Recently there has been several threads about steels and turning tools, both commercial and shop made. Many here came to the wood lathe by way of the engine lathe or other single point cutting machine tools and/or have backgrounds in engineering and construction, but not everyone here came by these paths.
We are a diverse group and many of us are limited in our understanding of steels. I thought to offer a brief primer to bring some of you up (and most of you down) to my level of understanding. It won't make anyone into a machinist or metallurgist and might not even be correct, but it works for me so I can at least read and partially understand catalogs, ads and rcw threads ...and as with this post, I can occasionally put on airs and "assume the mantle when I have it not". I urge your corrections and additions and hope those who really know something about these things will just laugh and not groan or throw up. :)
A steel tool must be harder (resist deformation) than wood in order to cut it. Any steel usually is. The steel must be tough (less brittle) enough to withstand mechanical shocks so as not to chip or fracture. It must not be abraided easily (wear resistance). If the steel has these attributes the tool still needs to have the proper shape to do a particular job and a sharp enough edge to penetrate the surface of the wood.
For a turning tool, the steel needs the above three qualities; hardness, toughness and wear resistance. They are each important, but they often work against each other (when one improves the other worsens). Additionally they reach a point (threshold) where more is not any better. Like most of our lives, it's the same ole compromise between hardness, toughness and wear resistance.
I've never seen a tool get red hot while turning wood, but grinding and sharpening a tool is part of the turning process and must be taken into account. Grinding can get steel very hot so there is another quality of steel to consider called "red hardness"; namely the ability to keep its hardness during and after very high temperatures.
The steel rounds and flats you buy at Lowes are useless as turning tools. This steel is fairly tough, but it's woefully lacking in hardness and wear resistance. It can be formed while hot or cold, but the cold rolled is smoother and cleaner and cold forming makes any steel harder.
Adding more carbon to these steels makes "high carbon steel" that has some of the three qualities, but its red hardness is poor and except for being fairly tough so as to take a fine edge without chipping or fracture, the other qualities are nothing to write home about. So those old 'high carbons' are fine if you don't mind renewing the edge frequently while keeping the tool nice and cool. I still use mine. Actually, resharpening is the only way I can tell if _any tool is as sharp as it can be. Some of you may be more clairvoyant than I. :)
There's a limit to what adding carbon alone can do, but adding alloys will make "high speed steel" which is a much better upgrade and compromises the 'qualities' far less than plain high carbon. It's hard and stays that way after getting red hot. It's tough and doesn't break when you get a catch and it takes and keeps a good edge. It isn't so easily abraided either.
The alloy materials added to the carbon alloyed steel to make high speed steel are variously tungsten, molybdenum, chromium and vanadium. Those with the letter 'T' are high in tungsten and you know what those with the higher molybdenum are called. Yep 'M' and ole faithful M2 is a good compromise for a high speed steel turning tool and except for special needs like maybe cobalt for inserted bits, I think it's good enough for most all woodturning use. IMHO we don't need to be overly concerned about the different high speed steels, whether cheap or expensive. imported or domestic.
I know there are some so called "super high speed" steels with special alloying that increases hardness without excessive brittleness or a tendency for the edges to chip. They are great for saving time for professionals and for affluent hobbyists to brag about. I don't own any so I can't comment, not that that usually stops me.
BUT WAIT! Some caring (enterprising) entrepreneurs powdered and froze those super steels to improve the 'qualities' (profits) and save the professionals even more time and give the hobbyists even more expensive toys to brag about. Of course, time is money for some, so it's back to the same old compromise.
That's my take on tool steels or rather steels for tools. Hope this helps, but your mileage may vary, so don't take these musings too seriously. :)
Turn to Safety, Arch Fortiter