1. What are the commonly used quenching methods and the principles for choosing different quenching methods?
Quenching method 1. Single-liquid quenching-a process of cooling to the end in a quenching medium. The single-liquid quenching structure has relatively large stress and thermal stress and large quenching deformation. 2. Double-liquid quenching-purpose: rapid cooling between 650 and Ms, so that V>Vc, and slow cooling below Ms, in order to reduce the tissue stress. Carbon steel: water first, oil later. Alloy steel: oil first, then air. 3. Graded quenching-after taking the workpiece out, stay at a certain temperature to make the inside and outside temperature of the workpiece consistent, and then air cooling process. Graded quenching is the process of M phase change during air cooling, and the internal stress is small. 4. Austempering-refers to isothermal in the bainite temperature zone, bainite transformation occurs, the internal stress is reduced, and the deformation is small.
The principle of quenching method selection should not only consider meeting the performance requirements, but also reduce the quenching stress as much as possible to avoid quenching deformation and cracking.
2. What is the difference between chemical vapor deposition and physical meteorological deposition technology, and their main applications?
Chemical weather deposition is mainly the CVD method. The reaction medium containing the coating material elements is vaporized at a lower temperature, and then sent to the high-temperature reaction chamber to contact the surface of the workpiece to produce a high-temperature chemical reaction, and precipitate alloys or metals and their compounds to deposit on the workpiece Coating is formed on the surface.
The main characteristics of the Cvd method: 1. It can deposit various crystalline or amorphous inorganic thin film materials. 2. The purity is high and the collective bonding force is strong. 3 Dense deposition layer, few pores, 4 good uniformity, simple equipment and process. 5 The reaction temperature is relatively high.
Application: Films for various purposes are prepared on the surface of steel, cemented carbide, non-ferrous metals, inorganic non-metals and other materials, mainly insulator films, semiconductor films, conductor and superconductor films, and corrosion-resistant films.
Physical Meteorological Deposition: The process of direct deposition of gaseous substances into solid films on the surface of the workpiece. Called the PVD method. There are three basic methods, vacuum evaporation, sputtering coating and ion plating. Application: wear-resistant coating, heat-resistant coating, corrosion-resistant coating, lubricating coating, functional coating and decorative coating. ,
3. Explain the microscopic morphology and macroscopic morphology of the fatigue fracture.
Micro: It is a striped pattern observed under a microscopic electron microscope, called fatigue strips or fatigue stripes. There are two types of fatigue strips: ductility and brittleness. The fatigue strips have a certain distance. Under certain conditions, each stripe corresponds to a stress cycle. Macro: In most cases, it has the characteristics of brittle fracture, without macroscopic deformation visible to the naked eye. A typical fatigue fracture is composed of the crack source area, the crack propagation area, and the final instantaneous fracture area. The areas of fatigue sources are relatively small and flat, sometimes showing bright mirror surfaces. The crack propagation area is in the shape of river beaches or shell patterns. Some fatigue sources with unequal intervals are parallel arcs with the center of the circle. The microscopic morphology of the instantaneous fracture zone depends on the characteristic load mode and size of the material. It may be dimples or quasi-dissociation, dissociation along crystal fractures or mixed shapes.
4. Point out three kinds of quality problems that often occur in induction heating quenching, and try to analyze the reasons.
1) Cracking: excessive heating temperature, uneven temperature; improper selection of quenching medium and temperature; delayed tempering and insufficient tempering; high hardenability of materials, segregation of components, defects, and excessive inclusions; parts The design is unreasonable. 2) Uneven surface hardness: unreasonable induction structure; uneven heating; uneven cooling; poor material organization (banded structure, local decarburization) 3), surface melting: unreasonable sensor structure; sharp parts Angles, holes, holes, etc.; heating time is too long, etc., there are cracks on the surface of the workpiece.
5. What are the characteristics of the new high-temperature tempering process for high-speed steel bottoms? (Take w18Cr4v as an example) Why is it better than ordinary tempered mechanical properties? W18Cr4v steel 1275 heating quenching +320*1h+540 to 560*1h*2 times of tempering.
1) The M2c type carbides of the bottom high temperature tempered high speed steel are more precipitated than the ordinary tempered high speed steel. The M2c, V4c and Fe3c type carbides have large dispersion and good uniformity, and there are about 5% to 7% of bainite. , This is an important organizational factor that makes the performance of the bottom high-temperature tempered high-speed steel superior to that of ordinary tempering.
6. What types of commonly used controllable atmospheres are there? Briefly describe the characteristics and applications of each atmosphere. There are endothermic atmosphere, dripping atmosphere, straight-body atmosphere, other controllable atmosphere (nitrogen machine atmosphere, ammonia decomposition atmosphere, exothermic atmosphere), etc.
1). The endothermic atmosphere is to mix the raw material gas with air in a certain proportion, pass through the catalyst at high temperature, and react to form an atmosphere mainly containing co, H2, N2 and trace amounts of co2, o2 and H2O. Because the reaction needs to absorb heat, So it is called endothermic atmosphere or RX gas. It is used for carburizing and carbonitriding. 2) The dripping atmosphere is to directly pour methanol into the furnace for cracking to generate a carrier containing co and h2, and then add enrichment agent for carburizing; carbonitriding at a lower temperature Infiltration, protection, heating, bright quenching, etc. 3) Mix the infiltrating agent such as natural gas and air in a certain proportion and pass it directly into the furnace. At high temperature, 900 reacts to directly generate a carburizing atmosphere. Ammonia decomposition gas is used for nitriding carrier gas, steel or non-ferrous metal low-temperature heating protective atmosphere. Nitrogen-based atmosphere has a good protection effect for high carbon steel or bearing steel. The exothermic atmosphere is used for bright heat treatment of low carbon steel and copper or decarburization annealing of malleable cast iron.
7. What is the purpose of austempering for ductile iron? What is the structure after isothermal temperature and austempering?
Answer: Purpose: Austempering in the bainite transformation zone of ductile cast iron after austenitization can obtain good mechanical properties and small distortion. Isothermal temperature: 260~300℃ to obtain lower bainite structure; 350 to 400℃ to obtain upper bainite structure.
8. Briefly describe the main characteristics of the commonly used chemical heat treatment (carburizing, nitriding, carbonitriding and nitrocarburizing) process, the structure and performance characteristics after heat treatment, which materials or parts are mainly applicable to?
Answer: Carburizing: It is mainly the process of infiltrating carbon atoms into the surface of the workpiece. The surface layer is tempered with martensite, residual A and carbides. The purpose of the core is to increase the carbon content of the surface. It has high hardness and high wear resistance, and the core has a certain degree of hardness. The strength and high toughness make it withstand large impact and friction. Low-carbon steel such as 20CrMnTi, gears and piston pins are commonly used.
Nitriding: Infiltrate nitrogen atoms into the surface to improve surface hardness, wear resistance, fatigue strength, corrosion resistance and thermal hardness. The surface layer is nitride, tempered sorbite at the core, gas nitriding, liquid nitriding, etc., commonly used 38CrMoAlA, 18CrNiW.
Carbonitriding: Carbonitriding has low temperature, high speed and small deformation of parts. The surface layer structure is fine needle tempered martensite + granular carbonitride Fe3 (C, N) + a small amount of retained austenite. It has high wear resistance, fatigue strength and compressive strength, and has a certain degree of corrosion resistance. It is often used in heavy and medium load gears made of low and medium carbon alloy steel.
Nitrocarburizing: The nitrocarburizing process has a faster co-carburizing speed, and the surface hardness is slightly lower than that of nitriding, but the fatigue resistance is good. Mainly used for parts and molds with low impact load, high wear resistance, high fatigue limit and small deformation. General steel parts Carbon structural steel, alloy structural steel, alloy tool steel, gray cast iron, ductile iron and powder metallurgy can all be nitrocarburized.
9. Briefly describe the principles of heat treatment process design
1. The advanced nature of the process 2. The process is reliable, reasonable and feasible 3. The economics of the process 4. The safety of the process 5. Use mechanized and highly automated process equipment as far as possible
10. What issues should be considered in the optimal design of the heat treatment process?
1. Fully consider the connection between hot and cold processing techniques, and the arrangement of the heat treatment process should be reasonable; 2. Use new technologies as much as possible, briefly describe the heat treatment process, and shorten the production cycle. Under the condition of ensuring the required organization and performance of the parts, try to combine different processes or processes with each other; 3. Sometimes in order to improve product quality and extend the service life of the workpiece, it is necessary to increase the heat treatment process.
11. Briefly describe the principles to be followed in sensor design
1. The coupling distance between the inductor and the workpiece should be as close as possible. 2. For the workpiece that relies on the outer wall of the coil to be heated, a flow guide magnet must be added. 3. The design of the inductor for the workpiece with sharp corners should avoid the sharp corner effect. 4. Avoid the lines of magnetic force. The offset phenomenon of 5, the inductor design should try to satisfy the workpiece can rotate when heated.
12. What are the basic principles that designers should consider when selecting materials
1. Select materials according to the working conditions of the parts, including load types and sizes, environmental conditions and main failure modes; 2. Consider the structure, shape and size of the parts and other factors, and select hardening for those that are prone to quenching distortion and cracking. Good properties, can be treated with oil quenching or water-soluble quenching medium; 3. Understand the structure and performance of the material after heat treatment. Some steel grades developed for various heat treatment methods will have better structure and performance after treatment. ; 4. Under the premise of ensuring the performance and life of the parts, the materials that can simplify the heat treatment process should be selected as much as possible, especially the materials that can be saved.
13. What process performance should be considered when selecting metal materials to make parts
1. Casting performance 2. Pressure processing performance 3. Mechanical processing performance 4. Welding performance 5. Heat treatment process performance.
14. How many types of wear failure are there? How to prevent various wear failures of parts?
Wear type: adhesive wear, abrasive wear, corrosive wear, contact fatigue. Prevention method: For adhesive wear, choose the matching materials of the friction pair reasonably; use surface treatment to reduce the friction coefficient or increase the surface hardness; reduce the contact pressure stress; reduce the surface roughness. For abrasive wear, in addition to reducing the contact pressure and sliding friction distance during the design, in addition to improving the lubricating oil filter device to remove the abrasive particles, it is also necessary to select high-hardness materials reasonably; use surface heat treatment and surface work hardening to improve the friction pair The surface hardness of the material. For corrosion and wear, select oxidation-resistant materials; surface coating; select corrosion-resistant materials; electrochemical protection; add corrosion inhibitors to reduce the stress concentration of tensile stress when designing; perform stress relief annealing; select materials that are not sensitive to stress corrosion; Change the media conditions. For contact fatigue, increase the hardness of the material; improve the purity of the material and reduce the inclusions; increase the strength and hardness of the core of the part; reduce the surface roughness of the part; increase the viscosity of the lubricating oil to reduce the oil wedge effect.
15. What is the basic process of chemical heat treatment of steel? P127 What are the main ways to accelerate chemical heat treatment? What are the advantages of P127-128 "carburizing segmented control process"? What is the structure of the surface and core of low carbon steel after carburizing and quenching under normal circumstances?
Decomposition, adsorption and diffusion; the application of segmented control method, compound infiltration treatment, high temperature diffusion, the use of new materials to accelerate the diffusion process, chemical infiltration, physical infiltration; prevent the surface oxidation of the workpiece, which is conducive to diffusion, so that the three processes Fully coordinate, reduce the process of forming carbon black on the surface of the workpiece, speed up the process of carburization, and ensure a wide and gentle transition layer of high-quality infiltration layer; from the surface to the center, there are hypereutectoids, eutectoids, and excessive hypoeutectoids. Primitive sub-eutectomies.
16. What is granular bainite?
It is composed of massive (equaxial) ferrite and high carbon A zone.
17. Explain the type, purpose and purpose of ball retreat?
Ordinary ball retreat: increase hardness, improve machinability, and reduce quenching distortion and cracking. Isothermal ball retreat: used for high carbon tool steel and alloy tool steel. Recirculating ball retreat: used for carbon tool steel and alloy tool steel.
18. The quenching temperature of hypoeutectoid steel is usually selected above Ac3, and why is the quenching heating temperature of hypereutectoid steel selected between Ac1-Acm, try to analyze it theoretically?
(1) Due to the low content of hypoeutectoid steel, the original structure P+F, if the quenching temperature is lower than Ac3, there will be undissolved F, and soft spots will appear after quenching. For hypereutectoid steel, if the temperature is too high, too much k'will dissolve, which will increase the amount of flake M, which is easy to cause deformation and cracking, increase the amount of A', and excessive k'will dissolve, which will make the steel wear resistant. Sexual decrease. (2) For hypereutectoid steels whose temperature is too high, the tendency of oxidative decarburization increases, which makes the surface composition of the steel uneven, and the MS level is different, resulting in quenching cracking. (3) Selecting the quenching temperature Ac1+ (30-50 degrees) can retain undissolved k'to improve wear resistance, reduce the carbon content of the matrix, and increase the strength, plasticity and toughness of steel.
19. The new technology of low-temperature and high-temperature tempering of high-speed steel will increase the life of high-speed steel quenched and tempered parts. Is it theoretically analyzed?
Uniform precipitation of ε and M3C makes M2C and MC more uniformly precipitate in the secondary hardening temperature range, promotes the transformation of part of the retained austenite into bainite, and improves the strength and toughness.
20. Point out the following alloy types
ZL104: cast aluminum, MB2: deformed magnesium alloy, ZM3: cast magnesium, TA4: α-type titanium alloy, H68: brass, QSn4-3: tin brass, QBe2: beryllium brass, TB2: β-type titanium alloy.
21. What is fracture toughness? How to judge whether a part has low-stress brittle fracture according to the fracture toughness K1c of the material, the working stress σ of the part and the crack radius length α in the part?
The performance index indicating the ability of a material to resist fracture is fracture toughness. According to the formula: δ=δs/k K1=δ√πα K1C=δ√πα If K1>K1c, the material will undergo low-stress brittle fracture
1. Compared with the traditional gray cast iron, the phase transformation characteristics: (1) Cast iron is a Fe-C-Si ternary alloy, and the eutectoid transformation is in a wide temperature range, in which there is ferrite + austenite + Graphite; 2) The graphitization process of cast iron is easy to proceed. Control this process to obtain ferrite matrix, pearlite matrix and ferrite + pearlite matrix cast iron; (3) Heating, heat preservation, The cooling conditions can be adjusted and controlled within a large range of A and the carbon content of the transformation products; (4) Compared with steel, the diffusion distance of carbon atoms is longer; (5) The heat treatment of cast iron cannot change the shape and distribution of graphite. Can change the organization and performance of the collective.
22. The basic process of A forming when steel is heated? What factors affect the grain size of A?
Formation process: the formation of A crystal nucleus, the growth of A crystal grain, the dissolution of residual cementite, the homogenization of A; factors: heating temperature, holding time, heating speed, steel composition, original structure.
23. What are the main ways to accelerate chemical heat treatment? Compare the characteristics of one-stage carburizing, two-stage carburizing and dynamic carbon potential control?
Ways: segmented control method, composite infiltration treatment, high temperature diffusion, new materials that accelerate the diffusion process, chemical infiltration, and physical infiltration.
24. What are the three basic heat transfer methods? Give examples of the application in energy saving of heat treatment furnace?
Heat transfer method: conduction heat transfer, convection heat transfer, radiation heat transfer; not found (radiation heat transfer for vacuum furnaces above 700 degrees)
25. What is the black structure that appears in carbonitriding? How to prevent it from occurring?
Black tissue refers to black spots, black belts, and black nets. In order to prevent the appearance of black tissue, the nitrogen content in the infiltrated layer should not be high enough, generally more than 0.5%, it is easy to appear dotted black tissue; the nitrogen content in the infiltrated layer should not be too low, otherwise it is easy to form a troostite network. In order to suppress the trostanite network, the amount of ammonia added should be moderate. The amount of ammonia gas is too high, and the dew point of the furnace gas decreases, which will promote the appearance of black tissue.
In order to suppress the appearance of tostenite network, it can also be appropriately increased.
