Metal slurry color while sharpening

N

Nenum

Joined
Sep 28, 2014
Messages
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Hi all. I've only recently started taking knife sharpening serious after 42 years of living with dull knives. After discovering the wonders of the Edge Pro Apex I now find myself rather hooked so I've been borrowing knives from friends and the local church to sharpen up for practice. I've noticed that metal slurry seems to be darker on some blades and lighter on others.

Is this darker color caused by having more carbon in the steel or is it simply that more steel is being removed by the stone?

Also, I understand that the heat treatment is largely responsible for the hardness of the steel but I was taught long ago that adding more carbon to steel will increase the hardness. Is this true?

Thanks.
 
Nenum said:
Hi all. I've only recently started taking knife sharpening serious after 42 years of living with dull knives. After discovering the wonders of the Edge Pro Apex I now find myself rather hooked so I've been borrowing knives from friends and the local church to sharpen up for practice. I've noticed that metal slurry seems to be darker on some blades and lighter on others.

Is this darker color caused by having more carbon in the steel or is it simply that more steel is being removed by the stone?

Also, I understand that the heat treatment is largely responsible for the hardness of the steel but I was taught long ago that adding more carbon to steel will increase the hardness. Is this true?

Thanks.
Click to expand...

I can't comment about the black on there but my "guess" would be more material is being removed.

Carbon plays really the biggest role in knives, the more carbon the "harder" it will be be, (harder for lack of a simpler term).

Heat treating is a way to bring out carbon's maximum potential and vastly increase the blade's usefulness.

Glad to have you here and hope you enjoy your stay. :thumbup: :D
 
Harden Knives said:
I can't comment about the black on there but my "guess" would be more material is being removed.

Carbon plays really the biggest role in knives, the more carbon the "harder" it will be be, (harder for lack of a simpler term).

Heat treating is a way to bring out carbon's maximum potential and vastly increase the blade's usefulness.

Glad to have you here and hope you enjoy your stay. :thumbup: :D
Click to expand...

Thanks for the explanation. I suspect you are correct about the darkness of the metal being ground off. I was sharpening an old Carvel Hall kitchen knife this evening that had loads of really dark slurry being removed. I suspect the blade was probably just a really soft metal so the amount of metal being removed just made it look like it was extra dark.

Thanks for the welcome.
 
Darker = more steel swarf less stone slurry/binding. Commonly soft and low alloy blades easier to abrade and abrasive in stone fracture less. Note: Some SiC stone with dark binder + grayish SiC, hence darker slurry too.

Practical steels with 0.5%C - 0.8%Carbon can get maximum martensite matrix up to 67rc. Those steels with ubber alloy/carbide volume (e.g. cpm-rex121) can add extra hardness to surpass 70rc. Excess carbon usually goes to alloying or form cementite but won't make a perfect martensite matrix any harder than 67rc

Nenum said:
Hi all. I've only recently started taking knife sharpening serious after 42 years of living with dull knives. After discovering the wonders of the Edge Pro Apex I now find myself rather hooked so I've been borrowing knives from friends and the local church to sharpen up for practice. I've noticed that metal slurry seems to be darker on some blades and lighter on others.

Is this darker color caused by having more carbon in the steel or is it simply that more steel is being removed by the stone?

Also, I understand that the heat treatment is largely responsible for the hardness of the steel but I was taught long ago that adding more carbon to steel will increase the hardness. Is this true?

Thanks.
Click to expand...
 
bluntcut said:
Darker = more steel swarf less stone slurry/binding. Commonly soft and low alloy blades easier to abrade and abrasive in stone fracture less. Note: Some SiC stone with dark binder + grayish SiC, hence darker slurry too.

Practical steels with 0.5%C - 0.8%Carbon can get maximum martensite matrix up to 67rc. Those steels with ubber alloy/carbide volume (e.g. cpm-rex121) can add extra hardness to surpass 70rc. Excess carbon usually goes to alloying or form cementite but won't make a perfect martensite matrix any harder than 67rc
Click to expand...

Great that helps explain what I am seeing. Thanks for your detailed explanation.
 
Darker = softer steel
Lighter = harder steel

(Most of the time)
 
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I've often wondered if the steel's tendency to oxidize faster or slower plays into the 'dark' appearance of the swarf. Non-stainless carbon steel obviously oxidizes faster than stainless, and it turns black when it does; higher carbon content accelerates oxidation as well. Stainless oxidizes, but the chromium in the steel is what forms the oxide layer first (forms chromium oxide), before the iron in the steel can react to oxygen, and the chromium oxide layer is extremely thin and essentially 'clear' or colorless.


David
 
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