Exam 2 Me 306 University Of Kansas

Front 1

________ - The brittle phase of Fe3C found in steel alloys

Back 1

cementite

Front 2

About 90% of all mechanical failure occurs by _______ _______.

Back 2

fatigue failure

Front 3

_______ - Simple carbon steel alloys with more than 0.022% C but less than 0.79% C

Back 3

hypoeutectoid

Front 4

_______ A steel microstructure composed of layers of alpha-iron and Fe3C

Back 4

pearlite

Front 5

The two prime factors that control crack propagation in brittle fracture are ______ ____ ______ and _______ _______.

Back 5

crack tip radius and crack length

Front 6

The temperature at which the ductility of many metals drops dramatically upon cooling is _________.

Back 6

DBTT - ?

Front 7

_________ - A metastable, brittle, body-centered tetragonal phase that can be formed in carbon steel materials

Back 7

martensite

Front 8

_________ - the phase formed by very rapidly quenching carbon steels to room temperature

Back 8

martensite

Front 9

___________ is continuous strain deformation that can occur under low stress at elevated temperatures, until failure occurs.

Back 9

creep

Front 10

A type of phase transformation in which one solid phase transforms into two different solid phases ________.

Back 10

eutectoid

Front 11

alpha iron, with a body-centered cubic crystal structure. It is the crystalline structure which gives steel and cast iron their magnetic properties.

Back 11

ferrite

Front 12

Also known as gamma-phase iron (y-FE), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Back 12

austenite

Front 13

acicular microstructure or phase morphology (not an equilibrium phase)

Back 13

bainite

Front 14

containing the minor component in excess of that contained in the eutectoid

Back 14

hyperuetectoid

Front 15

_____ _______

_____| |_______ fracture

Back 15

brittle failure

Front 16

specifies the temperature above which a material is completely liquid, and the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium.

Back 16

liquidus

Front 17

The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid.

Back 17

brittle fracture

Front 18

(of a state of equilibrium) stable provided it is subjected to no more than small disturbances.

Back 18

metastable

Front 19

lots of cystaline structure

Back 19

solidus

Front 20

A piece of non-alloy steel of eutectoid composition is rapidly cooled for 800 deg C to 575 deg C and held at this temperature. How long will it take for conversion of asustenite to start, when will it be 50% complete, and when will all austenite be gone? What Phases will be present and will the microstructure be?

Back 20

Conversion starts at 1 sec., is 50% completed at ~4 sec., and is complete at ~7 sec. (see heavy red dashed line and red arrows). The phases present will be ferrite (alpha-Fe and cementite (Fe3C), and the microstructure will be fine pearlite.

Front 21

A piece of non-alloy steel of eutectoid composition is rapidly cooled from 800 deg C to 575 deg C and held at this temperature for 10 seconds, and then is quickly cooled to room temperature. What will be present?

Back 21

The austenite will have converted to fine pearlite in 10 seconds, and upon quenching no further changes will occur, since the pearlite is composed of stable phases (see blue dashed line).

Front 22

A piece of non-alloy steel of eutectoid composition is rapidly cooled from 800 deg C to 575 deg C and held at this temperature for 100 sec. What will be present?

It is then reheated to 650 deg C and held for 25 hours before it is cooled to room temperature. What will be present.

Back 22

The austenite will have completely converted to fine pearlite (green arrow)

Holding for this extended period of time at an elevated temperature will cause the pearlite to convert to spheriodite (see green dashed lines).

Front 23

A piece of non-alloy steel of eutectoid composition is rapidly cooled from 800 deg C to 575 deg C and held at this temperature for 3-4 seconds. It is then rapidly cooled to 375 deg C and held at this temperature for 1000 seconds, and then cooled to room temperature. What will be present?

Back 23

In 3-4 seconds, 50% of the austenite will convert to fine pearlite. During the hold at 375 deg C, the remaining unstable austenite will convert to bainite. These are both composed of stable phases and will remain upon quenching to room temperature (see maroon red lines).

Front 24

A piece of non-alloy steel of eutectoid composition is rapidly cooled from 800 deg C to 575 deg C and held at this temperature for 3-4 seconds. It is then very rapidly cooled to room temperature. What will be present?

If this piece is now heated back to 650 deg C and held for 24 hours, what will be present?

Back 24

In 3-4 seconds, 50% of the austenite will convert to fine pearlite. Upon quenching, the remaining unstable austenite will convert to martensite. Thus, the material will be 50% fine pearlite and 50% martensite (see purple line).

At this long hold at elevated temperature, the pearlite will convert to spheroidite and the martensite will convert to tempered martensite.

Front 25

A piece of non-alloy steel of eutectoid composition is rapidly cooled from 800 deg C to 300 deg C and held at this temperature for 10 sec. It is then very rapidly cooled to room temperature. What will be present?

If this piece is now heated back to 650 deg C and held for 24 hours, what will be present?

Back 25

The hold at 300 deg C is too short to allow any transformation to start, so the material remains 100% unstable austenite. Upon quenching to room temperature, the austenite converts to mairtensite (see lighter weight dashed red lines)

The martensite will convert to tempered martensite during the long hold at high temperature.

Front 26

The conversion of austenite to pearlite starts much sooner at 575 deg C than at 675 deg C, even though diffusion and groth rates are much higher at 675 deg C. At the higher temperature, very coarse grain pearlite forms while at lower temperature the pearlite is fine grain. Explain why these phenomena occur.

Back 26

Conversion of austenite to pearlite requires both the nucleation and the growth of ferrite and cementite grains. At 575 deg C the nucleation rate is much higher than it is at 675 deg C, so more nuclei form and form much sooner. Thus, conversion starts sooner at the lower temperature. The growth rate is higher at 675 deg C, but there are fewer nuclei upon which crystals can grow, so the number of grains grow to large size, giving a coarse (large grain) structure.

Front 27

Discuss the curves shown below and their significance in selecting steel alloys

Back 27

A number of metals, but not all, exhibit a sometimes dramatic decrease in impact energy (toughness) as temperature is decreased. The temperature at chich this occurs is called the ductile-to-brittle transition temperature, or DBTT. Common carbon steels are included in this group. As shown above, low carbon plain steels exhibit high impact energies above roughly -50 deg C but below such temperatures have lost most of this toughness and exhibit very low impact energy values. Higher carbon steels are stronger but less tough, and at temperatures in the 0-150 deg C range lose most of their toughness. The higher carbon steels are, of course, stronger and less ductile than the low carbon steel alloys. Steels commonly used in many applications, such as ships, loose much of their toughness at temperatures around the freezing point of water, going from tough to brittle. When these steels are subjected to temperatures that are not uncommon in the environment, they can be brittle enough to fail in brittle fracture, and this behavior must be considered in engineering design.

Front 28

the fatigue life for each alloy at 150 MPa

Back 28

Red Brass: 10^5 cycles

Al Alloy: ~ 3*10^7 cycles

1045 steel: infinite number of cycles (theoretically)

(see red dashed lines)

Front 29

The fatigue life for the steel and aluminum alloys at 350 MPa

Back 29

Steel: ~ 5*10^5 cycles

Aluminum: 10^4 cycles

(blue dashed lines)

Front 30

The fatigue strength (stress amplitude) for each alloy at 300,000 cycles

Back 30

Red Brass: ~ 140 MPa

Aluminium: ~ 240 MPa

Steel: ~ 360 MPa

Front 31

The fatigue strength for each alloy at 50,000,000 cycles

Back 31

Red Brass: ~ 85 MPa

Aluminum: ~ 310 MPa

Front 32

The fatigue limit for each alloy

Back 32

Only the 1045 steel shows a distinct fatigue limit at about 310 MPa. We can assume, with no evidence to the contrary, that the curves for brass and aluminum will continue to slowly decrease.

Front 33

Back 33

Front 34

a. What is the probability of fracture at 500 MPa?

b. What is the probability of fracture at 600 MPa?

c. What is the probability of fracture at 700 MPa

Back 34

a. ~0.0005, or 0.05%

b. ~0.0043, or 0.43%

c. ~0.0053, or 0.53%

Front 35

At 1400 deg C

% solid/liquid phase

Back 35

~ 77% solid phase, ~ 33% liquid phase

Front 36

%Liquid

Back 36

[%solid phase-50] / [%solid phase-%liquid phase]

Front 37

%Solid

Back 37

[%liquid phase-50] / [%solid phase-%liquid phase]

Front 38

T/F

A slip system is the combination of slip planes and slip directions on those planes.

Back 38

True.

Front 39

T/F

The stress level at fatigue failure is usually much lower than the failure stress for a static load.

Back 39

True.

Front 40

T/F

For metals, isomorphous binary phase diagrams are relatively rarely encountered

Back 40

True.

Front 41

T/F

Once a metal is cold worked, the mechanical properties are permanently changed and cannot be restored back to their original pre-cold worked values.

Back 41

False.

Front 42

T/F

In the iron-ion carbide system, cementite (Fe3C) is a hard, brittle compound.

Back 42

True

Front 43

T/F

There is good correlation between harness of a metal and its tensile strength.

Back 43

True.

Front 44

T/F

The crystalline structure of a metal influences whether or not a ductile-to-brittle transition behavior with decreasing temperature occurs.

Back 44

Ture.

Front 45

T/F

Fatigue failure is not a particularly common type of mechanical failure.

Back 45

FALSE!!!

Front 46

Recrystallization of a metal occurs more rapidly at lower recrystallization temperatures.

Back 46

FALSE!!!

Front 47

In General, equilibrium phase diagrams are excellent predictors of results for actual materials heat processing, since equilibrium is usually established very quickly at elevated temperatures and the diagrams show all phases that can occur.

Back 47

FALSE.

Front 48

Which of the following apply to the Titanic disaster?

a. the prime cause of the sinking was brittle steel.

b. the prime cause of the sinking was a collision with an iceberg.

c. the builders learned from the Titanic and built the sister ship Olympia with better steel.

Back 48

b.

Front 49

Phase Diagrams

a. Show all the phases that could ever appear during heat treatment of metals.

b. need to be augmented with transformation diagrams in order to understand heat treatment

c. Can be drawn for up to three components

d. Can be drawn for up to four components

Back 49

b. and c.

Front 50

The ductile-to-brittle transition temperature

a. Is higher for low carbon steels

b. Is higher for high carbon steels

c. was a major factor in the Titanic disaster

Back 50

b.

Front 51

Creep can occur

a. in ceramics at elevated temps

b. in metals at elevated temps

c. in most metals at room temps

d. is an interesting phenomenon but of little practical importance

Back 51

a. and b.

Front 52

Transformation rates:

a. always increase as temp decreases

b. always increase as temp increases

c. can either increase or decrease as temps change - depends on type of transition.

Back 52

c.

Front 53

List four strategies for strengthening metals and briefly describe the strengthening mechanism for each

Back 53

1. Reduce grain size - this increases the amount of grain boundary area, which makes dislocation movement more difficult and thus increases strength.

2. Produce a solid solution - introducing a second element, even one with very similar atomic size, hinders dislocation movement and thus increases strength.

3. Introduce a precipitated phase, called precipitation strengthening - very effective in blocking dislocation movement and can greatly increase strength

4. Cold work the metal - this introduces very large number of new dislocations (dislocation density goes up), which pile up and interfere with dislocation movement. This increases strength.

Front 54

Discuss the mechanism involved in overageing

Back 54

In overageing the precipitated phase particles grow larger, so there are fewer but larger grains of the precipitate. It is thus less able to block dislocation movement and slip on slip planes, so that the metal becomes less strong and more ductile.