essexalan":39m0ucad said:
I have a Gyuto made from HAP40 and it takes no more time to sharpen than any other steel just use the right stones. Of course hand grinding a big mortise chisel might be something else again but I will never have one so no problem, PM-V11 is no harder to sharpen than any other steel. PM steels have a distinct advantage over other high alloy steels in that they have a very fine grain structure and no big carbides. It is not how hard the steel is but how tough it is which will decide which sharpening medium to use so if you want to use Washitas for everything don't bother with PM steels unless you just want to burnish your Washita for razor use and suchlike.
I would say you don't have the terminology quite correct. Toughness is how good the steel can handle impact. It is usually meassured with a sharp swinging "hammer" which breaks the steel, and then the energy dissipated is meassured, the Charpy notch test. It takes a lot more energy to break a tough kind of steel, then a brittle steel which breaks at just the thought of swinging a hammer at it. For our tools toughness is important, not just because we don't want to end up with two chisels after some malleting action, it also determines the chippiness of the steel. A steel that is too brittle will wear out quickly because small chips are nicked out of the edge.
Hardness is meassured with an indenter puched into the steel under a certain load. The depth of the indent determines the hardness. More hardness increases the wear resistance, because the steel will resist deformation under the cutting action.
In simple steels it is all a matter of balancing the carbon content with the hardness. Making the steel harder usually also makes it more brittle. There is an optimum around 60 HRc for steels like O1, W1, white paper steel, cast steel. In a laminated iron, a carefull smith can go a little higher, because the softer lamination kind of protects the harder steel bit.
When you add alloys to the steel, especially the wear resistant ones like chromium, vanadium, moledenum, tungsten, cobalt, things become more complex. Those alloys mix with the carbon that hasn't been used up by the iron, and forms carbides. These are generally very hard. Way harder then the steel matrix around them. I don't think you really can meassure this extra hardness with a HRc tester, because the carbides are relatively small and well dispersed throughout the steel. But they sure make a difference in wear resistance. At the edge the steel around the carbides tends to wear off and those very hard carbides take the brunt of the cutting work. Because they are so hard the wear slowly. But they also wear slow on a grinding or honing stone. No free lunch! And when the carbides are harder then the stone's grit particles, like in Novaculite, then the stone is abraided more then the steel. The stone gets burnished which is really just a rounding of the grit particles.
In a traditionally made alloy steel, the carbides tend to form relatively big clumbs. These aren't very solidly attached in the steel matrix, so they brak out easilly. The edge tends to be chippy. Steel like A2 or D2 can have this problem. The nice thing about Powder Metal (PM) steels is that the carbides remain small. So you can get more of them into the steel without greatly reducing the toughness of the edge. They are still very hard, so the edge wears down slower, but they also are hard on the grinding stone, meaning it takes longer to grind.
There are many PM steels available today, all with a different mix of elements, optimised for a specific purpose. Many of them are for the industry. There they are looking for very long edge life to minimise the down time of the machinery. I wouldn't wonder if many of these tool bits are regarded to be disposable, so the grinding time isn't so important for them.
PMV-11 is a bit special. It is a stainless knife steel. It contains a lot of chrome (about 16%) and almost no vanadium. Chromium carbides are hard, but not the hardest. Vanadium carbides are a factor harder again. So the composition of this steel is a compromise between edge durability and grindability. It receives a lot of praise from the woodworking commun, so I guess the compromise is a good one. It grinds about twice as slow as O1, which is not an rediculous increase, but no free lunch again.
HAP40 is a PM steel with not a very high chromium content but with lots of vanadium, molebdenum and cobalt. That makes the steel very wear resistant, but also very hard to grind.