experienced teach. The wife and I were out at a Mexican style restaurant last night and the waitress set down the next table's order. The waitress admonished the couple "Careful. The plates are very hot." The male half of the couple had touched his plate with his finger before the waitress had finished setting down his wife's on the table. He grimaced,  shook his hand and looked at his wife, "That's hot!". The waitress and wife exchanged knowing glances and shook their heads. I'm afraid that, that guy and myself, share some DNA. Yes, I know what you're thinking but consider this; the waitress said that the plates were " very hot". Is that hot like a hot tub or hot like an exhaust manifold? Now he knows. It's somewhere in between.

Which brings me back to overheated edges. Even our young waitress understands the principles and most easily measured effect of friction. Things can get hot. Who hasn't seen a lawn mower blade turn blue while using a bench grinder? Very bad. But wait, blue simply means that the steel went to 550°F and that's not the end of the world hardness wise. Spring steel is tempered around 700°F and it's not exactly soft. Particularly if you ever tried to drill a hole in it.  Grinding steel is going to produce heat but how much heat? If it gets hot, how long does it stay hot? Does it make a difference how long it stays hot? If it gets hot and stays hot does it affect hardness just a little or a lot? Does introducing water to the process in one way or another actually help? It might seem that it would but then again underwater blow torches work just like above water blow torches. 

Lot's of questions. Let's touch the plate and see if we can come up with some answers.

[Kwakster]

I've just ordered this 73 C / 163 F Tempilstik for the same purpose:

https://www.ebay.nl/itm/TEMPILSTIK-Tempi...2749.l2649

BTW: Greetings to all on this forum from the Netherlands, 

[EOU]

You are the man Kwakster! Thanks for pitching in on this project!

[EOU]

Well its Wednesday isn't it! Actually received our 325°F (163C) Tempilaq and thinner yesterday so we dove right into it this morning. Tempilaq is painted on to a surface in order to determine if that surface temperature reaches the trigger point, in this case 325°. 325° was chosen because that temperature represents the low end of the knife tempering spectrum. 

                                                                  

The idea here was to attempt to judge if a belt grinder produces grinding temperatures at or above 325° while grinding a knife edge. Our approach here was, in the first stages of the test, to do those things that would lessen temperatures (minimal grinding duration and pressure) and then to increase both pressure and grind times in subsequent stages (assuming the Tempilaq wasn't triggered in the initial stage of the test). The belt grinder is a 1 x 42 Kalamazoo using a slightly used 150 grit Cubitron belt. No cooling means was used during the test other than the natural cooling that occurred between test segments. The knife is a cheap stainless santoku that had been sharpened previously to BESS 155. The bevel angle was approximately 19° and we maintained that angle with our Grepper designed knife rest.

                                                                         

We marked the test area on one side of the knife (the grind side) and then painted both sides with Tempilaq. Then allowed the Tempilaq to dry for 15 minutes. Here is what the grind side looked like prior to testing.

                                                                      

                                     

Trigger temperature attainment is indicated by turning Tempilaq from opaque pink to clear. Our first experiment was to only very lightly touch the knife edge on one side ("the grind side") and then inspect to see if the Tempilaq had been triggered on either the "grind side" or the "backside". Of course the abrasive belt would remove the Tempilaq from the grind area on the grind side but we still would be able to inspect immediately adjacent on both sides of the grind area for triggering and then the backside of the edge for same. Here's a micro pic of the center of the grind area after that first touch.

                                                                 

As expected the Tempilaq has been removed from the grind side bevel. You can clearly see opaque Tempilaq still sticking up from the backside however. Next we moved the scope to the side to look at the margins of the grind and still on the grind side. Then a micro pic on the backside of the edge.

                                                                    

We didn't get the nice clean delineation line that we expected between grind and adjacent Tempilaq. Its sort of a mixed bag but in any case, the Tempilaq is still opaque. The back side of the grind is still pure opaque Tempilaq.

With no sign of triggered Tempilaq we moved on but a little more aggressively.  Since we weren't going to get a well defined line between Tempilaq and grind area we reverted to more standard grinding techniques by moving the blade slightly left and then back to the right thereby extending grind time and area. Then we took more micro pictures of the grind side and backside.

                                                                  

We've undoubtedly created a burr by now and it looks as if some of it has been pushed onto the backside but Tempilaq is still opaque. So now a third grind.

                                                                  

Now we are beginning to create serious burr visible on the grind side while the backside demonstrates nothing but opaque Tempilaq. It is becoming obvious now that we are in the process of converting our Santoku knife into a chisel ground edge. And a fourth grind.

                                                                  

Even bigger better bur but nothing but opaque Tempilaq to be seen. So is this stuff even working? We pulled out a heat gun to see. Of course you can see the Tempilaq disappearing right before your eyes when the heat gun is trained on the blade. Here's what it looks like, grind and backside, after the heat gun and under the microscope.

                                                                 

That's burr baby and one section of our santoku knife is now pure chisel grind! See how the Tempilaq has pooled and turned clear on the back side with just a few specs of opaque remaining? The grind side pic is even more dramatic to us in that the there is no longer any Tempilaq sticking up from the apex and backside. We guess the stuff is working, the test procedure described and the results reported. Now we can discuss what this all means.