One steel, Two steel, White steel, Blue steel
Japanese chisels and plane blades are for the most part made of different tool steels than the ones used in making western chisels and plane blades. Here, the common choices are O-1 and A-2. For Japanese tools, most plane blades and chisels are made from either white steel or blue steel. Both types of steel come from Hitachi, and the colors really refer to the paper used to wrap the bundles of steel as they come from the manufacturer. To the eye, white steel and blue steel look identical, once you get the paper wrapping off. The composition of the steels, however, are different.
To make white steel and blue steel, start with a piece of JIS SK steel that undergoes a couple of purification processes to get rid of impurities. What you are left with is white steel #2. This is a nearly pure high carbon steel, containing 1.05-1.15% carbon, with very low levels of impurities (sulphur and phosphorus). If you further process white steel #2 and add more carbon, you get white steel #1. The additional carbon adds hardness, but decreases toughness.
If you take white steel #2 and add tungsten and chromium as alloying agents, you get blue steel #2. Relative to white steel #2, blue steel #2 will have increased abrasion resistance, but will also be harder to sharpen, as at its core sharpening is controlled abrasion of the tool. Take blue steel #2 and add some more carbon, tungsten, and chromium, and you get blue steel #1, which will increase hardness and decrease toughness in a similar manner as what happens with white steel.
Finally, if you take blue steel #1, add more carbon, more tungsten, and molybdenum and vanadium as additional alloying agents, you get super blue steel, which has even more abrasion resistance.
And just to dispel the idea that white and blue steel #2 are very inferior in their hardness characteristics, both of these steels have more carbon in them than either O-1 (0.9%) or A-2 (1.0%), and I have seen very few criticisms of O-1 and A-2 as being not hard enough for woodworking tools.
This diagram should help clarify the manufacturing process, and is based on a diagram that used to be on the Hida Tool website, but is no longer there, unfortunately.
The super-simplified explanation as to the differences between white and blue steel when using a tool is that blue steel is a bit tougher and holds its edge longer, while white steel takes a bit of a sharper edge and is easier to sharpen. It’s very analogous to the differences between O-1 and A-2.
For what it’s worth, my Japanese chisels tend to be white steel, and my Japanese plane blades tend to be blue steel. For my chisels, I want a really sharp edge and a quick way to get that edge back when it dulls. For plane blades, abrasion resistance is probably more important. I’m not sure this is the absolute best way to go, but it’s what I do. I think I would love to have a high quality white steel plane at some point, and my bet is at that level there won’t be a meaningful difference in its edge holding capabilities compared to a blue steel plane blade.
So why would Japanese toolmakers choose one steel over another steel for making a tool? From a manufacturing standpoint, it’s easier to work with blue steel than white steel. If white steel is properly processed, the range of temperatures that you can use for the annealing/hardening/tempering process is fairly narrow compared to blue steel. Since the tolerances are tighter, it takes more precision and skill when working with white steel.
On the other hand, white steel also is less expensive than blue steel, so it tends to show up more in cheaper Japanese tools. As you move up the ladder of price points of Japanese tools, I find white steel chisels and plane blades on the cheap end, because of the lower cost of materials, then inexpensive blue steel tools, and then finally high end chisels and plane blades of both white and blue steel, where the cost more is a reflection of the skills and experience of the tool maker.
In the end, trying to base a tool decision on the type of steel used to make the tool is probably not a real useful exercise overall. I’ve found that the differences between edge holding of white and blue steels are overwhelmed by the differences between how good the toolmakers are. I’ve drawn a similar analogy before, but worrying about the white steel vs. blue steel issue when buying a Japanese tool is like asking whether you should eat a chicken dish made with a decent fresh chicken at the grocery store or an organic free range chicken. To some extent it doesn’t matter — Jacques Pepin will still make a better chicken dish than I will.