[Metal Materials Function Master Practical] 39th Edition Solution

1. What is the test that measures the time-dependent change in deformation when a constant stress is applied to a material?
◑ Creep Test

2. List 5 mechanical properties that can be obtained from a tensile test.
◑ Tensile Strength
◑ Yield Strength
◑ Elongation
◑ Reduction in Area
◑ Elastic Limit

3. What is the material testing method mainly used for materials like cast iron, bearing alloys, bricks, and concrete that are primarily used for pressure resistance?
◑ Compression Test

4. In a Rockwell hardness test, if the indentation depth is h, what are the hardness scale used, the indenter material and shape, and the test load?

5. An ideal crystal with perfectly arranged atoms or molecules without defects is called a perfect crystal. However, real crystals have lattice defects due to various reasons. List 3 types of lattice defects.
◑ Point Defect
◑ Line Defect
◑ Interface Defect
◑ Volume Defect

6. Describe the characteristics of parts (a) and (b) in the following figure during quenching operation.

7. What is the name of the structure that is formed when 0.8%C γ-austenite decomposes at 723°C into pearlite, which is a eutectoid structure of ferrite and cementite, a mixture of α-austenite and Fe3C layered structure?
◐ Pearlite

8. Arrange the following microstructures in order of decreasing hardness:
① Sorbite ② Pearlite ③ Martensite ④ Troostite
③ > ④ > ① > ②

☆ Hardness in increasing order (Ferrite, Austenite, Pearlite, Sorbite, Troostite, Martensite, Cementite)
◐ Ferrite → Austenite → Pearlite → Sorbite → Troostite → Martensite → Cementite

★ Hardness in decreasing order (Cementite, Martensite, Troostite, Sorbite, Pearlite, Austenite, Ferrite)
◐ Cementite → Martensite → Troostite → Sorbite → Pearlite → Austenite → Ferrite

9. What is the mass effect?
◐ In materials with large mass, heat conduction takes time, causing a temperature difference between the inside and outside. As a result, while the outside hardens, the inside may not harden.

10. List 3 general characteristics of amorphous alloys.
◐ Mechanical Properties
-. High toughness, tensile strength, wear resistance, and no crystal defects.
-. Excellent temperature dependency.
-. Work hardening hardly occurs.
-. Has high fracture toughness.
-. Toughness and ductility change significantly with temperature.
◐ Chemical Properties
-. Excellent corrosion resistance.

11. What is the heat treatment shown in the figure below, which is an isothermal heat treatment in the Ar' (between Ms and Mf) transformation region, where the material is quenched to a temperature below Ms (100-200°C) after the isothermal holding until the undercooling austenite transformation is almost completed, and then air-cooled?
◐ Martempering

12. Describe the 3 stages of annealing a cold-worked metal and the driving force of each stage.
◐ 3 Stages of Annealing: Recovery → Recrystallization → Grain Growth
◐ Recovery: Stored energy within the metal.
◐ Recrystallization: Stored strain energy from cold working.
◐ Grain Growth: Reduction of grain boundary surface energy.

13. List 4 methods for spheroidizing cementite (Fe3C).

◐ Long-Time Heating Method
Heating just below A1 (650-700°C) followed by cooling.

◐ Repeated Heating Method
Heating and cooling repeatedly near the A1 transformation point.

◐ Network Carbide Dissolution Method
Heating above A3 and Acm temperatures, dissolving Fe3C, and then rapid cooling to prevent the precipitation of network Fe3C and promote spheroidization.

◐ Slow Cooling Method
Heating to a temperature above A1 transformation point and below Acm and then slowly cooling to A1 transformation point.

◐ Isothermal Holding Method
Heating above A1 transformation point and below Acm, holding at a temperature below A1 transformation point until the transformation is complete, and then cooling.

14. List 3 methods for measuring the types of phases present and the interfacial area between phases through microstructure examination.
-. Types of Phases
◐ Austenite
◐ Ferrite
◐ Cementite
◐ Pearlite
◐ Bainite
◐ Martensite

-. Measurement Methods
◐ Point Counting Method
◐ Linear Intercept Method
◐ Area Fraction Method

15. When steel is heated to 500-550°C in ammonia (NH3) for 20-100 hours, it decomposes into N and H due to 2NH3 → (2N) + (3H2). N diffuses into the surface layer of the steel and forms a hardened layer. Also, the best composition in the solid carburizing method is 60% charcoal + (30% BaCO3) + 10% NaCO3. One of the chemical reaction equations in the solid carburizing method is (C) + (CO2) → 2CO.

16. If the carburizing depth was 2mm after carburizing at 930°C for 4 hours, how long should the carburizing time (t) be to achieve a carburizing depth (D) of 4mm?
◐ Calculation Formula
D = K * √t ------ ①
2 = K * √4
2 = K * 2
2 / 2 = K , K = 1

4 = 1 * √t ------- Substituting K=1 into equation ①
42 = t ------ Squaring both sides to eliminate the square root and solve for t

◐ Answer: 16 hours

◐ Example) If the carburizing depth was 1.2mm after carburizing at 920°C for 9 hours, what would be the depth if carburizing at the same temperature for 16 hours?
D=K√t 1.2=K√9 , 1.2=K x 3 , 1.2/3 = K = 0.4
D=0.4√16 = 0.4 x 4 = 1.6 Answer: 1.6mm

17. What is the non-destructive testing method that utilizes the release of stored energy in the form of elastic waves when a solid undergoes fracture or plastic deformation? This method is used for material research, manufacturing management of welding, and structural integrity assessment.
◐ Acoustic Emission (AE) Testing

18. Metal wires such as Cu have electrical resistance, so when current flows through them, power is consumed and the temperature rises. Reducing the temperature decreases resistance, but even when cooled near absolute zero, some resistance remains. However, certain types of metals exhibit a phenomenon where electrical resistance completely vanishes at a specific temperature. What is this phenomenon called, and what alloy is currently used in practice?
◐ Phenomenon: Superconductivity
◐ Practical Alloy: Nb-Ti alloy