Presents various problem types and solutions related to the Metal Materials Master Craftsman Practical Exam, and provides explanations about the characteristics of metal materials and heat treatment processes, including cryogenic treatment, fatigue limit, and high-frequency quenching.
Covers a wide range of areas, including non-destructive inspection methods, hardness testing, metal properties, martensitic transformation, and provides theoretical and practical problem-solving to help prepare for the Metal Materials Master Craftsman Exam.
Specifically designed to help you acquire comprehensive knowledge in metal materials, including surface treatment methods such as segregation, carburizing, and nitriding, as well as inspection testing, heat treatment, and microstructure observation.
1. Draw a cryogenic treatment process flow diagram and list 5 advantages of cryogenic treatment. ※ Process Flow Diagram
◑ Dry ice + alcohol (-78°C) and liquid nitrogen (-196°C) are used as refrigerants, and it is performed immediately after quenching before tempering. The treatment temperature is maintained at 60~80°C, and the holding time is 30 minutes per 25mm.
※ Advantages of Cryogenic Treatment ◑ Martensite transformation of residual austenite ◑ Improvement of mechanical properties of stainless steel ◑ Strengthening of steel ◑ Prevention of aging distortion ◑ Prevention of dimensional change
2. Explain what fatigue limit is. ◑ The maximum stress value that does not cause permanent material failure even when a repetitive load is applied to the material.
3. What is the name of the microstructure when 75% Ni steel is heat-treated for 48 to 120 hours and then tempered?
4. List 5 characteristics of high-frequency quenching. ◑ Rapid heating and cooling Very fast heating is possible, and the rapid cooling process shortens the heat treatment cycle, increasing production volume. ◑ Precise heat treatment control It is possible to harden only the necessary parts precisely while maintaining the properties of the remaining parts. ◑ Energy efficiency Since it is performed through direct heat transfer, energy loss is low, and energy efficiency is high. ◑ Improved surface quality The surface hardening process increases the wear resistance of the component and ensures a long service life. ◑ Clean and safe working environment Since it does not use flames or harmful gases, the working environment is cleaner and safer. ◑ Easy integration with automation It can be easily integrated with automated systems, making it efficient for mass production processes.
5. Explain the non-destructive method for detecting surface microcracks in an automobile engine block and the reason for using it. ◑ Fluorescent penetrant inspection ◑ It is possible to inspect even complex-shaped test pieces and detect even minute cracks.
6. When testing non-metallic inclusions using the point count method, the field size (f) is 40, the total number of grid points on the glass plate in the field of view (p) is 20 x 20, and the number of grid point centers occupied by the inclusion (n) is 48. Calculate the area percentage (cleanliness). ◑ Calculation formula d = n / (p * f) = 48 / (20 * 20 * 40) * 100 = 0.3% Area percentage (cleanliness) = Number of grid point centers / (Number of horizontal and vertical points x Field size)
7. What is the meaning of 10/3000/30 in Brinell hardness? ◑ 10: Diameter of the indenter ◑ 3000: Test load ◑ 30: Test duration
8. Which non-destructive inspection method is good for detecting lamination but not suitable for detecting defects like voids? ◑ Ultrasonic testing
9. List 5 types of segregation that appear in the sulfur printing method. ◑ Normal segregation (Sn) ◑ Inverse segregation (Si) ◑ Linear segregation (Sl) ◑ Spot segregation (Sd) ◑ Center segregation (Sc)
10. List the types of magnetization methods in magnetic particle inspection and explain each method. ◑ Central conductor method A method of magnetizing the test piece by passing a current directly through the test piece in the axial direction and utilizing the circular magnetic field generated around the current. ◑ Prod method A method of testing by contacting two electrodes to a local part of the test piece and passing current only between two points close to the surface of the test piece to form a circular magnetic field of the required intensity. ◑ Direct current method A magnetization method in which a direct current is passed directly in a direction perpendicular to the axis of the test piece. Defects perpendicular to the axis are best detected, while defects along the axis are difficult to detect.
11. Nitriding is 2NH3 → (2N) + (3H3), and solid carburizing uses a solid carburizing agent consisting of 60% charcoal + 30% ( ) + 16% NaCO3 to harden the surface. Fill in the blank. ◑ 30% BaCO3
12. What is the steel with good machinability? ◑ Free-machining steel
13. Calculate the grain size using the Heyn method when the microscope magnification is 100x and the area of the micrograph is 5000mm2.
14. Calculate the maximum hardened layer depth when gas carburizing at 630°C for 5 hours under standard conditions.
15. List 3 factors that affect the S-curve of steel heat treatment. ◑ Stress ◑ Segregation ◑ Heating temperature ◑ Alloying element ◑ Heating rate
16. List 5 characteristics of metals. ※ Physical properties ◑ Exists in solid form at room temperature ◑ Has a high melting point and high strength. ◑ Has a specific density and is generally heavy. ◑ Excellent thermal and electrical conductivity. ◑ Requires a lot of processing time, equipment, and cost. ※ Mechanical properties ◑ Excellent hardness and wear resistance. ◑ Good castability and alloy recovery is possible.
17. List 3 reasons why martensite becomes harder during transformation. ◑ Due to the increase in internal stress caused by slip in the atomic lattice. ◑ Due to the precipitation of carbon atoms and Fe3C as cementite and the formation of a superlattice. ◑ Due to the volume change caused by diffusionless transformation.