How does quenching affect the fracture energy of a material?

[961008]

Is it true that quenching will decrease the toughness of material and tempering the quenched material will then increase the toughness?
I need an explanation in term of the effect of quenching and tempering on the structure of the material.

 

[Kevin McHugh]

I am not sure if tempered materials are quenched first, but quenched materials will suffer negatively in terms of physical properties. Impact resistance, tensile and flexural strength will all be impacted. Quenching induces stress into the materials, and they crystallize before they have had a chance to relax and assume their lowest energy conformations. Those stresses can be relaxed upon subsequent heat history, as long as crystallization is allowed to take place in thermal equilibrium.