Some hardness figures for different metals, taken from several sources (Matweb, Butterworth's Mechanical Engineer's Reference Book 11th edition 1981, Macready's Orange Book and others). All figures are quoted in Brinell 'b' scale. Jacob may be interested to note that none of the sources was found in an armchair.
BS1400 LB4 leaded bronze, sand cast - 55 to 75
BS1400 LG4 leaded gunmetal, sand cast - 70 to 85
Mild Steel (EN3, 0.2%C) - annealed 120, workhardened 180
O1 Tool Steel - 229 annealed, 550 to 680 hardened and tempered, depending on temper
Malleable Iron - 130 approx
Grey Cast Iron - 156 to 302 depending on grade
Ductile Iron - 143 to 187 depending on grade.
We can thus deduce that even annealed grey cast iron (approximating to malleable iron) is of comparable hardness to mild steel, and notably harder than cast bronze, another material used for fine plane bodies. The hardness of iron will depend on grade, and varies considerably, but even the hardest are not greatly better than annealed tool steel, and in nowhere near hardened and tempered tool steel.
This is somewhat comparative, and figures should not be taken as absolute. There are something like 4000 grades of steel commercially available in Britain, and perhaps several hundred grades of cast iron. The exact hardness of any given grade will depend on the conditions of manufacture and any subsequent working, so absolute figures can only be given in very tight specified circumstances.
This indicates that there may be some truth in Jacob's assertion that some older planes are harder than some newer ones. However, it also indicates that any plane of cast iron, irrespective of it's vintage, is unlikely to be significantly harder than a modern one of ductile iron, or of a dovetailed mild steel plane, and planes with bronze or gunmetal cast bodies will be noticably softer. It's also worth noting that beechwood (among others) works perfectly well as a plane body (there are several centuries of evidence to support that point), and beech is a lot softer than even the softest bronze.
I'm not really sure that this proves anything we hadn't already worked out from long experience. Planes of whatever iron - seasoned, annealed, ductile - work perfectly satisfactorily, as do dovetailed mild steel and cast bronze and gunmetal ones. If they are used in a hostile environment, they are likely to suffer more than if used in a relatively controlled one; hence the reason that many carpenters and joiners prefer not to take their best tools on site.
Thanks to Matthewwh and Vann for correcting my mistake in asserting that Clifton use a ductile iron. It would seem that they end up with an annealed grey cast iron, a material in a condition in which I have (so far) been unable to find any hardness data.
I hope this satisfies Jacob's wish for hard data rather than armchair theories. I now feel that I've wasted quite enough time on a matter of not much importance. No doubt our friend will have much to say about it, and will find holes to pick in it all. I rather suspect the majority of readers will yawn, wander out to the workshop, and get on with planing without worrying about the exact Brinell hardness figure of their plane body.