Forgings, and Why They`re Better
Author: Harold Sharon
Almost every metal product we use has a grain structure. This structure usually isn't visible to the eye as it is in wood, but the effect the grain direction has on the metal is pronounced. We're all familiar with wood grain, and would never expect a floor beam or a diving board to survive if made with "cross grain” wood.
Engineering metals start life as cast ingots, typically round or rectangular bars. As the cast ingot solidifies from molten metal, grains first appear at the outside, where it's coolest, and gradually grow inward. This ingot, then, is substantially "cross grained'”. Likewise, if the metal is poured into a mold to cast a finished product, the product is substantially cross grained. Examples are door handles, motor blocks, aluminum wheels, and transmission casings. Their strength properties are inferior to similar products in which the grains are coerced into alignment along the direction of the part. Forging is this coercive force.
Heating an ingot to “red heat” softens the metal to where modest force can shear and deform the metal. If the deformation tool is a strong steel die with an impression carved in it, like the shape of a Model A Ford front axle, as the metal squishes and flows to fill the die, grains break, shear, roll, and generally align themselves along the shape of the part. The resulting part is stronger but more importantly, more ductile. This just means it'll bend or stretch more before breaking. If the car hits a severe bump like a curb, the difference between bending and breaking an axle could mean an outcome difference like life or death to the occupants of the car. The difference between bending and breaking a connecting rod could mean a new block and crank vs. a modest repair job.
There are other ways to forge besides pounding in an "impression" die. Rolling between cylindrical rollers to reduce the casting to flat plate or sheet produces the very strong and ductile sheet metal fenders. Rolling between form rollers produces products like I beams and railroad tracks, both of which are strong and ductile in the direction of usual loading. Music wire is "drawn", which means it’s pulled through a hole smaller than the wire diameter (usually at room temperature). This forging process gives exquisite grain refinement, alignment and enormous strength along the wire length. The tensile strength of this wire transverse to the wire axis is very modest, but that's ok because we don't load the wire in that direction.
Incidentally, forging does not have to be done at red heat If the alloy is mild enough, it can be forged at room temperature. Nail and some bolt heads are forged cold.
In a nutshell, forging enhances the properties of a part in some directions. If the parts are properly used and we don't ask the impossible of the part in its inferior direction, we reap the benefits..
Heat "treating” can easily double or triple the strength of a part but it can't enhance the properties in one direction. Forging and heat treating are the best of what’s obtainable and that's what's used for highly loaded parts. Prime examples are coil and leaf springs. In these application castings wouldn't be worth a dime if they were not both forged and heat treated. They would be ductile but not strong enough to hold the car up. You know how good springs are when they are both forged and heat treated.
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