PAST QUESTIONS AND PROBLEMS 2012-2013: I. True or False

F The solute concentration is lower in those regions that solidify last in a casting.

-Sections which freeze last contain higher solute concentration (since the liquid becomes progressively richer in solute as freezing progresses) resulting to solute segregation in the finished casting

T Strengthening of alloys in precipitation hardening is believed due to coherent precipitates capable of being sheared by matrix dislocations.

F The higher the working stress though below the elastic limit, the bigger the relative size of the fatigue zone in fatigue failure.

- High stress and/or low fracture toughness promote small fatigue zoneT The higher the temperature and the higher the stress, the faster the creep rate.

T Without the surface indentation which served as a stress raiser in a normally ductile material, it would not have fracture in a brittle manner.

T The degree of supercooling is lesser in heterogenous than homogenous nucleation.

F The low expenditure of energy in a brittle fracture results to a pronounced macroscopic gross plastic deformation.

-no appreciable macroscopic gross plastic deformation

T In precipitation hardening, hardness increases with time during the solution treatment.

T For materials loaded in tension, a tensile residual stress is far detrimental compared to compressive critical radius.

T In a vapor to liquid transformation, the maximum free energy of the liquid droplet occurs at the critical radius.

The structure of martensite is closer to ferrite than austenite.

F Steel of high hardenability is suitable for welding

-no kay brittle ang steels with high hardenability

F A normalized pearlitic structure has a lower hardness than a spheroidized carbide structure.

- spheroidized carbide structure is the softestF Cobalt is added to steel to increase hardenability.

-cobalt decreases the hardenabilityT The resulting structure of austempering is bainite.

T Process annealing of steels is done below the eutectoid temperature.

F The higher the working stress though below the elastic limit, the bigger the relative size of the fatigue zone in fatigue failure.

T The higher the temperature and the higher the stress, the faster the creep rate.

T Without the surface indentation which served as a stress raiser in a normally ductile material, it would not have fracture in a brittle manner.

F The low expenditure of energy in a brittle fracture results to a pronounced macroscopic gross plastic deformation.

T For materials loaded in tension, a tensile residual stress is far detrimental compared to compressive critical radius.

T The resulting structure of austempering is bainite.

T Process annealing of steels is done below the eutectoid temperature.

F The quenched structure of martensite is closer to the tetragonal rep. of aus. than ferrite.-ferrite than austenite

T The coarse aus. grain size is recommended to inc. hardenability

T The higher the carbon content of mart., the higher the hardness.

T As long as the cooling rate is slower than the critical, a certain amount of pearlite is always produced.

F Steel of high hardenability is suitable for welding

F A normalized pearlitic structure has a lower hand than a spheriodized carbide structure.

T Agitation inc. severity of the quench

F An inc. conc. Of any alloying element dec. the pearlite interlamellae spacing.

-replace any with specificT The addition of alloying elements would achieve hardening with a less rapid quench

F Partition of alloying elements in pearlitic growth is favored by a large supercoling.

-partitioning is favored by low degree of supersaturation or small undercooling

II. Multiple Choice 1. Which is a characteristic of a precipitation hardenable alloy? a. Precipitates impenetrable by matrix dislocations

b. Solvus line indicating partial solubility which is decreasing with decreasing temperature

c. Strength is increased by cold working

d. All of the above

2. The temperature in which freezing is completed under non-equilibrium conditions. a. Same as equilibrium conditions

b. Higher than equilibrium condition

c. Lower than equilibrium conditions

d. Constant from start to completion of freezing

3. An iron aluminum specimen fracture along the grain boundaries without microvoid coalescence, what is the fracture mechanism? a. Ductile transgranular

b. Brittle transgranular

c. Ductile intergranular

d. Brittle intergranular

4. A nucleus is considered stable and capable of growing because a. Its free energy decreases as it further grows

b. Its size is at least equal to the critical radius

c. The negative volume energy is greater than the positive surface energy

d. All of the above

5. An instantaneous failure mode a. Fatigue

b. Creep

c. Ductile

d. None of the above

6. Aluminum alloy rivets for use in the aircraft industry are stored in deep freeze refrigerators a. For solution treatment

b. To attain maximum hardening

c. To prevent hardening

d. Just for temporary safekeeping

7. The initiation zone of a fatigue fracture a. Characterized by an overload fracture surface

b. Its local stress is above the yield stress

c. Characterized by the presence of beach marks

d. All of the above

8. Temperature inversion during freezing is characterized as a. Release of heat of fusion at the interface

b. Temperature decreases in a direction away from the interface

c. Dendrites should forward in advance of the interface

d. All of the above

9. Strain energy in solid solid transformation a. Function of the surface area of the solid precipitate

b. Increases with decreasing lattice mismatch between precipitate and matrix

c. Strains both the precipitate and matrix, thus, opposing the formation of a solid nucleus

d. None of the above

10. Factors simultaneously present in low carbon fine-grained steels to cause brittle instead of a ductile fracture. a. Low temperature

b. Stress concentration due to manufacturing defects and service conditions

c. A relatively high tensile stress to cause microscopic plastic deformation

d. All of the above

11. The crystal structure of martensite a. FCC

b. BCC

c. BCT

d. Orthorhombic

12. Surface tensile stresses lead to quench cracking due to a. Carbon diffusion in the austenite to martensite transformation

b. Thermal contraction prevailing over martensitic expansion at the surface

c. Martensitic expansion prevailing over thermal contraction in the interior

d. None of the above

13. The stress below which no matter how many times the working stress is applied does not result to failure a. Fatigue limit

b. Fatigue strength

c. Maximum stress

d. None of the above

14. Which is not hardening by diffusion treatment? a. Carburizing

b. Carbonitriding

c. Nitriding

d. Full annealing

15. Increasing the carbon content of steels a. Lowers the Ms

b. Lowers the Mf

c. Increases hardness

d. All of the above

16. Which is not a heat treatment intended for softening? a. Spheroidizing

b. Process annealing

c. Flame hardening

d. None of the above

17. Factors simultaneously present in low carbon fine-grained steels to cause brittle instead of a ductile fracture a. Low temperature

b. Stress concentration due to manufacturing defects and service conditions

c. A relatively high tensile stress to cause microscopic plastic deformation

d. All of the above

18. Critical diameter is a function of

a. steel composition

b. austenitic grain size

c. severity of the quench

d. all of the above

19. Tempered martensite has a structure of

a. cementite spheroids in a matrix of ferrite

b. martensite spheroids

c. pearlite with a proeutectiod constituent depending on the carbon content

d. none of the above

21. Much larger pearlite nodules form just below the eutectoid temp. bec. of a

a. relatively high nucleation rate

b. max. growth rate

c. relatively low nucleation rate

d. either of the above

22. For steels, continuous cooling is characterized asa. pearlite transformation faster than isothermal cooling

b. no bainite is formed for the slowest cooling rate

c. a negligible amount of pearlite is formed for the most rapid cooling rate

d. none of the above

23. The most severe quenchant

a. Oil

b. water

c. brine

d. neither of the above

24. For a hypo eutectoid steel, non equilibrium cooling results to the ff.

a. carbon content of pearlite lowered

b. amount of the pro eutectoid ferrite is suppressed

c. at room temp. two constituent are present

d. all of the above

25. The austenite to martensite transformation

a. goes to completion at room temperature regardless of composition

b. athermal and diffusionless

c. the c-axis decreases with inc. carbon content

d. all of the above

26. A quench crack is a result of the surface tensile stressed due to

a. carbo diffusion in the austente to martensite

b. thermal contraction prevails over martensitic expansion in the interior

c. martensitic exp. Prevails over thermal contraction in the interior

d. thermal conctraction prevails over mary.expan. in the surface.

An iron aluminum specimen fracture along the grain boundaries without microvoid coalescence, what is the fracture mechanism?

Ductile transgranular

Brittle transgranular

Ductile intergranular

Brittle intergranular

An instantaneous failure mode

Fatigue

Creep

Ductile

None of the above

The initiation zone of a fatigue fracture

Characterized by an overload fracture surface

Its local stress is above the yield stress

Characterized by the presence of beach marks

All of the above

Factors simultaneously present in low carbon fine-grained steels to cause brittle instead of a ductile fracture.

Low temperature

Stress concentration due to manufacturing defects and service conditions

A relatively high tensile stress to cause microscopic plastic deformation

All of the above

Surface tensile stresses lead to quench cracking due to

Carbon diffusion in the austenite to martensite transformation

Thermal contraction prevailing over martensitic expansion at the surface

Martensitic expansion prevailing over thermal contraction in the interior

None of the above

The stress below which no matter how many times the working stress is applied does not result to failure

Fatigue limit

Fatigue strength

Maximum stress

None of the above

Which is not hardening by diffusion treatment?

Carburizing

Carbonitriding

Nitriding

Full annealing

Which is not a heat treatment intended for softening?

Spheroidizing

Process annealing

Flame hardening

None of the above

A proeutectoid nucleus grows in the direction of

Both adjacent austenite grains

The austenite grain defining the habit plane

The other austenite grain with the non-coherent high energy boundary

None of the above

The phase transformation when austenite transforms to ferrite and cementite

Eutectic

Eutectoid

Peritectic

None of the above

In what type of steel is the carbon content of austenite enriched upon cooling

Eutectoid

Hypoeutectoid

Hypereutectoid

All of the above

After the quench in precipitation hardening, the alloy is

Supersaturated with solute

Supersaturated with vacancies

Precipitates do not form yet

All of the above

A nucleus is considered stable and capable of growing because

Its total free energy decreases as it further grows

Its size is at least equal to the critical radius

Absolute value of volume energy is greater than the strain energy

All of the above

Aluminum alloy rivets incorporated in an aircraft are stored in deep-freeze refrigerators prior to use

For solution treatment

To attain maximum hardening

To prevent hardening

Just for temporary safekeeping

Which is a characteristic of a precipitation hardenable alloy?

Precipitates impenetrable by matrix dislocations

Solvus line indicating solute partial solubility decreasing with decreasing temperature

Strength is increased by cold working

None of the above

In order to minimize the strain energy, an incoherent nucleus blablabla the shape of a

Needle

Sphere

Disk

Any shape

Near the transformation temperature, the nucleation current is controlled by

Rate of jumping of atoms towards the nucleus

Energy barrier to form the nucleus

No. of embryos at the transformation temperature

All of the above

Consider the interference of adjacent growing precipitate particles, it is characterized by

Fall of solute concentration in the matrix far away from the particles

Concentration gradient adjacent to the particles remain the same

Growth of precipitates increase with time

All of the above

III. Fill in the blanks (20 points) 1. ______________ Creep? Temperature at which steels pass from ductile to brittle fracture.

2. Homogenization Heat treatment to remove segregation or differences in solute concentration in a single - phase cored casting.

-Homogenization of the cored structure by diffusion in solid-state involves reheating the alloy to a temperature below the solidus line allowing a more rapid rate of diffusion and homogenization

3. creep Occurs under sustained loading at a constant temperature above one half of the melting point in 0K.

4. fatigue limit The stress below which the life of a fatigue specimen seems to become infinite.

5. ______________ A very narrow zone of constitutional supercooling (microsegregation) as a result of a stable interface movement.

6. ______________ Spherical bubbles trapped inside the casting.

7. ______________ The strain prior to primary creep in a creep curve.

8. ______________ Type of boundary between precipitate and matrix which contains dislocations to counteract the elastic strains.

9. incubation period The period before actual precipitation starts in precipitation hardening.

10. heterogeneous nucleation Type of nucleation due to the presence of accidental impurity particles such as occurring at the mold surfaces.

11. incubation period. The period before actual precipitation starts

12. 6.7 % - Carbon content of cementite

13. Pearlite - Alternating lamellae of ferrite and cementite

14. martensite - Transformation product of austenite after a water quench

15. age hardening. Another name for precipitation hardening

16. guinier preston zones (GP zones). Form in the initial development of the precipitates involving the local clustering of blablablabla blablabla

17. growth. Occurs after a group of atoms has exceeded the critical rise and becomes a stable nucleus

18. volmer-weber theory. The theory relating the number of embryos to the number of atoms in an embryo in the supersaturated blablabla

19. diffusion controlled. Type of growth dependent on the migration of solute atoms from the matrix to the precipitate

20. interphase controlled. Type of growth dependent on the ability of solute atoms to cross over from the blablabla to the precipitate

21.high speed steels - type of steals which undergo secondary hardening due to carbide forming elements

22.50% martensite and 50% pearlite - the microstructure used as a criterion for hardenability

23.n=2^n-1 - equation for grain size number

24.hadfield Mn steel a very tough, hard and abrasion-resistant metal for buckets and teeth of power shovels

25.Mgo Temperature which indicate 90% of martensitic transformation

26.bainite Microstructure in steels which forms above the Ms temperature

27.c-65 Rockwell hardness of 100% martensite

28.ideal critical diameter Hardenability depth referred to hypothetical cooling medium

29.cobalt The only alloying element added to steel known to decrease hardenability

30.04%C The minimum amount of carbon needed for a marked degree of hardening

IV. Matching Type 1. Titanium alloys - Find applications requiring high strength-to-weight ratio

2. Solution heat treatment - Another term for precipitation hardening often employed with aluminum alloys

3. Process annealing - HT to restore ductility of a material while being worked at

4. Cyaniding - HT involving carbon and nitrogen diffusion into the surface layer of steel conducted in a liquid salt bath

5. Carbonitriding - HT involving the simultaneous absorption of carbon and nitrogen for steels heated in a gaseous atmosphere

6. Carburizing - HT to increase surface hardness of low carbon steels by carbon diffusion followed by quenching and tempering

7. High-speed steels- Type of steels which undergo secondary hardening due to carbide forming element

8. Hadfield manganese steels - A very tough, hard and abrasion-resistant metal for buckets and teeth of power shovels

9. Creep - Occurs under sustained loading particularly at high temperatures

10. Overload zone - Instantaneous failure in fatigue fracture

V. Essay At the microscopic level, briefly discuss the mechanisms of ductile fracture (10 points).

Examination of the fracture surface at a high magnification using a scanning electron microscope (SEM) reveals a dimpled surface. Under a normal tensile stress, these dimples (Figure 3 left) are usually round or equiaxed (having the same dimensions in all directions) while if shear stress has been dominant, the dimples are oval-shaped or elongated, with the ovals pointing towards the origin of fracture (Figure 3 right).

2. In a recent warehouse fire, a shipment of aircraft rivets was subjected to heat which was intense enough to overage the rivets but not intense enough to affect their size, shape or appearance. The quality control engineer of the aircraft manufacturer rejected the rivets for use. If you were their metallurgical engineer and the president of the firm asked you to comment, would you consider the rivets scrapped? Is there an alternative? (20 points)

No. There is another alternative since the size, shape and appearance is not affected. The overaged rivets can again undergo precipitation hardening. In so speaking, it can undergo again solution treatment in which the rivets will be heated above its solvus temperature until it is homogenized. It will be then quenched to retain its homogeneous structure but now it is supersaturated with its unprecipitated component. It will be heated again in temperature below its solvus curve for the precipitate to form. So as not to be overaged again, the temperature should be moderate to facilitate faster diffusion and nucleation, achieving maximum strength just needed to its purpose. In summary, it will undergo three process, solution treatment, quenching and aging process to produce the desired property of rivets needed.

Why is there a need for tempering? Discuss the effect of temperature and time on the properties of tempered steels? (10 points)

What is the common principle behind flame induction and laser and electron beam hardening? Would you recommend a 0.1% C iron-carbon alloy to be subjected to either type of heat treatment? (10 points)

Flame hardening is a type of surface hardening. Surface hardening is the process of hardening the surface of a metal object while allowing the inner metal to remain soft. In order for the process of hardening the steel to occur, you need to make sure there are at least 0.3% C and preferably more than 0.35% C but not greater than 0.55% C. Induction hardening on the other side is also a process used for surface hardening of steel and other alloy components. Carbon and alloy steels with carbon content range of 0.40% - 0.45% are most suitable for this process. And Laser and Electron Beam hardening are two of the engineering methods also for surface hardening of steels that required applied energy. The process is only applied for 0.4 % C to 0.6%C. As discussed above, 0.1% C cannot be applied in any of the three heat treatment processes because of its low Carbon content. The heat treatment that I can recommend is the Carburizing or Case Hardening. It is a heat treatment process that produces a surface that has high wear resistance yet maintaining toughness and strength at the core. It is only applied to low carbon steels, maximum o.2% C, due to its reliance to diffusion of carbon atoms.

5. The continuous transformation diagram for a 1.13 wt. % C iron-carbon alloy is shown below. Redrawn in your test booklet, sketch and label the continuous cooling to yield the following microstructures: (10 points)

a. Fine pearlite and proeutectoid cementite

b. Martensite

c. Martensite and proeutectoid cementite

d. Coarse pearlite and proeutectoid cementite

e. Martensite, fine pearlite and proeutectoid cementite

6. With the aid of appropriate diagram(s), explain that for solid state reactions the elastic strain energy makes nucleation more difficult.

A bigger G is needed to overcome due to the presence of strain energy. As shown in the diagram, nucleation is more difficult in solid state reaction because of the presence of strain. Strain energy and surface energy opposes the formation of nuclei, represented by the equation (above). The absolute value of gs for a nucleation to be possible. With the presence of strain energy, larger free energy must be overcome for a certain embryo to grow, to its critical radius. In summary, strain energy adds up the G that must be overcome for a critical radius and nucleus to grow. Therefore, the presence of strain energy makes the nucleation more difficult.

8. Given hypoeutectoid steel (0.4 % C) to undergo equilibrium cooling, what phases, composition, and amount of each phase would exist slightly above and below the eutectoid temperature? After cooling to room temperature, what constituents are found in the structure? Give the amounts and compositions of each; also, indicate the amount of proeutectoid and eutectoid ferrite.

Compare the diagram of quenching and tempering, martempering and austempering.

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