STEEL 367 to the use of steel for the construction of the bridge over the Mississippi at St. Louis : CONDITION OF STEEL. Tensile strength, Ibs. per square Inch. Crushing strength, Ibs. per square inch. No. 1. In its original condition . 109 478 to 131 864 100 080 to 112 400 No. 2. Heated to bright red and cool- ed in oil at 78 No. 3. Heated to bright red, harden- ed in water at 72, and tempered at blue heat 201,841 to 227,542 152,533 to 176,084 178,200 to 199,200 825 400 to 400,000 No. 4. Heated to bright red and hardened in water at 72 132,659 to 150,480 275,640 to 381,680 The change of molecular structure result- ing from working steel when cold has lately demanded attentive consideration from engi- neers, owing to the increased use of steel for construction and for the permanent ways of railroads. All violent mechanical treatment of steel after it has become cold, such as roll- ing, hammering, punching, notching, &c., is found to impair its strength seriously. Sand- berg has stated that the strength of steel rails notched on the flange was decreased from 50 to 97 per cent. ; the former where the notch was semicircular, the latter where the notch was square. It is evident that this decrease of strength is not alone due to the removal of so much material, but that there must be a local tension of the particles which leads to rupture, and annealing is found to remove this tension. The variety of opinions entertained by engi- neers as to the principal causes of fracture of steel rails is shown in the following summary of answers recently obtained from the admin- istrations of 24 German railways in response to the request of a commission appointed to investigate this subject. The figures in pa- rentheses indicate the number of administra- tions mentioning the prefixed cause. 1. The employment of too brittle metal (8). 2. Manu- facture at too high temperature (2). 3. Roll- ing at too low temperature (3). 4. Cooling irregularly or too rapidly after rolling (5). 5. straightening cold, producing fissures which enlarge and result in fracture (15) ; producing a change of structure (1). 6. Notching the flange (14) (only two denied this cause). 7. Manner of piercing the holes (6). 8. Reduc- tion of area of section of rails by the holes (1). 9. Bending the rails for laying on curves (3). 10. Rough handling of rails, such as throwing from cars to the ground, giving rise to fissures which result in fracture (9). It will be noticed that the majority of answers agree in attribu- ting the fracture of rails to improper treat- ment of the steel when cold. The cause of the brittleness and impaired strength in steel and iron consequent upon punching has been in- vestigated in Lorient, France, by J. Barba, who has found that cold punching induces a local hardening and tension of the metal, in a 764 VOL. xv. 24 zone less than 0'04 in. wide, around the hole, and that when this hardened portion is filed or cut away, or softened and relaxed by anneal- ing, the metal regains its original strength and extensibility. He thinks the hardening due to the combination of carbon and iron, as is also supposed to be the case when steel is hardened by heating and sudden cooling. Indeed, this heating and cooling is what undoubtedly occurs to the immediately adjacent metal in punching. The diminished strength of punched plates is caused by this narrow hardened portion, wiiich, owing to its decreased extensibility, receives the full effect of the stress, a rupture being produced in this portion and then extending throughout the whole mass of metal. The same effect, in an enhanced degree, would follow blows or shocks. The following are among the results obtained by Barba : TERRE NOIRE BESSEMKR STEEL. SIZE AND CHARACTER OF HOLE. TENSILE STRENGTH IN TONS PEE 8Q. IN. Bar T96 in. Bar 1'771 wide. in. wide. Cylindrical hole punched, 0' 669 in Hole enlarged to 0'774 in 25-86 27-76 32-20 84-61 Cylindrical hole drilled 0-669 in Cylindrical hole punched, 0*590 in., en- larged to 0*669 in 88-98 The effect of annealing after punching is shown in the following : CHARACTER OF BAR. inltoiu? Ts^hj Punched bar . ... 24-47 29-48 " " annealed Drilled bar .29/98 Punched enlarged" and annealed . 80"80 The effect of temperature on the strength of steel has already been considered under IKON, More recently Joule has experimented on the tensile strength of steel bars, and confirms the result of previous investigations, that the tensile strength is not impaired by reduction of temperature. In determining the effect of blows at reduced temperatures, he experiment- ed on cast-iron nails, and found that as many nails broke at ordinary as at freezing tempera- tures when exposed to a falling weight. These results must not be regarded as contradicting those of Sandberg on iron rails, nor does it follow that the same effect would have been produced had steel bars been used instead of cast iron. Uses of Steel. The industrial _ ap- plications of steel, formerly confined mainly to tools, weapons, and springs, have been widely extended since the introduction of the Bessemer and Martin processes. Among the principal modern uses of steel are rails, boil- ers, machinery, bridge construction, and ship building. The fact must not be overlooked that the term steel is now generally applied to all homogeneous, malleable compounds of iron, and includes products of all degrees of hard-
