Re: How to obtain a mirror edge?

[Jende Industries]

Well, I forgot to answer the ZDP question 😳

ZDP-189 – As far as I know, it is an extremely hard steel, RC 64 and abrasion resistant, so I’d go with diamonds all the way.

This is where things get even more fun. I’m not a dedicated steel junkie, so please bear with me if I mess up some of the finer details.

You have hard steel, and then you have abrasion resistant steel.

Hard steel is a result of heat treatment of a given steel, which varies greatly from knife maker to knife maker. Rockwell 60 and below is considered softer steel, and Rockwell 60 and up is considered hard steel. You can loosely apply 1/10 of the RC hardness to the Mohs scale directly, as most conventional steel is between 5 and 6 on the Mohs.

The factors that influence which sharpening medium to use here are based upon the corresponding hardness of the abrasive. Diamonds are 10 on the Mohs scale, and abrade all knife hardnesses (so far!). Silicon Carbide and Aluminum Oxide (which Shaptons and Choseras use) are in the 9’s and can abrade steel every bit as well, so there is really no surface issue with these types of abrasives cutting into RC 64 steel. Sandpaper is usually feldspar, quartz or emery, which averages in the 6’s on the Mohs, so there is definitely some more “evenness” between the fight of steel vs. abrasive scratching action, which can be positive in certain applications, mainly surface polishing.

The binding agents play a larger role here in determining which brand of abrasive medium to use. The general rule of thumb is a harder steel needs a softer stone, and a softer steel needs a harder stone. The softer stone allows for more abrasive to be released, thus cutting more effectively into the harder steel. On softer steel, you can easily cut through the steel with the abrasive relatively intact, so there is no real need to release abrasive at a high rate.

Diamonds negate this theory, since they still cut through everything, but then practicality enters the equation – do you really need diamonds to cut a RC55 steel? Not really, it’s a little overkill, but at the same time, there is no need to ponder if it will sharpen a certain steel type.

Abrasion Resistant steels pose different problems. These use more exotic materials, usually powdered steels, which when treated, form certain carbides in the steel. Most Carbides are generally in the 9 range on the Mohs, so we have a serious competition happening for scratch ability now. Ironically abrasion resistant does not equal “Hard” as in RC 90. Because of the powdered steel, the hard carbides are essentially embedded in a matrix of “softer” steel, which can be as low as RC 55-60. On most conventional sharpening stones, you can abrade the matrix enough to produce “carbide popout” which essentially gives you a serrated edge where the carbides literally fall out, not abraded through. You can still polish the bevels, although going is very slow to get through the carbides.

Diamonds cut through everything, and on abrasion resistant steels, they are king for doing any profiling or serious repairs. Keep in mind that the fight is still there….

Bringing these abrasion resistant steel to higher refinement is also tricky – it is subject to carbide popout as you make the edge of the edge too thin for the steel to hold the carbides.

Anyway, I’m starting to ramble…:whistle: