eaisaacs:
<font face="Verdana, Arial" size="2">if you were ordering a Custom folder which would you prefer D2 or CPM 420V</font>
Both of these are among the most brittle of the current high end cutlery steels. D2 has less than half the impact toughness of A2 at the same RC (60). A2 is hardly the toughest steel either, O1 is tougher than it, 5160 tougher than O1, L6 tougher than 5160 etc. CPM-420 is probably slightly more brittle than D2.
In regards to use, well it depends on what you are cutting and how much force you are using. On soft materials like foods, rope, wood and cardboard you will not have any problems, but they will chip out easily on even soft metals (staples) especially if there are twisting motions. Gritty or dirty material can also cause blade fractures as well.
Of course you can get around the low toughness by altering the heat treat (basically lowering the RC), or the geometry (make it thicker), but then you end up with a weaker steel that impacts readily and a blade that cuts poorly because of the excessive thickness. You are much better off switching to another steel.
They are however for much the same reasons very high in regards to wear resistance and strength. CPM-420V having a large lead in regards to wear resistance and probably strength unless you get someone who heat treats their D2 at about 62 RC, as 420V is hard to get over 59. If the D2 is also about 60 RC then CPM-420 will probably be stronger due to the much finer grain and thus will have a large lead in edge retention unless chipping is a factor.
420V also has a large lead in regards to corrosion resistance. D2 while resting rust better than many tool steels will still rust readily if in contact with seawater for example. 420V can resist damage under these conditions for a much longer period of time.
The finer grain structure of CPM-420V also means that it will take a higher edge polish. So if you prefer ultra polished edges this could be a factor. You can easily get D2 to nicely shave however but the large grain size and carbides will prevent it from obtaining the same level as 420V. However you need considerable skill in sharpening to polish and edge to such a level so as to be able to tell the difference.
In regards to what I would pick, I would chose the maker over the steel, but if offered a steel choice, and the maker was familiar with both steels I would pick CPM-420V for the better edge retention and corrosion resistance. D2 however is likely to be a *lot* cheaper.
-Cliff
