This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Hard and Soft Steel.
"Soft mild steels have a tenacity and resistance to compression, and an elastic limit somewhat "(proportionately much)" higher than wrought-iron. They can be hardened and tempered, but not easily. They are weldable and easily forged, and afford a very (?) reliable and ductile material adapted for structures subject to sudden shocks.
"Steel is more easily oxidized than wrought-iron, and far more easily than cast iron.
"It is not adopted for engineering structures to anything like the fullest extent of which it is capable, but is required by the engineer for tools, rails, boilers, machinery, wheels, etc., and is coining into use for some of the larger roofs and bridges."
The process of welding different pieces of iron together is a very important one and as it occurs frequently should be well understood and carefully watched. All iron to be forged to any shape should be thoroughly heated to an almost white heat, but not too much for fear of burning it ; the heating should be as uniform as possible and the iron should be worked as quickly as possible. The heat destroys the fibrous nature of the iron, and crystallizes it, but the fibres are again restored by hammer-ing. Care must therefore be taken to thoroughly hammer all parts. All angles should be avoided in forgings. Welding is the same pro-cess as forging, except that two separate parts are joined together. The metal in each is heated and the two are thoroughly hammered together. It is very easy to weld two pieces of wrought-iron together, as the iron becomes sticky at a comparatively low temperature.
Pieces of steel can be welded together if both are low ill carbon.
Those high in carbon cannot be welded. Iron and steel can be welded together if the steel does not contain too much carbon. Where the amount of carbon contained in the two pieces to be welded varies, the one with least carbon should be heated to a greater temperature than the other. With cast-iron no welding is possible. Welded joints in tension are considerably weaker than the original metal.
Sometimes layers of steel and iron are piled together, heated and rolled out into one mass, called three-ply steel and iron, five-ply, etc., according to the number of plates in the pile. These are used for safes and vault-work; after being rolled the steel is tempered or softened, the iron not being affected by the process. The plates are now fitted, drilled, etc., and then are hardened, when it becomes impossible to drill them on account of the extreme hardness of the steel layers, nor can they be broken by sledge hammers or hydraulic rams, on account of the elasticity of the iron layers, which are not affected by the hardening process. In hardening steel the colder the water and the more suddenly the heated metal is plunged into it, the harder will be the quality. By using oil for the cooling plunge instead of water the steel is less likely to crack or warp during the process. To remove the brittleness from hardened steel it is tempered as already explained, by reheating it to a much lower temperature, however, than before, and again plunging it in a water bath to cool or allowing it to cool slowly. Hardening steel adds greatly to its tensile strength and increases the limit of elastic-ity, but on the other hand greatly reduces its ductility.
To soften very hard steel it is annealed, that is, heated to a red heat and then allowed to cool very slowly in an annealing oven. The latter is a brick air-tight chamber in which several pieces of heated steel are placed to cool. The process of cooling is, of course, very slow, as the heat is retained by the brick walls and the radiations of heat from the different pieces greatly retard each other in cooling. In this way steel can be made to regain its original softness.
"Case hardening" is a process by which wrought-iron retains its tough and elastic nature but changes only its external skin to a depth of from 1/16" to 3/8" into hard steel so as to better resist wear and tear from external causes. This is accomplished by placing the iron in a box with bone dust and heating it to a red heat; it is kept thus for from half an hour to eight hours, according to the depth of skin desired and then quenched in cold water.
If the whole is to remain malleable it is not quenched in water but allowed to cool very slowly which leaves a skin of soft or mild steel. This is worked as desired and on completion the whole is re-heated and quenched suddenly, which hardens the steel skin, but does not affect the iron interior.
Welded Steel and Iron.
Hardening and Tempering Steel.
Softening or Annealing Steel.
Case Harden- • ing.
The protection of iron after erection is a very important subject to the architect. If iron is exposed to the air, particularly alternating damp and dry air, or to wet mortar, it will surely rust unless protected. Rust means the gradual scaling, corrosion, and crumbling of the whole. Cast-iron is the least liable to rust; wrought-iron, however, is very liable to it, and steel even more so. In cast-iron the skin formed during the cooling of the metal by fusion with the mould sand and absorption of gases, is not only supposed to give great strength to the metal, but offers its best protection against the weather. When this is removed or damaged, the iron is not only supposed to be greatly weakened in strength but it is certainly very much more liable to rust.
Rust is like a disease, and must be entirely cleaned off the metal before covering it, or it will continue to eat its way in. Where sand-paper is not sufficient for its removal, a bath of diluted sulphuric, muriatic, nitric or other strong acid should be resorted to.