Heat treatment question and answer

1. What are the commonly used quenching methods? Explain the principles of selecting different quenching methods?

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

Dual liquid quenching - Objective: To cool V > Vc rapidly between 650 and Ms, and to cool V > Vc slowly below MS, so as to reduce tissue stress. Carbon steel: First water, then oil. Alloy steel: oil before air.

Stage hardening - Staying at a certain temperature after the workpiece is taken out to make the internal and external temperature of the workpiece consistent, then air-cooled process. Stage hardening occurs when air-cooled M phase changes, with low internal stress.

Isothermal quenching - refers to the isothermal transformation of bainite occurring in the bainite temperature zone, which reduces the internal stress and reduces the deformation.

The selection principle of quenching method should consider not only meeting the performance requirements, but also reducing the quenching stress as far as possible to avoid quenching deformation and cracking.

2. What are the differences between chemical vapor deposition and physical meteorological deposition, and their main applications?

Chemical meteorological deposition is mainly CVD method. Reaction medium containing coating material elements is gasified at low temperature, and then sent into high temperature reaction chamber to contact the workpiece surface to produce high temperature chemical reaction. Alloys or metals and their compounds are precipitated and deposited on the workpiece surface to form coating.

The main characteristics of CVD method are as follows: 1) Inorganic thin film materials of various crystalline or amorphous states can be deposited. 2) High purity, strong collective binding force. 3) The deposits are dense with few stomata. 4) Good homogeneity, simple equipment and process. 5) Higher reaction temperature.

Applications: Thin films for various purposes are prepared on the surfaces of steel, cemented carbide, nonferrous metals, inorganic nonmetals 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 on the surface of a workpiece into a solid film is called PVD method. There are three basic methods: vacuum evaporation, sputtering and ion plating. Application: Wear-resistant coating, heat-resistant coating, corrosion-resistant coating, lubrication coating, functional coating decorative coating.

3. Explain the micro-morphology and macro-morphology of fatigue fracture.

Microscopic: A strip pattern observed under a microscopic electron microscope is called a fatigue band or a fatigue glow pattern. Fatigue strips have ductility and brittleness. Fatigue strips have certain spacing. Under certain conditions, each strip corresponds to a stress cycle.

Macroscopic: It is brittle fracture in many cases, and macroscopic deformation is not visible to the naked eye. Typical fatigue fracture is composed of crack source zone, crack propagation zone and final instantaneous fracture zone. The fatigue source area is less flat and sometimes is a bright mirror. The crack propagation area is in the shape of river beach or shell. Some fatigue sources with different spacing are parallel arcs of the center of the circle. The micro-morphology of the instantaneous fracture zone can be characterized by dimple or quasi-dissociation, intergranular fracture or mixing.

4. Point out three kinds of quality problems which often occur in induction heating quenching, and try to analyze the reasons.

1) Cracking: excessive heating temperature and uneven temperature; improper selection of quenching medium and temperature; untimely tempering and insufficient tempering; high hardenability of materials, segregation of components, defects, excessive inclusions; unreasonable design of parts.

2) uneven surface hardness: irrational induction structure; uneven heating; uneven cooling; poor material structure (banded structure, partial decarbonization).

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

5. What are the characteristics of the new low-temperature tempering process for high-speed steel? (Take w18Cr4v for example) Why is it better in mechanical properties than ordinary tempering?

W 18Cr4v steel 1275 is quenched by heating + 320*1h+540 to 560*1h*2 tempering.

1) The precipitation of M2C carbides in low-temperature tempered high-speed steels is more sufficient than that in ordinary tempered high-speed steels. The dispersion and homogeneity of M2c, V4c and Fe3c carbides are greater, and about 5% to 7% bainite exists. This is an important structural factor for the performance of low-temperature tempered high-speed steels to be better than that of ordinary tempered steels.

6. What kinds of controllable atmosphere are commonly used? The characteristics and applications of each atmosphere are briefly described.

There are endothermic atmosphere, drip atmosphere, straightforward atmosphere, other controllable atmosphere (nitrogen machine atmosphere, ammonia decomposition atmosphere, exothermic atmosphere) and so on.

1) The endothermic atmosphere is to mix the feed gas with air in a certain proportion and react with catalyst at high temperature to form an atmosphere mainly containing CO, H2, N2 and trace CO2, O2 and H2O. Because the reaction needs to absorb heat, it is called endothermic atmosphere or RX atmosphere. Used for carburizing and carbonitriding.

2) Drop-injection atmosphere is to crack methanol directly into the furnace to form carrier containing CO and H2, then add enriching agent for carburizing, carbonitriding at lower temperature, protective heating and bright quenching, etc.

3) The carburizing agent, such as natural gas and air, is mixed in a certain proportion and directly into the furnace. The carburizing atmosphere is directly generated by 900 reaction at high temperature. Ammonia decomposition gas is used to protect the atmosphere of nitriding carrier gas, steel or non-ferrous metals by low temperature heating. Nitrogen-based atmosphere has good protection effect for high carbon steel or bearing steel. Exothermic atmosphere is used for bright heat treatment of low carbon steel and copper or decarburization annealing of malleable iron.

7. What is the purpose of isothermal quenching of ductile iron? What are the isothermal temperature and the structure after isothermal quenching?

AIM: Isothermal quenching of austenitized ductile iron in bainite transformation zone can obtain good mechanical properties and small distortion.

Isothermal temperature: Lower bainite structure was obtained at 260 ~ 300 and upper bainite structure was obtained at 350 ~ 400 C.

8. Briefly describe the main characteristics of chemical heat treatment (carburizing, nitriding, carbonitriding and nitrocarburizing), the structure and properties of heat treatment, which materials or parts are mainly suitable for?

Carburizing: It is mainly the process of infiltrating carbon atoms into the surface of workpiece, surface tempered martensite, residual A and carbide. The purpose of the center part is to increase the carbon content of the surface layer, with high hardness and wear resistance. The center part has certain strength and toughness to withstand large impact and friction. Low carbon steel such as 20CrMnTi, gear and piston pins are commonly used.

Nitriding: Nitrogen atoms infiltrate into the surface, which improves the surface hardness, wear resistance, fatigue strength, corrosion resistance and thermal hardness. The surface layer is nitride, tempered sorbite at the heart, gas nitriding, liquid nitriding, etc., commonly used 38CrMoAlA, 18CrNiW.

Carbonitriding: Carbonitriding has low temperature, fast speed and small deformation of parts. 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. It is often used in heavy and medium load gears made of low and medium carbon alloy steel.

Nitrocarburizing: Nitrocarburizing process is faster, surface hardness is slightly lower than nitriding, but fatigue resistance is good. It is mainly used for parts and dies with small impact load, high wear resistance, high fatigue limit and small deformation. Carbon structural steel, alloy structural steel, alloy tool steel, gray cast iron, ductile iron and powder metallurgy are all nitrocarburized.

9. Principles of heat treatment process design

1) Advancement of Technology

2) The process is reliable, reasonable and feasible.

3) Economy of process

4) Process safety

5) Making the best use of mechanized and highly automated process equipment

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

1) Considering fully the connection between cold and hot processing technology, the arrangement of heat treatment process should be reasonable.

2) Adopt new technology as far as possible, outline heat treatment process and shorten production cycle. Under the condition of guaranteeing the required organization and performance of parts, the different processes or processes should be combined as far as possible.

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

11. Brief description of the principles to be followed in the design of inductors

1) The coupling distance between inductor and workpiece should be as close as possible.

2) Driving magnet must be added to the workpiece heated by the outer wall of the coil.

3) Avoiding sharp-angle effect in the design of work-piece inductors with sharp-angle

4) To avoid the offset phenomenon of magnetic lines 5. The design of inductors should satisfy the requirement that the workpiece can turn when heated as far as possible.

12. What basic principles should designers consider when selecting materials

1) Select materials according to the working conditions of the parts, including load type and size, environmental conditions and main failure modes; 2) Considering the structure, shape and size of the parts, the materials which are easy to produce quenching distortion and cracking should have good hardenability, and can be treated by oil quenching or water-soluble quenching media;

3) Understanding the structure and properties of materials after heat treatment. Some steel grades developed for various heat treatment processes will have better structure and properties after heat treatment.

4) On the premise of guaranteeing the service performance and service life of the parts, the heat treatment process should be simplified as far as possible, especially the material that can be saved.

13. What technological properties should be considered when choosing metal materials to make parts

1) Castability

2) Pressure Processing Performance

3) Machinability

4) Weldability

5) Heat treatment process performance.

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

Wear types: adhesion wear, abrasive wear, corrosion wear, contact fatigue.

Preventive methods: for adhesive wear, reasonable selection of friction pair material; surface treatment to reduce friction coefficient or improve surface hardness; reduce contact compressive stress; reduce surface roughness. For abrasive wear, besides reducing contact pressure and sliding friction distance in design, improving lubricating oil filter device to remove abrasive particles, high hardness materials should be selected reasonably, and surface hardening and heat treatment methods should be adopted to improve the surface hardness of friction pair materials. For corrosive wear, choose antioxidant materials; surface coating; select corrosion resistant materials; electrochemical protection; add corrosion inhibitor design to reduce the stress concentration of tensile stress; carry out stress relief annealing; select materials insensitive to stress corrosion; change medium conditions. For contact fatigue, improve the hardness of materials; improve the purity of materials, reduce inclusions; improve the strength and hardness of the core parts; reduce the surface roughness of parts; improve the viscosity of lubricating oil to reduce the role of oil wedge.

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 subsection control method, composite infiltration treatment, high temperature diffusion, new materials for accelerating diffusion process, chemical infiltration and physical infiltration are adopted.

17. What are the advantages of "Carburizing Section Control Process"?

Prevent the surface oxidation of the workpiece, facilitate diffusion, make the three processes fully coordinated, reduce the formation of carbon black on the workpiece surface, speed up the carburizing process, and ensure that the transition layer is wide and smooth.

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

From the surface centripetal part, hypereutectoid, eutectoid, hyper-hypoeutectoid and primitive hypoeutectoid are in turn.

19. What is granular bainite?

It is composed of massive (equiaxed) ferrite and high carbon zone A.

20. Describe the type, purpose and use of ball retreat?

Ordinary ball back: Increase hardness, improve machinability, reduce quenching distortion cracking.

Isothermal ball regression: Used in high carbon tool steel and alloy tool steel.

Circulating ball regression: used for carbon tool steel and alloy tool steel.

21. The quenching temperature of hypoeutectoid steel is usually above Ac3. Why the quenching temperature of hypereutectoid steel is between Ac1-Acm? Try to analyze it theoretically.

(1) As the content of hypoeutectoid steel is low, the original structure P+F, IF quenching temperature is lower than Ac3, there will be undissolved F, and after quenching, there will be soft spots. For hypereutectoid steels, if the temperature is too high, excessive k'dissolution will increase the amount of flake M, easily cause deformation and cracking, increase the amount of A', excessive k' dissolution, and reduce the wear resistance of steels.

(2) When the temperature of hypereutectoid steel is too high, the tendency of oxidative decarbonization increases, which makes the surface composition of the steel uneven, and the height of MS is different, leading to quenching cracking.

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

22. The new low-temperature and high-temperature tempering technology of high-speed steel will increase the service life of quenched and tempered parts of high-speed steel.

Homogeneous precipitation of epsilon and m3C makes M2C and MC precipitate more uniformly in the range of secondary hardening temperature, promotes transformation of some retained austenite into bainite, and improves strength and toughness.

23. Point out the following types of alloys

ZL104: Cast Aluminum

MB2: Deformed Magnesium Alloy

ZM3: Cast Magnesium

TA4:alpha Titanium Alloy

H68: brass.

QSn4-3: tin brass

QBe2: Beryllium Brass

TB2:beta titanium alloy

24. What is fracture toughness? How to judge whether low stress brittle fracture occurs in parts according to fracture toughness K1c of materials, working stress_of parts and crack radius length alpha of parts?

It shows that the fracture toughness is the performance index of the material's fracture resistance.

According to the formula:

If K1 > K1c, low stress brittle fracture occurs.

25. Phase Change Characteristics of Gray Iron Compared with Rigid Cast Iron

(1) Cast iron is a Fe-C-Si ternary alloy, eutectoid transformation occurs in a wide temperature range, where ferrite+austenite+graphite exists.

(2) The graphitization process of cast iron is easy to carry out, and iron with ferrite matrix, pearlite matrix and ferrite + pearlite matrix can be obtained by controlling the process.

(3) By controlling austenitizing temperature, heating, holding and cooling conditions, the carbon content of A and transformation products can be adjusted and controlled in a considerable range.

(4) Compared with steel, the diffusion distance of carbon atoms is longer.

(5) Heat treatment of cast iron can not change the shape and distribution of graphite, but can only change the collective structure and properties.

26. The basic process of A formation when steel is heated? Influencing factors of A grain size?

Formation process: nucleation of A, growth of A grain, dissolution of residual cementite, homogenization of A;

Factors: heating temperature, holding time, heating speed, steel composition and original structure.

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

Approaches: Segmental control method, composite infiltration treatment, high temperature diffusion.

Characteristic: Using new materials to accelerate diffusion process, chemical and physical catalysis.

28. What are the three basic heat transfer modes? Examples are given to illustrate the application of energy saving in heat treatment furnaces.

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

Application:

1) Conduction and heat transfer are mainly applied to heat transfer of furnace wall.

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

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

29. What is the black structure in carbonitriding? How to prevent it from happening?

Black tissue refers to black spots, black belts and black nets.

1) In order to prevent the appearance of black tissues, the nitrogen content in the nitriding layer should not be high enough, usually more than 0.5% is prone to spot-like black tissues, and the nitrogen content in the nitriding layer should not be too low, otherwise it is easy to form a Torstrite network.

2) In order to inhibit the toxoplast network, the appropriate amount of ammonia, the excessive amount of ammonia and the decrease of dew point of furnace gas will promote the appearance of black tissue. In order to inhibit the appearance of the toroidal network, the quenching heating temperature can be properly increased or the cooling medium with strong cooling ability can be used. When the depth of black tissue is less than 0.02 mm, spraying is used to strengthen the remedy.

30. Brief description of the selection principle of induction heating quenching process parameters

Heating method: Induction heating quenching has two methods: simultaneous primary quenching and continuous quenching by moving heating, which can be used to select equipment conditions and parts. At the same time, the specific power of heating is generally 0.5-4.0 kw/square centimeter, while the specific power of moving heating is generally greater than 1.5 kw/square centimeter. Longer shaft parts, tubular inner hole quenching parts, medium modulus gear with wide teeth and strip parts are continuously quenched, while super large gears are continuously quenched by single teeth.

Heating parameters: 1) Heating temperature, due to the fast induction heating speed, in order to make the structure change fully, quenching temperature is 30-50 degrees higher than general heat treatment; 2) Heating time: according to the technical requirements of parts, materials, shape, size current frequency, specific power and other factors.

Quenching Cooling Method and Quenching Medium: Quenching Cooling Method of Quenching Heating Usually adopts Jet Cooling and Invasive Cooling

31. Matters needing attention in tempering

Tempering must be done in time. After quenching, the parts should be tempered within 4 hours. The common tempering methods are self-tempering, in-furnace tempering and induction tempering.

32. Adjustment of Electrical Parameters of Induction Heating

The purpose is to make the high and medium frequency power supply work in a resonant state and make the equipment play a higher efficiency.

1) Adjustment of electrical parameters for high frequency heating (under 7-8 kV Low voltage load condition, adjust coupling, feedback handwheel position to make the ratio of grid current to anode current 1:5-1:10, then raise the anode voltage to use voltage, further adjust the electrical parameters, make the cell voltage adjust to the required value, match the best).

2) The electrical parameters of medium frequency heating are adjusted, and the appropriate turn ratio and capacitance of quenched transformer are selected according to the size of parts, the length of hardening zone and the structure of inductor to make it work in resonance state.

33. What are the common cooling media?

Water, brine, alkali water, mechanical oil, nitrate, polyvinyl alcohol, trinitrate solution, water-soluble quenching agent, special quenching oil, etc.

34. Trial analysis of factors affecting hardenability of steel?

(1) The influence of carbon content: the stability of hypoeutectoid steel increases with the increase of carbon content A, the C curve moves to the right; the unmelted carbide increases with the increase of carbon content in hypereutectoid steel, the stability of A decreases, and the C curve moves to the right.

(2) The influence of alloying elements: all solid-soluble metal elements except Co are shifted to the right of C curve.

(3) A temperature and holding time: The higher the A temperature, the longer the holding time, the more complete the dissolution of carbides, the bigger the A grain and the right shift of the C curve.

(4) The influence of primitive tissue: the thinner the primitive tissue is, the easier it is to get uniform A, to move C curve to the right, and to move MS down.

The effect of stress and strain: shifting C curve to the left.

35. Why should the gauge be stabilized? What is the conventional instrumentation stabilization process?

By treatment, the squareness of M can be reduced to be more stable M, and the transformed A'ageing can be achieved, and the residual stress after quenching and cryogenic treatment can be reduced, which has a good effect on dimensional stability.

36. What are the two methods of bearing superfine treatment and what are their purposes?

(1) The purpose of hot forging quenching pretreatment is to make the residual K of A'11.9%~12.1% 7.11% and the grain size of A'9~10 grades.

(2) Purpose of bearing double refinement treatment: After treatment, the carbide grain size of grade 1.5-2.0 can be refined by less than 0.6 um compared with the original grain size, which is beneficial to improve the fine needle-like M structure after quenching, and to improve the toughness, wear resistance and fatigue strength.

37. What problems should be considered in the formulation of heat treatment heating process?

(1) The advancement of technology fully adopts new technological methods and new heat treatment technologies and new technological materials.

(2) The reliable, reasonable and feasible process should be very reliable and stable.

(3) The economic process of technology should make rational use of energy, and the energy-saving process equipment should make full use of the existing equipment and adopt auxiliary tooling to meet the technological requirements of different parts.

(4) Safety of the process should be safe and reliable, and necessary safety precautions should be taken.

The adoption of mechanized and highly automated process equipment as far as possible can not only improve labor productivity, but also benefit the control of process and ensure the quality and reliability of heat treatment.

38. What is spheroidizing annealing? What are the process characteristics?

The so-called spheroidizing annealing of steel is an annealing process for spheroidizing carbides in steel.

1) The spheroidizing heating temperature of carbides in ordinary spheroidized annealing steel Ac1+20-30 (?) C depends on the time of workpiece burning through, but it is not suitable for long time. The cooling rate is generally 10-20 C/H in the furnace, and the air cooling rate is below 550 C.

2) Isothermal spheroidizing annealing is mainly used for alloy tool steels of high carbon tool steels. The spheroidization of the process is sufficiently easy to control, the cycle is short and the matter is big. The heating temperature is Ac1+20-30 C, and the holding time depends on the burning time of the workpiece. The holding temperature is Ac1+20-30 C, and the isothermal time depends on TTT curve.

39. Why can hypoeutectoid steel obtain higher strength and hardness than annealed steel after normalizing?

Both annealing and normalizing results in pearlitic structure. However, when normalizing and annealing are compared, normalized pearlite is obtained at a higher undercooling. Therefore, for hypoeutectoid steels, less pre-eutectoid ferrite is precipitated, more pearlite is produced, and the pearlite sheet spacing is smaller. In addition, the size of pearlite clusters is smaller because of the lower transition temperature and the higher nucleation rate of pearlite. Due to organizational differences, performance is different. Compared with annealing, normalizing has higher strength and hardness and similar plasticity.

40. What is martensite stepped quenching?

Stage quenching is to cool the workpiece directly from quenching temperature to a temperature above Ms point, and take out air-cooling to obtain martensite structure after holding for appropriate time. The general temperature is about 200 C (higher than the Ms point of the material), which is suitable for carbon steel and alloy steel workpieces with smaller effective size and more complex shape and size. Sometimes, the stepped quenching below Ms point is used, and the stepped temperature is 130-160 C. It is suitable for the workpiece with low hardenability and large size.

41. What is bainite isothermal quenching?

Austenitizing of steel or steel parts is heated and then rapidly cooled to isothermal holding in the bainite transformation temperature range (260-400 C), so that austenite is transformed into bainite quenching process. It is a common quenching process.

42. What is shot peening? What is the effect on the surface morphology and properties of the material?

The surface of the sprayed workpiece is impacted by a small projectile sprayed at high speed at room temperature, resulting in elastic-plastic deformation of the surface material at recrystallization temperature and large residual compressive stress, thus improving the surface strength, fatigue strength and stress corrosion resistance of the workpiece. Plastic rheology and work hardening are produced on the surface of the workpiece, and the hardness of the material surface is greatly improved. Reducing the surface roughness of the material while retaining the residual compressive stress on the surface of the workpiece can greatly improve the fatigue strength, fatigue life and stress corrosion resistance of the material.

43. What are the common quenching methods? Talk about the principles of selecting quenching methods.

1) Single medium quenching: simple carbon steel workpieces are water-cooled, alloy steel and alloy tool steel are oil-cooled;

2) Dual media; workpiece with complex and changeable shape;

3) Precooling quenching is used for tool grinding tool rigidity, which can reduce its deformation and cracking.

4) Graded quenching is used for tool steel to reduce deformation and cracking.

5) Isothermal quenching; for alloy steel workpieces requiring small deformation and high strength and toughness.

44. What is the essential grain size of steel?

Essential grain size refers to the grain size determined by standard test method after holding for a sufficient time (3-8 hours) at about 930 C.

45. What measures should be taken to obtain fine austenite grains when steel is heated?

Measures: Reduce temperature, reduce holding time, increase heating speed, and add strong carbon and nitride forming elements to steel.

46. How is the Widmanstatten structure formed in low alloy steels? What are its structural characteristics?

For carbon steel or low carbon alloy steel whose carbon content is less than 0.6% Wc, the eutectoid ferrite precipitates in flake or thick feather shape at a certain cooling rate and a coarser austenite grain, i.e. the so-called Widmanstatten structure.

47. What effect does Widmanstatten structure have on the properties of steel? How to avoid Widmanstatten structure in heat treatment?

1) The mechanical properties decrease, such as toughness.

2) Cooling speed and supercooling degree of austenite in a certain temperature range are guaranteed by adopting "cooling isothermal" process.

48. Analyse the quenching operation methods of shaft parts, long plate parts, parts with large difference in cross section, sleeve and thin-walled circular parts, and parts with concave surface.

Shaft parts should be quenched vertically with coolant. The long plate workpiece should be quenched with coolant on the lateral side. The parts with large section difference should be quenched into coolant first. The sleeve and thin-walled circular parts shall be quenched with coolant along the axis. Workpieces with concave faces shall be quenched upward with coolant.

49. What is the basic principle of induction heating? How to choose the best current frequency according to the depth of hardening layer required by parts?

Induction heating surface hardening is a kind of surface hardening which uses the induction current of a certain frequency produced by the heating inductor connected to AC. The skin effect of the induction current makes the surface layer of the workpiece heated rapidly to the austenite region, and then sprays water to cool immediately, so that the surface layer of the workpiece can obtain a certain depth of hardening layer. The higher the current frequency, the shallower the hardened layer.

Generally speaking, when the depth of hardening layer is 0.5-2 mm, the high frequency power supply of more than 10 kHz should be selected: when the depth of hardening layer is 1.0-4.0 nm, the power supply of 8-3 kHz should be selected; when the depth of hardening layer is over 4.0 mm, the power supply of 1-2.5 kHz should be selected. When the area or diameter of the part is larger, the lower frequency can be chosen. On the contrary, the higher frequency can be chosen for the parts with smaller heating area or diameter.

50. What is the purpose of isothermal quenching of ductile iron? What is the isothermal temperature and the structure after isothermal quenching?

AIM: To obtain good mechanical properties and small distortion of ductile iron after austenitizing and isothermal quenching in bainite transformation zone.

Isothermal temperature: the isothermal temperature of lower bainite isothermal quenching is 260 - 300 C; the isothermal temperature of upper bainite isothermal quenching is 350 - 400 C.

51. Brief Introduction to the Principle and Characteristics of Transmission Electron Microscope Imaging

The structure and imaging principle of TEM are basically the same as that of optical microscope, except that the visible light is replaced by electron beam and the optical lens is replaced by electromagnetic lens. After acceleration, the electron beam emitted by the electron gun converges into a very fine high-energy electron beam spot through the concentrator. The electron beam passes through the sample, and the details are imaged by an imaging system consisting of objective lens, intermediate mirror and projector. The image is finally projected onto the fluorescent screen to form a visible image for observation or photography. The auxiliary system of electron microscopy is complex, including vacuum, voltage stabilization, pneumatic circulation, control and computer systems.

52. What are the characteristics of phase transformation of cast iron compared with steel?

Unlike steel, carbon in cast iron is often diffused over a long distance during phase transformation. Its diffusion rate is influenced by temperature and chemical composition, and has a considerable influence on the phase transformation process and carbon content of phase transformation products.

53. What are the main measures and process methods to reduce heat treatment distortion of parts?

Reducing stress concentration, slowing down heating and cooling speed, reasonable placing of parts, and selecting appropriate tooling.

54. Discuss the main causes and solutions of brittleness of steel during heat treatment.

(1) The brittleness of hypereutectoid steel increases when the secondary carbide network appears at a slower cooling rate after austenitizing, and the plastic phase is separated by the secondary carbide network brittleness in space, which makes it impossible to exert its deformation ability. Solution: Reheating normalizing, increasing cooling rate, restraining the precipitation of brittle phase.

(2) The first kind of tempering brittleness occurs when quenched martensite is tempered at low temperature, and the second kind of tempering brittleness occurs when tempered at high temperature. The first kind of tempering brittleness is unavoidable. The second kind of tempering brittleness can be reheated to the original tempering temperature, and then the toughness can be restored by rapid cooling.

(3) The toughness of the workpiece decreases when the upper bainite appears during isothermal quenching, and the lower bainite can be solved by lowering the isothermal temperature after re-austenitizing.

(4) When austenitizing temperature is too high, grain coarseness and toughness decrease. For example, the toughness of hypereutectoid steel decreases when quenching temperature is high and grain size is coarse, and the brittleness increases when austenite grain is coarse and Widmanstatten structure appears. It can be solved by grain refinement.

55. This paper tries to point out three kinds of defects often occurring after heat treatment of carburized parts, and analyses their causes and preventive measures.

1) Low hardness after quenching: mainly due to low carbon concentration in deep surface layer or surface decarbonization; unreasonable quenching process, no quenching or excessive retained austenite.

2) Insufficient depth of carburizing layer: mainly due to low furnace temperature, short time, or poor circulation of atmosphere in the furnace, unclean surface of parts, carbon deposition on the surface of workpieces with excessive carbon potential; increase the temperature and time of carburizing, clean the surface of workpieces before charging, and reasonably control carbon potential.

3) Massive Wangzhuang carbides appear in the carburized layer, mainly because the surface carbon concentration is too high, the activity of carburizing agent is reduced, and the carbon potential is strictly controlled.

56. Automobile and tractor gears are made of 20CrMnTi material. The processing routes are as follows: blanking, forging, normalizing, machining, carburizing, quenching + low temperature tempering, shot peening and grinding finished products. The role of each heat treatment process is analyzed.

Normalizing - Eliminate the forging stress, make the structure uniform, adjust the hardness and improve the machinability.

Carburizing - Increasing the Concentration of Carbon on Tooth Surface (0.8-1.05%C)

Quenching - Increasing the hardness of the tooth surface and obtaining a certain depth of hardening layer, resulting in the surface of M tempered + alloy carbide + gamma with high hardness (58 ~ 62HRC), high wear resistance, high strength and a certain degree of toughness. To improve the wear resistance and contact fatigue strength of the tooth surface, M tempering + F is obtained in the center of the tooth, which has high toughness.

Low temperature tempering - eliminating quenching stress, preventing grinding cracks and improving impact resistance.

57. What is steel quenching?

The quenching of steel is a heat treatment process that heat the steel to the temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) for a period of time to austenitize it completely or partially, and then cool it to the temperature below Ms (or near Ms) at a cooling rate greater than the critical cooling rate for martensite (or bainite) transformation.

58. Taking carbon steel as an example, the possible microstructures obtained during quenching and their formation temperature range, microstructures, sub-structures and properties are pointed out respectively.

Hypoeutectoid steel: heating temperature Ac3+ (30-50)C, structure A+insoluble K, fast cooling below 550 C, normal structure above 350 C is dislocation, properties: high strength, high hardness, good plasticity and toughness.

Hypereutectoid steel: heating temperature Ac1+ (30-50) (?) C), structure A+insoluble K, fast cooling to below 200 (?) C. Normal structure obtained is flaky M+residual A+insoluble K, its sub-structure is twin, and its properties are high hardness and brittleness.

59. MARTENSITE STAGE QUENCHING

The steel or workpiece is austenitized by heating and then immersed in a liquid medium (salt bath or alkali bath) slightly higher or lower than the upper martensite point of the steel. The quenching process, also known as stepped quenching, is maintained for an appropriate time until the inner and outer layers of the steel reach the medium temperature and then air-cooled to obtain martensite structure. For alloy tool steel and small cross-section carbon tool steel, it can reduce deformation and cracking.

60. Hot Bath Quenching

The workpiece is cooled only by immersing in nitrate, salts and caustic soda at 150 - 180 C. The residence time is equal to one third to one half of the total heating time. Finally, the workpiece is taken out and cooled in air.

61. Isothermal Quenching of Bainite

The austenitizing of steel or steel parts is heated and then rapidly cooled to the bainite transformation temperature region (260-400 C) to maintain the isothermal temperature, so that the austenite is transformed into bainite quenching process. Sometimes it is also called isothermal quenching. The process can be used for alloy steel workpiece with small deformation and high toughness.

62. The known processing technology of GCr15 steel precision bearing is as follows: blanking, forging, ultra-fine treatment, machining, quenching, cold treatment, tempering, stabilization treatment. The main heat treatment parameters (heating temperature and cooling method) including superfine treatment, quenching, cooling treatment, tempering and stabilization treatment, and the purpose of adopting this process are briefly described.

Preparatory heat treatment: 1050 20 ~ 30 min, 320 340 2 h isothermal solution, 735 740 3 h furnace cooling and 600 outlet air cooling, is conducive to improving the fine acicular martensite structure after quenching, and improving impact toughness, wear resistance and fatigue strength.

Quenching: 835-850 45-60 min heated in protective atmosphere, cooled in No. 10 oil at 150-170 for 5-10 min, then cooled in oil at 30-60 C.

Cold treatment: Deep cooling treatment at - 40 ~70 C * 1 ~ 1.5 h after cleaning.

Tempering: tempering at 160-200 C for 3-4 h.

Stabilization treatment: 140-180 C 4-12 h after rough grinding and 120-160 C 6-24 h after fine grinding.

63. What is the actual grain size? Based on the actual production of heat treatment, what kind of grain steel should be selected?

Actual grain size: refers to the grain size obtained under a certain actual heat treatment heating condition.

From the point of view of heat treatment production, in order to obtain fine austenite grain, essential fine grain steel should be selected. In this way, the grain growth tendency is small, the quenching temperature range is wide, and it is easy to master in production.

64. What is the black structure during carbonitriding? How to avoid it?

Black structure refers to black spots, black belts and black nets in the surface layer of carbonitriding.

In order to prevent the occurrence of black address, the nitrogen content in the infiltrated layer should not be too high, usually more than 0.5% of Wn is prone to spot-like black tissue, and the nitrogen content in the infiltrated layer should not be too low, otherwise it is easy to form a tussah network. Therefore, the amount of ammonia should be moderate, the amount of ammonia is too high, and the dew point of furnace gas decreases, which will promote the appearance of black tissue.

In order to restrain the appearance of the towed net, the cooling medium with strong cooling ability can also be used to properly increase the quenching heating temperature. Shot peening can also be used to remedy the black tissue when the depth of light rain is 0.02 mm.

65. What kinds of pearlite are there? What are their morphological and performance characteristics?

The morphology of pearlite can be divided into two types: flake pearlite and granular pearlite.

1. Flake pearlite

It is composed of cementite and ferrite arranged alternately.

(1) Formation of flaky pearlite: At first, the nucleus of cementite precipitates on the grain boundary of austenite and grows in flaky direction. Carbon-poor austenite appears on both sides of the austenite, which promotes the nucleation and growth of ferrite on the interface of austenite and cementite, and makes the austenite near the flaky ferrite rich in carbon, and promotes the nucleation of cementite along the interface of austenite and ferrite. Big. When the above way of pearlite develops horizontally, the carbon in the austenite at the front of the flake ferrite diffuses to the front of the cementite, which promotes the growth of the extender along the longitudinal direction, resulting in the formation of the pearlite field. In an austenite grain, several pearlite domains can be formed.

(2) Pearlite interlamellar spacing: Pearlite interlamellar spacing refers to the average distance between two adjacent cementites in pearlite, and its size mainly depends on the transition temperature (supercooling). The lower the transition temperature is, the smaller the lamellar spacing is, the finer the pearlite structure is, and the greater the dispersion of cementite is.

2. Granular pearlite

The formation of granular pearlite is also a process of alternate precipitation of cementite and ferrite, in which the precipitation of cementite is a non-spontaneous nucleus of undissolved carbide in austenite grains in the fire-rich zone. As the growth of each grain is approximately the same, it eventually becomes a granular pearlite with granular (spherical) cementite evenly distributed on the ferrite matrix. Generally, it is considered that the austenitizing temperature is low. It is advantageous to form granular pearlite.

3. Mechanical properties of pearlite

The strength and hardness of lamellar pearlite increase with the decrease of lamellar spacing, while the strength and hardness of granular pearlite are lower, plasticity and toughness are better.

66. What measures can be taken to obtain fine austenite grain size in steel during heating?

1) Heating temperature and holding time: The higher the temperature, the longer the holding time, the faster the austenite grain growth and the bigger the grain size. The growth rate of austenite grain increases exponentially with the increase of temperature, while the effect of holding time on grain growth is greater at high temperature than at low temperature.

2) Heating speed: The higher the heating speed and superheat, the higher the actual forming temperature of austenite, because the ratio of nucleation rate to growth rate increases. Therefore, small initial grains can be obtained. It also shows that fine austenite grains can be obtained by rapid heating.

3) Chemical composition of steel: With the increase of carbon content in steel, but not enough to form undissolved carbides, austenite grains are easy to grow and coarsen. Therefore, eutectoid carbon steel is more sensitive to overheating than hypereutectoid carbon steel.

4) The original structure of steel: When the original structure is finer or the original structure is unbalanced, the bigger the carbide decomposition degree is, the smaller the initial austenite grain is, but the tendency of grain growth of steel increases, and the superheat sensitivity increases. For this reason, it is not appropriate to use too high heating temperature and too long holding time for steel with very fine original structure.

67. How does the first and second tempering brittleness come into being? How to eliminate temper brittleness after tempering?

The first kind of tempered brittleness (tempered martensite brittleness): When carbon steel is tempered in the temperature range of 200 ~400 degree C, the impact toughness at room temperature will decrease, resulting in brittleness, that is, the first kind of tempered brittleness or tempered martensite brittleness. For alloy steels, the temperature range of brittleness is slightly higher, ranging from 250 to 450 degrees.

If the first kind of tempering brittleness occurs after tempering, it can be eliminated by reheating and quenching.

The second type of tempering brittleness (high temperature tempering brittleness or reversible tempering brittleness): The impact toughness of some alloy steels decreases when tempered slowly and passed through the above temperature range in the temperature range of 450-650 degrees. The brittleness of these brittle steels will disappear if they are reheated again to a predetermined tempering temperature (slightly higher than the temperature range for brittleness) and then cooled rapidly to room temperature. Therefore, it is also called reversible tempering brittleness.

68. What is the hardenability of steel?

The ability of steel to acquire martensite during quenching is called hardenability. The hardenability of steel depends on the critical cooling rate of steel. The more right the position of C curve is, the smaller the critical cooling rate is and the greater the hardenability is.

69. What are the factors affecting hardenability?

1) The influence of carbon content: With the increase of carbon content in austenite, the stability increases and the C curve shifts to the right.

2) The effect of alloying elements: alloying elements (except Co) can improve the hardenability of steel.

3) The effect of austenitizing temperature and holding time: the higher the austenitizing temperature, the longer the holding time, the more complete the dissolution of carbide, the bigger the austenite grain, the smaller the total area of boundary and the nucleation, thus delaying the right shift of C curve to pearlite transformation. In a word, the faster the heating speed, the shorter the holding time, the smaller the austenite grain, the more uneven the composition and the more undissolved second phase, the faster the isothermal transformation speed and the left shift of the C curve.

70. Austenite grain growth should be controlled during heat treatment. Factors affecting austenite grain growth and measures to control austenite grain growth should be analyzed.

1) Heating temperature and holding time: The higher the heating temperature is, the longer the holding time is, the bigger the austenite grain is, and the heating temperature is the main factor.

2) Heating speed: The faster the heating speed is, the higher the superheat is. The higher the ratio of nucleation rate to growth rate is, the finer the grain is, and the higher the actual grain size of austenite is.

3) Chemical composition of steel:

(1) Carbon steel-eutectoid steel is easier to overheat than hypereutectoid steel;

(2) The addition of carbon and nitride forming elements such as Ti, V, Vr, Nb, W, Mo and Cr into alloy steel strongly hinders the migration of austenite grain boundaries and refines the grain size. The grain size of Steel Deoxidized with Al is fine, while that of Steel Deoxidized with Si is coarse.

4) Primitive structure: When the original structure is finer or unbalanced, the grain size of steel tends to increase, and the grain is easy to coarsen.

71. What are the general categories of cast iron? The existing forms of carbon in these cast iron and their effects on the properties of cast iron are pointed out respectively.

1) Grey cast iron: It has high compressive strength, excellent wear resistance and vibration attenuation, and low notch sensitivity.

2) Ductile iron: It not only has a little gray iron, but also has the tensile strength, bending fatigue strength and good plasticity and toughness of medium carbon steel.

3) Malleable cast iron: Graphite is flocculent and has little cutting effect on the matrix, so its strength, plasticity and toughness are higher than those of gray cast iron, especially pearlitic malleable cast iron, which is comparable to cast steel, but can not be forged.

4) Vermicular graphite cast iron: Vermicular graphite cast iron is superior to gray cast iron in tensile strength, plasticity and fatigue strength, while ductile iron is close to ferrite matrix. In addition, its thermal conductivity, castability and machinability are superior to nodular cast iron, which is similar to gray cast iron.

72. Give examples and briefly explain what effective heat treatment methods can be used to improve the service life of the die. Please give examples to illustrate more than five kinds.

1) Compound infiltration treatment process of 5CrMnMo steel hot forging die: C and N co-infiltration were used to quench at 850-900 (?) C, then tempered at 500 (?) C. After tempering at 500 (?) C, 540 (?) C co-infiltration was carried out at 540 (?) h under the premise of maintaining the heat resistance, wear resistance and certain hardness of the steel.

2) The process of strengthening and toughening and surface compound infiltration of 5CrNiMo steel hot forging die: quenching oil at 950 and pre-quenching oil at 180-260 followed by isothermal quenching at 280 and tempering at 450.

3) Strengthening and Toughening Heat treatment process of 45Cr2NiMoVSi steel hammer forging die: the die is put into the furnace at 500 ~C, preheated at 650 ~2.5h, preheated at 850 ~2h, heated at 970 ~5.5h, precooled to 780 ~oil-cooled at 200 ~C, preheated at 290 ~4h, tempered at 635 ~10h, tempered at 640 ~8h for the second time.

4) Strength and toughness heat treatment process of 5Cr2NiMoVSi steel press module and hammer forging die insert: heating 1150-1200 C, initial forging 1150 C, final forging 850 C, heap cooling after forging.

5) Gas nitrocarburizing treatment of 3Cr2W8V steel die casting die: after gas nitrocarburizing treatment, 580 C * 4.5h, 50% methanol plus 50% ammonia, oil-cooled.

73. The known processing technology of GCr15 steel precision bearing is as follows: blanking-forging-ultra-fine treatment-machining-quenching-cold treatment-stabilization treatment. The main heat treatment parameters (heating temperature and cooling method) including superfine treatment, quenching, cooling treatment, tempering and stabilization treatment, and the purpose of adopting this process are briefly described.

1) Ultra-fine heat treatment process: high temperature heating at 1050 20-30 min, isothermal salt bath at 250-350 350 2 h, air cooling at 690-720 3 h with furnace cooling to 500.

2) Quenching: heating in protective atmosphere at 835-850 45-60 min, cooling in oil at 150-170 for 5-10 min, then cooling in oil at 30-60 C.

3) Cold treatment: after cleaning, cryogenic treatment is carried out at - 40 70 C * 1 ~ 1.5 h.

4) Stabilization heat treatment: roughly grinded for 140-180 C 4-12 h, and finely grinded for 120-160 C 6-24 H.

74. Why is 45 steel usually used for machining machine tool gears, while 20CrMnTi is used for automobile gears. Please work out the processing route and the purpose of adopting heat treatment process separately.

(1) Machine tool gears work smoothly without strong impact, load is not large, speed is medium, the requirements for the strength and toughness of the gear core are not high, generally made of 40 or 45 steel. The working conditions of the gears of automobiles and tractors are worse than those of machine tools. The gears are subjected to heavy loads. They are frequently impacted when overloading and starting, braking and speed changing. The requirements for wear resistance, bending fatigue strength, contact fatigue strength, core strength and toughness are relatively high. Surface quenching with medium carbon steel or medium carbon low alloy by high frequency induction heating can not guarantee the service performance.

(2) Processing technology of machine tool gears: blanking-forging-normalizing-tempering-semi-finishing-high frequency induction heating surface hardening+low temperature tempering-finishing grinding-finished products. Normalizing can homogenize the structure, eliminate the forging stress, adjust the hardness and improve the machinability. The quenching and tempering treatment can make the gears have higher comprehensive mechanical properties, improve the strength and toughness of the tooth center, make the gears withstand larger bending stress and impact load, and reduce quenching deformation; high frequency induction heating surface quenching can improve the surface hardness and wear resistance of the gears, and improve the contact fatigue of the tooth surface; low temperature tempering can eliminate quenching stress without reducing the surface hardness. To prevent grinding cracks and improve the impact resistance of gears.

(3) Processing technology of automobile gears: blanking-forging-normalizing-machining-carburizing, quenching+low temperature tempering-shot peening-grinding-finished products. Normalizing treatment can make the structure uniform and adjust the hardness to improve the machinability; carburizing is to increase the mass fraction of carbon on the tooth surface (0.8% - 1.05%); quenching can improve the hardness of the tooth surface and obtain a certain depth of hardening layer (2.8-1.3 mm), improve the wear resistance and contact fatigue strength of the tooth surface; low temperature tempering can eliminate quenching stress, prevent grinding cracks and improve impact resistance; shot peening treatment can improve the impact resistance. In order to improve the hardness of the tooth surface about 1-3 HRC and increase the residual compressive stress of the surface, the contact fatigue strength is improved.

75. Types and solutions of tempering brittleness.

Tempering brittleness: When quenched steel is tempered, with the increase of tempering temperature, the impact toughness of the steel decreases obviously in a certain tempering temperature range, and the brittleness increases obviously. It can be divided into the first and the second categories.

The first is irreversible tempering embrittlement of quenched steel tempered at 250 ~ 400 C, and the second is reversible tempering at 450 ~ 650 C.

Method: The first generation can not be eliminated, Si can be added to increase the brittle transition temperature to more than 300, and then temper at 250; the second type: temper at brittle temperature for a short time, fast cooling does not occur, slow cooling occurs. Reheating and tempering at brittle temperature for a short time can eliminate rapid cooling.

76. The purpose of micro-fine heat treatment of cold-working die steel? CYCLIC ULTRAFINE TREATMENT OF Cr12MoV STEEL?

OBJECTIVE: Micronization heat treatment includes refinement of matrix structure and carbide of steel. Tissue refinement can improve the strength and toughness of steel, carbide refinement is conducive to strengthening the strength and toughness and wear resistance.

Process: 1150 heating quenching + 650 tempering + 1000 heating oil quenching + 650 tempering + 1030 heating oil quenching 170 isothermal 30 min air cooling + 170 tempering.

77. How many kinds of martensite are common in quenched steel? Substructure? Performance characteristics? Formation conditions?

Slabs and sheets. Slab substructure is dislocation with high strength, hardness, plasticity and toughness; forming condition is low carbon steel, temperature above 200 C. Slab medium and high carbon below 200 C, substructure is twin, performance: high hardness, high brittleness.

78. What are the main defects in ingots?

1) Shrinkage, porosity and inclusions, etc.

2) Segregation: macrosegregation (normal segregation, reverse segregation, specific gravity segregation); microscopic segregation.

79. What principles should be followed in the solid solution treatment process for manufacturing cast aluminium alloys?

1) Quenching temperature: generally slightly lower than the maximum solubility temperature.

2) Quenching heating: In order to prevent casting from overheating and deformation, it is better to enter the furnace at low temperature below 350 C, and then slowly heat to the quenching temperature with the furnace.

3) Insulation time: Insulation time is longer, generally 3-20 hours.

4) Cooling mode: generally cooling in hot water.

80. Explain the means of strengthening different aluminium alloys? Which heat treatment is used to improve the strength of ZL104 gasoline engine?

Deformed aluminium alloy strengthening: cold deformation strengthening (work hardening), heat treatment strengthening (solid solution + aging strengthening), cast aluminium alloy strengthening: modification treatment (fine structure), solution + aging.

ZL104 aluminium alloy was treated by solid solution treatment at (535 +5) *3 h, (175 +5) *9 H. The process is based on sand casting and has a long aging time. When ZL104 aluminium alloy is aged at 175 ~5h by sodium modification and low pressure casting with metal mould, GP zone is formed in the matrix, and the strengthening operation is remarkable.

81. In order to control the grain size of metal during crystallization, what methods are usually used to refine the grain in industrial production?

1) Increase environmental cooling capacity.

2) Chemical modification.

3) Increase liquid flow.

82. In order to improve the cutting performance, what heat treatment should be carried out for 15Cr, 20Cr2Ni4, 40Cr, 5CrMnMo, GCr15 and W18Cr4V steels?

15Cr: normalizing;

20Cr2Ni4: normalizing + tempering;

40Cr: tempering;

5CrMnMo: annealing;

GCr15: spheroidizing annealing;

W18Cr4V: Spheroidizing annealing.

83. The hardenability and hardenability of 20, 45, 40Cr, T8, 65 steels under normal quenching conditions were compared.

Hardenability ranges from high to low: 40Cr, T8, 65, 45, 20;

Hardenability ranges from high to low: T8, 65, 45, 40Cr, 20.

84. Can we use W18Cr4V steel to make cold stamping dies? Why?

It can be used as a die, but it is generally used in dies requiring high strength, high wear resistance and less impact. However, because of its poor toughness, brittle material characteristics and high price, it is not recommended to be used as a cold punching die.

85.The hardness of 45 steel is 217HB~255HB after quenching and tempering treatment, but it is found that the hardness is higher after heat treatment. Can the hardness be reduced by slowing down the cooling rate during tempering? If the hardness is low after heat treatment, can the hardness be improved by lowering the tempering temperature? Explain the reasons.

1) No, tempering temperature should be adjusted.

2) No, it is necessary to reduce tempering temperature after re-quenching.

86. The crankshaft of medium-sized tractor engine requires high strength and good toughness, and the crankshaft journal requires good wear resistance (50HRC~55HRC). (1) Choose material and write out steel number; (2) Make a concise process route for manufacturing; (3) Explain the structure of crankshaft and crankshaft journal surface in service.

Alloy carburized steel: 20CrMnMo, 20CrMnTi, 20MnVB

Process route: blanking - forging - normalizing - machining - carburizing, quenching + low temperature tempering - shot peening - grinding - finished products

Heart Tissue: Fine Pearlite; Surface Tissue: Tempered Martensite

87. What safety measures should we pay attention to when using nitrate bath furnace?

Attention must be paid to safety measures such as explosion protection. In a nitrate bath furnace, any local temperature above 595 C may ignite or explode, and the operating temperature should be strictly controlled below 550 C. Nitrate mixtures are oxidized and should not be mixed with materials that are easily oxidized. Fine carbide materials should not be used as cover for nitrate, and pollution of nitrate bath furnace caused by corruption and blackness accumulated at the discharge end of carburizing furnace must be avoided. When dealing with light metals of magnesium alloys, the maximum temperature of salt bath is specified.

88. How to calculate the effective thickness of the workpieces with the following shapes when making heat treatment process?

1) Round rod shape: calculated by diameter;

2) Flat workpiece: calculated by thickness;

3) Solid cone: calculated according to the diameter of the 1/3 height of the large end;

4) Workpiece with abrupt change of stepped shaft or section: calculated according to larger diameter or section.

89. What principles should be followed in formulating solid solution treatment process for cast aluminium alloy?

(1) Quenching temperature selection: Quenching temperature is generally slightly lower than the maximum solubility temperature, in order to avoid overburning or cracking.

(2) Quenching heating mode: In order to prevent casting from overheating and deformation, it is usually carried out in air circulation furnace. In order to prevent deformation, it is better to use low temperature below 350 degrees into the furnace, and then slowly heat to quenching temperature with the furnace.

(3) Holding time: The grain size of cast aluminium alloy is large, the dissolution of excess phase is difficult, and the holding time is long. Generally, it is 3-20 hours, and the holding time has little relationship with the thickness of the workpiece.

(4) Cooling mode: Because of the complex shape of the castings, more internal defects, lower strength and plasticity, too fast cooling rate will cause serious deformation of the castings, so after quenching, the castings should be cooled in hot water.

90. What is martensitic transformation plasticity? What is "TRIR" steel and what are its performance characteristics?

In the process of phase transformation, when the plasticity of metals and alloys increases, it is often lower than the yield limit of parent phase, that is, plastic deformation occurs, which is called phase transformation plasticity.

Several steels with Ma higher than room temperature and Ms lower than room temperature are designed by martensitic transformation plasticity. When they deform at room temperature, they will induce the formation of M, which in turn will induce the increase of plasticity. This kind of steel has both high strength and plastic deformation, so it is called transformation induced plasticity (TRIR) steel.