Why Cooling Things Quickly Makes It Permanently Harder?

[Carl Fredrik Ahl]

Hi,

I wonder why cooling things very quickly makes it permanently harder. Why won't it get back to normal hardness when the temperature goes up again?

Toughened glass used in car windshields is made by cooling molten glass very quickly to make it much harder.

[swansont]

Slow cooling allows certain structures to grow larger, especially if there is any kind of crystal structure to it. Quenching drastically shortens that time frame. You have to heat the material up past a certain point for this to work

https://en.wikipedia.org/wiki/Quenching

"A type of heat treating, quenching prevents undesired low-temperature processes, such as phase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable, and kinetically accessible; for instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness."

[Bufofrog]

8 hours ago, Carl Fredrik Ahl said:

Hi,

I wonder why cooling things very quickly makes it permanently harder. Why won't it get back to normal hardness when the temperature goes up again?

Toughened glass used in car windshields is made by cooling molten glass very quickly to make it much harder.

Glass is a bit different as to why it is quickly cooled to harden.  When glass is cooled rapidly the outside hardens first and then the inside of the glass cools and contracts.  This puts the surface of the glass into compression.  Glass breaks under tension, not compression, so the glass is much tougher.  Since there is so much stress in the tempered glass, when it does break it goes into a gazzilion (technical term) pieces.  Gorilla glass (think cell phones) is hard because large ions are exchanged for smaller ions in the glass and it again puts the glass surface in compression.  If you are familiar with Corelle brand dishes they are so strong because they are made with a glass laminate process and the inner laminate has a higher CTE so when the glass cools after the lamination step the inner glass shrinks more and places the surface in compression.

Tempered glass is not permanently hard, if the glass is taken up to the glass transition temperature the stresses will be relieved and the hardness will drop back to 'normal'.

I work for a glass/ceramic company and my wife is a glass technologist - could you tell? 

Edited February 20, 2019 by Bufofrog