Under another topic, Bookie mentioned an antique Remington barrel that failed, and appeared to be "crystallized"
This is exactly what a fracture of coarse-grained steel looks like.
All steel is made of many small crystals of iron all stuck together. Usually the crystals are too small to see with the naked eye. Generally they can be seen under the microscope very nicely at 100X. When the steel has been heated very hot, which happens during the forging process, the crystals can grow quite large. A heat treatment, such as anneal, or normalize (heat about 1650F and air cool) causes new, smaller grains to grow.
Metallurgists usually call a metal crystal a "grain", in English, "korn" in German.
The point here is that coarse grained steel, any kind, any hardness, tends to be brittle. So when it breaks one often sees those bright shiney broken crystals. If it is fine grained, and breaks for whatever reason, the crystals (grains) are too small to see. The surface of, say, a fractured file, has a silky appearance.
The practical aspect for guys making odd gun parts by forging is, do anneal or normalize that forging to refine the grains. The finished product will be much tougher. This is "book learning" it is also very true. It is important that any high carbon item such as a spring, knife, or axe meant for actual use beyond throwing, be annealed after forging to refine the grain. Then it may be hardened by your usual process.
And, Remington obviously did not anneal that barrel after forging it. Bad practice! Walter Cline, in The Muzzle-Loading Rifle Then and Now says: "After the barrel was welded, it was annealed and made as soft as possible . . . place the barrel on a level piece of ground . . .then pile a long heap of dry chestnut wood over it, and then set fire to the wood and leave it until all the wood had been consumed and the barrel had slowly cooled."
Starting with a billet of steel at Remington, the normal way to make an octagonal barrel would be to forge an octagonal blank. In the '60's I spoke with an older gentleman at Allegheney Ludlum's Leechburg, PA, plant. He recalled when they forged octagonal blanks for Winchester.
