Our most serviceable constituent, martensite, then, is a solid solution of carbon or iron carbide in beta iron. It is magnetic but this probably results from its containing some alpha iron through incomplete stoppage of the change by the quenching.
As has been stated, “tempering,” which means careful reheating to 400° F., 500° F., or 600° F., allows the slight “slipping” of enough of the beta solution, always eager at temperatures below the point of recalescence to return to alpha condition, to relieve the excessive brittleness of the hardened steel.
No. 72. The “Sorbite” Grain Is Produced by Cooling in a Blast of Air after Annealing. It Gives Good Wearing Properties
(Magnification 70 Diameters)
Annealing is the complete release of beta iron and the “trapped” carbon which allows of their return to the normal condition of pearlite with alpha iron. To accomplish this, the hardened steel has to be heated above its point of recalescence and cooled more or less slowly. Different speeds of cooling give different grain size, structures and physical properties.
This explanation of hardening, which is known as the “allotropic” theory, is not universally accepted, conclusive evidence being lacking at more than one point.
It must be stated also that some who hold the “allotropic theory” of hardening doubt the existence of beta iron. These contend that the so-called beta iron and martensite are only decomposition or transition forms of gamma iron and austenite.
Two or three other theories have been more or less strongly advocated but these also suffer from lack of evidence. The one which perhaps ranks next in number of advocates is the “carbon theory.” Its supporters contend that by quenching, alpha iron is made to hold carbon or iron carbide in solid solution and that it is this solution of carbon or carbide which gives to the steel hardness in proportion to its carbon content. Others hold that the great density or strain under which quenched steel exists accounts for the hardness.
Like many other great problems of the universe this one is not yet conclusively or satisfactorily solved, so reluctantly does Nature yield her secrets. But while it may not have explained all of the “whys” and “wherefores,” the work which investigators have done has brought about great improvement in methods of manufacture and quality of the alloys which are making our civilization greater and this Age more wonderful.