Steel Forging and Heat Treatment
Clang──! A metallic screech that could tear eardrums filled the forge. The blacksmith's hand tingled from the hammer's recoil, and he scowled, shaking it out.
"Ugh! This...! What kind of iron is this hard?!"
The blacksmith rubbed his numb wrist and set the hammer handle down.
"That's why I said steel is different from wrought iron."
"Yeah, but this is ridiculous."
"Just follow my instructions for now."
The blacksmith eyed the ingot on the workbench suspiciously. This was the quality check process—we'd brought the ingots from the crucible to test them by hammering.
Out of the seven ingots from the crucible, four had failed as high-carbon steel. At first, the blacksmith had tapped them lightly, tsked at the dull thud, and given me that "I knew it" look. He'd grumbled about wasting precious metal, saying they'd all turned into pig iron. Probably because the wrought iron chunks had too much slag, lowering the iron-to-charcoal ratio and overloading them with carbon.
Iron overloaded with carbon is called cast iron or pig iron—brittle under impact, shattering easily despite its hardness. But the remaining three, with reduced charcoal, were different. I'd urged him to test the rest without argument.
Grousing, the blacksmith hefted the hammer and swung with all his might. And the result was what we saw now: metal that was hard yet unyielding.
True to his expertise, after that one solid hit, he'd quickly grasped the value of high-carbon steel. Now he stared at the ingot in a daze, caressing it like a lover.
"This... is really steel?"
"Told you."
"It's not like any I've known."
"It's still steel. Even wrought iron varies by source, right? Same idea here."
I prodded the dazed blacksmith to keep moving. "Don't just stand there—let's continue. We need to break up the failures anyway."
We reheated the pig iron on the spot. Dunked the glowing red chunks in water for a rapid quench.
Hiss──! Steam exploded, stinging my nose with acrid fumes. The suddenly cooled pig iron warped internally, becoming even more brittle. We covered it with cloth and hammered it to smithereens.
Crack, clatter! It sounded like shattering glass muffled inside the fabric.
These high-carbon fragments would mix with low-carbon wrought iron pellets in the crucible—no charcoal this time—to melt into medium-carbon steel. Everything recyclable.
Now for the three successful high-carbon steel ingots. I pointed to one. "This one's yours for practice."
"What? This precious stuff?"
"In exchange, process it exactly as I say."
To make proper goods, he needed new forging and heat-treatment techniques. Treating steel like wrought iron would ruin the material or yield subpar results.
Strictly speaking, this was tech transfer. But without high-carbon steel itself, the knowledge was useless. In a cheap-labor medieval world, forging tech was less valuable than raw material production anyway. Even if spread widely, no ingots meant no mastery—and thus limited spread.
Better if it did spread; steel demand would grow, aiding my plans. I couldn't personally train every smith, though. Best to toss ingots their way whenever processing was needed.
Once they tasted it, they'd buy my carbon steels later, even if sparingly.
The blacksmith swallowed hard and tonged the ingot.
"Crank the furnace hotter first."
"It's plenty hot already."
"Not enough. This stuff only listens at higher temps."
I signaled his bellows assistant. The flames roared fiercer.
Normal wrought iron forges at around 750°C—cherry red. High-carbon steel demanded more.
"More! Until the color shifts!"
The ingot went from red to orange, then blinding yellow—nearly white-hot. Over 1,000°C.
"Now! Pull it!"
The blacksmith tonged it onto the anvil. He raised the hammer instinctively, then hesitated—remembering the earlier recoil.
"Don't chicken out—hit it. It's soft now."
Clang! Claaang! His eyes widened. The feedback was totally different—high heat had loosened the steel's structure, malleable even to wrought-iron hammers.
"Huh, who knew... just feed it more fire."
Excited now, he hammered away. The hot steel stretched and flattened with thunderous booms. I watched, explaining the next phase.
"Once shaped, grind the edge with a file when it's cooled enough. Then the real key step."
"Quenching?"
"Heat treatment."
The pinnacle of metalwork was heat treatment—what smiths called "quenching," but my knowledge broke it into finer steps. Think of it as enchanting iron products with hardness or softness.
Medieval folk made usable tools from soft wrought iron thanks to empirical heat treatments. But steel's were on another level.
"Three phases: annealing, quenching, tempering."
"...I get the gist."
He grasped it as subdividing his usual quench but tilted his head at the terms. I explained simply.
"First, the freshly hammered piece has internal stresses—leave it, and it'll shatter in use."
"Hence quenching."
"Heating before the water dunk is annealing."
Annealing reheated the shaped metal, relieving stresses for uniform structure—like resetting its heat-treat state.
"Next, quenching—you know this."
"Cool in water."
"Right. But not for this hard steel."
"Then what?"
"Oil."
Quenching rapidly cools annealed metal to boost hardness. I pointed to my prepared tub—preheated to ~50°C beside the furnace, filled with raw linseed oil from the general store.
Linseed oil, pressed from flax seeds (flax grown for linen), was village-common.
"Oil? Fry the iron?"
"Close. Wrought iron takes water fine, but this cracks like pig iron in it."
Water cooled too fast—high-carbon steel couldn't handle the surface-core temp gap, cracking from stress. Oil's higher boiling point and lower conductivity slowed it just right.
The forge used foul black water—rotted leather scraps, horns, bones—for wrought iron: dissolved carbon/nitrogen carburized/nitrided the hot surface, hardening it. Useless for homogeneous steel, so I shared 8-10% brine recipes for low-to-medium carbon steels too.
Skeptical but compliant, the blacksmith finished the sickle-blade shape, reheated to bright cherry (~800°C) for annealing.
"Now! Dunk it!"
Shiiiish──! The red-hot steel plunged; white smoke billowed. Nutty roasting-oil scent filled the air. No wild boil like water, but bubbles roiled the surface.
"Three! Four! Five! Out!"
He pulled it—surface charred black, oil dripping.
"...It works."
We wiped with cloth.
"Now tempering."
He knew tempering vaguely but not temps or cycles. Tempering reheated low to add toughness—quenched steel was hard but brittle, glass-like: prone to chipping or snapping.
Tempering toughened it against impacts. Hold 180°C (bright yellow) to 300°C (blue) for 1-2 hours, air-cool slowly; repeat 2-3 times. Lower temps preserved more hardness, lost some toughness; higher sacrificed hardness for toughness. Blades/tools: low temp. Hammers/springs: high.
Oil-rubbed cloth before final temper seasoned it like cast-iron pans, rust-proofing too.
"Got it. Pick up the sickle in three days."
That wrapped the all-day forging lesson from dawn. No charge for the practice ingot and knowledge—fee waived as "tech fee." Too cheap, so I took his scrap pile: nails, horseshoes, broken blades, busted tools—~10kg worth a few silver.
The rest? He'd buy carbon steel later anyway.