Re: Will changing the hardness change the density of glass or steel? If so why?

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