I know the conventional wisdom is that certain kinds of stainless steel work great for knives (particularly modern designs), but are too hard and brittle to make long-bladed swords. What I'm curious about is: does anybody have empiric evidence (or has anybody conducted an experiment) on how long a stainless steel blade could go before this excessive hardness becomes a problem? I'm asking this because the better sorts of stainless steel (things like ATS-34, perhaps) seem like attractive materials for fantasy blades where historical accuracy isn't a problem, but I have no ideas about whether any of them would be any good for practical cutting/thrusting blades over, say, 20cm (8") long.

Cutting isn't the same as combat. Would you be comfortable parrying an incoming strike with a stainless katana? I personally wouldn't use stainless on anything long enough to hit with. So for my druthers, a five inch knife in stainless would be fine, but for anything longer than say eight, I want carbon. The longest stainless steel knife own is a Buck 119, and I am careful with it. I sure wouldn't baton through sticks with it.

Even if it would work, a carbon steel would work better, so why mess with a less than ideal material?

Stainless steel is not "harder" than carbon steel. Hardness depends on heat treatment after all. But stainless steel is more brittle and tends to "get tired" very quickly. In other words, where a carbon steel blade would bend, a stainless one would break. And where a carbon steel blade would break after 10 000 blows a stainless one would break after just 1 000. As Timo said, "normal" stainless steels used for making knives would also be OK for making longer blades. Stainless khukri would be perfectly OK, a heavily built machete would probably also be. But I wouldn't make a training sword of stainless steel. Such sword would either be too heavy of would break too quickly, it's just not worth it. However a stainless katana (and these tend to be a little beefier than an average European sword of comparable length) would probably be OK for cutting some soft and medium targets.

Parrying an incoming strike with a stainless katana? Something like a 440A or equivalent stainless blade, why not? Historically, there have been people who have been happy enough to parry with the equivalent of 1095 or T10 blades, but with more likelihood of defects than modern steels. As brittle as 440A with reasonable heat treatment. If it has been good enough for people fighting for real with sharp swords, to the death, it appears to be good enough.

If I can hit rocks with it and have the blade survive, why shouldn't parrying work?

Or look at the numbers for fracture toughness for various steels/heat treats. Stainless might be relatively inferior (to carbon steels), but they do appear to be a workable solution. Carbon steels will be better. So what? A Ferrari might be better than your car. Does that mean your car isn't good enough? Good enough is good enough. Are all of your swords magnificently heat-treated L6? Why put up with inferior ordinary spring steel when you can have supersteel L6? Because good enough is good enough. If there are other design considerations that favour stainless, stainless might even be the best choice.

But the point about training swords is well made. Avoid stainless steels for training swords! A training sword is not a fighting sword or a cutting sword. Much more demand on toughness! Also on hardness - you don't want your training blade being turned into a saw by contact with your opponents' blades.

Barry Dawson used to do katana length blades in 440c. There is a story of him scaring the customer when clamping a blade in a vice and leaning it over to extreme.

Fast forward and you will see Hank Reinhardt endorsing stainless 420 series slab handled katana for demonstrations and then search out Bill Tsafa pounding on car tires with the United Cutlery example in the same 420 blend. Keep in mind that Buck knives has used a 420 blend in their knives back to the 1960s.

One of the steels Jerry Hossom used to use for his espadas was ATS-34

While some of the corrosion resistant steels can and have been used in sword length to satisfactory levels, 420 in the slab handle katana just means a thick soft blade.

Cheers

GC