Heat Treatment

Question 1

Define Microstructure

Answer 1

Microstructure refers to the arrangement of phases and defects such as grain boundaries

Question 2

At what stage is heat-treatment applied and what is the main hardening mechanism?

Answer 2

Heat treatment is a secondary process step which is applied to shaped components. The main hardening mechanism is precipitation hardening.

Question 3

What phases must heat treatable alloys be at high and low temperature

Answer 3

1. At high temperatures they must be a single phase solid solution
2. At low temperatures they must be two phase

Question 4

How does precipitation hardening work?

Answer 4

Precipitation hardening relies on the pinning of dislocations by material that has precipitated out of the two-phase mixture. All hardening mechanisms are inversely proportional to the spacing of obstacles.

Question 5

What are the two criteria for a heat treatment process

Answer 5

1. Correct starting point: the material must be heated to the correct temperature and held to form a single phase solid solution (solution heat treatment l)
2. Subsequent thermal history: on cooling, effective hardening is determined by the distribution of the second phase.

Question 6

What is the assumption made when slow cooling

Answer 6

When slow cooling we assume the phases and their proportions follow equilibrium

Question 7

Under what conditions is the Time Temperature Diagram valid

Answer 7

The time temperature diagram is valid only for isothermal processes so the specimen must be kept at a constant temperature

Question 8

What is the critical cooling rate

Answer 8

The CCR is the cooling rate the just avoids the onset of diffusional phase transformations. A material cooled faster than the CCR will form a Super Saturated Solid Solution

Question 9

What is the final microstructure and material properties of slow-cooled Aluminium Alloys

Answer 9

Slow cooling gives a coarse microstructure, the precipitates are well spaced. So the yield stress remains low and ductility is high

Question 10

What is the final microstructure and stability of rapid cooled Aluminium Alloys

Answer 10

If the Aluminium is quenched (rapid cooling) the precipitation phase transformation will not occur. At room temperature the material is a single phase solid solution. This material is known as a supersaturated solid solution and is metastable.

Question 11

What hardening mechanism takes place in slow-cooling of Al alloys

Answer 11

Precipitation hardening, but precipitates are coarse and widely spaced.

Question 12

What hardening mechanism takes place in rapid cooling Al alloys and how effective is this hardening mechanism?

Answer 12

Solid solution hardening occurs but the solute atoms provide weak pinning

Question 13

How can higher hardness be achieved in Al alloys

Answer 13

Age hardening can be used to increase hardness.

1. Artificial Ageing: reheat after a few hours.
2. Natural Ageing: leave for a few days at room temperature

Question 14

Micro-structural origin of age hardening

Answer 14

Age hardening starts which solution heat treatment, followed by quenching then finally ageing.
The quenching forms metastable precipitates which have a different crystal structure to the alloy. The precipitates are initially coherent with the Al lattice (aligned in the same direction). Over time the coherency reduces but the precipitates coarsen leading to a peak aged state

Question 15

Why is there a peak in the artificial age in curve for Al alloys

Answer 15

During ageing the solid solution hardening is replaced with precipitation hardening. The combination of progressive precipitate coarsening and loss of coherency result in a peak in the ageing curve.

Question 16

Why is there no peak in the hardening curve for natural ageing of Al alloys

Answer 16

In natural ageing that microstructure evolution stops at initial precipitation. The precipitates do not coarsen as there is insufficient energy.

Question 17

Which phases of Iron are involved in the heat treatment of steel alloys

Answer 17

Initially solution heat treatment is achieved with austentite. On cooling the austentite forms ferrite and cementite in a three phase reaction.

Question 18

Define Pearlite

Answer 18

Pearlite is a combination of ferrite and cementite. It is a two phase eutectoid microstructure. It has a lamellar (layered) appearance. It forms below 723C

Question 19

Explain the significance of the carbide line on the time-temperature transition diagram

Answer 19

Above the nose of the C-curves (diffusional phase transformations) the carbide line marks the transition from ferrite to pearlite nucleation. Below the nose of the C-curves (diffusion inhibited) the carbide line marks the transition from ferrite to bainite nucleation. It is represented with a dotted line.

Question 20

What do C Curves Represent

Answer 20

The C curves show the total % of material formed. We can interpolate between the curves. In Fe-C Phase Diagrams - they represent the fraction of austenite formed.

Question 21

How does the the time-temperature transition vary with carbon content

Answer 21

1. The proportion of Ferrite to Pearlite
2. The C-curves move to longer times - the greater amount of Carbon to redistribute delays the diffusional transformations
3. The martensite start and finish temperatures decrease - more carbon in supersaturated solid solution means more lattice strain this requires greater undercooling to achieve martensite transformation

Question 22

Describe the micro-structural changes that occur during the slow-cooling of iron-carbon alloys

Answer 22

1. The iron begins as single phase austentite.
2. The material enters the two phase ferrite+austentite region. The ferrite nucleates on the grain boundaries. The ferrite rejects carbon into the austentite .
3. At the eutectoid the austentite transforms to Pearlite. Nucleation again occurs on the grain boundaries.

Question 23

What are the properties of slow cooled steels

Answer 23

For low carbon steels they have a relatively high yield stress (200MPa) when compared to our metals and remain ductile.
For higher carbon steels they form Pearlite which obstructs dislocation motion increasing the yield stress significantly.

Question 24

What are the different phases formed in diffusion controlled transformations in Fe-C at various temperatures.

Answer 24

1. If we quench to a temperature greater than the eutectoid ferrite nucleates in the austentite grain boundaries
2. If we quench to a temperature below the eutectoid ferrite begin to nucleate in the austentite grain boundaries but the transformation switches from ferrite to pearlite.
3. If we quench below the nose of the C-curves diffusion is inhibited. Austentite forms ferrite and cementite directly in a fine scale dispersion. This is bainite

Question 25

What is Bainite and how does it form

Answer 25

Bainite is a fine scale dispersion of ferrite and cementite that forms at lower temperatures (~450C) when diffusion is inhibited. Bainite nucleates on austenite grain boundaries with simultaneous growth of ferrite and cementite. This results in a fine scale dispersion of Iron Carbide needles in a ferrite matrix.

Question 26

Compare and contrast Bainite and Pearlite

Answer 26

Bainite and Pearlite have the same chemical composition (ferrite and cementite) but are different phases as they have a different atomic structure. Bainite is a fine scale dispersion whereas Pearlite has a plate-like lamellar structure.

Question 27

Describe the microstructure of diffusion-less transformations (lower temperature) in Fe-C

Answer 27

If we quench to low temperatures (below the c-curves). The austentite forms martensite (metastable). This involves cooling faster than the Critical Coooling Rate. This prevents any diffusional phase transformation from taking place. Quenching produces a supersaturated solid solution (SSSS). So there is more carbon trapped in the Fe lattice than in the equilibrium case. Also the SSSS is in the wrong atomic packing compared with the equilibrium as it FCC at high temp and BCC at low temp. The martensite phase transformation allows the material to change to BCC without diffusion - through shear.

Question 28

Why does the fraction of martensite formed depend on the temperature

Answer 28

Martensite begins to form at the Martensite start temperature, reducing the temperature further increases the fraction of Martensite formed. The thermodynamic reason for this is as the temperature falls, the Gibbs free energy of the martensite phase becomes lower than the austenite phase. A certain degree of undercooling is needed in order to start the transformation.

Question 29

What are the properties of Bainite

Answer 29

Bainite has the ideal microstructure for precipitation hardening. Fine scale dispersion of hard precipitates and all grains have the same microstructure and strength.

Question 30

What are the properties and microstructure of Martensite

Answer 30

The BCC lattice is heavily saturated with Carbon. This gives solid solution hardening. This is further enhanced by the distortion of the lattice caused by interstitial spaces being filled by Carbon atoms too large to fit. Hence the higher the carbon content the higher the hardness.

Question 31

What are the advantages of tempering martensite

Answer 31

Tempering martensite increases the fracture toughness ambit decreases yield stress. The fine scale iron carbide precipitates formed during tempering provide excellent precipitation hardening.

Question 32

What micro-structural changes occur when tempering

Answer 32

- The Carbon in the SSSS diffuses to form Fe3C precipitates and the Fe-lattice relaxes to undistorted BCC. - The diffusional phase transformations require the sample to be heated following quenching.

Question 33

What is harden-ability and how can it be measured

Answer 33

Harden-ability is a measure of how easily a steel forms martensite on cooling. The following measures are used: 1. Time taken for diffusion phase transformations to begin. The longer the time the greater the harden-ability 2. The critical cooling rate gives the slowest rate at which 100% martensite is formed 3. The bar diameter that gives a 100% martensite core after quenching.

Question 34

When is a steel with low harden-ability required

Answer 34

When martensite is unwanted upon cooling. For example welding.

Question 35

What can we do increase harden-ability

Answer 35

1. Increasing the Carbon content as it delays diffusional phase transformations 2. Further alloying as it delays diffusional phase transformations

Question 36

What function is used to model carburisation for steels

Answer 36

1-erf{x/2*sqrt(DT)}

Question 37

How do you find the initial C0 and surface CS concentrations

Answer 37

The initial concentration is the composition of the steel being hardened The surface concentration is the maximum amount of carbon that can be accommodated as a single phase solid solution

Question 38

How do you find the value of D the diffusion coefficient in the error function expression

Answer 38

D=D0exp{-Q/RT}

Question 39

What is carburising

Answer 39

Carburising is a technique that involves immersing steel in a high temperature high carbon environment. Carbon diffuses into the surface making it easier to form martensite. The added carbon further distorts the existing martensite lattices making it harder.

Question 40

What symbol denotes Martensite

Answer 40

Alpha Prime

Question 41

Explain the shape of the C Curves

Answer 41

The rate of transformation depends on the rate of nucleation of the new phase which in turn depends on the undercooling. However, undercooling restricts diffusion which is a thermally activated process. Hence the peak rate

Question 42

What is the martensite phase transformation

Answer 42

The martensite phase transformation is a diffusion-less transformation during which the martensite unit cell can get closer to BCC. This is achieved by shear across the austenite grain leaving a needle of martensite within the austenite grain.

Question 43

Describe the visual appearance of Martensite

Answer 43

Martensite has a needle like appearance. At lower temperature the proportion of Martensite formed increases so for steels with high harden-ability the final microstructure will appear as a dense matrix of martensite needles.

Question 44

Draw a graph of the variation between Carbon content and Hardness (Vickers)

Answer 44

Initially there is a linear increase in Vickers Hardness for an increase in Carbon content, however after 0.6 wt% C, the curve levels off.