374: IRON Results of Experiment* on Hie effect of Hardening on the EtrtensltnUty and Strength of Iron, and Steel. (From Knut Slyfffs " Iron and Steel.") KINDS. Treatment of bar before experiment. Amount of carbon per cent. Breaking wtlght per sq. inch, original area, ibi. Ratio befn arer of fracture and orig. mean area. P 1 ft- TI " h 0-33 79873 0-42 Uchatius steel from Wik- ) Heated to redness and slowly cooled in warm coal dust. Strongly heated and hardened in oil 0-83 1-22 51,259 101,351 0-27 1-00 Heated to redness and slowly cooled in warm coal dust. 1-23 0'20 84,814 68757 0-50 0-79 0-20 46,730 0-86 0'08 62581 0-33 Heated to redness and slowly cooled in hot coal dust. . 0-08 44,877 0-32 Results of Experiments on the Strength of Iron rolled cold. (Fairbairn.) CONDITION OF BAR, Br'king weight per sq. in., tons. Strength (untouched= rooo). Untouched (black) 26-178 1-000 Boiled, cold 89-838 1-500 Turned 27-119 1-036 The resistance of cold rolled iron to tension, compression, and transverse strain, and also its hardness, are increased in nearly the same ratio as its breaking weight. The following table ex- hibits the results of experiments to determine the strength of the iron from two exploded boilers, compared with other brands of Ameri- can iron and English Lowmoor boiler plate : KINDS. Average breaking weight per iq.in.,lba. Hlgb.,1. Lowest. Latter le than former per cent. 41 <>.' 50182 29012 42 54.128 67,012 51 813 9-1 42144 53277 j 48,881 42011 58,174 48525 86,111 85679 82 28 1 41,249 49000 52.277 60.000 83,003 42000 88 25 68,984 64000 55800 14 The great variation in strength in the iron from the exploded boilers was supposed to be due to the wrenching and twisting accompany- ing the explosion. From the foregoing tables it will be seen that the physical properties of iron, strength, elasticity, &c., vary according to composition and treatment. The following are some of the conclusions of Knut Styffe ("The Elasticity, Extensibility, and Tensile Strength of Iron and Steel," translated by C. P. Sandberg, London, 1869): "The limit of elasticity, the absolute strength, and the exten- sibility are to a great extent dependent, in both iron and steel, on the mechanical treatment to which the material has been submitted, and on the temperature to which it has been exposed, either during working or subsequently. By cold-hammering, cold-rolling, and other forms of mechanical treatment applied at a low tem- perature, both the limit of elasticity and the absolute strength are increased ; while by the same treatment the extensibility is diminished. In these respects heating produces an opposite effect. When the proportion of carbon in iron or steel is increased, while the other conditions remain the same, the limit of elasticity, as well as the absolute strength, is to a certain extent increased ; but the extensibility, on the contrary, is diminished. The absolute strength, which in good soft iron may be estimated in round numbers at 48,034 Ibs. or 21'44 tons per square inch, seems to attain its maximum in steel con- taining about 1'2 per cent, of carbon, and is then in good cast steel or Bessemer steel about 137,240 Ibs. or 61'26 tons per square inch. A small proportion of phosphorus in iron gener- ally raises the limit of elasticity and the abso- lute strength, and therefore also the hardness of the metal ; but at the same time it diminishes its extensibility, provided that the iron during its manufacture has been so much drawn out that on slow rupture it exhibits a fibrous fracture. By admixture, however, of slag (which always makes the iron unsound and difficult to be re-formed when heated, but which facilitates the development of a fibrous structure), an iron containing 0'25 per cent, of phosphorus seems capable of acquiring nearly the same extensibility as an iron which con- tains only traces of phosphorus. The presence of slag also seems to oppose the tendency of the iron to become when strongly heated crys- talline, and therefore cold-short. By heating and sudden cooling (hardening), the limit of elasticity is raised, while the extensibility is di- minished, not only in steel, but also in iron. The absolute strength likewise is increased by hardening, if this be performed in a manner adapted to the quality of the material. Hard- ening in water without subsequent moderate heating (tempering) generally diminishes the strength of hard steel to a very considerable ex- tent ; while hardening in oil does not occasion this inconvenience, provided the heat previous to hardening has not been too high." Styffe likewise gives the result of an elaborate series of experiments on the strength of iron and steel at different temperatures from iO F.
