And to further help educate anyone interested in the topic, from Kevin Cashen, who knows more about metallurgy than just about anyone on the planet.....
I have been asked by a couple of different folks to help with questions regarding the same topic. This is a tough one to fully grasp by most folks mostly because of misleading or confusing terminology. The topic is that of how forging effects the grain direction in steel.
I know how this one trips people up because we have already made the fatal error on the topic, and that is the loose use of the word grain. When metallurgy, or one with even a rudimentary knowledge of it, speaks of grain they are referring to a basic unit of common crystalline orientation in the steel. Most often this is in reference to an austenite grain since it is the prior austenite grain boundary that defines the shape and size of the units of other phases. This is an example of grains defined by prior austenite boundaries:
This is the grainy texture that we see on the ends of broken steel, and this is the unit that metallurgists mean when they mention grain size. Since this is based in austenite grain, it is very transitory, and is remade on every heat that exceeds the recrystallization temperature (non-magnetic, just to keep it simple). Because of this these grains are isotropic for the most part. What this means is that they have no directional properties in relation to the shape of the steel, you can deform the grains lengthwise in the direction of cold working but they will completely reset in a new isotropic state as soon as you reheat the steel, this is why annealing works.
Where we run into trouble is when we confuse these actual grains with the metaphorically applied term grain direction of the steel, as in going with the grain on a piece of wood. Entirely independent and separate from austenite grains is the fibrous direction of a bar of traditionally milled steel. This condition is the result of flaws and inconsistencies in the poured ingot being reduced and drawn out lengthwise in the direction of the rolling process. This is anisotropic, i.e. it is directional in nature and it is not transitory but a permanent property in the steel. You cannot change it by heat treating alone like you can with austenite grains. This is an example of the fibrous flow lines in steel forgings:
The only way to affect the orientation of crystalline (prior austenite) grains with a hammer is to work the snot out of it cold and then never reheat it. The orientation of the anisotropic fibrous nature of poured and rolled steel can, however, be affected by forging because it is not undone by mere heating. Think of it as super fine damascus defined by millions of weld zones. The actual grains can form, recrystallize and do their thing all around and within these lines as they are two different things.
Aha! So directional properties of steel are improved by forging! Well, hold on, not so fast. In the case of a complex part with abrupt changes in direction and cross section, such as a crank shaft, yes. But with a shape as flat and simple as a knife blade, not really. The only way to even notice this with a knife blade would be to shear off your stock from a large sheet with the anisotropic fibrosity going transverse to the blade length, i.e. from spine to edge instead of from tip to tang. Then you could measure its effects on impact and tensile strengths or perhaps edge stability. But with the direction running parallel to the blade length I would have a hard time thinking of how it could be measured even in a lab. Do the proponents of the forged tip versus the ground tip really envision the end splintering out like wood because the flow lines run off the edge from being ground? If so they havent bent much steel.
And as for the persistent idea of bladesmiths compressing all of these flow lines along the edge
well stop and really think about this. In the reduction from the ingot to the typical finished stock used by knifemakers the mill reduces this material from several feet down to ¼, and then the bladesmith comes along and hammers a slight bevel down the side and wants to take credit for reduction??? Really?
As we have seen, these concepts are very easy to confuse due to the reckless use of terminology, but no matter how many times this is clarified the same old faces continue to muddy the water with the same misinformation. At what point do mistakes become willful ignorance? And how much willful ignorance does is take to equal dishonesty?
I just got my Christmas present to myself a moment ago- a library copy, in excellent condition, of Recrystallization, Grain Growth and Textures by ASM. It has plenty of good stuff regarding these issues. But I would also recommend:
Plasticity of Metals by Kurrein
Forging and Related Technology by The Institute of Mechanical Engineers
Metal Forming by Hosford and Caddell
The Inhomogeneity of Plastic Deformation by ASM
