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.
Straightening the Pieces.
For very important tension-members in trusses or arches, or for important hangers it is very desirable to use the double refined-iron. But, as a rule, the best results will be obtained by being satisfied with whatever kind of iron the mill is accustomed to roll. It stands to reason that a mill will not upset its regular shop practice and line of work for a single order unless well paid for it, so that, as a rule, it is more speedy and more economical to take their regular iron and use a larger size of cross-section. Then, too, where double refined-iron is ordered, the most constant supervision will be needed to get it. The men will probably prefer to relapse into their regular ways even if the principals do not. Then, too, in a report to the Government of a large series of experiments made at many mills and tested at the Watertown Arsenal, it was found that the result of the double refining was harmful at times, and that its value depended entirely upon the nature of the muck bar.
Where the muck bar was of a decidedly poor quality, showed air holes, blisters, and a general crystalline appearance on fracture, or spluttered greatly while being run through the rotary squeezer or the steam-hammer, denoting the presence of much slag and impurities, that is, when the quality of the muck bar resembled that of a dirty and poor cast-iron, the double refining was of great service in adding strength to the material. Where, however, the muck bar was of a good quality, the puddle ball spluttering but little in the rotary squeezer or in the steam-hammer, and the muck bar itself showing on fracture a goodly part of fibrous section and the balance with but little impurities, the double refining was found to positively injure and weaken the iron, to dry it up, as it were, leaving the successive layers distinctly visible, when the iron is tested by eating away its edge with muriatic acid, or in circular pieces distinctly showing and opening the successive layers on fracture. This in the case of double refining of poor-quality muck bars was not nearly so marked.
Result of Several Rollings.
It concludes, therefore, that some impurities help to amalgamate the final layers, and that where the muck bar is already of a partly wrought-iron nature - partly fibrous on fracture - that double refining works an injury. For important work, therefore, there should be a constant mill supervision by an inspector stationed there for that purpose, and the question of double refining or not should be left for decision to tests made on the premises.
Extra refined-iron, however, can safely be used at all times, the only drawback being its additional cost, and the doubts whether it will be furnished as specified unless the work is closely watched and inspected.
Instead of using muck bars to make up the bundles for the final heating, the mills sometimes use the cut off and otherwise waste ends of beams and other rolled-iron ; there is no objection to this if the iron is not too dry, but, as a rule, it is a very good practice, as the iron thus gets an extra rolling or double refining. Nor is there any other objection than above to the use of old rails, beams, or scrap iron, provided always that all such material is free from all rust and is thoroughly cleaned ; where mixed with muck bars, and thoroughly cleaned, and perfectly welded, it is greatly to be recommended.
The frequent rolling of English iron within a reasonable mit, according to Sir W. Fairbairn, improves its quality and strength; if, however, it is rolled too often, re-heated more than five times, he says it quickly loses its strength.
Pare iron, that is, iron [practically] free from carbon, consists when heated to a white heat of a mass of crystals of cubical form. The effect of rolling is to elongate these cubical crystals into long silky fibres which interlace and bind themselves together and form the mass of all good wrought-iron. A repeated sudden shock or blow, when cold, or continuous heavy vibrations, tends to restore the fibres to their original form, that is, crystallize them, the iron becoming very brittle and weak. It is for this reason, in testing wrought-iron. that if it shows crystalline specks on fracture, it is considered to be of a poor quality, provided of course the; fracture has not been made by sudden blows; if. however, the broken section is entirely fibrous the nature of the wrought-iron is correct and considered good. If wrought-iron contains sulphur it is red short." or cold short. and difficult to roll.
Use of Old Iron.
Wrougkt-iron is usually sold by the pound, and each rolled section is known by its weight per yard. The reason of this is that the weight in pounds of a section of wrought-iron one yard long is exactly ten times its area of cross-section in square inches. One-tenth of the weight per yard in pounds is then the area of cross-section in square inches. Steel is weighed up the same as iron and then two per cent added to what would be the weight of iron.
In the above the writer has purposely used the words "wrought" and "rolled" iron interchangeably. They are really the same thing. Their nature is supposed to be exactly identical. If any distinction can be drawn, it would be that in rolled-iron the elongated fibres are formed by rolling, while in wrought-iron they are formed by hammering, either by hand or machinery.
Where a large number of pieces of equal sections are needed, rolling is to be preferred; but for small orders (unless the order is for some regular mill section), hammering by hand, or by steam hammer and die, is generally preferable ; these latter are frequently also known as forged-iron. Welding is the process of heating two separate pieces of iron and uniting them together, while hot, by hammering or forging, the fibres of the two parts interlacing and forming a joint nearly as strong as the material. Any iron that can be forged, rolled or welded is said to be malleable.