Salutations to Stone Sharpeners!

[Jan]

It is good to distinguish between the tempering colours discussed in previous posts and the red hot colour sequence valid for steel at temperatures above 750°F.

Tempering colours are caused by a tiny oxide layer formed on freshly ground steel surface. Day light interferes in this film and the colour of the reflected light tells us about layer thickness and indirectly about steel temperature. This method works up to some 700°F.

The red hot colour sequence is caused by thermal radiation - glowing of the steel. It starts with heat red (more than 750°F) and continues to cherry red (1000-1400°F) to orange (1500-2000°F) to yellow, white and bluish.
   
Colours were successfully used to judge the temperature for centuries by master blacksmiths working with wrought iron, but it is good to know how imprecise they can be.


Jan

[Jan]

One final thought on the grinding heat topic.  A fair amount of research has been done into how much heat is generated while grinding.  The most reliable study I've read shows a temperature of somewhere near 1000 F generated during grinding.  Some other first hand conversations indicate the temperature can (not is or will be, but can) be much higher.  I know GE regularly does tests to see if grinding their parts has resulted in excess grain growth in nickel alloys meant for high temperature applications.

German steel expert R.Landes considers as proven that the peak grinding temperatures may be for some small fraction of a second as high as 2000°C.

Some five years ago I have read about technology called grind-hardening, which utilises grinding heat to harden the workpiece surface. During this short-time HT process the martensitic hardening is achieved by self-quenching mechanism.

It is easy to imagine that when the heat generation in the contact zone between grinding wheel/belt and the blade is not sufficient for surface grind-hardening, it may be sufficient for tempering.

Jan

[me2]

That's the personal communication l was referring to earlier. It is worth noting those temperatures from Roman are for dry sharpening by hand, not power grinding. I have not seen the study, but he said it came from old German razor blade research

[grepper]

 Do you guys think that dragging a blade across a stone by hand sharpening can possibly raise the edge temp to 3600°F (2000°C) even for a few milliseconds?

I guess I can imagine that maybe, possibly, a molecule of steel might freak out and get really hot as it was torn from the edge, but even it that was actually the case, would it be significant?

[Jan]

Mr. Me2, it is good to know that we rely on the same information source.

Mr. Grepper, you ask a good question concerning the significance of the temperature spikes. If the temperature spike is very limited in time and space there may be not enough heat available for martensitic phase transformation.

In some heat partitioning studies it is assumed that maximum wheel grain temperatures are quite near to the melting point of the workpiece.

A different issue are the colours we observe on the steel surface. In this case the temperature flashes may contribute to the growth of iron oxide layer, which then appears as coloured spot thanks the interference of the day light in this thin layer.

Jan

[grepper]

Hypothetically and just for grins,  imagine that a blade is sharpened on a Kally and the blade gets just barely warm.  Not hot in the slightest, just warmed enough so that maybe it’s just possible to tell that the blade is warmer than ambient air temperature.  Not even close to changing the color of the steel.

So what do you think is happening there?  Is the edge apex getting super hot, like 3,000° and then warming the rest of the blade, or is the entire blade/belt contact area warming slowly at the same time?  Or maybe both?

I’m having difficulty imagining that we really could be causing anything like 3,000° heating with very light sharpening.  

I can understand that surprising things may happen on the molecular level, but if this super heating is on that level then, like I mentioned before, I would think that heating would of extremely short duration and the real world significance would be minimal.  

That said, there has been some empirical evidence reported here that supports the idea edge heating significantly impacts blade performance.  Mr. Me2 observed that blades power sharpened dented more easily when hacking through wood (bamboo?) dowels, and I’ve noticed that sometimes burrs seem far more malleable I would have thought suggesting that the steel hardness had been reduced.  Hence my previous post about why are burrs so malleable.

Regardless, I'm not giving up my Kally!    It may be something I just have to live with whilst making a determined effort to keep heating with its associated deleterious ramifications to a bare minimum.

[Jan]

Following L. Zhang (2012)** the current understanding of heat generation during grinding is following:

"Flash heating: Heat enters the grinding contact in short bursts of intensive energy leading to flash temperatures. The flash temperatures occur in the extremely short time it takes for a grain to pass a point on the workpiece. A point on the workpiece has contact with an individual grain for approximately 1 micro-second.
 
Grain heating: A grain is heated at the grain-workpiece contact for much longer than a point on the workpiece. A grain typically moves across the whole contact length in 100 micro-seconds. The grain therefore experiences a heat pulse for a period approximately 100 times longer than a point on the workpiece. It can be shown that this allows the surface of the grain to reach quasi steady-state temperatures. The maximum grain temperature is close to the workpiece melting temperature.
 
Background heating: Numerous flash contacts gradually heat up the whole workpiece contact area. It is usual therefore to make a distinction between flash temperatures at a grain contact and background temperatures over the whole contact area. The overall duration of energy pulses in the contact area that provides the background temperatures is of the order of 10,000 micro-seconds. This is the time it takes the wheel to move through the contact length."
 
Jan
 
** https://www.intechopen.com/books/heat-tr...n-grinding

[me2]

Seems like the flash heating stage could generate very high temperatures.

I was actually cleaning up brush in the yard when I noticed it the first time. The branches were quite small, 1/8 to 1 inch diameter. The smaller ones would cause small dents in the edge that did not happen on water wheel sharpened edges. I only suggest the bamboo skewer chopping as a repeatable way to check for denting before actual use.

[Ken S]

Grepper,

At the risk of sounding heretical, some people have been known to substitute variable speed motors with their Kallys....... Naturally, I not mean to imply that anyone on the BESS exchange would be so gosh as to remove a legendary Baldor.

Ken

[grepper]

Mr. Jan – Thanks for the definitions.  Most interesting to know the various types of heating are well defined. 
 
I am left wondering how this relates to real world impact on a blade, and how much the blade is actually heated, especially with light pressure sharpening where the overall background heating is very low.
 
Flash heating would seem to be heat that equals or exceeds the original HT temperatures.  I would think that grain heating would add to the background heating and/or be more or less completely responsible for it.
 
In my original post I was talking about a situation where background heating is minimal. 
 
I wonder what this all really means.  How much does belt speed matter, how much flash heating actually matters, and what the real world ramifications of fairly low level background heating during sharpening is.
 
Mr. Me2, Other than my malleable burr suspicions, your observations of blade denting are the only real world evidence that I have seen posted about the deleterious affects of minimal blade heating.  I say minimal because I doubt any sharpener that would be concerned about this stuff would ever significantly overheat a blade during sharpening.
 
Mr. Ken, What??? Remove my actual Baldor labeled motor?  You are right.  Gosh Mr. Ken, that would indeed be heresy!  A variable speed motor?  What a novel idea!
 
Mr. Me2 mentioned that flash heating of 3,000° F can be produced by hand sharpening on a dry stone.  If that is true, (really?) how much would a variable speed motor matter?  How much does flash heating actually affect the edge?  How far does flash heating penetrate into the blade? 
 
Mr. Jan has shared the current understanding of heat generation during grinding.  I’m sure that those definitions apply to any type of grinding process, but I’m still left wondering what it all means in real world sharpening with Kally.