Paper wheels edge heat treatment

[BluntCut MetalWorks]

After went thru all posts on Richard J thread about paper wheels, I bought one. I used/tested it for 3 weeks - sharpened various knives of diff steel types (vg-10, 10xx, s30v, zdp,cpm-m4, d2, blue, 420hc/j, 440A/C, ..), resulted in uneven bevel line (lack of steady hands) but very sharp edge - almost able to smoothly push cut cross grain phonebook paper. I was quite surprised that some blades with soft+low alloy got quite sharp and edge retention seem surpassed my freehand (for these blades: finished on 0.8 to 2K grit, strop on basal loaded with 3 micron cbn). why so? and why 8" recommended at such high speed 3600rpm? So I noodle on this speculations for a week before float it by you for inputs:

Could thermal (post-kinetic) from 8" @ 3.6K rpm ~ 171miles/h or 276km/h heat the edge apex to 900c+ for carburization to occur(draw carbon from wax in solid or gas form) then temper around wax flash point 204c? Essentially, the cutting edge surface harden/coated with newly carburized alloys/carbides?

Knut or plausible?

 

[Bernoulli]

I have achieved better edges on some knives with paper wheels than with other systems. I can't account for it though. I don't use the faster motor.

 

[HeavyHanded]

Very interesting question. I've wondered myself why burr formation becomes less using a belt sander than when I use the same belt by hand. Have come across references to metal elongation tests where it was noted testing duration increases elongation of a given sample and increases the load it can take before separating. Applying the load to a sample faster causes it to elongate less and break under smaller load. I can only speculate that softer low-alloy steels might be more prone to these forces? Could a similar effect be happening on paper wheels - metal is removed with such speed the remaining metal is less disturbed/worked by the abrasive?

 

[Obsessed with Edges]

If I'm not mistaken, most of the grinder/wheel fans around here consistently recommend a lower speed (1750 rpm is the common number, I believe). Too easy to overheat edges with the faster grinders, and most warn against using the 3600 rpm machines on knives. I would've sworn even Richard J has recommended the lower speed, but I might be mistaken.

 

[BluntCut MetalWorks]

If I'm not mistaken, most of the grinder/wheel fans around here consistently recommend a lower speed (1750 rpm is the common number, I believe). Too easy to overheat edges with the faster grinders, and most warn against using the 3600 rpm machines on knives. I would've sworn even Richard J has recommended the lower speed, but I might be mistaken.

Right, Richard J's rig is running at 1.7K rpm. But Razor Sharp System clearly recommended (seem insisted) 3.6K rpm, along with plenty of wax, perhaps/speculative continous/dynamic surface over heat and quench were intended for soft knives to achieve better sharpness via carburization.

 

[Obsessed with Edges]

Right, Richard J's rig is running at 1.7K rpm. But Razor Sharp System clearly recommended (seem insisted) 3.6K rpm, along with plenty of wax, perhaps/speculative continous/dynamic surface over heat and quench were intended for soft knives to achieve better sharpness via carburization.

That really surprises me. I'd think this would be a method best reserved for more experienced hands though, to avoid damaging blades. I suppose the wax, in bountiful quantity, should help to dissipate excess heat.

Interesting topic though.

 

[BluntCut MetalWorks]

Very interesting question. I've wondered myself why burr formation becomes less using a belt sander than when I use the same belt by hand. Have come across references to metal elongation tests where it was noted testing duration increases elongation of a given sample and increases the load it can take before separating. Applying the load to a sample faster causes it to elongate less and break under smaller load. I can only speculate that softer low-alloy steels might be more prone to these forces? Could a similar effect be happening on paper wheels - metal is removed with such speed the remaining metal is less disturbed/worked by the abrasive?

Yeah with speculation, white rouge contains 15um Al2O3 (I think) abrasive strike steel at 171mph would either tear or heat up steel molecules. does the very edge hot enough to carburize? if yes, could it mess-up ht for high alloy blade (cemenite and or change grain structure for the worse)?

 

[BluntCut MetalWorks]

I'd think this would be a method best reserved for more experienced hands though, to avoid damaging blades.

Quote from Red-Green show "If it ain't broken, you're not trying.", ack: I messed up a handful of high end knives edges (not blades), took a while to freehand reprofiled them back in shape - lost 0.5-1mm worth of blade height.

I suppose the wax, in bountiful quantity, should help to dissipate excess heat.

5th sense, there is something missing in Mike or Richard J assertion about wax role in thermal dissipation. because many other binders beside wax would also dissipate heat (25 inches worth of linear material per 8" disc revolution).

When in doubt - more tests:

1) Using paper wheel 8x1/4 disc coated with water-base (0.5 gradually polluted to 3um CBN, then continue with 3um to 9um mono diamond) to sharpen cheap kitchen 420j knife. The result seem less sharp and lower retention than when using wax and white rouge (on 8x0.75" disc).

2) Using paper wheel 8x1/4 disc coated with oil-base (0.1um gradually polluted to 9um mono&poly diamond). The result seem better than 1) but still not as good as wax and white rouge.

Grinder: 6" at 3.6K rpm.
Edge preparation: blunt & grind new edge using gritted wheel. Buff on slotted wheel.
Test sequences: slice printer paper, slice phonebook paper, cut cardboard, slice phonebook then printer paper. repeat until fail slicing printer paper.

 

[hardheart]

Seems overall quite unlikely. The temperature needed for carburizing is because the steel needs to be austenized, and is about twice what I recall being reported from an experiment as the temperature of a knife edge when sharpened on power equipment without coolant. Since any particular section of the edge is in contact with the wheel for only seconds at a time, there really seems to be no time to reach the temperature and to diffuse the carbon which would first have to be liberated from the atmosphere or wax. Also, since this is not done in a controlled environment, if you could reach such temperatures you would also face oxidation and/or decarb. And even to allow that you are reaching the necessry temperature, the time at temperature is so low that the carburizing depth would be so minimal that the polishing with the white rouge after grinding with the wax coated grit wheel would likely remove the carburized layer anyway. Plus effective carburizing is done on low carbon steels to start with and generally they bring the surface carbon levels to 0.8-0.9%, while most of your steels start off as hypereutectoid already.

 

[BluntCut MetalWorks]

:thumbup:

Seems overall quite unlikely. The temperature needed for carburizing is because the steel needs to be austenized, and is about twice what I recall being reported from an experiment as the temperature of a knife edge when sharpened on power equipment without coolant. Since any particular section of the edge is in contact with the wheel for only seconds at a time, there really seems to be no time to reach the temperature and to diffuse the carbon which would first have to be liberated from the atmosphere or wax. Also, since this is not done in a controlled environment, if you could reach such temperatures you would also face oxidation and/or decarb. And even to allow that you are reaching the necessry temperature, the time at temperature is so low that the carburizing depth would be so minimal that the polishing with the white rouge after grinding with the wax coated grit wheel would likely remove the carburized layer anyway. Plus effective carburizing is done on low carbon steels to start with and generally they bring the surface carbon levels to 0.8-0.9%, while most of your steels start off as hypereutectoid already.

Hardheart - thanks for your thoughtful & analytical post, it covered/answered many of my thoughts! My spouse spent a few years on thin-film research, triggered my speculation that high velocity particle trike can alter/deposit/cementite/? nm or um surface layer. Yes, carburize seem unlikely since there are many phrase changes (martensite->austenite + C) and long with heat equation, just too many high energies boundaries which inhibit reaction saddle-point.