1. Ms point decreases with the increase of C%
The Ms point decreases with the increase of C%. During quenching, the temperature at which the supercooled austenite begins to transform into martensite is called the Ms point, and the temperature at which the transformation is completed is called the Mf point. The higher the %C content, the lower the Ms point temperature. The Ms temperature of 0.4%C carbon steel is about 350°C, while that of 0.8%C carbon steel is reduced to about 200°C.
2. Quenching liquid can be added with appropriate additives
(1) Adding salt to the water can double the cooling rate: the cooling rate of brine quenching is fast, and there will be no quenching cracks and uneven quenching. It can be called the most ideal coolant for quenching. The proportion of salt added is preferably 10% by weight.
(2) Impurities in the water are more suitable as quenching liquid than pure water: adding solid particles to the water helps to clean the surface of the workpiece, destroy the vapor film effect, increase the cooling rate, and prevent the occurrence of quenching spots. Therefore, it is a very important concept to use the quenching technology of mixed water instead of pure water for quenching treatment.
(3) The polymer can be mixed with water to form a water-soluble quenching liquid: the polymer quenching liquid can be prepared according to the degree of water added to the quenching liquid with the cooling rate from water to oil, which is very convenient and free from fire, pollution and other public hazards. Yu is quite forward-looking.
(4) Dry ice and ethanol can be used for cryogenic treatment of liquid: adding dry ice to ethanol can produce a uniform temperature of -76 °C, which is a very practical low-temperature cooling liquid.
3. The difference between quenching and tempering cooling methods
There are three common cooling methods for quenching, namely: (1) continuous cooling; (2) constant temperature cooling (3) stage cooling. In order to reduce the occurrence of quenching cracks during the quenching process, it is appropriate to use rapid cooling higher than the critical cooling rate above the critical area temperature; when entering a dangerous area, the use of slow cooling is an extremely important key technology. Therefore, when such cooling methods are implemented, it is most suitable to use stage cooling or constant temperature cooling. The common cooling methods for tempering treatment include two cooling methods: rapid cooling and slow cooling. Among them, alloy steel generally uses rapid cooling; tool steel is preferably slow cooling. When the tool steel is quenched at the self-tempering temperature, cracks are easily generated due to the transformation of retained austenite, which is called tempering cracks; similarly, if the alloy steel adopts the slow cooling method, it is easy to cause temper brittleness. After quenching, the role of retained austenite is that there are often martensite and retained austenite in the quenched workpiece, and it is easy to cause cracks when placed at room temperature for a long time. This is due to the transformation of retained austenite. , caused by swelling, this phenomenon is most likely to occur in cold climates in winter. In addition, another major disadvantage of retained austenite is that the hardness is too low, which deteriorates the machinability of the tool. Cryogenic treatment can be used to promote the transformation of martensite so that the retained austenite cannot be transformed even if it is further cooled, or the unstable retained austenite can be transformed into martensite by external force processing to reduce the retained austenite. The effect of the body on the properties of steel.
4. Reasons for insufficient hardness after quenching
The purpose of quenching is to obtain satisfactory hardness on the surface of the steel. If the hardness value is not ideal, it may be caused by the following factors: (1) the quenching temperature or austenitizing temperature is not enough; (2) it may be caused by the insufficient cooling rate; (3) If decarburization occurs on the surface of the workpiece before heat treatment, the effect of surface hardening of the workpiece will be greatly reduced; (4) When there is rust or black skin on the surface of the workpiece, the hardness of the place will be obviously insufficient, so it is advisable to first After the surface of the workpiece is cleaned by shot blasting, quenching treatment is applied.
5. Reasons for Quenching Cracking
The main reasons that will affect the quenching crack include the size and shape of the workpiece, the level of carbon content, the cooling method and the pretreatment method. Heat treatment of steel will cause quenching cracks because the quenching process will generate transformation stress, and this transformation stress is related to the process of martensitic transformation. Usually, steel does not crack at the beginning of martensitic transformation, but in martensitic transformation. When the transformation is about 50% (the temperature is about 150 ℃ at this time), that is, it occurs just before the end of quenching. Therefore, in the quenching process, rapid cooling is required at high temperatures and slow cooling at low temperatures. If we can grasp the key to “fast first and then slow”, the quenching cracks can be minimized.
6. Overheating is prone to quenching cracks
Heating above the appropriate quenching temperature by more than 100°C is called overheating. When overheated, the crystal grains of austenite becomes coarse, resulting in the formation of coarse martensite after quenching and embrittlement, which is easy to cause transverse cracks (this is called martensite cracks) in the trunk of acicular martensite. Cracks can easily develop into quenching cracks. Therefore, when your workpiece is overheated at the austenitizing temperature, subsequent quenching and cooling cannot prevent the occurrence of quenching cracks, so some people call “overheating” the culprit of quenching cracks.
7. The structure before quenching will affect the quenching crack
The organization before quenching will of course affect the success or failure of quenching. The most normal pre-structure should be a normalized structure or annealed structure (pearlite structure). If the pre-quenching structure is a superheated structure and spheroidized structure, there will be different results. The overheated structure is prone to quenching cracks, and the spheroidized structure can be uniformly quenched to avoid quenching cracks and quenching bending. Therefore, it is one of the important quenching technologies to perform spheroidization before tool steel or high-carbon steel is quenched. At this time, spheroidizing annealing or tempering spheroidizing treatment can be applied to obtain spherical carbides. If carbides exist in a network structure, quenching cracks are likely to occur there.
8. Defects of quenched parts caused by placing at room temperature
If the quenched parts are placed at room temperature for a long time, there may be two defects: shelf cracks and shelf deformation. Shelving cracks are also called aging cracks, especially in cold nights in winter, as the temperature drops, the retained austenite transforms into martensite, resulting in cracks, also known as night weeping cracks. Shelving deformation, also known as aging deformation, is a phenomenon in which the quenched workpiece is placed at room temperature and causes changes in size and shape, mostly due to incomplete tempering treatment. In order to prevent deformation during storage, it is necessary to stabilize the structure of the steel, so the unstable retained austenite must first be eliminated (cryogenic treatment). Next, a tempering treatment at 200° C. to 250° C. is performed to stabilize the martensite.
9. Advantages of 100℃ hot water tempering
Low-temperature tempering is often used around 180°C to 200°C, and oil is used for tempering. In fact, if hot water at 100°C is used for tempering, there will be many advantages, including (1) Tempering at 100°C can reduce the occurrence of wear cracks; (2) Tempering at 100°C can slightly increase the hardness of the workpiece and improve the Wear resistance; (3) 100 ℃ hot water tempering can reduce the chance of cracks caused by rapid heating; (4) when cryogenic treatment is performed, the probability of cryogenic cracks in the workpiece is reduced, and the retained austenite has a buffering effect. Increase the strength and toughness of the material; (5) No oil coke will be produced on the surface of the workpiece, and the surface hardness is slightly lower, which is suitable for grinding by a grinder, and will not cause the phenomenon of overheating and dry burning in oil.
10. High-temperature tempering treatment for secondary hardening
For tool steel, both residual stress and retained austenite have adverse effects on the steel, and high-temperature tempering or low-temperature tempering is required to eliminate them. High-temperature tempering treatment will have a secondary hardening phenomenon. For SKD11, the hardness of steel tempered at 530℃ is slightly lower than that of low-temperature tempering at 200℃, but it has good heat resistance, will not cause aging deformation, and can improve the heat resistance of steel. It can also prevent the deformation of electric discharge machining, which has many benefits.
11. Temper cracks caused by tempering
When the quenched steel material is tempered, the cracks caused by rapid cooling, rapid heating or structural changes are called tempering cracks. Common high-speed steel, SKD11 die steel and other temper-hardening steels will also be quenched after high-temperature tempering. This kind of steel produces the first martensitic transformation during the first quenching, and the second martensitic transformation (retained austenite transforms into martensite) during tempering, resulting in cracks. Therefore, in order to prevent tempering cracks, it is best to cool slowly from the tempering temperature. At the same time, in the operation of quenching and tempering, it is also necessary to avoid the heat treatment method of tempering and then quenching in advance.
12. Temper brittleness caused by tempering
It can be divided into 300 ℃ brittleness and tempering slow cooling brittleness. The so-called 300°C brittleness means that when some steels are tempered at about 270°C to 300°C, carbides will precipitate on the grain boundaries due to the decomposition of retained austenite, resulting in temper brittleness. The so-called tempering slow cooling brittleness refers to the brittleness that occurs when the tempering temperature (500 ° C ~ 600 ° C) is slow cooling, and Ni-Cr steel is quite remarkable. Tempering slow-cooling brittleness can be prevented by self-tempering temperature quenching. According to various experimental results, the alloy steel used for mechanical structure is air-cooled at a self-tempering temperature, and the cooling rate above 10℃/min will not produce a backlash. Fire slow cold brittleness.
13. Common problems of high-frequency quenching
Common defects in high-frequency quenching treatment include quenching cracks, soft spots and peeling. High-frequency quenching is the most taboo for uneven heating to cause overheating in local areas, such as the sharp corners of the workpiece, key slots, and round holes, which are very easy to cause overheating, resulting in the occurrence of quenching cracks. The above situation can be filled with copper sheets to reduce quenching cracks’ possibility of occurrence. In addition, the uneven quenching process of high-frequency quenched workpieces will cause the disadvantage of low surface hardness of the workpiece, which is called a soft spot. This phenomenon is caused by uneven high-frequency quenching temperature, blocked water jet holes, or improper size and number of holes. The third defect is surface peeling. The main reason is that the hardness of the section varies greatly or the hardened layer is too shallow. Therefore, preheating is often used to deepen the hardened layer, which can effectively prevent peeling.
14. Why can’t stainless steel be tempered between 500℃ and 650℃?
After most of the stainless steel is solutionized, if the temperature is kept between 475 °C and 500 °C for a long time, the hardness and brittleness will increase greatly. This is called 475 °C embrittlement. The main reasons are: There are various theories, including phase decomposition, the precipitation of chromium-containing carbides on grain boundaries, and the formation of Fe-Cr compounds, etc., which greatly reduce the normal temperature toughness, and the corrosion resistance is also very poor. Generally, the heat treatment of stainless steel should avoid constant temperature holding for a long time. in this temperature range. In addition, maintaining the temperature between 600 °C and 700 °C for a long time will cause the precipitation of s-phase. This s-phase is a Fe-Cr intermetallic compound, which is not only hard and brittle but also depletes a large amount of chromium in the steel. The corrosion resistance and toughness of stainless steel are reduced.
15. Why does tempering deformation occur?
The main reason for the tempering deformation is the residual stress or structural changes generated during tempering and quenching, or shrinking due to the elimination of tensile stress due to tempering, and expansion due to the elimination of compressive stress, including the precipitation of e-carbides in the early stage of tempering. There are several shrinkages, the condensation process of iron carbide will shrink a lot, the retained austenite will transform into martensite and expand, and the retained austenite will transform into bainite and expand, which will lead to the deformation of the workpiece after tempering. The prevention methods include (1) implementing pressure tempering treatment; (2) reducing residual stress by means of hot bath or air quenching; (3) correcting by machining and (4) reserving deformation and other methods.
16. Types of tempering hardenability
(1) Embrittlement at 270℃~350℃: also known as low-temperature tempering hardenability, mostly occurs in carbon steel and low alloy steel. (2) Embrittlement at 400°C ~ 550°C: Usually, alloy steel for construction is prone to embrittlement in this temperature range. (3) Embrittlement at 475°C: In particular, ferritic stainless steel with a Cr content of more than 13%, when tempered between 400°C and 550°C, the hardness increases and the embrittlement occur, especially at around 475°C. . (4) Embrittlement at 500°C ~ 570°C: Commonly used in processing tool steel, high-speed steel and other materials, carbides will precipitate at this temperature, causing secondary hardening, but also leading to an increase in brittleness.
17. How can the workpiece obtain a fine wave body structure with excellent performance?
Annealing treatment will soften the steel, while quenching treatment will make the steel hard. In comparison, if the “normalization” treatment is applied, a layered pearlite structure can be obtained, which can effectively improve the machinability and wear resistance of the steel. At the same time, it has the advantages of no cracks, less deformation and convenient operation. However, normalization treatment is a difficult heat treatment technology, because it adopts air cooling, which will be affected by many factors. Also affects the cooling rate. Therefore, normalization uses various methods to maintain uniformity, which can be shading, enclosures, potholes, fans, etc.
18. Difference between normalization treatment and annealing treatment
The normalization treatment is a heat treatment procedure that is heated to the A3 point or above the Acm point at 40~60 °C for a period of time to make the steel structure into a uniform austenite structure and then cooled to room temperature in still air. For hypereutectoid steel, it can achieve the purpose of grain refinement and have good strength and toughness; for hypereutectoid steel, it can prevent the formation of network precipitation of iron carbide on the austenite grain boundary, to reduce the toughness of the material. The main purpose of the complete annealing treatment is to soften the steel and improve its machinability of the steel. After complete austenite structure (or austenite plus ferrite structure), at 50°C below the A1 point, the pearlite transformation takes place sufficiently to obtain softened steel. In addition, stress relief annealing is heating at 450~650℃ below the metamorphic point for a period of time and then slowly cooling to room temperature, which can eliminate the residual stress inside the steel during cutting, stamping, casting, and welding.
19. How to eliminate the residual stress of the workpiece?
Stress relief annealing is to heat for a period of time at 450~650 ℃ below the metamorphic point and then slowly cool to room temperature, which can eliminate the residual stress inside the steel during cutting, stamping, casting, and welding processes. For carbon steel, the reference heating temperature is 625±25°C; for alloy steel, the reference heating temperature is 700±25°C. The holding time will also vary. For carbon steel, the holding time is 1 hour per 25mm thickness; for alloy steel, the holding time is 2 hours per 25mm thickness, and the cooling rate is every 25mm. Cool it at a cooling rate below 275°C/hour.
20 How to prevent thermal deformation?
To prevent the occurrence of heating deformation, it is best to heat slowly and carry out preheating treatment. When selecting the preheating temperature of general steel, the preheating temperature can be selected according to the following criteria: (1) The preheating temperature should be below the transformation point, for example, the ordinary steel is about 650~700℃, and the high-speed steel is about 800~850℃ about. (2) The preheating temperature is about 500°C. (3) For two-stage preheating, the first stage is preheated at about 500°C, and after a period of sufficient preheating, the preheating temperature is increased to below the A1 metamorphosis point. (4) Three-stage preheating, for large steels containing high alloy content, such as high-speed steel, sometimes the third stage preheating is required at 1000~1050 °C.
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