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01 What are the commonly used quenching methods? Explain the principles for selecting different quenching methods Quenching methods: 1. Single-fluid quenching – a process in which cooling takes place in a single quenching medium; this method results in high mechanical and thermal stresses as well as significant quenching deformation. 2. Two-fluid quenching – Purpose: Rapid cooling between 650°C and Ms to ensure V > Vc, followed by slow cooling below Ms in order to reduce structural stress. Carbon steel: water first, then oil. Alloy steel: oil first, then air. 3. Step quenching – This process involves removing the workpiece and holding it at a certain temperature to ensure uniform temperature inside and outside the workpiece, followed by air cooling. In step quenching, the M-phase transformation occurs during air cooling, resulting in low internal stress. 4. Isothermal quenching – refers to maintaining the material at the bainite temperature range, where bainite transformation occurs; this results in reduced internal stresses and minimal deformation. The principles for selecting a quenching method should consider both meeting the performance requirements and minimizing quenching stresses, so as to prevent quenching deformation and cracking. 02 What is the difference between chemical vapor deposition and physical vapor deposition techniques, and what are their main applications? Chemical vapor deposition primarily involves the CVD method: a reaction medium containing elements of the coating material is vaporized at low temperatures, and then introduced into a high-temperature reaction chamber where it comes into contact with the surface of the workpiece, resulting in high-temperature chemical reactions that lead to the formation of alloys, metals, or their compounds on the workpiece surface as a coating. Main characteristics of the CVD method: 1. It can deposit various crystalline or amorphous inorganic thin film materials. 2. High purity and strong collective cohesion. 3. The deposited layer is dense, with very few pores. 4. It has good uniformity, and the equipment and process are simple. 5. The reaction temperature is relatively high. Applications: Preparation of films for various purposes on the surfaces of materials such as steel, cemented carbides, non-ferrous metals, and inorganic non-metals, mainly including insulating films, semiconductor films, conductive and superconductive films, as well as corrosion-resistant films. Physical vapor deposition: The process in which gaseous substances are directly deposited as a solid film on the surface of a workpiece is known as PVD. There are three basic methods: vacuum evaporation, sputter coating, and ion plating. Applications: wear-resistant coatings, heat-resistant coatings, corrosion-resistant coatings, lubricating coatings, functional coatings, decorative coatings. 03 Describe the microstructure and macrostructure of the fatigue fracture surface. Microscopically: These are strip-like patterns observed under a scanning electron microscope; they are known as fatigue striations or fatigue markings. Fatigue bands come in both ductile and brittle types. They are spaced at certain intervals, and under specific conditions, each band corresponds to one stress cycle. Macroscopically: In most cases, it exhibits brittle fracture characteristics, with no visibly macroscopic deformation occurring. A typical fatigue fracture surface consists of a crack initiation zone, a crack propagation zone, and a final instantaneous fracture zone. The areas of fatigue origins are relatively uneven; sometimes they appear as shiny, mirror-like surfaces. The crack propagation zones exhibit a riverbed or shell-like pattern, along with several parallel arcs centered on fatigue origins at varying intervals. The microstructure of the fracture zone depends on factors such as the material’s characteristic loading mode and magnitude, and may take the form of ductile pits or quasi-dislocation structures, grain-boundary fractures, or a mixed pattern. 04 Identify the three common quality problems that occur in induction heating quenching, and analyze their causes. 1. Cracking: Excessively high heating temperature, uneven temperature distribution ； Improper selection of quenching medium and temperature ； Delayed tempering and insufficient tempering ； The material has high hardenability, compositional segregation, defects, and excessive inclusions ； The part design is unreasonable. 2. Uneven surface hardness: unreasonable induction structure ； Uneven heating ； Uneven cooling ； Poor material structure (banding present, local decarburization). 3. Surface melting: Unreasonable design of the inductor ； The parts have sharp corners, holes, notches, etc ； Cracks appear on the surface of the workpiece due to excessive heating time, etc. 05 What are the characteristics of the new high-temperature tempering process for high-speed steel bases? Taking W18Cr4V as an example, why does it have better mechanical properties than those after ordinary tempering? W18Cr4V steel is heat-treated by quenching at 1275°C, followed by tempering at 320°C for 1 hour, and then at 540–560°C for 1 hour, twice in total. High-temperature tempered high-speed steel exhibits more complete precipitation of M2C-type carbides compared to normally tempered high-speed steel. The M2C, V4C, and Fe3C-type carbides are highly dispersed and uniformly distributed, and it contains approximately 5% to 7% bainite; these are the key microstructural factors that account for the superior properties of high-temperature tempered high-speed steel. 06 What are the common types of controlled atmospheres? Briefly describe the characteristics and applications of each atmosphere. There are endothermic atmospheres, drip-type atmospheres, straight-type atmospheres, and other controllable atmospheres (nitrogen-based atmospheres, ammonia decomposition atmospheres, exothermic atmospheres), etc. 1. An endothermic atmosphere is one in which the feed gas is mixed with air in a certain ratio and then passed over a catalyst at high temperature to undergo a reaction that produces an atmosphere mainly containing CO, H2, and N2, along with trace amounts of CO2, O2, and H2O. Since this reaction absorbs heat, it is called an endothermic atmosphere or RX gas. Used for carburizing and cyaniding. 2. The drip-type atmosphere involves directly injecting methanol into the furnace for pyrolysis, thereby producing a carrier containing CO and H2; thereafter, a enriching agent is added for carburization ； Carbidenitriding at lower temperatures, protected heating and bright quenching, etc. 3. The infiltrant, such as natural gas and air, is mixed in a certain ratio and then introduced directly into the furnace, where it reacts at a high temperature of 900°C to directly generate a carburizing atmosphere. Ammonia decomposition gas is used as a nitriding carrier gas and as a protective atmosphere for low-temperature heating of steel or non-ferrous metals. A nitrogen-based atmosphere provides good protection for high-carbon steel or bearing steel. Exothermic atmospheres are used for bright heat treatment of low-carbon steel and copper, or decarburizing annealing of malleable cast iron. 07 What is the purpose of isothermal quenching for ductile iron? What are the isothermal temperature and the microstructure after isothermal quenching? Objective: Isothermal quenching in the bainite transformation zone after austenitization of ductile iron can yield good mechanical properties and low distortion. Isothermal temperature: 260~300°C to obtain lower bainite structure ； Bainite structure is obtained at 350~400°C. 08 Briefly describe the main characteristics of the common chemical heat treatment processes (carburizing, nitriding, carbonitriding, and cyaniding), the microstructural and mechanical properties obtained after heat treatment, and the types of materials or components for which these processes are suitable Carburizing: It is primarily a process of introducing carbon atoms into the surface of the workpiece. The surface layer becomes tempered martensite, along with residual austenite and carbides. The purpose of this process is to increase the carbon content in the surface layer, thereby achieving high hardness and wear resistance. The core portion of the material possesses sufficient strength and toughness to withstand heavy impacts and friction. Low-carbon steels such as 20CrMnTi are commonly used for gears and piston pins. Nitriding: Involves the infiltration of nitrogen atoms into the surface, which improves surface hardness, wear resistance, fatigue strength, corrosion resistance, and heat hardness. The surface layer consists of nitrides while the core is composed of tempered sorbite. There are various methods for nitriding, such as gas nitriding and liquid nitriding. Materials commonly used include 38CrMoAlA and 18CrNiW. Carbidenitriding: Carbidenitriding operates at low temperatures, proceeds quickly, and results in minimal deformation of the parts. The surface layer consists of fine-grained tempered martensite + granular carbonitrides Fe3(C,N) + a small amount of residual austenite. It possesses high wear resistance, fatigue strength, and compressive strength, as well as a certain degree of corrosion resistance. Often used for heavy and medium-duty gears in the manufacturing of low- and medium-carbon alloy steels. Nitrocarburizing: The nitrocarburizing process features a relatively fast infiltration rate; the surface hardness is slightly lower than that obtained through nitriding, but the fatigue resistance is good. It is mainly used for manufacturing molds for parts that are subject to low impact loads, and require wear resistance, a high fatigue limit, and minimal deformation. Common steel parts, such as carbon structural steel, alloy structural steel, alloy tool steel, gray cast iron, ductile iron, and powder metallurgy, can all be subjected to nitrocarburizing. 09 Briefly describe the principles of heat treatment process design. 1. The advancement of the technology. 2. The process is reliable, reasonable, and feasible. 3. Economic efficiency of the process. 4. Safety of the process. 5. Make use of process equipment with a high degree of mechanization and automation wherever possible. 10 What factors should be considered in the optimized design of heat treatment processes? 1. Full consideration should be given to the connection between cold and hot working processes, and the arrangement of heat treatment steps must be reasonable. 2. Make use of new technologies wherever possible, briefly describe the heat treatment process, and shorten the production cycle. Under the condition of ensuring the required microstructure and properties of the parts, different processing steps or techniques should be combined as much as possible. 3. Sometimes, to improve product quality and extend the service life of workpieces, it is necessary to add heat treatment processes. 11 Briefly describe the principles that should be followed in sensor design. 1. The coupling distance between the sensor and the workpiece should be as close as possible. 2. Flow-inducing magnets must be added to workpieces that rely on heating from the outer wall of the coil. 3. The design of the sensor for workpieces with sharp corners avoids the effect of those sharp corners. 4. Avoid the cancellation of magnetic field lines. 5. The sensor design should be such as to allow the workpiece to rotate during heating. 12 What basic principles should designers consider when selecting materials? 1. Select the material based on the operating conditions of the part, including the type and magnitude of the loads, environmental conditions, and the main failure modes ； 2. Taking into account factors such as the structure, shape, and size of the parts, for those prone to quenching distortion and cracking, materials with better hardenability that can be treated using oil quenching or water-soluble quenching media should be selected ； 3. Understand the microstructure and properties of materials after heat treatment; some steel grades developed for various heat treatment processes exhibit improved microstructure and properties as a result of such treatment ； 4. While ensuring the performance and service life of the parts, efforts should be made to choose options that can simplify the heat treatment processes, especially those that can save materials. 13 What process properties should be considered when selecting metal materials for manufacturing parts? 1. Castability. 2. Forming properties. 3. Machinability. 4. Weldability. 5. Thermal treatment process performance. 14 How many types of wear failure are there? How to prevent various types of wear-induced failures in parts? Wear types: adhesive wear, abrasive wear, corrosive wear, contact fatigue. Prevention method: For adhesive wear, select the friction pair materials appropriately ； Use surface treatment to reduce the friction coefficient or increase surface hardness ； Reduce contact compressive stress ； Reduce surface roughness. To address abrasive wear, in addition to reducing the contact pressure and sliding friction distance during design as well as improving the lubricant filtration system to remove abrasive particles, it is also necessary to select high-hardness materials appropriately ； Methods such as surface heat treatment and surface work hardening are employed to increase the surface hardness of the materials in friction pairs. For corrosion wear, choose antioxidant materials ； Surface coating ； Use corrosion-resistant materials ； Electrochemical protection ； Reduce stress concentration caused by tensile stress when designing with corrosion inhibitors ； Perform stress-relief annealing ； Choose materials that are insensitive to stress corrosion ； Change the medium conditions. Regarding contact fatigue, increasing the hardness of the material ； Increase the purity of the material and reduce inclusions ； Increase the strength and hardness of the core part ； Reduce the surface roughness of the parts ； Increase the viscosity of the lubricating oil to reduce the oil wedge effect. 15 What is the basic process of chemical heat treatment of steel? Discuss the main approaches to accelerating chemical heat treatment What are the advantages of the segmented control process for carburizing? What are the microstructures of the surface layer and the core area in low-carbon steel after carburizing and quenching under normal conditions? Three steps: decomposition, adsorption, diffusion. Application of segmented control method, composite infiltration treatment, high-temperature diffusion, new materials utilizing accelerated diffusion processes, chemical induction of infiltration, physical induction of infiltration ； It prevents oxidation of the workpiece surface, facilitates diffusion, ensures proper coordination among the three processes, reduces the formation of carbon black on the workpiece surface, accelerates the carburization process, and guarantees the formation of a high-quality carburized layer that is wide and smooth ； From the surface toward the core, they are in sequence hypereutectoid, eutectoid, superhypoeutectoid, and prime hypoeutectoid. 16 What is granular bainite? It is composed of massive (isometric) ferrite and high-carbon region A. 17 Explain the types, purposes, and uses of ball retraction? Regular ball annealing: increases hardness, improves machinability, and reduces quenching distortion and cracking. Isothermal spheroidization: used for high-carbon tool steels and alloy tool steels. Cyclo ball regrinding: used for carbon tool steel and alloy tool steel. 18. The quenching temperature for hypoeutectoid steel is usually chosen above Ac3; why is the quenching heating temperature for hyper-eutectoid steel selected between Ac1 and Acm? Please analyze this theoretically 1. In hypoeutectoid steel, due to its low carbon content, the initial microstructure is P+F; if the quenching temperature is below Ac3, unreacted F remains, and soft spots will appear after quenching. For hypereutectoid steel, if the temperature is too high, an excessive amount of k’ dissolves, increasing the quantity of flaky M; this can lead to deformation and cracking. An increase in the amount of A’ also occurs, and excessive dissolution of k’ reduces the wear resistance of the steel. 2. In hypereutectoid steel, excessive temperatures increase the tendency for oxidation and decarburization, resulting in uneven surface composition and varying values of Ms, which leads to cracking during quenching. 3. Choosing a quenching temperature of Ac1+ (30–50°C) allows the undissolved k’ to remain, thereby improving wear resistance; it also reduces the carbon content in the matrix, increasing the strength, plasticity, and toughness of the steel. 19 New low- and high-temperature tempering processes for high-speed steel can increase the service life of high-speed steel parts that have been quenched and tempered – is this analyzed theoretically? The uniform precipitation of ε and M3C leads to a more uniform precipitation of M2C and MC within the secondary hardening temperature range, facilitating the transformation of some residual austenite into bainite and thereby improving strength and toughness. 20 Identify the following alloy types. ZL104: cast aluminum; MB2: wrought magnesium alloy; ZM3: cast magnesium; TA4: α-type titanium alloy; H68: brass; QSn4-3: tin brass; QBe2: beryllium brass; TB2: β-type titanium alloy. 21 What is fracture toughness? How can it be determined whether a part will experience low-stress brittle fracture, based on the material’s fracture toughness K1C, the working stress σ of the part, and the radius of the crack in the part, α? The performance parameter that indicates a material’s ability to resist fracture is known as fracture toughness. If K1 > K1C, the material undergoes low-stress brittle fracture. Phase transformation characteristics of gray cast iron compared to ductile iron: 1) Cast iron is a Fe-C-Si ternary alloy, and the eutectic transformation occurs over a wide temperature range; within this range, ferrite + austenite + graphite exist ； 2) The graphitization process of cast iron proceeds easily, and by controlling this process, cast irons with a ferritic matrix, a pearlitic matrix, or a ferritic+pearlitic matrix can be obtained ； 3) By controlling the heating, holding, and cooling conditions at the austenitization temperature, the carbon content of A and the transformation products can be adjusted and controlled over a fairly wide range ； 4) Compared to steel, carbon atoms diffuse over a longer distance ； 5) Heat treatment of cast iron cannot change the shape and distribution of graphite; it can only alter the microstructure and properties of the material. What is the basic process for the formation of A when steel is heated? What factors affect the grain size of A? Formation process: Formation of A nuclei, growth of A grains, dissolution of residual cementite, homogenization of A ； Factors: heating temperature, holding time, heating rate, steel composition, original microstructure. 23 What are the main approaches to accelerating chemical heat treatment? Methods: segmented control method, composite infiltration treatment, high-temperature diffusion; new materials that accelerate the diffusion process, chemical induction of infiltration, physical induction of infiltration. 24 What are the three basic modes of heat transfer? Heat transfer modes: conductive heat transfer, convective heat transfer, and radiative heat transfer (radiative heat transfer applies to vacuum furnaces above 700°C). 25 What is the black structure that appears in carburitriding? How to prevent it from occurring? Black tissue refers to black spots, black bands, and black networks. To prevent the appearance of black tissue, the nitrogen content in the case layer should not be too high; generally, a level above 0.5% makes it easy for dot-like black tissue to appear ； The nitrogen content in the case layer should also not be too low, otherwise a troostite network is likely to form. To suppress the troostite network, the amount of ammonia added must be moderate; an excessive amount of ammonia will lower the dew point of the furnace gas, thereby promoting the formation of black structures. To suppress the formation of troostite networks, it is also possible to appropriately increase the quenching heating temperature or use a cooling medium with stronger cooling capacity. When the depth of the black tissue is less than 0.02 mm, shot peening reinforcement is used as a remedy. 26 Briefly describe the principles for selecting process parameters in induction heating quenching. Heating method: Induction heating quenching includes two methods – simultaneous heating for one-time quenching and moving heating for continuous quenching – with the choice depending on the equipment conditions and the type of part. The specific power for simultaneous heating is generally in the range of 0.5~4.0 kW per square centimeter, while that for moving heating is usually greater than 1.5 kW per square centimeter. Long shaft components, tubular parts with internal cavities that require quenching, gears with a large tooth width and medium module, as well as strip-shaped parts are subjected to continuous quenching ； Extra-large gears are subjected to single-tooth continuous quenching. Heating parameters: 1. Heating temperature – due to the fast speed of induction heating, in order to ensure thorough structural transformation, the quenching temperature is 30-50 degrees higher than that used in conventional heat treatment℃ ； 2. Heating time: It is determined by various factors such as the technical requirements of the part, the material, shape, size, current frequency, and specific power. Quenching cooling methods and quenching media: The cooling methods for quenched parts after heating are typically jet cooling and immersion cooling. 27 What are the precautions for tempering? Tempering must be carried out promptly; the parts should be tempered within 4 hours after quenching. Common tempering methods include self-tempering, furnace tempering, and induction tempering. 28 Adjustment of electrical parameters for induction heating. The purpose is to keep the high and medium-frequency power supplies in a resonant state, thereby enabling the equipment to operate with higher efficiency. 1. Adjustment of high-frequency heating electrical parameters. Under low-voltage load conditions of 7–8 kV, adjust the coupling and set the position of the feedback knob so that the ratio of gate current to anode current is 1:5–1:10. Then increase the anode voltage to the operating voltage, and further adjust the electrical parameters to bring the tank voltage to the desired value for optimal performance. 2. Adjustment of the electrical parameters for medium-frequency heating: Based on the size and shape of the part, the length of the hardening zone, and the structure of the inductor, the appropriate turns ratio of the quenching transformer as well as the suitable capacitance are selected to ensure that it operates in a resonant state. 29 What are the common cooling media? Water, saline water, alkaline water, machine oil, nitrates, polyvinyl alcohol, trinitrate solutions, water-soluble quenching agents, specialized quenching oils, etc. 30. Analyze the factors that affect the hardenability of steel 1. Effect of carbon content: In hypoeutectoid steel, as the carbon content increases, the stability of A increases and the C curve shifts to the right ； In hypereutectoid steel, as the carbon content increases and the amount of unmelted carbides rises, the stability of A decreases, causing the C curve to shift to the right. 2. Effect of alloying elements: Except for Co, the solid-solution metal elements all shift the C curve to the right. 3. A-aging temperature and holding time: The higher the A-aging temperature and the longer the holding time, the more complete the dissolution of carbides, the larger the A-grains become, and the curve C shifts to the right. 4. Influence of the base tissue: The finer the base tissue, the easier it is to achieve a uniform A, which shifts the C curve to the right and lowers Ms. 5. The effect of stress-strain causes the C curve to shift to the left.