Surface Hardening Review Questions

Question 1

Why are Metals heat treated

Answer 1

To effect microstructural changes in a material which in turn afect its mechanical properties

Question 2

Identify the important reasons why metals are annealed.

Answer 2

• 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

Question 3

What is the most important heat treatment for hardened steel?

Answer 3

Martensite transformation by heating steel into the austenite region and quenching

Question 4

What is the mechanism by which carbon strengthens steel during heat treatment?

Answer 4

Martensite Formation

Extreme hardness from martensite results from lattice strain created by carbon atoms trapped innthe body-centered tetragonal structure, thus providing a barrier to slip

Question 5

What is conveyed in the TTT Curve

Answer 5

Time-Temperature-Transformation curve indicates what phases in the iron-carbide phase diagram will be produced under various conditions

Question 6

What function is served by tempering of martensite

Answer 6

Heat treatment applied to hardened steels to reduce brittleness, increase ductility and toughness, and relieve stresses in the martensite structure

Question 7

Define hardenability

Answer 7

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

Question 8

Name some elements that have the greatest effect on the hardenability of steel

Answer 8

chromium

mangnesium

molybdenum

nickel

Question 9

Indicate how hardenability alloying elements in steel affect the TTT curve

Answer 9

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

Question 10

Define precipitation hardening

Answer 10

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

Question 11

How does carburizing work

Answer 11

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