ME Metallurgy and Phase Diagrams 2

Question 1

Tempering of martensite is performed to:

Accepted Answer

Reduce brittleness while accepting some reduction in hardness

Answer

Tempering reheats quenched martensite to a temperature below the eutectoid to allow some carbon diffusion, reducing brittleness while sacrificing some hardness.

Question 2

Austenite in iron-carbon alloys has which crystal structure?

Accepted Answer

Face-centered cubic (FCC)

Answer

Austenite (gamma-iron) has an FCC crystal structure, which can dissolve significantly more carbon (up to 2.14 wt%) than the BCC ferrite phase.

Question 3

In the TTT (Time-Temperature-Transformation) diagram, the 'nose' of the curve represents:

Accepted Answer

The temperature and time of fastest austenite decomposition

Answer

The nose of the TTT curve is where decomposition of austenite is fastest because both the thermodynamic driving force and atomic diffusivity are balanced.

Question 4

Bainite microstructure in steel is produced by isothermal transformation of austenite at temperatures:

Accepted Answer

Between the nose of the TTT curve and the martensite start temperature

Answer

Bainite forms by isothermal transformation of austenite at temperatures between the pearlite nose and the martensite start (Ms) temperature.

Question 5

Which alloying element is added to steel primarily to improve hardenability?

Accepted Answer

Chromium

Answer

Chromium (and elements like molybdenum, nickel, and manganese) shift the TTT curve to the right, improving hardenability by allowing martensite formation at slower cooling rates.

ME - Materials Engineer Practice Test

ME - Materials Engineer Practice Test

ME Metallurgy and Phase Diagrams 2