Rather than asking, there is a search function in the upper right corner of the page you can use. Searching "Technology Progress" provided at least four topics specifically devoted to the notion. (Search terms must all be present for a hit to register, if you can't find anything, use fewer search terms per search.)
This one is the most recent, whose premise was pretty much exactly a "random tech evolution" as you described, which then went into the broader argument over whether there should be technological progress as we think about it with a modern mindset in a medieval world at all, much less of the sort that can be fit into the way that Dwarf Fortress is modeled. (A culture that exists for hundreds of years, in fact, does NOT have to progress. It is, in fact, entirely possible for cultures to backslide.)
I.E. Agricultural reforms are a massively important cultural event that can propel a nation from subsistance to wealthy, but which would, in-game, be represented by the actions of the player in how he manages his farmland.
The best you can do is have a system by which workshops have pre-requisite tools that can be improved and upgraded and which enable better quality work, or outright whether work on advanced armors like platemail can take place based upon having proper tool upgrades. (See
http://www.bay12forums.com/smf/index.php?topic=49802.15)
Having something like a set of reactions that creates different metals as gradual improvements of metallurgy is one of the few things that would be possible, however... it just requires that you put into the raws a dozen different formulas of "Steel" or "Bronze" or other alloys that indicate somehow being able finetune the process down to making a better alloy, although even this would require more advanced tools than simply a "Oh, hey, I've got a new idea! Let's try adding a little less carbon than the previous 1000 years of dwarven smithies that came before me! I'm sure they never thought of trying to experiment with that before!", and even then, the differences between having "Fine Dwarven Steel" versus "Damascus Steel" or even "Crude Steel" may not stand out terribly much in-game. (As long as steel is still the best metal besides the HFS, you will still put it on as many dwarves as you can, and you will still control dwarves in the exact same way.)