Study #3

During this type of nucleation, nuclei form at stable "nucleating surfaces "such as the wall of molds and impurities in the liquid phase.

heterogeneous nucleation

If you wanted to cool a liquid metal and form large, coarse grains what would you do?

small supercooling - slow nucleation rate - large grain growth

How is transformation rate defined (by convention)?

rate = 1/t0.5

Name the 3 classifications of phase transformations.

1. simple diffusion dependent transformations

2. diffusion dependent transformations

3. diffusion less transformations

What is the of critical radius a nuclei?

The radius above which a nuclei will grow and below which it will shrink.

Which microstructure of steel is metastable?

martensite

The critical cooling rate is the minimum rate of cooling (i.e., quenching) required to produce a steel microstructure that is 100% austenite. (True/False)

True

Which of the following is the hardest microstructure:
● spheroidite
● martensite
● bainite
● fine pearlite

martensite

If you wanted to create 100% coarse pearlite, how would you do it? What is going on at the atomic level?

Rapidly cool from above the eutectoid temperature to 650 C (ish) and hold at that temp until the phase transformation is complete (small supercooling). Carbon has time to diffuse longer distances so thicker layers of ferrite and cementite can form.

Describe what the addition of alloying elements such as chromium does to steel's isothermal phase transformation diagram?

1. shifts the transformation from austenite to pearlite to longer times

2. creates a separate nose for the formation of bainite

Rank the quenching media from most rapid cooling to least rapid.

water, oil, air

Process annealing allows recovery, recrystallization, and grain growth because a coarse microstructure is preferred. (True or False)

False. Grain growth isn't allowed because a fine microstructure is preferred.

The AISI / SAE designation for steel consists of a 4-digit number (xxxx). Explain the meaning of each digit.

1st digit - alloying element(s),
if applicable 2nd digit - amount of alloying element
3rd & 4th digit -amount of carbon e.g.) 1020 steel - plain carbon steel with 0.20 %C

Which annealing process austenizes the material, then cools in the furnace?

full anneal

Why is stress relieving performed?

To eliminate residual stresses that may lead to part warpage / geometrical changes.

Metals that gain an electron are undergoing a __________ reaction.

reduction

The site of an oxidation reaction is called:

anode

What is a galvanic couple?

two dissimilar metals connected electrically in an electrolyte

What is the Galvanic Series?

a ranking of the relative reactivity /inertness of common metals and alloys in seawater

What is passivity? Name a material that passivates in a room temperature environment.

a normally active metal/alloy lose their reactivity and become extremely inert; believed to be caused by a protective oxide layer; stainless steel, aluminum

This form of stress occurs due to the combined action of tensile stress and corrosive environment.

stress corrosion

This form of corrosion occurs when two dissimilar metals are in electrical contact in the presence of ________.

galvanic corrosion, electrolyte

This form of corrosion only affects solid solution alloys.

selective leaching

This is "probably" the most common form of corrosion. Where do the oxidation and reduction reactions occur?

Uniform attack; random locations across the surface

What is galvanizing? At the atomic level, how does it protect steel from corroding?

1. Dipping a metal (often steel) in liquid zinc.

2. This creates a galvanic couple, and since zinc is more reactive it will be the anode which gives up electrons that are electrodeposited on the steel. Zinc corrodes first. Also corrosion rate is slow since the anode-to-cathode surface area is large

What is overaging?

When a precipitation heat treatment is performed too long and the increased strength / hardness is lost.

Why type of materials are particularly susceptible to hydrogen embrittlement? Which are resistant?

high strength steels; FCC alloys

This form of corrosion arises from the combined action of chemical attack and mechanical abrasion / wear.

erosion corrosion

This "insidious" form of corrosion exhibits as small holes or pits in the surface. How can you make a part more resistant to it?

pitting; polish it ( pits are likely initiated at surface defects like scratches)

What is the underlying cause of crevice corrosion? Name 2 ways to prevent it.

concentration differences of ions or dissolved gasses in an electrolyte solution between two regions of the same metal.
(1) welded joints
(2) use non-absorbing gaskets
(3) removed accumulated deposits frequently
(4)avoid stagnant areas when designing

Explain how precipitation hardening works.

1. solution heat treat: material is heated until it is fully transformed to new phase ( in-class example - alpha phase)

2. rapid quench to room temperature (in-class example - alpha phase is "locked in")

3. precipitation heat treat: material is heated (lower than step 1) to grow small particles theta

How can you change the thickness of alternating layers of pearlite?

more supercooling - fine layers
less supercooling - coarse layers

How do you achieve homogeneous nucleation vs heterogeneous nucleation?

homogeneous - considerable supercooling ( typically 80 - 300 degrees C)
heterogeneous - slight supercooling ( 0.1 - 10 degrees C)

How large (in terms of atoms) are the small particles (or "nuclei) that form during the nucleation stage of phase transformation?

one particle = a few hundred atoms

What is supercooling?

The phenomenon whereby appreciable nucleation rates occur only when the temperature of a cooling liquid is "well" below the melting temperature. Also, the driving force behind nucleation.

What is the name of this equation? y = 1 - exp(-ktn)

Avrami Equation

What is another name for an isothermal transformation diagram?

time-temperature-transformation plot (TTT)

Which of the following is a metastable phase because the transformation is diffusionless:

1. bainite

2. pearlite

3. spheroidite

4. none of the above

none of the above; martensite

True or False. An isothermal transformation diagram is applicable for all compositions of steel.

False; only one composition

The addition of alloying elements (Cr, Ni, Mo, etc) to steel changes the positions and shapes of the curves on the T-T-T diagram. Name 2 of them.

1. shifts transformation from austenite to pearlite to longer times

2. formation of a separate bainite nose

3. increase the time window to form martensite

List the following microstructures of steel in order from most ductile to least ductile. (bainite, martensite, tempered martensite, spheroidite, coarse pearlite, fine pearlite)

spheroidite, coarse pearlite, fine pearlite, bainite, tempered martensite, martensite

What are three reasons you might select a non-ferrous metal over a ferrous metal?

In general they have:

1. lower density

2. better electrical conductivity

3. more resistance to corrosion

Does a phase transformation occur during a process anneal?

no; recovery, recrystallization, no phase change (not high enough temps)

Which heat treatment doesn't alter the effects of cold working?

stress relieve

Why is normalizing performed? How is it done?

1. Normalizing is done to produce fine, uniform grains (often after a cold-rolling process).

2. Austenize, cool in air.

Label annealing processes 1-5 on the diagram.

1. process anneal

2. stress relief anneal

3. spheroidizing

4. normalizing

5. full anneal

What chemical reaction occurs at the cathode?

reduction reaction

True or False) Cold worked material is less likely to corrode.

False; more likely

What manufacturing process uses electrodeposition to coat one metal in another metal?

electroplating

What is an electrolyte?

substance that allows electrical current to flow between the anode and the cathode in the form of ions

Aside from gold and platinum, why don't you find lumps of metal (like iron, aluminum, etc) in creeks or in rocks?

Because most metals corrode and form stable ionic compounds with nonmetallic elements (e.g., steels rust to form iron oxide - more stable; lower energy state)

What are the 5 general techniques to prevent corrosion?

1. material selection

2. environmental alteration

3. design changes

4. coatings

5. cathodic protection

Which of the following materials is considered already corroded:

1. metals

2. ceramics

3. polymers

4. none of the above

ceramics

Name the 3 environmental factors that influence corrosion rate? Explain if they increase or decrease corrosion rate.

1. fluid velocity (increase, increase)

2. temperature (increase, increase)

3. concentration of corrosive species(increase, increase)

Name the 3 forms of cathodic protection.

1. sacrificial anodes

2. galvanizing

3. providing a source of electrons from a DC power source

What form of corrosion is common when welding stainless steels? How would reduce the likelihood of this corrosion after welding?

intergranular corrosion
1) heat treat after welding to re-dissolve chromium carbide particles

This test (with a silly name) measures the ability of a steel to form martensite.

Jominy end quench test

Metals are often grouped into two broad classifications. What are they?

1. Ferrous metals (where iron is the primary element)

2. Non-ferrous metals

Name 2 factors that may affect the hardness of steels after quenching.

1. rate of cooling

2. ratio of surface area to volume

3. presence of alloying elements

Describe how precipitation hardening increases the strength of a material.

The growth of small precipitate particles during precipitation heat treatment impedes dislocation motion.

Why does hardness decrease with distance from the quenched end during a Jominy End Quench test?

cooling rate decreases

What is the most awesome AF manufacturing process ever!?

machining