While rolled/plate metal produce some grain structure, it does not have microstructures like a true forging. As you machine away metal to form the component you cut directly into that grain flow, where a forging reforms the flow into the geometry of the part. This is why forging will still produce a stronger, more reliable component. In addition to improved properties, there is also a conservation of raw material.
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The advantage forging has over producing finished parts from plate or bar is typically that you've got more efficient use of the raw material. You are going to get a forge component that is much nearer to net-shape and there will be much less machining. But there are other metallurgical advantages as well. Now both bar and plate, by being in a rolling process, that rolling process is actually... Because it's deformation in a solid state, it is a forging process and it does improve from the original cast structure. But when you take the material and then go through an additional forging process, you're further refining the microstructure. You are causing the flow of the bar to then follow the contour of the part and you're producing a part that's going to have better fatigue properties and be a tougher, stronger part.
Well, when making the finished part, if one is machining the part from bar, you've got to realize that in that bar stock there will be longitudinal grain structure from rolling that bar or even the plate through the rolling process. Now when you go and take that raw material and start to machine away everything you don't need, all that excess material, you're actually going to be cutting through that grain flow. You will be disrupting that grain flow. If you start with a forging, you take that bar, you reform that flow, you reform all that microstructure so it follows the contour of the part, so that when you come back and machine, you're not machining through that structure in a way that would potentially weaken the part.
And of course, if you're starting with a forged part that already has the contour, is already near to net-shape, you're going to be conserving material that otherwise you would machine away. You would literally remove and put it in the scrap pile, instead of having it be in your component part. So the forging process is shaping that material closer to your finished part and therefore preserving raw material.
So, for instance, if you're having to cut contours out of a plate, there's all the drop between all of those pieces as well as all of the material that you're gonna machine away to create the finished component part. With bar stock, maybe you'll get a little more efficiency, but again, with a forging, you will get much nearer to net-shape. You'll have much less scrap and you will be more effective and efficient in producing the parts you want.