Theory of Hardening and Tempering

THEORY OF HARDENING AND TEMPERING "Slip Interference."—These various actions which go on within solid steel at moderate temperatures are of the greatest commercial importance, for they control the important operations of hardening and tempering steels. In the simplest example these consist of slowly heating the steel to a temperature somewhat above line GSE, holding it there long enough for it to become a uniform solid solution, then quenching it rapidly thus retarding the reverse reactions as much as possible, and finally reheating the quenched bar moderately to produce troostite or sorbite, still hard but much less brittle than the freshly quenched structures. The most widely accepted explanation of the hardening phenom enon is known as the slip interference theory, definitely formu lated and systematized by the American metallurgists Zay Jeffries and R. S. Archer. It starts with the knowledge that metals are crystalline, and that metallic crystals when they are stressed be yond the elastic limit, do not tear apart irregularly, but one por tion slides past another portion along definite crystalline planes without appearing to let go. Such motion is speedily jammed and stops because it tends to disarrange all the neighbouring crystals. The slip plane also heals immediately. Much of the hardness of quenched steel is believed to be due to the sub-microscopic size of the constituent crystals; the boundaries of each one interfere with slip under stress in all the neighbours; since slip cannot occur, the metal resists deformation, and is therefore hard. In steel an other hardening factor supplements this slip interference at the grain boundaries, namely, the presence of a great number of car bon atoms, cementite molecules or even tiniest cementite crystals uniformly dispersed through the ferrite grains. Such particles harden the entire metal from two causes : first, they prevent the start of any slip between crystalline layers as would billions of tiny staples in the leaves of a book; and second, the slips once started despite this hindrance must drag against a multitude of anchors.

Hardening by Cold Work.

Since a ductile metal like iron deforms plastically under cold work or stress beyond the elastic limit by internal slip along crystalline planes, it would appear that this slip has the effect of breaking up the original crystals into numbers of small ones. For that reason cold work hardens the metal ; finer grained metals are harder. Under the microscope the slip planes are seen only in special circumstances, but the original grains appear to be flattened out and assume a shape more or less parallel to the extension. In extreme cases, as in wire for suspen sion bridge cables, a high carbon steel with original tensile strength of about 8o,000 lb. per sq.in. can be cold worked into a condition where it will carry 220,000 lb. per sq.in. before rupture. Micro structure of such a wire parallel to the direction of drawing shows the original crystalline boundaries flattened out until the structure seems to be fibrous (Plate III., fig. I I).

Increase in hardness is accompanied by an increase in brittle ness, so excessive cold work is liable to break the article, either during manufacture, or in its after history—season cracking. To avoid this danger, and to restore ductility, the piece may be an nealed (q.v.) or reheated moderately to a point which does not affect the microscopic appearance yet the added atomic mobility at higher temperature permits such rearrangements as neces sary to relieve internal stresses associated with work hardening.

Recrystallization.—A little higher heat on a cold worked piece of metal will cause the previous grain structure to be en tirely replaced by a new arrangement. This is known as recrys tallization. The recrystallization temperature varies with the amount of cold work previously done on the metal. Long stay at temperatures above this will cause the growth of large crystals. Such is to be avoided because it makes for a condition known as Stead's brittleness. Slip planes started across a large crystal find less supporting material at the grain boundaries, and the self healing action ordinarily occurring during plastic deformation is absent; the big crystal splits along the cleavage plane.

Refining the Grain.

Recrystallization by moderate heat after cold work is to be distinguished from recrystallization on a higher heating to above the line GSE. The alpha iron then trans forms into crystals of gamma iron. Apparently the new crystalline arrangement is almost entirely independent of the old; if the temperature is maintained for some time the new set of austenite crystals grow in average size as the temperature or time increases. On slow cooling through the transformation range these new aus tenite grains eject the excess constituent to the borders, and the kernels are the locus of the pearlite regions.

Thus by a heating and cooling through the transformation range, the grain size of steel can be changed, refined or enlarged, as de sired. Since previous effects of work or hardening are wiped out by such an anneal, the process is frequently called normalizing. Burning.—Excessive heating of steel ruins it by burning. A burnt steel is brittle and the brittleness cannot be removed by heat treatment. Microscopic examination shows that the grain size is unusually large, due to the high temperature experienced, and the grain boundaries contain considerable oxide. Burnt steel in fact has been heated beyond the solidus (line AE) and the high carbon regions in the austenite have actually melted. The melted metal absorbs furnace gases very quickly, which are responsible for the incurable damage done in the molten portions of the steel.

BIBLIOGRAPHY.—The fullest account of American practice is con tained in J. M. Camp and C. B. Francis, The Making, Shaping and Treating of Steel (1939); for British and Continental see F. W. Harbord and J. W. Hall, The Metallurgy of Steel (1923) ; H. P. Tiemann, Iron and Steel (1933), a pocket dictionary of terms and a brief encyclopaedia of the whole subject; American Society for Metals, Metals Handbook (1939). Some fine text books cover only portions of the subject, such as Robert Forsythe, The Blast Furnace and the Manufacture of Pig Iron (1922) ; Walter Lister, Practical Steelmaking (1929) ; A. W. and Harry Brearley, Ingots and Ingot Moulds (1919) ; D. K. Bullens, Steel and its Heat Treatment (1939); Z. Jeffries and R. S. Archer, The Science of Metals (1939). Professional societies which devote much attention to the manufacture of iron and steel, and whose journals contain accounts and symposiums on the current state of the art are: (British) Iron and Steel Institute. Ameri