Argthtjtr

Heat up the products above the eutectoid temperature to form austenite and then rapidly quench it to induce the formation of martensite. This will induce the formation of hardened steel which is more brittle.

Martensite is formed in carbon steels by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite. [...] Too much martensite leaves steel brittle; too little leaves it soft.

A skilled artisan will use this process to make the outer of the product hard, but he would do it to the right point, so that the core is left resilient.

The saboteur would just move past this optimum and ruin the final result. A bit like baking a pizza 5 minutes more: from crust to char with the blink of an eye.

@JBH points out the difficulty. If you provide poor materials the smiths will detect problems because the metal will not act right. Quality control is integral to their line of work. Also, the smiths are not really bribable - it is their livelihood and if they get a reputation for turning out shoddy products that is the end of their work. A bribe would have to be enough to retire on.

Sabotaging the materials after they are made is really hard too. They are steel and steel is durable. Damage enough to make them fail on first use will be readily evident, certainly to the smiths but probably to anyone familiar with such items.

I can think of only one option.

Changeling.

Fairies take human children and leave in their place supernatural creatures who pretend to be children. The changeling children are sickly or weird or spooky. This has to be the solution for the armor and unfortunately, it probably requires magic. Magical entities (though probably not fairies, given they hate iron) steal the finished high-quality arms and armor and substitute pottery items, with a glamour cast upon them to make them look and sound like the originals. Of course, the changeling armaments fail immediately on use.

I like the idea that when someone figures out how to dispel the glamour, the crude pottery items are seen to have scrawled on them dirty words and obscene drawings labeled with the names of various knights.

Adding too much phosphorus to the iron makes it "cold short", i.e. brittle at low temperatures.

The effects of cold shortness are magnified by temperature. Thus, a piece of iron that is perfectly serviceable in summer, might become extremely brittle in winter. There is some evidence that during the Middle Ages the very wealthy may have had a high phosphorus sword for summer and a low phosphorus sword for winter (Rostoker & Bronson 1990, p. 22).

Iron ore

Use a STASIS spell to alter the supply chain, and later remove the spell to destroy the weapons.

Replace the Carbon used in the smelting and forging with Carbon-11, which has a half-life of about 20 minutes, but apply a stasis spell to the carbon to keep it from decaying. This could be done by a magic-user placed at the gates, watching for the ox carts filled with coal or charcoal.

The incorruptible smiths smelt the iron and forge the swords with the Carbon-11. It works in the forge the same way. The strength is tested, and the swords are pronounced excellent. They get the martensite profile just right. The swords are deployed to the troops. They work perfectly during training. Against other foes, they work as expected -- they are sharp and subtle, yielding to glancing blows, yet slicing through armor and bone. But -- when the critical battle starts, remove the stasis spell. The swords and armor will quickly become weakened as the carbon turns to boron and the crystal structure breaks down.

There may also be a benefit from the positron radiation emitted by the iron after the stasis is released.

Given the answer of Willk, it appears to be unlikely to manipulate the process without the smith noticing it. If you don't want to use magic directly, use something similar instead: Alchemy.

Some mixture has been used for the iron which takes over time with incrementing effect over days or weeks. The initiator could be the heat applied in the forge or the water to cool it off - which means the mixture could be applied even earlier. It would render the metal brittle until it unexpectedly breaks, but given the progress rate it would occur quite rapidly. The blacksmith could not notice anything because the effect is near non-existent in the first days or weeks.

The process wouldn't require some form of "intelligence" which magic would usually contain (do effect x when y is triggered, but not z). It would rather be a fictional chemical process. Maybe something in the real world exists which does something similar or works similarly.

The blacksmiths use a very sophisticated technique to produce their legendary swords that requires repeated forging and cooling. This produces a very special flexible and durable but also very sharp steel....
Unbeknownst to them, the process requires certain trace elements to be present in the ore,
and they get this (or more likely wootz steel made from this) special ore shipped from a neighboring country.

The adversary manages to capture some of these shipments and replace it with wootz steel from a different mine. To the blacksmiths everything looks just the same, but the swords are too brittle now. No one understands what has happened.

Note that this has a precedent in human history: There is a hypothesis that Damascus steel got its very special properties from the presence of trace metals in the ore combined with a special manufacturing process. At some point in history the blacksmiths lost the ability to reproduce that steel and this might be because the respective mines in India were exhausted.

Wikipedia: Damascus steel

Reibold et al., Carbon Nanotubes in ancient Damascus sabre (2006), Nature 444, 286

I'm assuming from your username and the setting details that you're running some sort of D&D or D&D-adjacent system. There's always one answer to that guy who got full plate before he should: the rust monster.

Rust monsters have strong corrosive materials in their antennae, and they're flimsy enough that an enterprising hostile kingdom or well funded crime syndicate could capture, breed, and harvest them.

Your saboteurs will need fairly lengthy access to the caravan to pull this off, but if the arms are coated in some kind of clear coat (varnish, lacquer, whatever - armor should come treated from the forge, but the weapons themselves may or may not be and would be the responsibility of the saboteur - shouldn't raise any alarms to find them coated) and stored with powdered rust monster antennae (in the sheath, on resin paper wrapping the armor, whatever), the arms themselves will:

1. Leave the blacksmith in good condition


2. Survive transit reasonably well, depending on the care taken by the saboteurs


3. Perform well in their initial inspection, if treated gently enough


4. Fail faster in the field if they were used in practice, but should fail eventually if used hard enough in battle regardless.


5. Show signs that look like water damage that could be blamed on an incompetent carman (wagon driver), marshal (officer in charge of transport), or lighterman (ship-to-shore ferryman)

The saboteurs entered the blacksmiths guild. With some help they ascended in ranks, becoming the guild masters. They transformed the guild in a company with employed blacksmiths. They added a huge bureaucratic top-layer: Accounting, human resources, quality control, sales, marketing, R&D, supply chain management, legal and controlling. The blacksmiths got gradually replaced with inexperienced workers. The work got compartmentalized. As a result workers are unable to understand the entire organization. Then they started with quality degradation: Controlling argued that the contracted quality could still be meet while introducing inferior raw materials (iron ore rich in phosphor and sulfur). R&D had is scope changed to focus on lowering costs. Supply chain management squeezed every penny from the suppliers, resulting in raw material degradation. Legal sued the local news herald when they mocked the inferior quality.

When the city council became concerned about the degrading quality sabotage got further.

The saboteurs argued that city help is needed as competing cities are advancing in iron processing. They asked for import tariffs and city subventions and got them. They introduced a city-financed institute for advancement in iron processing. The council pressed the city council to force independent manufacturers into the iron conglomerate. The blacksmith school got incorporated into the iron institute and replaced by cheap least minimum training. The institute declared that the brittle iron weapons/armor parts are state of the art, opposing opinions got suppressed by libel charges and by media campaigns, accusing them to be non-patriotic.

In the end the entire iron processing ability of the city got corrupted. Since nobody realizes the dimension of the degradation it is unlikely that the cities iron processing ability will ever raise to past quality.

Weaken essential parts instead of the whole thing

My source for all of this is that I work at a historical armoury and have actually assembled/disassembled quite a few pieces for restoration.

I know which damages are fixed easily and which ones take ages. I also know which ones render a weapon/armor useless and which ones don't.

It's pretty easy to discern accidental change from something a person did on purpose - in my case the deliberate changes were made by past restaurators but seeing as their goal is also to not have their work noticed I think the methods would be the same as a saboteurs - only the goal is the complete opposite.

I think it wouldn't be easy to sabotage the actual armor plates or blades. Even after centuries they are still basically unchanged and usable. Any sabotage that effectively weakens a plate would probably be visible or at least discernible as sabotage.
Likewise a good blacksmith would notice changed raw materials or sabotage during production.

Also all armor, weapons and especially guns were tested before being accepted by the army. This means that any major faults like a suit of armor breaking on the first hit would be noticed before they even leave the armourers workshop.

Therefor I would suggest to go for the things keeping it all together. After all what use is a suit of armor if you can't fix it to your body?

Leather straps

Sabotage: Armor, scabbards, guns

You could have them treat the leather straps with oil that has gone bad. This will turn the leather acidic which will cause it to go weak and break.

Upside: The chemistry of why the leather breaks wasn't yet known in the middle ages so it would probably be blamed on the blacksmith using poor materials.

Downside: It takes a few years.
Complexity of fixing it: You need to replace the leather straps - for armor that is complete disassembly and reassembly. Scabbards and guns: quick fix
Source: This actually happened accidentally after WWII when supplies were short and the armouries batch of petroleum had gone bad. This is the reason why there are so few original leather pieces left.

(btw: do NOT use petroleum for leather! Even if it hasn't gone bad it's not a good idea)

Rivets

Sabotage: Armor, scabbards, polearms, shields

Every piece of a suit of armor is connected to another part or a strap by rivets.
If you don't know how handmade rivets in armor works: It's basically a nail that is stuck through a tiny hole in the plate and let's say a leather strap and then flattened with a hammer on the other side.

Your saboteurs could file off that flattened piece of the rivet. Not completely so it doesn't fall out immediately but if it's just slightly bigger than the hole in the leather strap and plate it will eventually slip through with movement which leads to the armor falling apart.

Upside: it would look exactly like shabby workmanship but wouldn't be spotted by a rough inspection
Complexity of fixing it: Depending on which rivet breaks you might have to disassemble the armour completely. Scabbards and Polearms: quick fix
Source: I've seen this happen when we had to replace the leather strap and the new rivet wasn't done properly.

Hilts

Sabotage: Swords, sabres, daggers...

A sword isn't much use without a hilt. Mostly the blade has a smaller metal part protruding from the back. The crossguard is fixed to that piece and then leather/wood/wire is wrapped around it to form the actual handle.

If the leather/wire is loosened the handle will come apart leaving you with an acceptable blade you can't wield. If the handle is wood the saboteurs could saw off a tiny bit on one end so that it moves a bit between the crossguard and the pommel. It doesn't actually destroy anything but makes the sword awful to swing. If they saw off just a tiny bit it would look like the blacksmith fitted the handle poorly.

Complexity of fixing it: reasonably quick fix
Source: Quite a few handles have loosened by age and some of the wooden handles in the armoury have shrunk giving that loose effect.

Scabbards

Soldiers don't run around with drawn weapons all the time so sabotaging the scabbards is also worth a shot. You can do the same thing as I described above with the armor rivets.

You could also make slits or holes in the outer layer of leather. It normally prevents moisture from reaching the wooden part inside. If the slit is directly next to a metal band or the tip protector it might look like the blacksmith damaged the leather when assembling it.

Not so much for discrediting the blacksmith but still useful: You can also pour blood or salt water into the scabbard which will cause the sword to rust in its scabbard. (This is the reason why they have to be cleaned before being put back into the scabbard) Combined with a slit here and there and if you're lucky people assume the liquid got in through the slits which would be the fault of the blacksmith.

Upside: looks like the soldiers weren't careful with their weapons.
Downside: rusting takes a few days to weeks - depending on other circumstances.
Complexity of fixing it: if the blade is just rusty: reasonably quick fix, if it is rusted stuck in the scabbard: they would probably cut the scabbard off the blade and make a new one so reasonable to long fix (in the museum we have to preserve both)

Pole weapons

Most pole weapons are mounted to the wooden staff by metal bands that run down the wood and are nailed to it.

These nails can be loosened quite easily. The can also be filed off a bit like rivets.

Complexity of fixing it: quick fix
Source: Shrinkage of wood over time has caused lots of our poleweapons to have the nails come loose and they have become wobbly.

Gun powder

I don't know if there are firearms in your universe but if there are your saboteurs might want to refrain from doing the obvious and corrupting the lock. That's the first place one would look for sabotage. Seeing as the mechanism of a flintlock is actually pretty simple any damage would be quickly spotted by a blacksmith as soon as they take the lock off the gun.

Every gun was inspected and tested after being finished so again I would instead opt for the accessories. After all a gun without powder is useless.

It's hard to pin this on the blacksmith though. One way of doing that would be to temper with some of the black powder. If the powder is more powerful it might explode the barrel of the gun. This also happened with poorly crafted barrels. If too many barrels explode the blame will be put on whoever made the powder though.

Upside: if the gun explodes you also take out a soldier at the same time
Complexity of fixing it: an exploded barrel can't be fixed
Source: lots of people have tried shooting antique firearms but they used modern powder. Modern powder is more powerful and explodes old guns.

Splints

Sabotage: guns

Gun barrels are fixed to the wood by (most of the time) two splints and one two three screws. All of the screws are in the back next to the lock while the splints are placed along the length of the barrel.

Pull out those splints and all the strain is put on the screws. They are relatively small and sometimes go missing on their own. It could either be that the blacksmith forgot them in the first place or made them too thin so they fell out.

Complexity of fixing it: quick fix
Source: So many old weapons missing the splints.

Bolts and Screws

Sabotage: guns, some swords, some armor

Screw them in extremely tight so there is more pressure on the screw. This might snap off the head or damage the thread if there is some fault in the material already.
Damage the slot so it can't be used properly anymore. This usually happens by accident when fixing something but it could also be done on purpose.

Upside: soldiers don't usually touch the screws so any damage to them would only be noticed by a blacksmith who would then know that the one previously handling the object was an idiot who didn't use a fitting screwdriver - discredit the blacksmith among his peers!
Downside: mangled screws are only a problem once you want to fix the object because it has another problem
Complexity of fixing it: very annoying reasonable fix
Source: If the slot of a screw is mangled we actually have to fit a screwdriver to it so we can get the screw out

Mismatch equipment

Sabotage: swords, guns, armor

Mix swords and scabbards so they don't fit properly anymore. Same with armor parts like the front and back of a suit of armor.

With guns it's even easier because soldiers had to make their own bullets. All of that equipment is handmade and fitted to the barrel - switch it up with another and they don't fit properly anymore.

Also if you take two guns and switch out their locks it might even render the gun completely useless. There is a small hole in the barrel right were the pan on the lock is. When firing this is were the spark from the powder on the pan goes through to ignite the powder in the barrel. Because no two guns are exactly the same the small hole might be off or even covered by the pan if the wrong lock is fixed to the barrel.

If confronted about it the blacksmith himself will probably realise what happened but it's something you can't really prove. There are no serial numbers to show which parts belong together and to a layman they all look the same.

Conclusion

Some of these will be harder to pull off than others and some have the added benefit of annoying the army blacksmith who has to fix it.

All of these equipment failures would probably happen anyway so with your sabotage you are just going to increase the frequency. Of course if suddenly everything goes wrong all at once people will get suspicious especially if the blacksmith has always delivered quality work before.

None of these are particularly big but all of them show a lack of attention or skill if they happen to fairly new weapons.

So even though the blades and plates this blacksmith makes are actually pretty good he will earn a reputation of not taking care of details and slacking with the unglamorous parts.

One option could be a poor quality ore the blacksmith was working with. If the ore had an excess of one metal or another to give it unfavorable qualities, then it may be possible the final product would fail. If it had excessive amounts of lead, tin or copper, the weapon would be too soft and deform easily. If it contained excessive amounts of nickel, cobalt or tungsten, the armor would be too strong, thus brittle and may crack upon receiving a blow.

Now, any good blacksmith would recognize that the metal is not tuning out as expected, however, if he had several apprentices, they may not recognize the poor materials and force the piece into the desired form despite the material not wanting to play

Quench bath contamination leading to Stress Corrosion Cracking down the line

As other answers here indicate -- these smiths' reputations and livelihoods are on the line here, so their processes would be in good nick and unlikely to change suddenly, and they would likely notice issues with raw material changes, say to the ore, charcoal, or fluxes used. Furthermore, even if they didn't, proofing of the blades (acceptance testing) by the quartermaster would weed out blades that were, say, extremely cold short due to phosphorous contamination.

This means we need to get diabolical here, and look at the things a smith or quartermaster cannot see, or catch immediately with tests. In particular, preindustrial forging relied exclusively on static, water-cooled quenching and tempering processes, and these will leave traces of salts present in the quench bath on the blade as it is removed from the bath (simply from water boiling off upon contact with hot iron).

Normally, this isn't an issue, as the contents of most water sources aren't going leave behind anything that will cause serious trouble down the line. However, if the quench bath was "spiked" with something that was a potent promoter of Stress Corrosion Cracking (SCC) in carbon steels (carbonates may work, depending on their activity at or near body temperature) under mild conditions, this could effectively, yet insidiously, sabotage their efforts to produce quality blades.

As such a contaminated blade was handled and used, moisture from the environment that gets past whatever oiling is used to protect the blade from rusting would combine with the boiled-out SCC promoters in crevices in the weapon, such as at the blade-to-handle interface. Furthermore, such points would also be stress risers in the blade, and combined with residual stresses from forging and heat-treatment as well as the rigors of usage, this could lead to stresses at the risers exceeding the SCC critical stresses for the material. Finally, a well-kept sword is going to be in an environment that is only mildly corrosive (due to normal maintenance activity vs. being exposed to rain, blood, etc).

The end upshot would be swords that look fine, proof fine when received by the quartermaster, yet snap in half like twigs months or years later. A modern failure analysis would reveal that the broken sword was spiderwebbed with invisible yet lethal cracks in key spots, despite looking the part of a well-kept sword by any account chosen.

The iron used to build everything (weapons and armors) could've been saboted by adding more carbon inside, before being sold to the blacksmith. If more than 2,11% of carbon is added to iron, it becomes steel and isn't as flexible as the iron. Then, it will break.

But if you have magic in your world, why not think about another material than carbon added to iron, which degrade it slowly (for the blacksmith not notice it) and make weapons and armor bad quality faster (degradation appears after the first fight you said).

Some techniques for differential hardening of blades (wherein the edge is very hard, while the core of the blade is more flexible to provide strength) involve the application of clay along the center of the blade before the final hardening. If your saboteurs tampered with this clay and changed its characteristics, it could lead to a blade that looked right, but was actually very fragile.

There are quality control protocols that would catch the issue (most bladesmiths do a fairly rigorous bend test to determine the flexibility and strength of their blades), but if your smith is working past his capacity, those might fall by the wayside.

Not the most effective way but an interesting mythological occurrence of this by Loki can be read in places like here https://norse-mythology.org/tales/loki-and-the-dwarves/

The short version of this is Loki turned into a fly and distracted the dwarf who was working the bellows in an attempt to sabotage their attempts at crafting.

Transforming into a fly might not be a reasonable method but unleashing a plague of rats/vermin/sickness on the workers at the smithy could slow down their production, cause them to cut corners and release shoddy work.

Have you ever seen one of those amazing videos of aluminum being dissolved by gallium or mercury?

Your blacksmiths likely made perfectly good equipment, and your saboteurs intercepted it along the way to the army and treated it with mercury, greatly weakening its internal structure. These videos emphasize that the liquid metal won't be able to penetrate and react with the solid metal unless you remove the outer oxide layer first, but it seems you can do that with sandpaper or any other simple tool that is strong enough to scratch the metal. It only takes a few small scratches before the liquid metal can penetrate the solid metal and weaken it from the inside. This is why mercury and gallium aren't allowed on airplanes. It seems that the process can weaken the metal enough to fail under light stress in only a few minutes, according to some of these YouTube videos. It doesn't take much to tear these apart by hand, so you may want to just weaken a few parts in the center of the shield so it doesn't fail until it is in combat. They wouldn't even need to treat all of the equipment, even 10% failure rate would likely make them lose faith in the whole batch.

According to the Wikipedia page on liquid metal embrittlement,

The embrittling effects of mercury were first recognized by Pliny the Elder circa 78 AD.

I'm not a chemist so perhaps I don't know what to look for, but I mostly am finding details of how it affects aluminum in the context of airplane safety. Those are some modern alloys that wouldn't have been used in the middle ages, but if the effects were in fact noted in 78 AD, then it must happen to some extent in metals that were known and used at that time.

All your saboteurs would need is access to liquid mercury (not too hard to get then), unsupervised access to the weapons and armor (maybe they were the ones transporting them to the army, or they had secret access to the armory in the middle of the night after the shipment was delivered), and a few minutes with an alchemist to learn about the method.

I'm just imagining the villains loading up the perfectly good equipment into the back of a covered wagon, and during the transport one of them sits in the back scratching shields with a few nails while a second dunks them in a tub of mercury, like an assembly line. You can work out what the details are for your story, but as long as the basic flow is

Good metal -> Scratched -> Quickly coated in mercury around the scratch before oxide layer can completely reform -> Wash mercury off, handling the equipment very carefully from this point on -> Deliver to army

then you're in good shape.