. . . Fast forward to the Age of Firearms.
1. Did Rifling correctly placed in the Industrial Era ? I favor either 'Machine Tools' or 'Power Tools' instead since custom made rifles already exists well in the 16th Century. but power tools to make one were industrial era thing.
2. Where should Steel be? Middle Ages (Civ5, enables Longswordsman), Industrial Era (Civ4 maybe??) or Modern Era (Unlocks Battleship and Artillery). I favor its positions in the Industrial Era that it comes AFTER steam power and it should enabled Pre-Dreadnough Battleship and Protected Cruisers. (Chemistry will now enables Artillery and Combustions enabled AT guns/crew thing instead
My idea
3. And 'Replaceable Parts' being placed in Modern Era and enabled 'Infantry' ? I'd favor them to be in the mid or late Industrial Era, in place i'd like to see either 'metal cartridges' or 'repeating firearms'.
Brown Bess was the first weapon that has replaciable parts, and amongs the first to be converted to percussion caps muskets.
https://www.pinterest.com/pin/355854808034142974/
Rifling and Steel are two 'Technologies' that fit into our discussion of Needs and Prerequisites, and not understanding those is why both of them have been wandering through the various Civ Tech Trees.
Both Steel of various grades and Rifling were available, respectively, in the Classical and Renaissance Eras, but only as minor effects and not fulfilling any real Need in either case.
Let's take Steel.
There is a continuity from Iron to Steel which has more components than Civ has ever shown.
1. Wrought Iron - the first 'iron' was heated in medium-temperature forges and hammered into shape; 'wrought'. How much carbon was included, a measure of how close to Steel it got, was almost completely Random, because nobody understood the molecular processes taking place in the alloy. When a blacksmith (because everything was Individually Made - even the Roman 'arms factories' were just collections of individual blacksmiths and metalworkers, each working on his own) got it exactly right - either because he got a really good batch of Iron Ore or happened to have a carbon source mixed in when the iron was being forged - the result was one of the Legendary Weapons: Excaliber, Durendal, or the 'magical' weapons of the Gods. There's a reason all those myths and stories exist, but it's not the sort of thing you can build a Unit or an Army around. More common is the account of Celts bending their swords into an 'L' shape and having to brace them against a rock and stomp on them to straighten them - in the middle of a battle. That's low-carbon wrought iron, not any kind of Steel.
2. 'Wrought' Steel. As early as the 4th century BCE
Wootz Steel was being produced in India. This was a Historical Accident. On the one hand, there was a particularly 'pure' deposit of Iron ore which resulted in very pure Iron, on the other hand, chimney kilns for pottery were adapted to provide a 'blast' furnace for smelting iron using the steady, strong Monsoon winds and the 'kilns or furnaces were stoked with Wood because they had no coal yet. The result was a good carbon content in the mix, and actual Steel. Alexander the Great was given a 20 kilogram batch of it as a gift when he was in India, and may have been the first man in history to wear a genuine Steel Cuirass body armor. This is the 'pure' Iron/Steel which was traded into the Middle East and gave the 'Damascus' Steel Blades their reputation over a thousand years later. Again, though, this is a One Off - available in small quantities (20 kg was a 'Kingly Gift', which gives you an idea of the quantities available) from
one location in the world and still with no real appreciation of what was actually causing the exceptional nature of this 'iron'.
By the 10th century, Blacksmiths in Europe had discovered that carbon, in the form of various woody plants or charcoal fuel, made a huge difference in the quality of 'iron' objects. They started producing at least medium Carbon Steels with which they were able to produce real steel swords and other blades, and steel armor culminating in the late 13th century with articulated steel plate armor so sophisticated that NASA consulted the Tower of London armory which maintained the tool set used to make such armored suits when designing joints for Space Suits in the 1960s CE.
Still, this is all Hand Made. The best 'mechanization; you could get, by the Renaissance, was a waterwheel-driven trip hammer to help hammer the raw iron into rough ingots/plates before the individual Blacksmith/Swordsmith took over to do the 'real' work.
3. Cast Iron. By adapting relatively high temperature pottery kilns (required to fire Porcelain) the Chinese were making and using Cast Iron as early as the 6th century BCE (Zhou Dynasty). That's almost 2000 years before Europe was casting iron, but there's a reason: cast iron is almost useless for traditional weapons - too brittle. It was used extensively for household objects - lamps, bowls, cookware - and tools - iron plows, hammers and saws, for instance. In Europe there were other materials available for all of those, including wrought iron from the village blacksmith. No Need.
When Europe finally developed Cast Iron in the 14th century, it was by adapting the casting techniques used with Bronze to make Cathedral Bells (1 ton or larger castings) to casting Bombards - iron was a lot cheaper metal than Bronze or Brass, and the warring European States needed lots and lots of the new 'artillery' requiring 100s of tons of metal. Brand New Need = Brand New Development/Technology.
The big development that has not been in Civ (or any other 4X game) in the past is the transition from Hand Made metallurgy of all kinds to Industrial Mass Production metallurgy. This started on a relatively small scale in the mid-18th century with Iron Rolling Mills (water-powered) and the Puddling Process to produce steel ingots, but the 'take off' was in 1856 and 1861 CE, when the Bessemer and Open Hearth processes for making steel by the dozens of tons at a time were developed. After that, Steel was the basic material for nearly everything, but specifically the Needs that had developed were for Steel Cannon (various inventors and engineers were wrestling with the problem of breech-loading, rifled cannon, for which wrought iron or bronze was only marginally suitable) and - the Big One - Railroad Rails: required by the thousands of miles and thousands of tons, and again wrought iron was a grossly inadequate material.
Side Effect: Rise of the Krupp Empire, because Krupp was first to develop the techniques for working with cast and rolled steel to produce steel rails in quantity and cast steel cannon by the mid-1860s CE.
If you are looking for a simple, Civ VI-type Effect, Industrial Steel = rifled, breechloading cannon and Railroads.
BUT to summarize, the timeline is like this:
(Wrought)
Iron
Need: Better Weapons, Better Tools
Prerequisite Resource; Iron Ore
Other Prerequisites: Pottery Kilns (relatively high temperature firing), Bronze Working (basic metal forging and working)
Historically, first Iron Objects were iron-tipped Plows in Egypt and Assyria, about 2300 BCE - because Bronze was much too expensive to be used for simple agricultural tools. By 1400 BCE wrought iron was in use in Anatolia for weapons like swords, spear points and daggers.
Side Effect: Once people knew how to work Iron, it was a much cheaper metal than Bronze, so the 'middle class' army like the Greek Hoplites or Roman Legion became possible. Contrary to Civ, the Hoplite was not a Bronze Era Unit, he was Iron Age (post 600 BCE)
Cast Iron
Need: Better Tools, Decorative 'Luxury' Items (bowls, furniture), Bombards/cast iron Cannon
Prerequisite Resource: Iron Ore
Prerequisite Technology: Porcelain (high temperature blast furnace firing) OR Blast Furnace metallurgy
Steel
Need: Better Weapons, Better Tools
Prerequisite Resource: Iron Ore
Other Prerequisites: Charcoal Fired Furnace, Watermill-Powered Metal-working
Industrial Steel
Need: Railroads, mass-produced rifled weapons, ships, buildings, bridges
Prerequisite Resource: Iron Ore in Industrial Quantities (Deep Mining)
Other Prerequisites: Coal (Coke) Fired Furnaces, Cast Iron hearths and converters
Historical Note: after first Industrial Quantity Steel was available in 1856 CE, Krupp's first cast steel rifled cannon followed in the same year, steel railroad rails the following year (1857 CE), first steel-wire suspension bridge in 1870 CE, steel alloy ship armor in 1876 CE, first steel-hulled warship the same year, first steel-frame skyscraper in 1890 CE.
Oh, and a quick note on Replaceable Parts:
The idea actually dates back to Classical Athens and Carthage: both had stockpiles of interchangeable parts for warships stored in the Ship Sheds at Piraeas for Athenian Triremes and the Cothon at Carthage for their Quinqueremes in the 1st Punic War: pre-cast Rams, pre-built oars, stem and stern posts, and other 'interchangeable' parts.
NONE of the machines or mechanical weapons like muskets built before 1800 CE had truly interchangeable parts, because the required precision in machine manufacturing didn't exist yet. The Land Pattern Musket ("Brown Bess") in England AND the Charleroi Musket in France had parts made to 'standard' patterns, like the barrel and lock (firing) mechanism, but the locks themselves were individually made by craft workers: you could change one lock for another because the overall dimensions were roughly the same, but you couldn't switch out the parts of a lock, because they were all individually made. Eli Whitney's "interchangeable/replaceable' parts in 1801 CE was a Fake. He fooled the government purchasing agents by having all the parts pre-crafted by metalworkers so they apparently 'fit together', but each musket had to be made by hand and fitted individually.
Real Interchangeable/Replaceable Parts date to 1800 CE: Henry Maudslay's (who should be a Great Engineer) Screw-Cutting Lathe which allowed precisely identical screw threads to be cut by machine - meaning that all the things fastened together by screws could be made interchangeable for the first time. Within 5 years, machine-made precisely-interchangeable/replaceable parts were being used to build clocks (in the USA) and Pulley Blocks for the Royal Navy (Britain). In 1836 CE Samual Colt had machine tools whipping out interchangeable parts to mass-produce repeating pistols (revolvers) for the US military - the first really Interchangeable Part Firearms. By 1880 CE the principle was applied to manufacture everything from sewing machines to typewriters to agricultural machinery to railroad locomotives to cannon.
