Why do blacksmiths need to ‘hammer’ blades into their shape? Why can’t they just pour the molten metal into a cast and have it cool and solidify into a blade-shaped piece of metal?
In: Technology
Bronze blades were in fact made that way. Then simply needed cleaning up after being cast. Iron and steel don’t have good metallurgical properties when cast, though. They go through molecular changes when forged that give them the combination of hardness and springiness needed to be a good blade.
Cast metal is strong thick. Like a cast iron block in your classic car. Forged metal realigns molecules to be stronger thin, but is more labor intensive.
Because forging realigns the metals structure, making it tougher and more resistant to breaking. A cast knife would be so brittle as to be useless, I think., but this being Reddit, I am certain there is someone who knows much more than I do!
One important difference, you can’t cast steel if it has to be strong , because it gets so hot the carbon stops being incorporated in the steel and it turns brittle this is because it got melted once when it was turned into steel and bringing it to that point again stresses the “new” metal. Each metal has a certain grain because of the molecule/s that the metal/alloy is comprised of reacts differently to everything (typically within seemably reasonable norms (Sodium is a metal))
For an analogy (obviously not a perfect one), think about pizza dough.
Why not just mix the flour and water together a little bit and pour it onto a pizza-sized platter? Why spend all that effort kneading it and stretching it out? Because kneading and stretching the dough changes the form of the gluten molecules, making them all stretchy. It gives the pizza crust that nice chewy texture.
If you take a bunch of nails and throw them in a box, they are all tangled and pointing in different directions. Shake the box and they start to align themselves into rows, facing the same direction.
This is what heating and hammering does to metal… you heat it up to spread the molecules out and hammer it to get the molecules to fall in line.
These “grained” molecules have better, stronger connections to eachother and therefore the metal is stronger.
EDIT: adding an edit here because it has been brought up a number of times… quenching and tempering are used to make steel harder, that is correct. However, the question being asked here is casting vs hammer forging, that is the question I am answering.
EDIT 2: Thanks for the awards Reddit friends!
EDIT 3: https://youtu.be/A4zsoA5kjCo (video of shaking a box of nails)
EDIT 4: one last edit and I’m done. No they arent molecules they are arranged in lattices… this is ELI5… no wonder this sub usually sucks… too many people with inflated opinion of themselves to realize the point is to make the explanation as simple as possible. Does saying molecules vs atoms in a lattice formation make more sense easily to a 5 year old? You bet your sweet bippy it does. Does it matter in the grand scheme of the difference between the two? Not a single bit.
Lots of missinformation here.
Forging is ultimately much more efficient than casting thin pieces.
It’s much cheaper to hear a forge, than a kiln.
Failure rates of casting thin pieces is higher than forging them. You can’t fix a bad cast, but you can fix a blade being forced as you go along.
Everytime you heat metal above a certain degree it starts to oxidize and wasted, when it’s melted it’s called slag.
More waste is produced with melting than with hearing to forging temps.
Short answer, higher rate of success and it’s cheaper.
There’s a few reasons why forging works better
– casting is done at much higher temperatures than forging
– you need a mould that can handle molten steel (not impossible, but not easy either)
– the forging process helps drive out impurities in the material leading to a better quality steel with the technology available at the time.
– casting steel and irons tends to lead to higher carbon alloys which are more brittle
With modern materials technology, the most efficient way to make a good steel blade is often a blade shape from flat bar of the right thickness and then grind out the shape being careful not to overheat the blade then heat treat it. It’s a lot easier now though as I can just order a specific alloy with the right components in a consistent distribution and structure. They had to deal with whatever came out of the local smelters which was highly dependent on local ores. This is also one of the reasons why swords or armour from specific areas were better than others
Hammering out the metal breaks up the dislocations & helps distribute them out so that the blade can maintain its strength & some elasticity. The dislocations will keep a grain boundary crack from propagating throughout the blade. Casting would lead to a strong metal, but not as strong as forged & much more likely to fracture under impact.
It has to do with the molecular structure. Temperature will determine the phase, and cooling will determine how the phase is locked and aligned.
Hammering also allows for multiple phased metals to be put together to get better overall blade properties so it can flex where it needs to and be harder in other locations.
Here’s a chart to give you a glimpse of metallurgy phase structure.
https://www.imetllc.com/training-article/phase-diagram/
Latest Answers