Author: Harold Sharon
In the early thirties along came a revolutionary invention: "safety" glass. Two flat layers of glass were bonded under heat and pressure to a very tough, and quite transparent, layer of plastic. The bond was so strong that as the glass flexed in a collision it broke into lots of small pieces, many of which stuck to the plastic. The passenger might have 500 fragments of glass imbedded in his skin, but no severed arteries. Quite an improvement. By the forties the industry learned how to make tooling to produce curved panels for, ugh, streamlined cars. It was expensive but immediately adopted by all manufacturers, so it was worth the cost.
The next development was an economical way to make distortion-free flat glass. "Float" glass is made by pouring molten glass onto a vat of molten tin. As long as the floor doesn't shake and the tin has a clean surface this is a less expensive way to make flat glass, as clear as ground plate. This glass isn"t really flat. It follows the curvature of the earth, (Have you heard his question, "does a train track flex when a fly lands on it...) It still has to be laminated for safety.
Next came "tempered" glass, which has been exposed at high temperatures to chemicals which affect a few thousandth of an inch on the surface, causing the molecules to expand. The thin layer of larger molecules is not strong enough to stretch the core much. Both sides are trying to expand. The core is in a mild state of tension. The skins are quite strongly in compression, because of their constrained urge to expand. When any point on the surface fails the core cracks through and the crack propagates in all directions at lightening speed. So, flat or curved, the glass shatters into tiny blunt pieces. If they fall on you no cuts, but if they stay in place you cant see through the cracks, which is why it ain't so good for windshields.,
The current windshield technology comes from Europe, It is laminated safety glass with another plastic skin on the inside to contain the fragments,
The other recent development is not exactly high tech, although "tempered" glass helped to bring it along. This great invention is thinner, therefore, lighter glass, It gives a lower center of gravity, better gas milage. Your old car glass should be 1. safe, 2 clear and free of distortion, 3 light weight, if your horsepower is as scant as mine ( That last comment from a man who never had a windshield) Replacing the original glass on a Model T sedan would make it safer, faster, more economical, less tippy. How could you not do it?
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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