This property of resistance to pulling apart or bending is modified wonderfully both by the chemical nature of the particles of a metal, by their size, and by the way in which they are arranged. The purest and softest iron which has a tensile strength of some 40,000 pounds per square inch, can be brought, by alloying and working and right heating, up to a strength of 400,000 pounds per square inch in small sections. The proper heat treatment and working often can increase a metal's strength 100 per cent over an original cast condition.

The strengths of all metals vary so with composition and treatment that any specified strength has little meaning unless accompanied by the facts of composition, and of beat and mechanical treatments.

Metal Fatigue

All metals are susceptible to a gradual weakening by progressive internal rupture, when subjected to enough alternating strains to affect - although to not exceed - the elastic limit of the metal. This is called fatigue; many materials yield to it after a few thousand trials; those which require millions of alternations to rupture are said to be very resistant to fatigue.

Metal Plasticity

Plasticity, or the flow of metals under pressure, is by no means a general or uniform property of the metals. As already intimated, there are all degrees of malleability in the cold, the degree evidently being associated closely with the perfection of electrical and heat conductivities (gee Table II). What may be considered malleability at one temperature is the slipping of the atoms along crystal cleavage planes; at a higher temperature, a similar deformation may be the flow of an extremely viscid and tenacious solid solution.

Table II. Physical Constants

Conditions

Although this malleability is a cardinal property of the most useful metals, each metal or alloy requires specific conditions for making use of the property, if, indeed, it can be used at all. For cold working, a metal frequently is annealed, lest crystal ruptures be developed exactly as in an overdone fatigue test (rolling gold and silver). Compounds present dare not be overlooked (cementite present in tool steel in the cold is in solution at red heat). Temperatures and chemical actions must be within bounds (steel "burned" has been heated until actually deeply oxidized), while, finally, at the temperature of optimum workability, extreme care must be taken not to damage physically the weakened but still cohesive metal (in welding steel, slag and scale often are forced into the main body of the metal). In fact, the conditions under which each separate metal shall be worked, must be most carefully studied.

Table III. Chemical Source Of The Metals