Assuming ordinary carbon steel to be in the annealed condition, i.e., having been slowly cooled from a red heat or higher temperature, as the carbon increases to 0.90 per cent the tensile strength of the metal increases from some 50,000 to 125,000 pounds per square inch, the elastic limit remaining equal to not less than one-half the tensile strength. With further increase of carbon the tensile strength decreases while the elastic limit continues to increase. The elongation which meas ures the ductility of the metal decreases with the carbon content from some 25 or 30 per cent in steel containing very little car bon down to some 5 per cent in very highly carburized steel. The hardness of the metal increases continuously with the carbon content while the hardening power reaches its maximum at about 0.90 per cent carbon. Very low carbon steel can be readily welded but as the percent age of carbon increases the difficulty of weld ing increases rapidly. Carbon lowers appreci ably the melting point; while low carbon steel melts slightly below C., steel containing 1 per cent carbon melts in the vicinity of 1,300° C. Through suitable mechanical and heat treat ments the physical properties of steel can be deeply altered. Mechanical work performed
while the metal is red ur white hot in creases its physical soundness and gen erally raises its strength and elastic limit. Mechanical ‘1.or k performed while the metal is cold greatly 111LI C4JCS is tensile strength and elastic limit as well as its hardness, but decreases its ductility, eventually causing brittle ness. The heat treatment generally applied to steel may consist (1) in annealing, i.e., in cool ing slowly from a high temperature when it is desired to remove existing stresses, tp produce softness and ductility, but necessarily at the expense of strength and elastic limit; (2) in hardening, i.e., in cooling quickly (quenching) from a high temperature followed generally by reheating (tempering) to a temperature seldom exceeding 400° C. when it is desired to produce implements of very great hardness, but neces sarily- lacking in ductility to the point of being actually brittle and (3) in hardening followed by reheating to a temperature generally exceed ing 400° C. and which may be as high as C. when it is desired to obtain great strength and elastic limit combined with a fair amount of ductility according to requirements.