Master examination - iehk.rwth-aachen.de

Master examination

„Metallic Materials“

11.09.2017

Name, first name:

Matriculation number:

Declaration: I am healthy and able to take part in the examination.

Signature:

Task Points: achieved Points: Points after review

(additional Points)

1 10

2 4

3 5

4 8

5 7

6 7

7 8.5

8 7

9 5

10 7

11 6

12 3

13 7

14 6

15 4

16 5.5

Summe 100

The overall grade for the examination of „Metallic Materials“ will be weighted from the results of the

respective parts "Microstructure, Microscopy and Modelling" and "Metallic Materials” for a duration of

90 minutes each.

Metallic Materials 2

Head of Department: Univ. Prof. Dr.-Ing. W. Bleck Fon: +49 241 8095782 E-Mail: [email protected] Fax: +49 241 8092224

Task 1 Crystal Structure 10 Points

A feature of iron is that different crystal modifications can occur in its solid condition: the

body centred cubic (bcc) and the face-centred cubic (fcc) lattice.

a) In Figure 1 both a bcc-lattice (grid) and a fcc- lattice (grid) are given. Highlight in both

lattices an example for an octahedron gap and a tetrahedron gap and draw the

corresponding octahedron and tetrahedron in the respective lattice (4 Points)

Figure 1:

BCC-Lattice FCC-Lattice

b) Which differences exist between the two lattice types concerning the number and size

of the gaps? Which consequences result from this for the diffusion characteristics and the

solubility of C in Fe (6 Points)?



Task 2 Magnetic Properties 4 Points

a) Describe the different types of magnetism (Ferro-, Para-, Dia-magnetism)

occurring in metals. Sketch the magnetic moments for these types of magnetism

in Figure 1. (3 Points)

Figure 1:

b) Give a short explanation for the Curie temperature Tc. (1 Point)



Task 3 Elastic Properties 5 Points

An engineering stress-strain diagram is given in Figure 1.

a) Calculate the young’s modulus for this steel. (Assumption isotropic behavior).

(1 Point)

Figure 1:

b) What is the approximate young’s modulus for FCC and BCC iron at room

temperature? (1 Point)



c) Sketch the temperature dependency of the Young’s modulus for iron in Figure 2. (i)

Fill in the values of the y-axis and unit of the Young’s modulus. (ii) Consider the

changes of the curves at the curie- and A3-temperature. (3 Points)

Figure 2:



Task 4 Alloying Elements I 8 Points

a) Name all 10 phase fields in the metastable Fe-Fe3C phase diagram (Appendix 1)

in the temperature range from 400 °C to 1600°C and a carbon content from 0 % to

6,67 %. (5 Points)

Appendix 1: metastable system Fe-Fe3C



b) There are three different types of cementite. Name the phases from which the

different cementite types originate. Mark areas where the cementite types occur in

appendix 1. (3 Points)



Task 5 Phase Transformations Austenite 7 Points

The phase transformations of the supercooled austenite are utilized industrially for

microstructural adjustment of steels.

a) Name the three phase transformation stages of the supercooled austenite.

(3 points)

b) During which phase transformation can Carbon diffusion take place, but diffusion of

elements like Iron and Silicon is restricted? (1 Point)

b) What are the microstructures resulting from near equilibrium cooling for a steel

containing (i) 0.002, (ii) 0.4 and (iii) 0.8 mass% carbon (3 Points)



Task 6 Phase transformations Ferrite/Perlite 7 Points

a) How can you describe Perlit (phases, morphology)? (1,5 Points)

b) Describe (in key points) the pearlite transformation mechanism. (1.5 Points)



c) Sketch the C-concentration for both phases of pearlite during the growth into

Figure 1. (2 Points).

Figure 1:

d) What is the characteristic value to describe perlite? How does this characteristic

value change with increasing supercooling? (2 points)



Task 7 Phase transformations Martensite 8,5 Points

The phase transformation from austenite to martensite occurs at very high undercooling.

A characteristic of martensite is its higher strength in comparison with austenite.

a) Name four characteristic features of the martensite transformation.

(2 Points)

b) Name four factors, which lead to the high strength of martensite. (2 Points)



c) Sketch the dependency of the carbon content on the volume change during

martensite transformation in Figure 1. (1 Point)

Figure 1

d) Sketch the influence of the carbon content on the Ms-temperature in Figure 2.

Which carbon content is necessary that the Mf-temperature is equal to the room

temperature? (2,5 Points)

Figure 2



e) A steel with 1.0 mass.-% Carbon is quenched from 1100°C to room temperature

(no bainite transformation is observed). What is the microstructure of this steel

after quenching? (1 Point)



Task 8 Phase transformations Bainite 7 Points

Bainitic microstructures are characterized by a favourable combination of strength and

toughness. The bainitic transformation from the austenite has characteristics of the

diffusive/diffusible and the diffusionless transformation.

a) Label all phase spaces in the isothermal ZTU diagram in Figure 1. (2 points)

Figure 1

b) Sketch the approximate conversion temperatures for upper and lower bainite in

Figure 1. Sketch both microstructures afterwards. (3 points)



c) Which secondary phases can be present after a bainitic transformation in the body-

centered cubic matrix? Name at least two of these. (2 Points)



Task 9 Aging 5 Points

Bake-Hardening steels are used for the production of high-strength sheet metal for car

body construction.

a) Explain the advantage of bake-hardening steels with regard to the manufacture of

car body components (e.g., doors) (2 points)?

b) What is the content range of dissolved carbon in BH steels (1 point)?

c) Is it possible to exceed the absolute carbon content value given in part b)?

Explain your answer (2 points).



Task 10 CCT-Diagrams I 7 Points

Heat treatments can be used to control the microstructure and therefore the mechanical

properties of steels. In Appendix 1 there is a TTT-diagram for the bearing steel 100Cr6.

The following microstructures should be achieved:

100 % pearlite and carbides with maximum hardness

100 % bainite and carbides with lowest hardness.

Based on the TTT diagram given in Appendix 1, suggest heat treatment schedules of

100Cr6 for obtaining the two desired microstructures above by sketching the complete

temperature – time diagrams starting and ending at room temperature. Assume a small

sample size. Start from the room temperature and show the temperature and time period

for each step. What is the hardness after each annealing treatment? (7 Points)



Appendix 1:





Task 11 CCT-Diagrams II 6 Points

Figure 1 shows the standardized transformed amount in dependence of logarithm of time

for a diffusion controlled transformation process (e.g. ferrite formation). The results show

a sigmoidal curve.

Figure 1:

a) Explain the processes in the three given ranges briefly (3 Points).



b) State the equation for the isothermal ferrite formation and state required constants and

variables. (3 Points)



Task 12 Technical Heat Treatment I 3 Points

Figure 1 shows a section of the Fe-C diagram, where different regions for heat treatments

are marked.

Add the names of the different heat treatments in the corresponding boxes in the diagram!

(3 Points)

Figure 1: Section of the iron-carbon-diagram



Task 13 Technical Heat Treatment II 7 Points

During the carburizing of steels, a defined carbon content is introduced in the component

surface. For this purpose, a component with a very small carbon content is exposed to a

carbon-rich gas atmosphere for t = 10 min at T = 920 ° C.

Parameters:

D0= 0,2 cm²/s

Q = 130 kJ/mol

R = 8,314 J/(Mol*K

a) Why are steels carburized? Give a practical example. (1 Point)

b) Calculate the diffusion depth of the carbon (average diffusion path).

(2 Points)



The diffusion depth should now be doubled.

c) How long does it take to anneal if the annealing temperature should remain

unchanged? (2 Points)

d) Sketch the temperature profile for a (i) carbonizing and a (ii) nitriding process. (2

Points)

Figure 1



Task 14 Quench and Tempering 6 Points

According to the DIN EN 10052 standard quenching and tempering is a combined heat

treatment.

a) Sketch the quench and tempering process in Appendix 1. Add Ac-, Bs- or Ms-

temperature lines to indicate the general temperature regime and cooling rate during

the process. (4 Points)

Appendix 1:

b) Which mechanical properties are improved after the quench and tempering

treatment in comparison with the steels after quenching? (2 Points)



Task 15 Steel processing 4 Points

As can be seen from Figure 1, the element iron is known as the symbol Fe with the

atomic number of 26 and its atomic mass is 55.8. Since it has low chemical stability, it is

rarely found as a pure substance but rather as an oxide, sulphide, carbonate, or silicate.

With the advent of the Iron Age (~BC 1500), Fe has been introduced to a number of

application and it is universally used until now.

Why do you think Fe is universally, technically applicable? List four reasons based on

the information provided in Figs.1-3 and explain. (4 Points)

Figure 1: Position of iron in the periodic table of the elements and its physical properties.

Figure 2: Relative abundance of elements in the Earth’s upper crust with respect to the atomic number, Z (Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS).

Figure 3. Different phases of pure Fe with respect to temperature





Task 16 stainless steels 5,5 Points

a) What is the lattice structure of the following steels: (1 Point)

i) X6Cr17

ii) X5CrNi18-10

b) Sketch a stress-strain diagram for steel X6Cr17 and X5CrNi18-10. Consider yield

strength, strain hardening and total elongation. (2 Points)

c) Is the resistance against corrosion of Cr-alloyed steels affected due to welding?

Where is the weakest location in the microstructure of not properly welded Cr-

alloyed steels? Explain your answer briefly. (2,5 Points)

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