Week 6

Question 1

How does austenite grain size affect hardenability?

Answer 1

Larger grains increase hardenability by reducing nucleation sites for ferrite.

Question 2

Why do smaller austenite grains reduce hardenability?

Answer 2

More grain boundaries promote ferrite nucleation, shifting TTT/CCT curves left.

Question 3

Why isn’t austenite grain size used to increase hardenability in practice?

Answer 3

It reduces strength and toughness in the final product.

Question 4

Which elements are used to control austenite grain size?

Answer 4

Titanium (Ti) and Niobium (Nb) limit grain growth during reheating.

Question 5

How does carbon affect hardenability?

Answer 5

Higher carbon increases hardenability by stabilizing austenite and retarding ferrite formation.

Question 6

How does carbon affect martensite hardness?

Answer 6

More carbon increases strain in the bct lattice, raising hardness up to ~0.6 wt.% C.

Question 7

What issue arises with >0.6 wt.% C?

Answer 7

Ms falls below room temperature, causing retained austenite.

Question 8

How does increasing carbon content affect Ms and Mf?

Answer 8

Both are lowered, reducing transformation to martensite.

Question 9

Why can carbon reduce toughness?

Answer 9

Harder martensite becomes more brittle with increasing carbon.

Question 10

What are empirical relationships used for in steels?

Answer 10

To estimate martensite hardness based on composition and cooling rate.

Question 11

Why do empirical models fail at high carbon content?

Answer 11

They don’t account for retained austenite lowering hardness.

Question 12

What is the benefit of combining C with other alloying elements?

Answer 12

It enhances hardenability more effectively than carbon alone.

Question 13

Why is higher alloy content used in engineering steels?

Answer 13

To promote martensite formation and improve hardenability.

Question 14

What is the role of substitutional elements in hardenability?

Answer 14

They stabilize austenite, retard C diffusion, and reduce transformation driving force.

Question 15

How do austenite stabilizers increase hardenability?

Answer 15

By lowering transformation temperatures and pushing TTT/CCT curves down/right.

Question 16

How do ferrite stabilizers increase hardenability?

Answer 16

They form carbides that tie up carbon, slowing transformation to ferrite/pearlite.

Question 17

How does Ni affect hardenability?

Answer 17

Ni is an austenite stabilizer that lowers transformation temperatures and shifts TTT curves down/right.

Question 18

How does Mo affect hardenability?

Answer 18

Mo forms carbides that trap C, shifting TTT curves right and separating bainite/ferrite curves.

Question 19

Why does Mo not affect bainite transformation?

Answer 19

Bainite forms via displacive mechanism that doesn’t rely on C diffusion.

Question 20

How does Cr affect hardenability?

Answer 20

Cr delays ferrite/pearlite formation and increases bainite stability.

Question 21

What is separation of ‘C’ curves?

Answer 21

Distinction between ferrite/pearlite and bainite transformation regions in TTT/CCT diagrams.

Question 22

What is Di?

Answer 22

Ideal critical diameter representing hardenability under ideal quenching.

Question 23

How is Di estimated?

Answer 23

Using base hardenability (D₀,C) multiplied by factors for each alloying element.

Question 24

What standard defines Di estimation methods?

Answer 24

ASTM A255.

Question 25

Why are interstitial elements effective at increasing hardenability?

Answer 25

They are more soluble in austenite and segregate to grain boundaries.

Question 26

Why is boron especially effective?

Answer 26

It segregates to grain boundaries, suppressing ferrite nucleation.

Question 27

What happens if too much B is added?

Answer 27

It can excessively stabilize austenite and lower Ms below room temperature.

Question 28

Why is combining carbon with alloying elements more effective than just increasing carbon?

Answer 28

Carbon boosts hardness but alloying controls hardenability and phase transformation behavior.

Question 29

Why is boron effective even in very small amounts?

Answer 29

It segregates to grain boundaries and delays ferrite nucleation with minimal impact on toughness.

Question 30

What happens if Ms temperature drops below room temperature?

Answer 30

Martensite may not fully form, leading to retained austenite and reduced hardness.

Question 31

What is a practical downside of retained austenite?

Answer 31

It can transform during service, causing dimensional instability or cracking.

Question 32

What happens when multiple ferrite stabilizers are added?

Answer 32

They increase carbide formation and limit carbon mobility, further delaying ferrite/pearlite transformation.

Question 33

Why is molybdenum common in Cr-Mo steels?

Answer 33

It boosts hardenability and helps separate bainite from ferrite/pearlite curves.

Question 34

What issue can arise from excessive Mo addition?

Answer 34

It can promote hard, brittle phases and raise cost.

Question 35

Why does boron require careful control in steelmaking?

Answer 35

It’s highly potent in small doses, but excess can destabilize microstructure.

Question 36

Why is predicting Di useful for engineers?

Answer 36

It helps match steel grades to part sizes and cooling conditions in real-world processing.

Question 37

Why can empirical Di models be limited?

Answer 37

They assume uniform distribution and neglect real-world effects like segregation or geometry.

Question 38

What is the risk of high alloy content in weldable components?

Answer 38

Martensite formation in heat-affected zones can cause brittle welds.

Question 39

How do you mitigate brittle welds in high-hardenability steels?

Answer 39

Preheat, post-weld heat treatment, or use of filler metals with lower hardenability.