I thought that stainless is more ductile than carbon steel (I associate carbon steel with brittleness, possibly incorrectly - I was never a metallurgist. Probably because I know that cast iron is brittle)
This site[/URL] upholds what you say but then goes on to say the most durable edge of all the knives in his collection is the one made from carbon steel 
so I guess there's not a lot in it.
In any case, I stand corrected 
It can be a complex subject.
Ductility is the ability, or degree to which, a metal can be drawn out, as in a wire. Hardness is the resistance to indentation or wear by materials harder or more abrasive than the metal in question. Malleable is the word you are seeking here, the degree to which a metal can be formed (or deformed) before it breaks. Yes, generally the more ductile and malleable a metal is, the less hard it will be and the less tensile strength it will have. There is always a trade-off and it's impossible (though the developers try) to have the maximum of every quality in one grade of metal, be it steel or whatever.
All steels contain carbon. Steel isn't an element but an alloy, just like brass or bronze. Steel is then an alloy of iron and carbon. Complete elimination (extremely difficult) of all carbon and any other trace elements would leave pure iron, itself a relatively soft, malleable and ductile material but more or less useless for any kind of tool or implement. The original 'wrought-iron' would have been near pure iron and lends itself well to forming artistic shapes, especially when red hot. There is virtually no wrought-iron today in this original sense. All modern 'wrought-ironwork' will actually be mild steel which is a very low carbon content (0.03% typically) steel. Mild steel cannot be appreciably hardened by heat and quench methods as there isn't enough carbon content. It is relatively soft and can be quite easily worked hot or cold. In the higher carbon content ranges (up to about 3%) the steel can be hardened by heat and quench as the liquid carbon (in red hot steel) is trapped by sudden and rapid quenching and forms iron carbide which is dispersed within the structure. Iron Carbide is an extremely hard but also very brittle material.
The degree of hardness that can be attained then is more or less directly related to the carbon content. However even plain carbon steels are now rare as a multitude of trace elements are added for differing qualities. Manganese is the most commonly added element along with the carbon allowed to remain. There are over 2000 kinds of carbon and alloy steels available, probably now more as development continues all the time.
You are correct in saying that high-quality carbon steel is probably the better material for knives and such. It doesn't come cheap and the precision hardening (and then tempering) is a technical matter that also requires skill and precise equipment. High quality tools will generally have Manganese, Vanadium, Molybdenum or Nickel (or a combination of these and maybe others) added to a plain carbon steel mix. This is why high-quality tools are obviously expensive. You get what you pay for. Carbon steels of any quality require looking after. They will rust readily and can chip or even snap if exposed to excessive force. The higher the carbon, the harder they will tend to be (if hardened properly and then not overheated so 'drawing the temper') and the more brittle they will then be too.
Nonetheless, Stainless Steel is itself a high quality and expensive product. It is much harder than a lot of steels, certainly mild steel and many low carbon steels. Think cutlery and even sink unit tops. Typical mixtures include 18% Chromium and 8% Nickel as well as some carbon and trace elements. So much in fact that it isn't really a steel at all, and behaves differently so that hardening and forging methods tend to be entirely different. The major added bonus is also resistance to corrosion.
Forging (hot or cold) also imparts strength, or rather toughness, to steels, including stainless. Most steels, even mild steel, become harder and tougher when worked or hammered. This is known as 'work-hardening' or 'strain-hardening' and is often undesirable but is used to advantage in forging into shape.
Cast iron (typically 3% - 6% carbon) is indeed a brittle material, but is produced by pouring when molten (casting) and isn't drawn or forged as a steel would be. By careful control and by ageing it can be made fairly hard. It has low Tensile Strength but high Compressive Strength and is also quite thermally stable - doesn't change shape much when subjected to heat, though sudden high heat or cooling will crack it for sure.
Hardness - resistance to indentation or wear
Toughness - resistance to breakage or deformation by sudden shock or impact
Tensile Strength - resistance to shearing when put under tension (pulled from either end) - high quality bolts need high TS
Ductility - the ability to be drawn out into a wire - Brass is superb for this.
Malleability - the degree a metal can be formed before breakage - most metals are more malleable when hot.
As regards garden tools, I would say that forged stainless steel would be by far the better material to obtain and that's before the corrosion-resistant factor is considered. Given that, they win hands down. Knives and such are probably better in high-carbon steel, but expect to look after them and to pay more for them than their stainless counterparts. Cheap carbon steel tools are a waste of time and money, and can be dangerous when they suddenly snap or spend all their lives blunt.
Sharpening by high-speed grinding wheels (including an angle grinder), particularly with carbon steels, must be done carefully. Never overheat the steel, which can very easily occur at the business end which tends to be a thin edge. If a steel is overheated so much it turns blue, it's probably ruined. The temper has been drawn and the only cure is to re-harden all over again, more or less beyond most peoples' facilities. Have pauses between passes on the wheel to allow cooling, or even better, have a receptacle containing clean, cold water and very regularly dip the tool in the water to keep the edge cool. A file will generally be the best tool for sharpening things such as spades, hoes and even fork tines. For keener edges, a file if the edge is really ragged, followed by a hand sharpening stone.
The link you posted is a good 'un, thanks.
I think that's enough to be going on with. Sorry. :roll:
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July 24, 2013, 20:36