Why are Metals heat treated
To effect microstructural changes in a material which in turn afect its mechanical properties
Identify the important reasons why metals are annealed.
- reduce hardness and brittleness
- alter microstrucure so that desirable mechanical properties can be obtained
- soften metal for improved machining or formability
- recrystallize cold-worked metals
- relieve residual stresses from prior processes
What is the most important heat treatment for hardened steel?
Martensite transformation by heating steel into the austenite region and quenching
What is the mechanism by which carbon strengthens steel during heat treatment?
Extreme hardness from martensite results from lattice strain created by carbon atoms trapped innthe body-centered tetragonal structure, thus providing a barrier to slip
What is conveyed in the TTT Curve
Time-Temperature-Transformation curve indicates what phases in the iron-carbide phase diagram will be produced under various conditions
What function is served by tempering of martensite
Heat treatment applied to hardened steels to reduce brittleness, increase ductility and toughness, and relieve stresses in the martensite structure
Hardenability refers to the relative capacity of a steel to be hardened by transformation to martensite
Determines the depth below the quenched surface to which steel is hardened or the severity of the quench required to achieve a certain hardness penetration
Name some elements that have the greatest effect on the hardenability of steel
Indicate how hardenability alloying elements in steel affect the TTT curve
Alloying elements extend the time before the start of austenite-to-pearlite transformation in the TTT curve diagram
TTT Curve is essentially moved to the right, permitting slower quench rates during quenching
Define precipitation hardening
Formation of fine particles that act to block the movement of dislocations and thus strengthen and harden the metal
Primary heat treatment or alloys of nonferrous metals
How does carburizing work
Most common surface-hardening teatment
Heating a part of low carbon steel in the presence of a carbon rich environment so that carbon is diffused onto the surface.
Surface is converted into high carbon steel