When the carbon is practically all graphitic, as in over-annealed steel, then the solution may be compared to thoroughly muddy fresh water.
This hypothesis of solution agrees well with the saturation noted; then about 100 carbon is all that iron will dissolve without extraneous force; and higher carbon must be forced into solution by the work of hammers, presses, or rolls.
This gives reason to the experienced tool-maker’s well-known preference for well-hammered steel.
The hypothesis of tension, probably molecular, covers all of the phenomena of excessive hardness due to high heat, which means high molecular motion checked violently by sudden quenching. It accounts for the progressive softening due to every added degree of heat, and it accounts for rupture, cracking, due to excessive heat or to any unevenness of heat.
Without this hypothesis of tension it is difficult to understand why quenching should rupture a piece of steel, no matter what the degree of heat, or how uneven it might be.
Without it, too, it is hard to see how successive additions of heat can cause gradual changes from β to α iron, or from an unstable carbide to an imperfect solution. It would seem that the allotropic changes, or the decompositions of carbides, must be more marked than the gradual changes from hard to soft which we know to take place by slow and gentle accretions of heat.
There is no property of steel known to the author which is not covered by Langley’s hypothesis, and therefore it is put forward with confidence for engineers and steel-users to work by until the scientists shall have completed their investigations, and after that it is believed that it will be a safe working hypothesis, because science does not change facts, it only collates them and reveals the laws of action.
Under this hypothesis of Langley’s we may define hardness as tension, softness as absence of tension.
This is not stated as established fact; it is given as a simple definition to cover the known phenomena until the final solution of the problem shall lead to a better explanation.
Regarding steel as a solution of carbon in iron, one important fact may be set down as established thoroughly: that is, that the more perfect the solution under all circumstances the better the steel.