1. Describe the HRC 6066, STB2 heat treatment cycle to prevent distortion.
※ HRC 6066, STB2 (high carbon chromium steel)
◑ Q(quenching)-T(tempering) after soaking treatment
◑ Maintain for about 3 hours around 700 degrees below the quenching temperature, then air cool.
◑ Air cool after holding at 790820 degrees for 30 minutes, 25 minutes at Ms (170130 degrees).
◑ Water cool after 40 minutes, 25 minutes at 150~180 degrees.

2. Chloride salt bath was selected to obtain upper bainite structure for high speed engine shaft sensitive to deformation. Write down 3 ingredients of low temperature salt bath (Melting point: 2360 degrees, Operating temperature: 400 degrees).
◑ Low temperature salt bath: Sodium nitrate, Potassium nitrate, Sodium nitrite, Potassium nitrite

3. List the methods for obtaining the S-curve. (3 methods for obtaining the CCT curve)
◐ Metallurgical method
◐ Thermal analysis method
◐ Thermal expansion method
◐ Magnetic analysis method

4. Find the degrees of freedom using the phase rule at the invariant point.
◐ F=C+1-P
◐ C = 2 , P = 3 , F = 0

5. When a Rockwell hardness test is performed using a 120 degree diamond cone with a load of 150 kg and the depth of the indentation mark is 0.045 mm, what is the HRC?
◐ Calculation formula
HRC = 100 - 500h (h = indentation depth in mm)
100 - 500 x 0.045 = 77.5

◐ Answer
77.5

6. Define the fatigue limit and explain how to determine the fatigue limit in metals such as Al where it is difficult to determine the fatigue limit.
※ Fatigue limit definition
◑ The maximum stress value that does not cause permanent failure of the material even when a repeated load is applied to the material.

※ In the case of aluminum (Al) where it is difficult to determine the fatigue limit
◑ Define the stress amplitude at 10^7 cycles as the fatigue limit (determines the effective life of the product or component)

7. Carburizing was performed on C 0.25% carburized steel for 7 hours, including the diffusion time, to reach a target carbon concentration of 0.8%. If the surface carbon concentration was 1.15%, how long was the carburizing time?
◑ Harris equation
Tc = Carburizing time required, Tt = Carburizing time + Diffusion time
C = Target surface carbon concentration (%), Co = Carbon concentration during carburizing, Ci = Carbon concentration of the material itself (%)

◑ Calculation formula
Tc = Tt ((C - Ci) / Co - Ci))2
Tc = 7 * ((0.8-0.25)/(1.15-0.25))2

◑ Answer
Tc = 2.6 hours

8. In the Fe-C phase diagram, write down the (①) point of carbon at 4.3%, the degrees of freedom at the eutectic point (②), the self-transformation temperature (③) degrees, and the cubic lattices (④) of γ-iron, α-iron.
◑ ① : Invariant point
◑ ② : 0
◑ ③ : A2 (768 degrees)
◑ ④ : BCC

9. Describe what the mass effect is.
◑ Even in the same steel, the depth of hardening varies depending on the size, thickness, and mass shape of the product.

10. Explain hardenability.
◑ Even for products of the same size, the depth of hardening during quenching varies depending on the steel. This hardening depth is called hardenability, which is the performance of the steel itself.

11. Distinguish between linear magnetization and circular magnetization by magnetization method in magnetic particle inspection.
※ Linear magnetization method
◑ Pole method (M)
◑ Coil method (C)

※ Circular magnetization method
◑ Magnetic flux penetration method (I)
◑ Axial current method (EA)
◑ Prodding method (P)

12. Connect to the things that are related to the thermocouple (explain the thermocouple_alternative).
※ Thermocouple requirements (heat, resistance to heat)
◑ It should have a high thermoelectric power.
◑ It should be able to withstand high temperatures.
◑ It should have high heat resistance.
◑ It should have good corrosion resistance.
◑ It should have low thermal conductivity.

13. Write down 3 purposes of cryogenic treatment for bearing steel, precision machine steel, and gauge steel.
◑ Preventing dimensional deformation
◑ Preventing aging deformation
◑ Strengthen the steel
◑ Strengthen the carburized layer
◑ Improve the mechanical properties of stainless steel
◑ Martensite transformation of residual austenite

14. Explain the lamination inspection method.
※ Lamination
Internal defects in rolled steel, such as non-metallic inclusions, are stretched parallel to the rolling direction to form a laminated structure.

15. Describe 5 conditions and precautions for general heat treatment tooling materials.
◑ It should have good heat resistance.
◑ It should have very little high-temperature deformation.
◑ It should be easy to manufacture.
◑ It should be a corrosion-resistant material.
◑ It should maintain sufficient hardness against deformation.

16. Write down 4 advantages and 3 disadvantages of the eddy current inspection method.
※ Advantages (High-speed inspection)
◑ Inspection is possible even at high temperatures.
◑ Non-contact and fast inspection speed.
◑ Inspection records can be saved.
​◑ Excellent detection ability for surface defects.

※ Disadvantages (Internal inspection)
◑ Not suitable for measuring internal defects.
◑ Inspection is difficult for complex shapes.
◑ It is difficult to directly determine the type and shape of the defect from the indications obtained by inspection.

17. Explain the mass effect during heat treatment.
◑ Mass effect
The depth of hardening varies depending on the size, dimensions, and thickness even in the same steel.
It is a measure of the heat treatment effect based on the size and shape of the material.