This is a side topic to the Lobato bridge story.
Lots of posts have mentioned wrought iron, steel, even cast iron and maybe for those who are interested, a little tutorial would help. The three alloy families mentioned are similar in that they are basically iron plus other stuff. The other stuff is what determines what to call the piece of metal you are considering. Let's start with the family nobody has mentioned, pig iron. Pig iron is a really crude form of iron alloy that comes from the first smelting of iron ore in some kind of air blast furnace. Giant blast furnaces of Pittsburgh and Youngstown, and Pueblo produced this stuff. In the top of the towering furnace goes iron ore, coke, and a bit of limestone plus a strong blast of hot air from near the bottom and out comes molten pig iron. It is mostly iron but it has several percent silicon, carbon, and many other things that make it unusable as an engineering material, like sulfur and phosphorous. The only thing pig iron is used for is window sash weights.
Pig iron is used as the feedstock for the processes to make the real alloys of construction. Those processes are there to get out the excess silicon, sulfur, phos, and carbon. They also change the physical structure of the piece to give it better and more consistent properties.
The oldest of these processes is cast iron. This process is a repeat of the blast furnace on a small scale. The pig iron is added along with scrap iron a bit more limestone and coke in a vertical shaft furnace called a cupola, (same word as the caboose part). With the air blast, the metals melt and some chemical reactions take place to trap and collect the sulfer and silicon into the slag. Out comes cast iron. Cast iron has several percent of carbon. This makes the metal melt at a fairly low temp compared to steel and when liquid it flows like water and casts into intricate shapes. The bad side is that it is very brittle and simply cannot be used when the loads put it into any significant tension. In compression it is wonderful and has a great ability to damp out vibration. Wonderful stuff for making engine components like cylinders and blocks. Put a load on it and you are in business. Hang a load from it and you are asking for trouble.
The next step in iron evolution was wrought iron. This alloy is really interesting because with it the industrial revolution became real. Wrought iron, WI, starts with either pig or cast iron in a small furnace called a puddling furnace. The amount of metal involved is about the amount that would fit in a 5 gallon bucket. The moltel metal sits in a pocket with the hot flames blowing over it. A workman stirs the molten metal with an iron bar for many hours to expose all the metal to the oxidizing flames. This very gradually burns away the excess carbon and some of the sulfur and phos. As these burn away the melting point risies and the metal forms a ragged ball on the end of the bar. After ten hours more or less the white hot ball is pulled out of the furnace and taken accross the floor of the works to a steam hammer and it is squashed repeatedly. This squeezes our the slag that has been mixed into the mass by the stirring. The wad is now roughly bar shaped and it then fed through a rolling mill to flatten it. This bar is then sheared into pieces and the pieces staked into a cube alternating directions. The cube is reheated and steam hammered and rolled. This process is repated three times or more. Finally the metal is rolled to a final shape, bar, angle, plate, or whatever is desired. It might look like a steel bar but it is wrought iron. The stuff is an ancestor to modern composite materal. It has a very distinct grain structure made of layers of iron alternating with fibers of slag. It has a very low iron content. It is moderatly strong, it forges and rivets well, and it has great corrosion resistance. The downside is it is horribly labor intensive to make. The batches are tiny, and as a result it was always expensive. The alloy family of wrought iron is extinct. It is not made in practical quantities anywhere on Earth anymore. One or two museum operations demonstrate the process, but no real production. There are a copuple of firms that collect iron from brides and buildings and re-roll it to new forms. Repair materials for bridges like Lobato can be had for a price. Likewise if the bridge must be replaced, the iron has significant salvage value above that of just structural steel scrap.
Now we come to steel. WI was doomed by the Bessemer steel process in the late 19th century. Here molten pig iron was poured into an egg shaped vessel that could be tilted. When tilted upright the molten iron covered blast openings in the bottom of the vessel. When compressed air was blown upwards through the melt a rapid series of chemical reactions takes place. In effect it selectively burns ingedients of the melt. The silicon and carbon burn away and the iron just gets hotter. A melt of several tons could be processed in just a few minutes. The molten steel could be poured into ingot molds or even into prepared sand molds to cast parts. Now steel could be used for things like boilerplate and cannon barrels. Unfortunately the quality of the steel made this was was not that good. It had lots of nitrogen from the compressed air. This makes the steel crack when it is red hot and being forged. The chemical consistency varied too much and some steel was better and some worse unpredictably. It wasn't until the open hearth steel process got perfected and high quality steel became the universal replacement for wrought iron.
Open hearth steel was a grown up step child of the wI puddling furnace. Instead of a five gallon bucket, the open hearth could process many tons of steel. It was a gradual and controlled process that allowed the chemistry of the heat to be checked and modified to make the speicification boiler plate and structural steel that had to be just so.
All these processes are pretty much gone. (Except cast iron which still soldiers on in many ways, automobile engine blocks for example) They made the stuff we like to watch and ride. Today the scrap cycle is very important and most steel mills process scrap more than ore. The Bessemer process evolved into the oxygen lance and the argon oxygen degassing converter. Electric arc furnaces do most of the melting in the steel mills and elctric induction furnces do the work in precision foundries. The huge blast furnaces still work mostly in foreign places.
So that is a very sketchy review of iron & steel 101. I hope the metallurgical police don't read this for I have simplified and glossed over an awful lot of important stuff. I love this side of the preservation activity and the tragedy of Lobato gave me an excuse to post something....
