Ytdyklly

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

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.

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)

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

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.

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.

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.