Table 1 The basic structure of the iron and steel material. The meaning of the number name 1 The crystal grains and the small crystals formed after the crystallization of the grain boundary metal are inconsistent in shape and the internal character arrangement direction are the same are called crystal grains. The interface between the crystal grain and the crystal grain is called the grain boundary 2 phase and the phase boundary. In a metal or alloy, all the uniform components with the same composition, the same structure and the interface separated from each other are called phases. The interface between the phases is called the phase boundary 3 solid solution The solid phase formed by dissolving the atoms of another element in the crystals of one metal element composing the alloy is called the solid solution. Solid solution generally has higher strength, good plasticity, corrosion resistance and higher electrical resistance.
Table 1 Basic structure of steel materials
Serial number
Name
Meaning
1. Grains and grain boundaries
The small crystals with inconsistent appearance and the same internal character arrangement are called crystal grains after metal crystallization. The interface between the grain and the grain is called the grain boundary
2. Phase and phase boundary
In metals or alloys, all homogeneous components with the same composition, the same structure, and the interface separated from each other are called phases. The interface between the phases is called the phase boundary
3. Solid solution
A solid phase formed by dissolving atoms of another element in the crystals of one metal element constituting the alloy is called a solid solution. Solid solutions generally have higher strength, good plasticity, corrosion resistance, and higher electrical resistance and magnetism
4. Metal compound
A solid phase with metallic characteristics formed by the interaction of atoms of different elements in an alloy with a lattice type and properties completely different from its constituent elements, called a metal compound
5. Austenite
Austenite is a solid solution in which carbon and other elements are dissolved in y-Fe. Austenite has face-centered cubic crystals, good plasticity, and generally exists at high temperatures
6. Ferrite body
Ferrite is a solid solution in which carbon and other elements are dissolved in a-Fe. Ferrite has a body-centered cubic lattice and contains very little carbon. Its performance is very similar to that of pure iron. It is also called pure ferrite.
7. Cementite
Cementite is a compound of iron and carbon, also known as iron carbide (#c), with a carbon content of 6.69% and a complex character structure. Its performance is hard and brittle, almost no plasticity
8. Pearlite
Pearlite is a lamellar structure between ferrite and cementite. It is named for its fingerprint-like pearly luster in its microstructure. Its performance is between ferrite and cementite, moderate strength and hardness, and good plasticity and toughness
9. Sorbite
Also known as fine pearlite, it is a mixture of ferrite and cementite decomposed by austenite at a temperature lower than the formation temperature of pearlite. Its layers are thinner than pearlite and can only be distinguished under a high-power microscope. Hardness, strength and impact toughness are all higher than pearlite
10. Troostite
Also known as ultra-fine pearlite, a mixture of ferrite and cementite decomposed by austenite at a temperature lower than the formation temperature of pearlite. The layers are thinner than sorbite. Its hardness and strength are higher than sorbite
11. Bainite
Bainite is a mixture of supersaturated ferrite and cementite. Bainite is divided into upper bainite and lower bainite. The "upper bainite" formed at a higher temperature is called "upper bainite", and the "lower bainite" formed at a lower temperature is called needle-like or bamboo leaf-like. Compared with upper bainite, lower bainite has higher hardness and strength, and maintains certain toughness and plasticity.
12. Martensite
Martensite usually refers to a supersaturated solid solution of carbon in a-Fe. The hardness of martensite in steel increases with the increase of carbon content. High-carbon martensite has high hardness and brittleness, while low-carbon martensite has higher toughness. Martensite has the highest hardness among austenite transformation products
13. Ledeburite
It is a kind of eutectic structure in carbon alloy. At high temperature, it is composed of austenite and cementite; at low temperature (below 727°C), it is composed of pearlite and cementite. The carbon content is 4.3%, and the structure contains a large amount of cementite, so the hardness is high, and the plasticity and toughness are low
14. Fracture inspection
Fracture structure is one of steel quality marks. After notching or breaking the sample, check the fracture with the naked eye or a 10 times magnifying glass, which is called fracture inspection. The metal defects can be seen from the fracture
15. Tower turning hair
Pattern inspection
The steel car is turned into a prescribed tower or stepped sample, and then the hairline is inspected by the acid etching or magnetic powder method, referred to as the tower inspection
Note: Carbon content refers to mass fraction
Table 2 General heat treatment of steel materials
name
Heat treatment process
Heat treatment purpose
1. Annealing
Heat the steel to a certain temperature, keep it for a certain period of time, and then slowly cool it to room temperature
①Reduce the hardness of steel and increase plasticity to facilitate cutting and cold deformation processing
② Refine the grains, uniform the structure of the steel, improve the performance of the steel and prepare for the subsequent heat treatment
③ Eliminate internal stress in steel. Prevent deformation and cracking of parts after processing
retreat
fire
kind
do not
(1) Fully annealed
Heat the steel to the critical temperature (the critical temperature of different steels is also different, generally 710-750℃, and the critical temperature of individual alloy steels can reach 800-900℃) 30-50℃, keep it for a certain period of time, and then slowly cool with the furnace (or Buried in sand to cool)
Refine grains, uniform organization, reduce hardness, fully eliminate internal stress, complete annealing is suitable for carbon content (mass fraction) in O. Forgings or steel castings below 8%
(2) Spheroidizing annealing
The steel parts are heated to 20～30ºC above the critical temperature, and after heat preservation, they are slowly cooled to below 500℃ and then air-cooled out of the furnace
Reduce the hardness of steel, improve cutting performance, and prepare for subsequent quenching to reduce deformation and cracking after quenching. Spheroidizing annealing is suitable for carbon content (mass fraction) greater than O. 8% carbon steel and alloy tool steel
(3) Stress relief annealing
Heat the steel parts to 500～650ºC, keep it for a certain period of time, and then slowly cool down (generally use furnace cooling)
Eliminate internal stress generated during welding and cold straightening of steel parts, eliminate internal stress generated during cutting of precision parts, to prevent deformation during subsequent processing and use
Stress relief annealing is suitable for all kinds of castings, forgings, welded parts and cold extruded parts, etc.
2. Normalizing
Heat the steel to 40～60ºC above the critical temperature, keep it for a certain period of time, and then cool it in the air
①Improve the organization structure and cutting performance
②Normalizing is often used as the final heat treatment for parts that do not require high mechanical properties
③ Eliminate internal stress
3. Quenching
Heat the steel to the quenching temperature, keep it for a period of time, and then rapidly cool it in water, salt water or oil (individual materials in the air)
①Make steel parts obtain higher hardness and wear resistance
②Make the steel parts get some special properties after tempering, such as higher strength, elasticity and toughness, etc.
Quench
fire
kind
do not
(1) Single liquid quenching
The steel parts are heated to the quenching temperature, and after heat preservation, they are cooled in a quenching agent
Single-liquid quenching is only suitable for carbon steel and alloy steel parts with simple shapes and low technical requirements. When quenching, for carbon steel parts with a diameter or thickness greater than 5-8mm, choose salt water or water cooling; alloy steel parts choose oil cooling
(2) Double liquid quenching
The steel parts are heated to the quenching temperature, after heat preservation, they are quickly cooled in water to 300-400ºC, and then moved to oil for cooling
(3) Flame surface quenching
Spray the flame of mixed combustion of acetylene and oxygen on the surface of the part to quickly heat the part to the quenching temperature, and then immediately spray the surface of the part with water. The flame surface quenching is suitable for single-piece or small batch production, and the surface requires hard and wear-resistant, and can Large medium-carbon steel and medium-carbon alloy steel parts that can withstand impact loads, such as crankshafts, gears and guide rails, etc.
(4) Surface induction hardening
fire
Put the steel part in the inductor, the inductor generates a magnetic field under the action of a certain frequency of alternating current, the steel part generates an induced current under the action of the magnetic field, so that the surface of the steel part is rapidly heated (2-10min) to the quenching temperature, then immediately Spray water onto the surface of the steel.
The surface induction hardened parts have a hard and wear-resistant surface, while the core maintains good strength and toughness.
Surface induction hardening is suitable for medium carbon steel and alloy steel parts with medium carbon content
4. Temper
Heat the quenched steel to below the critical temperature, keep it for a period of time, and then cool it in air or oil
Tempering is carried out immediately after quenching, and is also the last process of heat treatment
① Obtain the required mechanical properties. Under normal circumstances, the strength and hardness of the parts after quenching are greatly improved, but the plasticity and toughness are significantly reduced, and the actual working conditions of the parts require good strength and toughness. After selecting the appropriate tempering temperature for tempering, the required mechanical properties can be obtained
②Stable organization, stable size
③ Eliminate internal stress
(1) Low temperature tempering
Heat the hardened steel parts to 150-50ºC, keep them at this temperature for a certain period of time, and then cool them in the air. Low temperature tempering is mostly used for cutting tools, measuring tools, molds, rolling bearings and carburized parts, etc.
Eliminate internal stress caused by quenching of steel parts
kind
do not
(1)Medium temperature tempering
Heat the quenched steel parts to 350-450%, and cool down after holding for a period of time. Generally used for parts such as various springs and hot stamping dies
Make steel parts obtain higher elasticity, certain toughness and hardness
(1) High temperature tempering
The quenched steel parts are heated to 500～650ºC and cooled after heat preservation. They are mainly used for important structural parts requiring high strength and toughness, such as main shafts, crankshafts, cams, gears and connecting rods.
Make steel parts obtain better comprehensive mechanical properties, that is, higher strength, toughness and sufficient hardness, and eliminate internal stress caused by quenching of steel parts
5. Tempering
High temperature (500～600ºC) tempering of quenched steel parts is mostly used for important structural parts, such as shafts, gears, connecting rods, etc. Quenching and tempering are generally performed after rough machining
Refine the grains to make the steel parts obtain higher toughness and sufficient strength, so that it has good comprehensive mechanical properties
6.
Timeliness treatment
(1) Artificial aging
The quenched steel parts are heated to 100～160℃, after a long time of heat preservation, and then cooled
Eliminate internal stress, reduce part deformation, stabilize size, and it is more important for parts with higher accuracy requirements
(2) Natural aging
Place castings in the open air; steel parts (such as long shafts, lead screws, etc.) are placed in sea water or suspended for a long time or lightly tapped. Parts that have undergone natural aging are best to be roughed first
7. Chemical heat treatment
Put the steel part in a chemical medium containing some active atoms (such as carbon, nitrogen, chromium, etc.), and make certain atoms in the medium penetrate into the surface of the steel part through heating, heat preservation, cooling and other methods, so as to achieve change The chemical composition of the surface layer of the steel part makes the surface layer of the steel part have a certain special performance
change
learn
hot
Place
reason
(1) Steel infiltrated carbon
Infiltrate carbon atoms into the surface of steel
Commonly used for wear-resistant and impact-affected parts, such as wheels, gears, shafts, piston pins, etc.
Make the surface have high hardness (HRC60～65) and wear resistance, while the center still maintains high toughness
(2) Nitriding of steel
Infiltrate nitrogen atoms into the surface of steel parts
Commonly used for important bolts, nuts, pins and other parts
Improve the hardness and wear resistance of the surface of steel parts,
Corrosion resistance
kind
do not
(3) Cyanide of steel
Infiltrate carbon and nitrogen atoms into the surface of steel parts at the same time. Suitable for low-carbon steel, medium-carbon steel or alloy steel parts, and can also be used for high-speed steel tools
Improve the hardness and wear resistance of the surface layer of steel parts
8. Blackening
The metal parts are heated and oxidized in a strong alkali and oxidant solution to form a magnetic ferroferric oxide film on the surface of the metal parts. Commonly used in low-carbon steel and low-carbon alloy tool steel
Due to the influence of materials and other factors, the color of the film of the blackening layer is blue-black, black, red-brown, tan, etc., and its thickness is 0.6～O. 8µm
Anti-rust, increase the beauty and luster of the metal surface, and eliminate the stress in the quenching process
The crystal nucleus of carbon body grows into the territory in the form of flakes, and carbon-poor austenite appears on both sides of it, which promotes the nucleation and growth of ferrite on the austenite and cementite interface. Lamellar ferrite makes the nearby austenite rich in carbon, and promotes the nucleation and growth of cementite along the austenite-ferrite interface. Such repeated alternations finally form flaky pearlite. When the above-mentioned method of pearlite develops laterally, the carbon in the austenite at the front of flaky ferrite diffuses to the front of cementite, prompting the transformation Growing up vertically, the result is a pearlite field. In an austenite grain, several pearlite fields can be formed.
(2) The distance between pearlite sheets The distance between pearlite sheets refers to the average distance between two adjacent cementite sheets in pearlite, and its size mainly depends on the transition temperature (degree of supercooling). The lower the transition temperature, the smaller the lamellar spacing, the finer the pearlite structure, and the greater the dispersion of cementite. B. Granular pearlite, the formation of granular pearlite is also a process of alternate precipitation of cementite and ferrite. The precipitation of cementite is the non-dissolved carbide fire carbon-rich zone in the austenite grains. The spontaneous nucleus, due to the similar growth of each item, will eventually become granular pearlite with granular (spherical) cementite uniformly distributed on the ferrite matrix. It is generally believed that the lower austenitization temperature is beneficial to the formation of granular pearlite. The mechanical properties of C pearlite, the strength and hardness of flaky pearlite, increase with the decrease of lamella spacing; granular pearlite has lower strength, hardness, plasticity and toughness.
57. What measures can be taken to obtain fine austenite grain size during the heating process?
A: Heating temperature and holding time: The higher the temperature and the longer the holding time, the faster the austenite grains grow and the coarser the grains. The growth rate of austenite grains increases exponentially with the increase in temperature; while at high temperatures, the effect of holding time on grain growth is greater at low temperatures.
B: Heating rate: The greater the heating rate and the greater the degree of overheating, the higher the actual formation temperature of austenite, because the ratio of the nucleation rate to the growth rate increases. Thus, small initial crystal grains can be obtained. This also shows that rapid heating can obtain fine austenite grains.
C: The chemical composition of steel: As the carbon content of the steel increases, but it is not enough to form undissolved carbides, the austenite grains are easy to grow and coarsen. As a result, eutectoid carbon steels are more sensitive to overheating than hypereutectoid carbon steels.
D: Original structure of steel: Generally, the finer the original structure or the non-equilibrium structure of the original structure, the greater the degree of carbide decomposition, the smaller the initial austenite grains obtained, but the tendency of steel grains to grow , The overheating sensitivity increases. For this reason, steel with extremely fine original structure should not be used with too high heating temperature and too long holding time.
58. How are the first and second types of temper brittleness produced? How to eliminate temper brittleness after it has occurred?
The first type of temper brittleness (tempered martensitic brittleness): tempering carbon steel in the temperature range of 200~400°C will cause a decrease in impact toughness at room temperature, resulting in brittleness. This is the first type of temper brittleness or called brittleness. Tempered martensite is brittle. For alloy steel, the temperature range where brittleness occurs is slightly higher, about 250 to 450 degrees.
If the first type of temper brittleness occurs after the part is tempered, it needs to be reheated and quenched to eliminate it.
The second type of temper brittleness (martensitic high temperature temper brittleness or reversible temper brittleness): When some alloy steels are tempered in the temperature range of 450 to 650 degrees and slowly cooled through the above temperature range, the impact toughness will decrease The phenomenon. If this kind of brittle steel has been reheated to the predetermined tempering temperature (slightly higher than the temperature range that caused the embrittlement) and then quickly cooled to room temperature, the brittleness will disappear. For this reason, it is also called reversible temper brittleness.
59. What is the hardenability of steel? What are the factors that affect hardenability?
A: The ability of steel to obtain martensite during quenching, that is, the depth to which the steel is hardened, is called hardenability. The hardenability of steel depends on the critical cooling rate of steel. The more to the right of the C curve, the lower the critical cooling rate and the greater the hardenability.
B: 1. The influence of carbon content: As the carbon content of austenite increases, the stability increases and the C curve shifts to the right.
2. The influence of alloying elements: alloying elements (except Co) can improve the hardenability of steel.
3. The influence of austenitizing temperature and holding time: the higher the austenitizing temperature, the longer the holding time, the more complete the dissolution of carbides, the larger the austenite grains, the reduction of the total boundary area and the reduction of nucleation, thus Shifting the C curve to the right delays the pearlite transformation. In short, the faster the heating rate, the shorter the holding time, the smaller the austenite grains, the more uneven the composition, and the more undissolved second phases, the faster the isothermal transformation speed, which shifts the C curve to the left.
60. It is necessary to control the growth of austenite grains during heat treatment. Try to analyze the factors that affect the growth of austenite grains and the measures to control the growth of austenite grains.
Heating temperature and holding time: the higher the heating temperature, the longer the holding time, the coarser the austenite grains, and the heating temperature is the main one.
Heating rate: The faster the heating rate, the greater the degree of overheating, which increases the ratio of the nucleation rate to the growth rate to refine the grains, and the actual grain size of austenite is higher. Chemical composition of steel:
1. Carbon steel-eutectoid steel is easier to overheat than hypereutectoid steel;
2. Alloy steel—The addition of carbon and nitride forming elements such as Ti, V, Vr, Nb, W, Mo, Cr, etc. to the steel strongly hinder the migration of austenite grain boundaries and refine the grains. The grains of steel deoxidized with Al are fine, and the grains of steel deoxidized with Si are coarser;
Original structure-when the original structure is finer or non-equilibrium structure, the grain size of steel tends to increase, and the grains are easy to coarsen.
61. How many categories are cast iron usually divided into? Respectively point out the form of carbon in these cast irons and their influence on the properties of cast irons?
Gray cast iron: has high compressive strength, excellent wear resistance and vibration absorption, and low notch sensitivity.
Nodular cast iron: It has the advantages of gray cast iron, but also has the tensile strength, bending fatigue strength and good shape and toughness of medium carbon steel.
Malleable cast iron graphite is in the form of flocs, which has little cutting effect on the matrix, so its strength, plasticity and toughness are higher than gray cast iron. In particular, pearlite malleable cast iron is comparable to cast steel, but cannot be forged.
Vermicular graphite cast iron: The tensile strength, plasticity, and fatigue strength of vermicular graphite cast iron are better than gray cast iron, and ductile iron alloy cast iron close to the ferrite matrix. In addition, its thermal conductivity, castability, and machinability are better than ductile iron and similar to gray cast iron.
62. Give examples and briefly explain which effective heat treatment processes can be used to increase the life of the mold. Please give examples to illustrate more than five kinds.
The known processing route of GCr15 steel precision bearing is:
Cutting-forging-ultra-fine processing-machining-quenching-cold treatment-stabilization treatment. The heat treatment process includes:
①The ultra-fine heat treatment process is 1050℃×20～30min high temperature heating, 250～350℃×2h salt bath isothermal, 690～720℃×3h with furnace cooling to 500℃ and air cooling.
②Quenching: heating at 835～850℃×45～60min in protective atmosphere, cooling in oil at 150～170℃ for 5～10min, and then cooling in oil at 30-60℃.
③Cold treatment: Cryogenic treatment at -40—-70℃×1～1.5h after cleaning
④ Stabilization heat treatment: 140～180℃×4～12h after rough grinding; 120～160℃×6～24h after fine grinding.
63. Why is the material for processing machine tool gears usually selected from 45 steel, etc., while the material for automobile gears is 20CrMnTi, etc. Please separately formulate the processing route and the purpose of the heat treatment process. P90
(1) The gears of the machine tool work smoothly without strong impact, the load is not large, the speed is medium, and the requirements for the strength and toughness of the gear core are not high. Generally, 40 or 45 steel is used. The working conditions of automobile and tractor gears are worse than those of machine tool gears, and they are subject to greater stress. They are subject to frequent impacts during overload and starting, braking and shifting, and are resistant to wear resistance, bending fatigue strength, contact fatigue strength, heart strength and toughness, etc. The requirements are relatively high, and the use of medium-carbon steel or medium-carbon low-alloy surface quenching by high-frequency induction heating can no longer guarantee the performance.
(2) Machine tool gear processing technology route: blanking-forging-normalizing-quenching and tempering-semi-finishing-high frequency induction heating surface quenching + low temperature tempering-fine grinding-finished product. Normalizing can homogenize the structure, eliminate forging stress, adjust hardness and improve machinability. Quenching and tempering treatment can make the gear have higher comprehensive mechanical properties, improve the strength and toughness of the tooth core, so that the gear can withstand greater bending stress and impact load, and reduce quenching deformation; high-frequency induction heating surface quenching can increase the hardness and surface hardness of the gear Wear resistance, improve tooth surface contact fatigue; low temperature tempering is to eliminate quenching stress without reducing the surface hardness. Prevent the generation of grinding cracks and improve the gear's impact resistance.
The processing route of automobile gears: blanking-forging-normalizing-machining-carburizing, quenching + low temperature tempering-shot peening-grinding-finished product. Normalizing treatment can make the structure uniform, adjust the hardness to improve the machinability; carburizing is to increase the mass fraction of tooth surface carbon (0.8-1.05%); quenching can increase the tooth surface hardness and obtain a certain hardened layer depth (2.8-1.3mm) ) To improve the wear resistance and contact fatigue strength of the tooth surface; the role of low-temperature tempering is to eliminate quenching stress, prevent grinding cracks, and improve impact resistance; shot peening can increase the hardness of the tooth surface by about 1-3HRC and increase the surface residual compressive stress , Thereby improving the contact fatigue strength.
64. Types and solutions of temper brittleness
Tempering brittleness: When quenched steel is tempered, as the tempering temperature increases, the impact toughness of the steel is obviously reduced within a certain tempering temperature range, and the brittleness is obviously increased. Divided into the first and second categories.
The first category: the irreversible temper brittleness of quenched steel at 250-400 tempering; the second category: 450-650 reversible.
Method: The first type of production cannot be eliminated, and si can be added to increase the brittleness transition temperature to above 300, and then tempered at 250; the second type: tempering at the brittle temperature for a short time, rapid cooling does not occur, slow cooling occurs. Reheat and temper at brittle temperature for a short time, and rapid cooling can be eliminated.
65. The purpose of miniaturization heat treatment of cold work die steel? What is the cyclic ultrafine treatment process of Cr12MoV steel?
Purpose: The miniaturization heat treatment includes the refinement of the steel matrix structure and the refinement of carbides. The refinement of the structure can improve the strength and toughness of steel, and the refinement of carbides is beneficial to enhance the toughness and wear resistance.
Process: 1150 heating quenching + 650 tempering + 1000 heating oil quenching + 650 tempering + 1030 heating oil quenching 170 isothermal 30min air cooling + 170 tempering.
66. How many kinds of martensite are common in quenched steel? Substructure? Performance characteristics? Formation conditions?
Slatted and flaky. The substructure of the lath is dislocation, performance: high strength, hardness, good plasticity and toughness; formation condition: low carbon steel, temperature above 200℃. flake medium-high carbon below 200℃, substructure is twin crystal, performance: high hardness, Brittle
67. What are the main defects in ingot casting?
Micro segregation. Macro segregation 1) normal segregation 2) reverse 3) specific gravity. Inclusions and pores. Shrinkage and porosity
68. What principles should be followed in the solution treatment process of manufacturing cast aluminum alloy?
1. Quenching temperature: generally slightly lower than the maximum solubility temperature. 2. Quenching heating: In order to prevent overheating and deformation of the casting, it is best to enter the furnace at a low temperature below 350, and then slowly heat it to the quenching temperature along with the furnace. 3. Holding time: The holding time is longer, generally 3~20h. 4. Cooling method: generally cooling in hot water.
69. Explain the methods of strengthening different aluminum alloys? What kind of heat treatment is used to improve the strength of the ZL104 gasoline engine?
Deformation aluminum alloy strengthening: cold deformation strengthening (work hardening), heat treatment strengthening (solid solution + aging strengthening) Cast aluminum alloy strengthening: modification treatment (refining the structure), solid solution + aging, ZL104 aluminum alloy adopts (535±5) ℃ *3h solution treatment, (175±5)℃*9h. This process is based on sand casting and has a longer aging time. Adopt sodium metamorphism and low-pressure metal casting. When ZL104 aluminum alloy is aged at 175℃*5h, the GP zone is formed in the matrix, and the strengthening operation is remarkable.
70. In order to control the crystal grain size during metal crystallization, what methods are usually used to refine the crystal grains in industrial production?
1. Increase the environmental cooling capacity. 2. Chemical modification method. 3. Increase liquid flow.
71. In order to improve the cutting performance, what kind of heat treatment should be carried out for 15Cr, 20Cr2Ni4, 40Cr, 5CrMnMo, GCr15, W18Cr4V steel?
15Cr: normalizing; 20Cr2Ni4: normalizing + tempering; 40Cr: quenching and tempering; 5CrMnMo: annealing; GCr15: spheroidizing annealing; W18Cr4V: spheroidizing annealing.
72. Try to analyze and compare the hardenability and hardenability of 20, 45, 40Cr, T8, 65 and other steels under normal quenching conditions.
Hardenability from high to low: 40Cr, T8, 65, 45, 20;
Hardenability from high to low: T8, 65, 45, 40Cr, 20.
73. Is it possible to use W18Cr4V steel to make cold stamping dies, and why?
It can be used as a mold, but it is generally used in molds that require high strength, high wear resistance and less impact. However, due to its poor toughness, brittle material properties, and high price, it is not recommended to be used as a cold stamping die.
74 and 45 steels are required to have a hardness of 217HB～255 HB after quenching and tempering, but the hardness is found to be high after heat treatment. Can I reduce the hardness by slowing down the cooling rate during tempering? If the hardness after heat treatment is low, can the hardness be increased by lowering the temperature during tempering? Explain the reason.
No, you need to adjust the tempering temperature; no, you need to lower the tempering temperature after re-quenching.
75. The crankshaft of the medium-sized tractor engine requires high strength and good toughness, and the crankshaft journal requires good wear resistance (50HRC~55 HRC). (1) Select the material and write the steel number; (2) Develop a concise manufacturing process route; (3) Explain the organization of the crankshaft and the surface of the crankshaft journal in use.
Alloy carburized steel: 20CrMnMo, 20CrMnTi, 20MnVB
Process route: blanking-forging-normalizing-mechanical processing-carburizing, quenching + low temperature tempering-shot peening-grinding processing-finished product
Heart organization: fine flake pearlite; surface organization: tempered martensite
76. What safety measures should be paid attention to when using a nitrate bath furnace?
Attention must be paid to safety measures such as explosion-proof.
In the nitrate bath furnace, any local temperature exceeding 595°C may catch fire or explode. The operating temperature should be strictly controlled below 550°C.
The nitrate mixture is oxidizing and should not be mixed with easily oxidized materials.
The fine carbonized material should not be used as the nitrate cover, and the pollution of the nitrate bath furnace by the black accumulated at the discharge end of the carburizing furnace must also be avoided.
When processing magnesium alloy light metals, the maximum salt bath temperature has certain regulations.
