Hi, Stacy. Thanks for the input. I'm new here and new to knives.
I have ground & polished metals before, but those were relatively soft metals (thermocycled SAC alloys) for electron microscopy.
But I have never ground/polished knives (not counting kitchen knives). Unlike SAC alloys, knives, particularly those of exotic high-end stainless steels, such as M390, have a high content of carbides or so I read.
I dug around for a comprehensive source of info on polishing M390 or any other metals that are high in vanadium carbide content, but I haven't been able to find any. There are some scant pieces of information spread here and there, but overall, there are still many missing pieces of the puzzle to give anyone who has yet to polish a knife a reasonably complete picture of how to go about bringing that mirror-finish to M390 surface.
So below is an impromptu general process of how I plan to go about my very first project. If you find any errors, please feel free to correct me.
Below is a micrograph of Bohler M390 annealed at 2140°F (from
knifesteelnerds website):
Most of the microstructures shown in the photo above appear to be greater than 1 micron. I am going to hazard a guess that most of those white-silvery intermetallics are vanadium carbides. Vanadium carbides have approx. 89 HRC, not something that can be easily ground down using sandpapers - attempting to do so will probably result in an uneven surface contour where the peaks are those of unground carbides and the valleys are those of softer metals. Such an uneven surface contour can cause incident light to scatter off in all directions, thus giving off that characteristic cloudy sheen as reported by several members here who tried to polish M390.
I am going to guess that a truly mirror-like surface requires an average height difference between the peaks and valleys to be less than the shortest wavelength in the visible spectrum. In other words, I need a flat surface for that mirror-finish. Flatter the surface, better the mirror-finish.
To render the surface flat or flatter, I need to grind down those carbides somehow. I need to grind them so that the average height difference between the peaks and valleys is less than 0.400 microns, the shortest wavelength in the visible spectrum. What will I use to achieve this level of flatness? Sandpapers can't do it. I think diamond paste is the only viable option here.
Making an educated guess, I approximate the width of a very fine scratch mark visible to the naked eye is about 10 microns. The corresponding grit size to 10 microns is approximately 1500. So I plan on starting with 1500-grit diamond paste on my M390 knife surface which is reasonably flat and shiny (for a stone-wash finish).
Using a wool-felt polishing wheel mounted to a Dremel at 5000 rpm (the lowest possible setting), I will start polishing in a series of pulses. This is to minimize heat build-up. The material removal rate is initially expected to be high on the lower grits, thus a lot of heat is expected. From here, it's a simple matter of gradual grit progression all the way down to 0.25 microns for that true mirror-finish. Each grit size compound will have its own dedicated polishing wheel to minimize cross-contamination. Once I reach sub-micron grit size, I will switch to muslin wheels to bring about that final shine.
you could try diamond lapping film to 1-3 microns, that should do it. keep us posted!
