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Basic knowledge of heat treatment required for material heat treatment engineers

1. What are the commonly used quenching methods, indicating the principle of using different quenching methods?

Single-liquid quenching - a process of cooling to the bottom in a quenching medium, the stress thermal stress of the single-liquid quenching structure is relatively large, and the quenching deformation is large.

 

Two-liquid quenching - purpose: to quickly cool between 650 ~ Ms, so that V> Vc, slowly cooling below Ms to reduce tissue stress. Carbon steel: first water and then oil. Alloy steel: air first after oil.

 

Graded quenching - a process in which the workpiece is taken out at a certain temperature to make the temperature inside and outside the workpiece uniform, and then air-cooled. The step quenching occurs when the M phase occurs during air cooling, and the internal stress is small.

 

Austempering - refers to the isothermal temperature in the bainite temperature zone, the bainite transformation occurs, the internal stress decreases, and the deformation is small.

 

The principle of quenching method selection should consider both the performance requirements and 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 application?

The chemical meteorological deposition is mainly a CVD method. The reaction medium containing the coating material element is vaporized at a lower temperature, and then sent to a high temperature reaction chamber to contact the surface of the workpiece to generate a high temperature chemical reaction, and the precipitated alloy or metal and its compound are deposited on the workpiece. The surface forms a coating.

 

The main features of the CVD method: 1) can deposit various crystalline or amorphous inorganic thin film materials. 2) High purity and strong collective strength. 3) The sediment layer is dense and there are few pores. 4) Good uniformity, simple equipment and process. 5) The reaction temperature is high.

 

Application: Films for various applications on the surface of steel, cemented carbide, non-ferrous metals, inorganic non-metals, etc., mainly insulator films, semiconductor films, conductors and superconductor films, and corrosion-resistant films.

 

Physical Meteorological Deposition: The process of depositing a gaseous substance directly on the surface of a workpiece into a solid film is called the PVD method. There are three basic methods, vacuum evaporation, sputter coating and ion plating. Application: wear resistant coating, heat resistant coating, corrosion resistant coating, lubricious coating, functional coating decorative coating.

 

3. Explain the microscopic morphology and macroscopic morphology of the fatigue fracture.

Microscopic: It is a strip pattern observed under a microscopic electron microscope, called a fatigue strip or a fatigue glow. Fatigue strips are both ductile and brittle. The fatigue strips have a certain spacing. Under certain conditions, each stripe corresponds to a stress cycle.

 

Macro: In other cases, it has brittle fracture characteristics, and no macroscopic deformation can be seen by the naked eye. The typical fatigue fracture is composed of the crack source region, the crack extension region, and the final instantaneous break region. The area of ​​the fatigue source is small and flat, sometimes it is a bright mirror surface, and the crack propagation area is a river beach or a shell pattern, and some fatigue sources with different pitches are parallel arcs of the center. The microscopic shape of the instantaneous fault zone takes the characteristic load mode and size of the material, which may be dimple or quasi-dissociation, dissociation along the crystal fracture or mixed shape.

 

4. Point out the three quality problems that often occur in induction heating quenching, and analyze the reasons.

1) Cracking: heating temperature is too high, temperature is uneven; quenching medium and temperature are not properly selected; tempering is not timely and tempering is insufficient; material hardenability is high, composition segregation, defects, excessive inclusions; part design unreasonable.

 

2) Uneven surface hardness: unreasonable induction structure; uneven heating; uneven cooling; poor material structure (banded structure, local decarburization).

 

3) Surface melting: The structure of the inductor is unreasonable; the parts have sharp corners, holes, and grains; the heating time is too long, and the surface of the workpiece has cracks.

 

Heat treatment production line

 

5. What are the characteristics of the new high-speed steel tempering process? (take w18Cr4v as an example) Why is it better than ordinary tempering?

 

W18Cr4v steel 1275 heat quenching +320 1h + 540 to 560 1h 2 times tempered.

1) Low-temperature tempered high-speed steel has sufficient precipitation of m2c type carbides than ordinary tempered high-speed steel. M2c, V4c and Fe3c type carbides have large dispersion and uniformity, and about 5% to 7% of bainite exists. This is an important organizational factor for the performance of low-temperature tempered high-speed steel over ordinary tempering.

 

6. What kinds of commonly used controlled atmospheres are there? Briefly describe the characteristics and applications of each atmosphere.

There are an endothermic atmosphere, a drip-type atmosphere, a straight-type atmosphere, and other controlled atmospheres (nitrogen atmosphere, ammonia decomposition atmosphere, exothermic atmosphere).

 

1) The endothermic atmosphere is to mix the raw material gas with air at a certain ratio and pass through a catalyst at a high temperature to form an atmosphere mainly containing CO, H2, N2 and trace amounts of CO2, O2 and H2O. Since the reaction absorbs heat, Called an endothermic atmosphere or RX gas. Used for carburizing, carbonitriding.

 

2) The drip-type atmosphere is to directly dissolve the methanol into the furnace to form a carrier containing CO and H2, and then add a rich agent for carburizing; carbonitriding at a lower temperature, protection heating and bright quenching.

 

3) The infiltration agent, such as natural gas and air, is mixed in a certain ratio and directly introduced into the furnace, and the carburizing atmosphere is directly formed at 900 at a high temperature. Ammonia decomposition gas is used to protect the atmosphere of nitriding carrier gas, steel or non-ferrous metal under low temperature heating. The nitrogen-based atmosphere is good for the protection of high carbon steel or bearing steel. The exothermic atmosphere is used for decarburization annealing of low carbon steel, copper bright heat treatment or malleable cast iron.

 

7. What is the purpose of isothermal quenching of ductile iron? What is the tissue after isothermal temperature and austempering?

OBJECTIVE: Austempering of ductile iron after austenitizing in the bainite transformation zone can obtain good mechanical properties and small distortion.

 

 Isothermal temperature: 260~300 °C to obtain lower bainite structure; 350~400 °C to obtain upper bainite structure.

 

8. Briefly describe the main features of the commonly used chemical heat treatment (carburizing, nitriding, carbonitriding and nitrocarburizing), the microstructure and performance characteristics after heat treatment, which materials or parts are mainly applicable?

Carburizing: mainly the process of infiltrating carbon atoms into the surface of the workpiece, surface tempering martensite, residual A and carbide, the purpose of the core is to increase the surface carbon content, with high hardness and high wear resistance, the core has a certain strength And high toughness, it is subject to large impact and friction, low carbon steel such as 20CrMnTi, gears and piston pins are commonly used.

 

 Nitriding: Nitrogen atoms are infiltrated into the surface, which are surface hardness, wear resistance, fatigue strength and corrosion resistance, and thermal hardness. The surface layer is nitride, core tempered sorbite, gas nitriding, liquid nitriding, etc. 38CrMoAlA, 18CrNiW.

 

Carbonitriding: The carbonitriding temperature is low, the speed is fast, and the deformation of the parts is small. The surface 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 certain corrosion resistance. Commonly used in heavy and medium load gears made of low and medium carbon alloy steel.

 

Nitrocarburizing: The nitrocarburizing process has a faster co-filtration rate, and the surface hardness is slightly lower than that of nitriding, but the fatigue resistance is good. It is mainly used for parts and tools that require less impact load and require 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 be nitrocarburized.

 

9. Briefly describe the principles of heat treatment process design

1) Advanced technology

2) Process is reliable, reasonable and feasible

3) Economics of the process

4) Process safety

5) Try to use mechanical and automated process equipment

 

10. What problems should be considered in the optimization design of the heat treatment process?

1) Fully consider the connection between the hot and cold processing processes, and the arrangement of the heat treatment process should be reasonable;

2) Use new technology as much as possible, briefly describe the heat treatment process, and shorten the production cycle. Under the conditions of ensuring the required structure and performance of the parts, try to combine different processes or processes into each other;

3) Sometimes in order to improve product quality and extend the life of the workpiece, it is necessary to increase the heat treatment process.

 

11. Briefly describe the principles that should be followed in sensor design.

1) The coupling distance between the sensor and the workpiece should be as close as possible

2) For the workpiece heated by the outer wall of the coil, it is necessary to add a flow-guiding magnet.

3) Design of workpiece sensors with sharp corners to avoid sharp corner effects

4) Avoid the offset phenomenon of magnetic lines. 5. The sensor design should try to satisfy the workpiece's rotation when heating.

 

12. What are the basic principles that designers should consider when selecting materials?

1) According to the working conditions of the parts, including the type and size of the load, environmental conditions and main failure modes, etc.; 2) Considering the structure, shape and size of the parts, it is necessary to use hardening for quenching distortion and cracking. Good properties, materials that can be treated with oil-quenched or water-soluble quenching media;

3) Understand the microstructure and properties of the material after heat treatment, and some steel grades developed for various heat treatment processes will have better microstructure and performance after treatment;

4) Under the premise of ensuring the performance and life of the parts, materials should be selected to simplify the heat treatment process, especially the materials that can be saved.

 

13. What process properties should be considered when selecting metal materials for manufacturing parts?

1) Casting performance

2) Pressure processing performance

3) Machinability

4) Welding performance

5) Heat treatment process performance.

 

14. How many types of wear and failure are there? How to prevent all kinds of wear and tear of parts?

Type of wear: adhesive wear, abrasive wear, corrosion wear, contact fatigue.

 

Prevention method: For the adhesive wear, the friction pair matching material is selected reasonably; the surface treatment is used to reduce the friction coefficient or increase the surface hardness; the contact compressive stress is reduced; and the surface roughness is reduced. For abrasive wear, in addition to reducing contact pressure and sliding friction distance during design, the lubricating oil filtering device is improved to remove abrasive particles, and high-hardness materials should be reasonably selected; surface friction treatment and surface hardening are used to improve friction pair. Surface hardness of the material. For corrosion and wear, select anti-oxidation materials; surface coating; use corrosion-resistant materials; electrochemical protection; stress concentration to reduce tensile stress when corrosion inhibitor is designed; perform stress relief annealing; select materials that are not sensitive to stress corrosion; Change the media conditions. Contact fatigue, improve material hardness; improve material purity, reduce inclusions; improve part core strength and hardness; reduce part surface roughness; increase lubricant viscosity to reduce oil wedge effect.

 

15. What is the basic process of chemical heat treatment of steel?

Decomposition, adsorption, and diffusion.

 

16. What are the main ways to accelerate chemical heat treatment?

The application of the segmentation control method, composite infiltration treatment, high temperature diffusion, new materials using accelerated diffusion process, chemical infiltration, physical infiltration.

 

17. What is the superiority of the “Carburizing Segmentation Control Process”?

Prevent the surface oxidation of the workpiece, facilitate the diffusion, fully coordinate the three processes, reduce the process of forming carbon black on the surface of the workpiece, accelerate the carburizing process, and ensure a wide and smooth transition layer of the transition layer.

 

18. What is the structure of the surface layer and the core after low carbon steel carburizing and quenching under normal circumstances?

From the surface to the heart, there are, in turn, hypereutectoid, eutectoid, excessive hypoeutectoid, and original hypoeutectoid.

 

19. What is granular bainite?

It consists of massive (equal) ferrite and high carbon A zone.

 

20. What is the type, purpose and use of the ball?

Ordinary ball retreat: increase hardness, improve machinability, and reduce quenching distortion cracking.

Isothermal ball retreat: for high carbon tool steel, alloy tool steel.

Recirculating ball retreat: used for carbon tool steel and alloy tool steel.
 

21. The quenching temperature of the hypoeutectoid steel is often selected above Ac3, and the quenching heating temperature of the hypereutectoid steel is chosen between Ac1-Acm. Is it theoretically analyzed?

(1) Because of the low content of the hypoeutectic 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' dissolves, so that the amount of flake M increases, which is easy to cause deformation and cracking, increase A' amount, excessive k' dissolution, and make steel wear-resistant. Reduced sex.

 

(2) For the hypereutectoid steel temperature is too high, the oxidative decarburization tendency is increased, the surface composition of the steel is not uniform, and the MS level is different, resulting in quenching cracking.

 

(3) Selecting the quenching temperature Ac1+ (30-50 °C) can retain the undissolved k' to improve the wear resistance and reduce the carbon content of the matrix, which is an increase in the strength plasticity and toughness of the steel.

 

22, high-speed steel low temperature, high temperature tempering new technology, will increase the life of high-speed steel quenching and tempering parts, try to analyze from the theory?

Uniform precipitation of ε and M3C causes M2C and MC to precipitate more uniformly in the secondary hardening temperature range, which promotes the transformation of partially retained austenite to bainite and improves the toughness.

 

23. Identify the following alloy types

ZL104: cast aluminum

MB2: Deformed magnesium alloy

ZM3: Magnesium

TA4: α-type titanium alloy

H68: Brass

QSn4-3: tin brass

QBe2: 铍 brass

TB2 : β-type titanium alloy

 

24. What is fracture toughness? How to judge whether the 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 the material to resist fracture is the fracture toughness.

According to the formula:

If K1>K1c, the material has a low stress brittle fracture.

 

25. Phase transition characteristics of gray cast iron compared with just

(1) Cast iron is a Fe-C-Si ternary alloy, and the eutectoid transformation is carried out over a wide temperature range, in which ferrite + austenite + graphite is present;

2) The graphitization process of cast iron is easy to carry out, and the process is controlled to obtain a ferrite matrix, a pearlite matrix and a ferrite + pearlite matrix cast iron;

(3) Adjusting and controlling the carbon content of A and the conversion product in a considerable range by controlling the austenitizing temperature heating, heat preservation and cooling conditions;

(4) Compared with steel, the carbon atom has a longer diffusion distance;

(5) The heat treatment of cast iron does not change the shape and distribution of graphite, but only changes the collective organization and properties.

 

26. When the steel is heated, A forms the basic process? Factors affecting the size of A grain?

Formation process: formation of A crystal nucleus, growth of A crystal grain, dissolution of residual cementite, homogenization of A;

Factors: heating temperature, holding time, heating rate, composition of steel, original structure.

 

27. What are the main ways to accelerate chemical heat treatment? Compare a section of carburizing, two-stage carburizing and dynamic carbon potential control characteristics?

Pathway: segmentation control method, composite infiltration treatment, high temperature diffusion.

Features: New materials that accelerate the diffusion process, chemical osmosis, physical osmosis.

 

28. What are the three basic heat transfer methods? Examples of applications in energy saving in heat treatment furnaces?

Heat transfer mode: conduction heat transfer, convection heat transfer, radiation heat transfer;

application:

1) Conductive heat transfer is mainly used for heat transfer such as furnace walls.

2) Convective heat transfer is mainly manifested in four forms: furnace gas, molten salt in salt bath, flow particles in the flow particle furnace and workpiece surface, furnace wall surface and workshop air.

3) The vacuum furnace above 700 degrees is radiative heat exchange.

 

29, what is carbon
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