Explain why when we heat treat materials like we did with st

Explain why when we heat treat materials (like we did with steel in lab) that the modulus of elasticity stays (for the most part) constant for every heat treatment but yield stress varies wildly.

Solution

Yield stress:

Ordinary materials (steel for instance) contain defects (points, lines and walls) that produce internal stresses. Heat treatments may modify the density and arrangement of defects thus changing the magnitude of the overall internal stress. So yield stress changes during heat treatment

Modulus of elasticity:

Plastic properties of metals, which are heavily influenced by dislocations, are significantly influenced by heat treatment. These processes change the structure, and the dislocations.

Elastic properties on the other hand are affected by the interactions between the subatomic particles. Heat treatment does not change the atoms themselves - only the arrangement. So modulus of elasticity stays constant for heat treatment.