I know iron is valuable, but I was wondering if iron was extremely rare, rare enough that it could not be used for construction, weapons, or anything our society uses iron, what this society would look like.
Would they be able to find a good alternate or several alternates in order to accomplish the same things as our society, or will they be stuck on the planet?
The biggest problem would be magnets. Iron has the ability to keep magnetic fields contained in a much smaller volume. An air core transformer is much larger. Motors too would be very large. Other materials are magnetic, but not nearly so much as iron. Alloys? Maybe.
C.M. Kornbluth wrote a story, "That Share of Glory" about a planet that had little or no transition metals on it. (Copper, Iron, Nickle, ....) Their answer was ceramics. Given how well our Corelle dishes wear, this has a possibility.
Another short story was rebuilding after an nuclear war. All the easy ore deposits were used up. What to rebuild with? Their answer was a form of tempered glass.
Not having steel would be a huge handicap. The replacement of wooden hulled ships would be much later. How would you railroad? Sure, bronze. But the price of bronze compared to the price of steel would mean that you would use it sparingly. Do a price comparison on a bronze hammer compared to a steel one.
Aluminum is a good replacement. Aluminum Magnesium alloys are even better. And it's cheap enough in bulk for ship building. The MHS Sheffield was an aluminum alloy frigate(?) in the Falklands war. Got hit by a single exocet missile and burned. I didn't realize that aluminum could sustain a flame, much like magnesium.
Titanium is another expensive replacement. When external frame packs were still in when I was a boy, you could get a steel pack frame for 20 bucks, and aluminum one that was about half the weight for about 35, and a titanium frame that took another 4 oz. off it for $300.
Getting to the Aluminum Age from the Bronze age would be hard. Cheap steel made the industrial revolution possible. Without iron, you'd be stuck with bronze/brass. https://en.wikipedia.org/wiki/Bronze
This page: http://www.roymech.co.uk/Useful_Tables/Matter/Costs.html gives typical comparison costs. This gives you a way to look at alternatives, but with no iron the demand for everything else skyrockets. For story purposes I would take the multiplier and do a further multiplication by a factor of 5. So for example, a steam locomotive made of bronze would be a factor of 6 for bronze, and a factor of 5 because no one has steel, so 30 times the price.
Of course if copper, zinc, tin are more common due to iron's absence, this will affect these factors.
Once you get to the Aluminum age, shortages disappear. Aluminum is very common. It's just expensive to refine. At this point, you get down to things that are only a factor of 4 or so more expensive. I think your industrial age will limp along until the aluminum industry gets going.
Look at bicycle technology. Steel alloy frames are in the 30 pound mark, and you can still get a serviceable $150 buck bike. With carbon fiber you can get the weight down below 12 pounds, and a price tag comparable to a good used car. Note however that the chain and gears and spokes are still steel. There are bronze alloys that would work for this, but at a price factor of 6 to 30 times as much.
From a historical perspective, iron kinda sucks. It's actually very comparable to bronze in terms of mechanical characteristics.
Steel on the other hand is so incredibly useful, we've only recently begun to come across things that work as well as it.
As far as replacements go, it really depends on what you need to do. Titanium alloys can go toe to toe with most steel alloys in terms of physical performance, but the but the best steels are still better than the best titanium. Additionally, it's MUCH harder to machine or smelt titanium alloys as they don't conduct heat very well and are comprised of more component elements. Whereas steel could be (and probably was) discovered by accident.
If you're looking for a replacement for use in spaceships and the like, Aluminum is your best bet. It's not as strong, so it requires more skilled engineering, but it's got great properties for its weight.
On Earth, natural aluminum is incredibly rare and we were only able to mass produce it once we got the ability to pump tens of thousands of volts through bauxite. If your society had plenty of copper, tin, and bauxite, it's possible they could skip the iron and steel stages of development and get to electricity and make the jump straight to aluminum, but there would be several differences. They wouldn't have skyscrapers. Their buildings would, in all likelihood, cap out at about 600 or so feet tall and have to be made of concrete or the like. Bridges would likely be causeways instead of suspension bridges like we have today, so inland shipping would have to happen on smaller, flatter ships.
I suppose it's possible in theory they could largely bypass metals and discover polymer chemistry and eventually materials like carbon fiber.
At the very least they'd probably focus on composite materials for construction applications and possibly an alternative metal alloy for electrical uses. Fairly common copper is probably a minimum requirement for quickly developing electrical theory. We've actually started to explore Plastic Magnets, so that's a possibility down the tech road for your civilization. And superconductors don't actually require iron at all.
The problem would be that no iron age and no industrial revolution with abundant iron would probably slow down development a lot.
But there are no total barriers.
Bamboo for scaffolding is still used, although it's use is now declining. I think we should not leap to the conclusion that we would not find materials that do the job we need. We used to build warships from wood and particular woods were valued highly because they were exceptionally strong.
You might find this Wikipedia list of copper alloys useful. There are lots of copper alloys which are more important than iron.
There's also quite an interesting page on magnetic metals which gives a good overview of possibilities.
For ferromagnetism the next most important metals to iron would be Nickel and Cobalt, I think. As you'll notice from the list of copper alloys, nickel and cobalt turn out to be quite useful in making interesting alloys, along with zinc.
"Happily" gunmetal does not require iron at all. :-) Not only does this mean we can keep killing each other with more sophisticated means on your iron-depleted planet, it means we can build lots of other high performance industrial equipment.
Iron available in relatively small quantities is useful in making some alloys.
The biggest value of iron is that it is cheap. It is very common, and relatively easy to work.
Something will always be the winner in strength per cost, and that will be what everyone uses when they care about getting something done rather than how it is done.
Everything generically 'metal' is steel because because it's cheap. If iron wasn't cheap something else would be the cheapest, but not being as cheap products wouldn't be as cheap, and since the runners up (tin and aluminum I think) aren't as strong you'd get less performance. There isn't really anything magic about iron, if it was too expensive people would make due, the same way we make due without adamantium or nano tubes.
Thinking on how technology evolves, I believe this society would not find something to replace iron in things that we make from iron, but they would instead create other things with other materials to deal with the same things we use iron-based things to.
Would they be able to find a good alternate or several alternates in order to accomplish the same things as our society (...) ?
They certainly would find ways to develop things according to their needs. They would accomplish the exact same things our society did?
If you mean material things, certainly not... if you mean development, science, technology, politics, economy, religion, art, etc., I believe so.
As others have mentioned, magnetism is an important property of iron, though any of the ferrous metals (iron, nickel, cobalt) would work for that.
The other fairly unique property is that its used for steel, and steel's fatigue limit means it can effectively be exposed to low levels of fatigue indefinitely. This is critical for things like bridges, and the reason airplanes have a lifespan of takeoff/landing cycles before the aluminum is too fatigued to be safe. Apparently titanium alloys have this same property.
Two novels by nowadays sort of famous Russian sci-fi/fantasy author Lukianenko had a quite similar setting. The names and "Cold shores" and its sequel "Narrowing morning". I don't know it they were translated in other languages. It is a retelling of a biblical story in the more or less modern world, with apostles and travel and doubt if the leader is brining genuine good.
The world is purely technological with a single fantasy addition and a same-natured large-scale fantasy event roughly 2000 years ago. The basic idea is that a heavily implied analogue of Jesus blamed the iron in all the conflicts the mankind led and vanquished all known to him sources and forged iron artefacts from that world.
So, they had normal Iron Age and diverged from our historical timeline 2000 years ago.
The actual fantasy addendum is the ability to hide and produce again items to a personal wormhole dimension. The problem is that these dimensions are hierarchical and the root one holds all the iron. For typical people the size of the dimension storage they can use is low, so it's more a portable bank storage cell and less a teleport for large items. It plays an important role in the plot, but not really in the world building.
The actual novel happens in 19xx, but the technological level is more akin to the 19th if not 18th century with few additions / novelties. So, basically a steampunk setting. Iron is very rare, mostly from artefacts made more than 2000 years ago and hidden at the time of vanquishing of the iron. Is is mostly used for jewellery, similar to gold in our world.
For production use brass and copper are used. They did not reach to aluminium or titan in large scale production. The creative use of rockets, however, enabled take off-assisted gliders, mostly from wood.
According to Lukianenko, if the Iron-era humanity suddenly looses all the iron sources known to it in, say, 30 AD, we would be slowed down for 100-150 years of progress and have Queen Victoria's steampunk with brass steam-powered battleships and wooden glider planes in 20th century.
There's be no civilization as we know it because we wouldn't be here. Iron is a critical nutrient to vertebrate life, necessary for the creation of red blood cells to transport Oxygen throughout the body. Its commonness is why sufficient amounts of it dissolve in soils world wide to be absorbed into plants for us to consume.
The iron age started not because bronze was mechanically inferior but because it was relatively expensive. There are bronzes, those made with noticeable amounts cobalt in particular, that are in fact still superior to steel. Iron's advantage is that it is over 20,000 times more common than tin, which is needed as an alloying material with copper (833 times rarer than iron), in the Earth's crust. So a world with as much bronze ore (90% copper, 10% tin) as we have iron might skip the iron age altogether. As for the industrial revolution basic bronzes could have been used with difficulty, everything would have taken longer and cost more. The age of electricity would depend on the availability of large amounts of the other ferrous metals, nickel and cobalt, for making permanent magnets, aluminium would then be available in quantity, aluminium has a number of alloys that can take the place of steel in structural applications and is of course very important in aerospace.
So that's alternatives at various stages, where are you really going to miss steel? Jet aircraft, and rocket motors, bronzes will get you a long way because they're very good at dissipating heat but where you need to concentrate heat, such as in steam engines, jet engines and rocketry, bronzes are not as useful. So you need rhenium, niobium, titanium, tungsten, neodymium and a host of other relatively rare, on Earth, elements that do some things iron does better at the expense of being a good "all-rounder" for example tungsten is very strong but very hard to work because of it's extremely high melting point and quite dense so weight dependent applications need not apply. The space industry already relies heavily on non-ferrous metals like aluminium and titanium for light weight construction and even more heavily on non-metallic ceramics and polymers for heat shields, solar panels, insulation, etc... so if these materials were easier to come by there's no reason people wouldn't still engineer a space race and all the good and bad that has come from it.
Ok. So let's look at what man did have in the bronze age. They had enough technique by that time to mine and process both tin and copper and to combine them in the correct ratios to produce bronze. The would have used bronze for things like weapons for hunting or war, utensils like pots, locks and yes, plows.
What needs to be borne in mind is that the discovery of bronze was probably not a deliberate action. Someone accidentally did something stupid or thoughtless, and this resulted in (ooooh it's so much sharper than my flint kinfe) the start of the bronze age. This age does not merely cover the use of bronze in ever expanding ways, which led to the invention of metallurgy, but also saw the first writing, farming and the evolution of social structures. If iron were scarce, some other accidental discovery might have kicked off another age. Considering the shortage of iron, your next step might have been kicked of by the metallurgical discovery of a better, stronger metal (here is a list of the most common alloys, though some of them are modern).
Alternatively, small quantities of iron might have been used sparingly (like on the edge of a plow blade, to allow it to keep its edge longer).
If iron were rare, it would certainly hinder the development of an emerging society. Iron is fairly easy to extract from iron ore, can be done with fire and modest air blasts. Aluminum, for example, requires electricity to extract from bauxite, which is why aluminum wasn't used in industry until after electricity could be generated in sufficient volume, the late 1800's. The earliest form of steel, which is iron plus carbon, dates back to the 9th century, the Ulfbehrt swords built from handmade Damascus steel. It wasn't until the Bessemer furnace and crucible steel process was developed, lowering the cost considerably, that steel use became widespread.
In a modern technical society, scarcity of iron can be overcome, because that society has the knowledge base to find substitutes. But to a pre-industrial society, the absence of a metal that is strong, plentiful, and can be smelted and worked with using crude technology would slow advancement considerably. Developing the advanced tech to extract other metals is hard to do, if you don't have a simple to extract metal to build that technology with.
Just remember: there's a reason that humans switched to iron from bronze asap: it's:
If you want to know what a world without much iron is like, take a look at the Bronze Age, and imagine metallurgy stopped there.
Many consequences ensue, including:
