Transformations - Applications

Question 1

What is annealing?

Answer 1

A general term, most frequently used for heat treatments in the high austenite region where diffusion is very quick even over long distances. High tempearture, for a long time.

Question 2

What is normalizing?

Answer 2

Low austenite heat treating to produce a uniform fine-grained ferrite/pearlite structure.

Question 3

Define spheroidizing

Answer 3

Heat treatment just below the eutectoid temperature - it makes a coarse pearlite structure with low strength.

Question 4

What’s process/stress relief annealing?

Answer 4

Sub-austenitic heat treatment that induces recystalization or relaxes internal stresses via plastic flow or diffusion.

Question 5

Define hardening (in the context of heat treatment)

Answer 5

Formation of martensite (i.e. quench from austenite)

Question 6

Define tempering

Answer 6

A softening heat treatment done after hardening - it makes martensite less brittle and tough at the cost of some strength.

Question 7

Why would you want to anneal a component?

Answer 7

To eliminate solute gradients, relieve residual stresses, refine the grain size, and/ord improve ductility/toughness

Question 8

How does normalizing differ from annealing?

Answer 8

Normalizing is distinguished by a relatively fast cool to rom temperature - produces a finer grain size than annealing: gives a higher strength steel at the cost of some ductility/toughness.

Question 9

What’s autenitizing?

Answer 9

Heating steel to the austenite region and allowing it transform completely to austenite.

Question 10

Why might normalizing be preferable to annealing for a hypereutectic alloy?

Answer 10

Normalizing’s faster cool would prevent carbon from cementite that dissolved during austenisation from diffusing to form a brittle carbide network (which would severely reduce the steel’s toughness)

Question 11

What is the Hall-Petch effect?

Answer 11

Strengthening a material by decreasing its grain size (i.e. grain boundaries impede dislocation movement)

Question 12

Why is spheroidizing a subscritical anneal?

Answer 12

Because it takes place below the eutectoid temperature (727C) - there is no change in phase fractions.

Question 13

How did spheroidizing get its name?

Answer 13

Heating steel for a long time (>24hrs) allows carbon to diffuse significantly and iron-carbide (cementite) to form into spheres (to reduce their surface-to-volume ratio).

Question 14

What properties does spheroidizing achieve?

Answer 14

Low strength and hardness, substantially increased ductility (w.r.t. pearlite).

Question 15

Why is martensite heavily distorted?

Answer 15

Because it is supersaturated with trapped carbon.

Question 16

How does alloying steel with elements like Mn, Ni and Cr affect steel’s hardenability?

Answer 16

It improves it - these elements push the pearlite bay to longer times thereby making it easier to form martensite.

Question 17

What is austempering?

Answer 17

Heating steel to the austenite phase, then quench and holding to form bainite.

Question 18

What is case hardening used for?

Answer 18

Getting wear resisting benefits of martensite with the toughness of ferrite/pearlite in the same component.

Question 19

Give two methods for surface hardening.

Answer 19

Heat and quench a low-hardenability steel, carburization

Question 20

What is carburization?

Answer 20

Infiltrating carbon into a steel’s surface by heating the steel in a carbon-rich environment.

Question 21

Give the expression for the diffusion coefficient for carburisation, D

Answer 21

D = D_{0}exp(-Q/RT)

Question 22

What does the C_x in the expression pertaining to carburization stand for?

Answer 22

Carbon content at depth x

Question 23

How is carburisation depth defined?

Answer 23

The depth where the carbon content is equal to the mean of the surface and nominal carbon contents

Question 24

Give an expression for the case depth of a carburized steel, indicating the meaning of each variable

Answer 24

d = sqrt(Dt)
d = case depth
D = diffusion coeff. (D=D_0exp(-Q/RT))
t = carburization time