|MadSci Network: Engineering|
Hi Jay, Good question. Changing the hardness or steel or glass will indeed change the density, but not significantly. Exact numbers would depend on the hardening process and how much of the part was affected by it. The two main methods for hardening (aka toughening) glass is tempering and second phase precipitation. Tempering is when you quickly cool (aka quench) the outermost layer to apply a compressive forces on the inner layer. This will prevent crack propagation through the material. This is different than a thermal anneal, which will heal cracks in the surface, increasing the strength of the glass. 2nd phase precipitation is when you process the glass in such a method as to leave zones or bubbles of different material in the glass matrix. The glass will then behave like a composite material, with the precipitates as obstacles to the cracks. In some cases, such as Corelle, the 2nd phase is "pressurized" so it will flow into a crack tip and blunt it. In either case, the local densities will be different with thermally hardened glass. If the piece is large enough, the overall density will not be significantly altered, but it depends on the size of the piece, and how much of it was affected by the process. Work hardening steel by hammering it will change the density about as much as tempering steel changes the density of glass. When you strain the steel, you move and tangle dislocations within grains. This prevents further strain, and results in a harder steel. Again, the density changes will be very small, and only on a local scale. (right where you hit the steel, and only a few hundreds of millimeters deep) This will probably not cause a measurable change if your part is big enough. If you heat treat the steel, then you have a higher chance of causing a density change. You can temper steel in the same fashion as glass, but you can also entirely change the internal structure by how you heat and cool it. Martensite vs. Austenite, Large grains vs small grains, and so on... Hope this helps. Jeff Yap Mad Scientist References: Reed-Hill, R; Abbaschian, R; Physical Metallurgy Principles; PWS Publishing; 1994 Varshneya, A; Fundamentals of Inorganic Glasses; Academic Press; 1994 Corning Glass Works
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