Quenching specification includes a) quenching mode, 2) the heating temperature, 3) holding time, 4) the cooling medium and cooling methods. Determine quenching specification is based on the workpiece drawing and technical requirements, the use of material grades, the phase transition point and supercooled austenite isothermal or continuous cooling transformation curve, Jominy curve before quenching process route and the original organization. Only fully grasp these raw materials, in order to correctly determine the quenching process specifications.
First, the quenching methods and principles to determine the heating temperature
Quenching is usually the final heat treatment processes. Therefore, the use of protective atmosphere furnace heating heating or salt. Only some of the blanks or bar stock quenching (quenching, tempering) can be heated in normal air medium. Yet because the quenching mechanical cutting, you can remove surface oxidation, decarbonization and other heating defects. But with little development without cutting, only after quenching some small amount of finishing machining, and thus require no oxidation, decarbonization heating.
Quenching furnace is generally charge. However, the larger size of the workpiece, the geometry of complex high-alloy steel parts, should be based on the size of the production batches, using preheated oven (cycle operation) preheating, or partition (continuous furnace) heating mode for heating.
1: quenching temperature:
Quenching temperature, mainly determined by the phase transition point of steel. For sub-eutectoid steel, generally used in quenching temperature Ac3 + (30-50 ℃), eutectoid steel was Ac1 + (30-50 ℃). The reason for this is determined, because of the sub-eutectoid steels, if the heating temperature is lower than Ac3, the heated state of austenite and ferrite two phases, ferrite after quenching preserved, making parts after quenching hardness uneven, strength and hardness decreased. Higher than the Ac3 point 30-50 ℃ purpose is to make the central portion of the workpiece within a predetermined heating time guarantee the achievement of a temperature above the Ac3 point, ferrite physical completely dissolved in austenite, the austenite composition is relatively homogeneous, and Ostwald coarse grain without falling. For eutectoid steel, the quenching temperature is between Ac1 ~ Ac3, fine austenite grain heating state and undissolved carbides, spherical carbon material obtained after quenching hidden martensite and evenly distributed. This organization is not only high strength and hardness, high wear resistance, but also good toughness. If the quenching temperature is too high, carbide dissolution, austenite grain growth, after quenched plate martensite (twins Ma Di body), which microcracks increases, increased brittleness, quench cracking tendency also increases great. Due to the dissolution of carbides, austenite carbon content increases, increased amount of residual austenite after quenching, hardness and wear resistance of steel decreases. 30-50 ℃ higher than Ac1 point goals and sub-eutectoid steel Similarly, in order to ensure that all parts of a workpiece temperature is higher than the Ac1.
2: Note: When determining the quenching temperature, there should be consideration of the shape of the workpiece, the size of the original tissue, heating rate, the cooling medium and cooling and other factors.
In the workpiece size large, fast heating case, the optional quenching temperature was higher. Because large workpieces, heat slowly, easily heated enough so that not all of the quenched martensite or hardened layer thinning. Heating speed, workpiece temperature, and also less prone to heating. Further, heating speed, fine starting grain, also allows the use of higher heating temperature. In this case, the quenching temperature desirable Ac3 + (50-80 ℃), for fine grain steels sometimes take Ac3 + 100 ℃. For more complex shapes, cracking of the workpiece is easily deformed, the heating rate is slow, the lower limit of the quenching temperature is taken.
When considering the original organization, such as the first eutectoid ferrite is relatively large, or pearlite spacing is large, in order to accelerate the process of homogenization of austenite, quenching temperature get higher. Eutectoid steel in order to accelerate the dissolution of alloy carbides, and the homogenization of alloying elements should also be taken to a higher quenching temperature. For example Ac1 point high-speed steel is 820-840 ℃, quenching temperature up to 1280 ℃.
When considering the selection of the quenching and cooling medium, the cooling rate in the use of lower quenching medium and the quenching method in the case, in order to increase the stability of supercooled austenite too, due to the low cooling rate to prevent the pearlescent leaving the workpiece during quenching Body changes often take a slightly higher quenching temperature.
Second, determine the principles of quenching time
Quenching time should include the entire cross section of the workpiece is heated to a predetermined quenching temperature, and allowed to complete tissue changes at this temperature carbide dissolution and austenitic time required uniform composition. Thus, quenching temperature and holding time includes two periods. In actual production, only large workpieces or installed furnace capacity in many cases, only then the heating time and holding time were considered. Under normal circumstances the heating and holding two periods known as quenching time. When the heating time and holding time are considered, due to the quenching temperature is higher than the phase transition temperature, so the heating time includes a phase change recrystallization time. In fact as far as the holding time and austenitic components carbide dissolution time required to homogenization. Under specific conditions of production, quenching time empirical formula used by the test finalized. Empirical formula is used
Where
T - heating time, (min);
a-- heating coefficient, (min / mm);
K-- installed furnace correction factor;
D-- Parts effective thickness (mm).
Heating coefficient mouth represents heating time required for unit thickness of the workpiece, the size of a workpiece size, the chemical composition of the steel and related to the heating medium. X installed furnace capacity correction factor is to consider how much of stove determined. When loading large furnace, K should be to achieve greater value, generally determined by experiment; D calculate the effective thickness of the workpiece, and according to the following principles: take the diameter of the cylinder, take the side length of the square cross-section, take the short side length of the rectangular cross-section , take the plate thickness, the wall thickness of the sleeve workpieces take, take away from the cone head 2/3 of the length of the diameter of the ball to take the ball diameter 0.6 times as effective thickness D.
Third, the quenching medium and cooling mode selection and determine
Select quenching medium, the materials used should first workpiece and its requirements hardenability depth, according to the Jominy curve of the kinds of materials, through some charts to choose. Its selection method has been thoroughly described in the section of this chapter quenching. If only hardenability depth standpoint, any quenching intensity greater than the intensity of the quenching medium quenching hardenability by the required depth can be used. But from the stress and deformation hardening cracking viewpoint quenching quenching medium intensity smaller the better. Integrated these two requirements, select quenchants first principle should be to meet the workpiece quenching premise penetration depth required to select the lowest intensity quenching quenching medium.
Four: quenching method and its application:
The most commonly used are the following methods;
1) single liquid quenching method
It is the simplest method of quenching. Heated to the temperature of the workpiece quenching quenching people a quenching medium to cool completely. This method is commonly used in the shape of a simple carbon and alloy steel workpiece. For carbon steels, 3-5mm diameter larger than the workpiece to be quenched in water, the smaller can be quenched in oil. For various grades of steel, places commonly used oil quenching medium.
By supercooled austenite transformation (such as temperature or continuous cooling) kinetics seen supercooled austenite and, near the temperature region is relatively stable. In order to reduce the temperature difference between the workpiece and the quenching medium, reducing stress, can quench the desire of the workpiece, before quenching the quenching medium, the first air-cooled for some time. This method is called pre-cooling quenching.
2 interrupt quenching method (double quenching medium quenching)
The method is to heat the workpiece to the quenching temperature, the cooling ability of the first quenching medium is cooled to near Ms point, and then transferred to the slow cooling of the quenching medium is cooled to room temperature, to achieve different quench temperature range, and has ideal quench rate. This will ensure a higher level of hardness and case depth and reduce stress and prevent quench cracking. General water as fast quenching medium, oil or air as the slow cooling quenching medium, but less by air.
The disadvantage of this approach are: it is difficult to determine for various parts should stay in a cold medium to fast time, and for the same kind of workpiece, it is also difficult to control the time. Cooled in water for too long, the cold will make some parts of the workpiece to martensite point, the martensite transformation, which may result in deformation and cracking. Conversely, if the residence time in the water is not enough, the workpiece has not cooled below the temperature of the most unstable austenite, pearlite transformation occurs, resulting in insufficient quenching hardness. Also, consider: When removing artifacts from the water, because the heart is always higher than the temperature of the surface temperature of the Department, if taken early, excessive heat storage core part, will prevent surface cooling, the surface temperature rise, causing already quenching into martensite, austenite unconverted pearlite or bainite transformation occurs. Because so far yet to find a combination of water, oil quenching medium advantages, so even though this method is to keep the time in the water more difficult to determine and control, but only in the water hardened carbon tool steel is still more use of this method. Of course, this requires the operator to have sufficient hardening cooked refining technology. The residence time in the water for the effective thickness of each of 5 ~ 6mm for about 1 second.
Interrupt quenching method can also be another way, i.e. the workpiece directly quenched into water from the austenitizing temperature, to maintain a certain time, to stay out in the air, due to the heat of the central portion of the surface has been heated rumor tempered, while section along the workpiece temperature difference decreases, and then quenched into the water to keep the workpiece for a short time, then remove to stay in the air, and so forth several times, and finally cooled in air or oil. This method is mainly used for large workpieces made of carbon steel, to reduce the internal stress in the water quenching. Clearly this method does not give very high hardness.
3. Jet quenching
This method is the method of quenching a workpiece water jets. Water vary in depth hardening soap may be required. In this way quenching, not vapor film formed on the surface of the workpiece, so it can guarantee to get deeper than the past through the water quench hardened layer. In order to eliminate unevenness due to the cooling capacity between the different cooling water flow caused by the water flow should be fine, preferably while up and down movement or rotation of the workpiece. This method is mainly used for local quenching. When used for local hardening, due to the partially cooled without water cooling is slow, in order to avoid the influence of the residual portion has been quenched in part by the heat untempered workpiece once all black, the entire workpiece immediately quenched into water or oil.
4. Quenching method.
The artifacts from the austenitizing temperature is higher than the kind of person quenched steel martensite start temperature quenching medium, in which the various parts of the workpiece is cooled until the temperature reaches the temperature quenching medium, and then slowly cooled to room temperature, the occurrence of martensite change. This approach not only reduces the small thermal stress, and because before the martensitic transformation temperature of each part of the workpiece has become uniform uniform, and therefore martensite transformation is not the same phenomenon is also reduced. Quenching is only applicable to small size of the workpiece. For larger workpieces, due to the higher temperature of the cooling medium, slow cooling the workpiece, making it difficult to achieve the critical quench rate. Some critical quenching rate of small steel hinge no need to use this method, because the oil quenching still will not cause great stress. Conversely, if instead of using quenching oil quenching efficiency of its production does not significantly increase the string.
Temperature of quenching medium may be higher or slightly lower than the martensite point, when below the martensite point, due to the relatively low temperature, cooling more severe, it may be used to quench the larger workpiece. Cable various carbon tool steel and alloy tool steel (Ms = 200-250 ℃) when the quenching temperature selection grading near 250 ℃, but more often the choice of 120-150 ℃, or 100 ℃. Classification temperature selection below Ms point, whether still that of quenching, is still open to question. Because the general concept of hierarchical classification after quenching is isothermal temperature, slow cooling martensite transformation when removed, but after a temperature below the Ms point has undergone a great deal of isothermal martensitic transformation. Polarized hold time should be shorter than the incubation period under the classification's body temperature isothermal decomposition smell, but should try to make the inner and outer strength of the workpiece uniform.
After the classification of the workpiece in the austenitic state, with great plasticity (phase transformation superplasticity), thereby creating a workpiece in the straightening and correction of the condition. This has particular significance for the tool. Thus higher than the Ms temperature rating martempering, widely used tools for manufacturing. Carbon steel, this quenching is suitable for a diameter of 8 lOmm tools.
If the quenching temperature is lower than the Ms point, because when you remove the self-hardening artifacts agent, has been part of austenite into martensite, straightening above austenitic state can not be used. However, this method for large workpiece dimensions (up to 10-15mm diameter tool steel), the crack does not cause stress and hardening, it is still widely used.
5. Isothermal quenching
After quenching heating, if long-term to maintain the temperature of the lower bainite region, making it complete isothermal transformation of austenite to obtain lower bainite, this hardening is called isothermal hardening. Austempering difference is that the former and martempering get lower bainite. Austempering purpose is to obtain less deformation, high hardness and toughness of both the good parts. Because the higher hardness and toughness lower bainite good, and so disheartened when they slow quenching, bainite specific volume is relatively small, thermal stress, organizational stress are very small, so the shape of the deformation and deformation volume small.
Isothermal quenching medium quenching and quenching the same. Isothermal temperature mainly by the organization and performance of the steel workpiece C curve may be required. The lower the temperature and other temperatures, the higher the hardness, the specific volume is increased, a corresponding increase in the volume of deformation. Therefore, the adjustment isothermal temperature can alter the mechanical properties and deformation of hardened steel, is generally considered Mc ~ Ms point + 30 ℃ ju-degree range of isothermal get good strength and toughness
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2015-04-03
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