For the general knowledge of metal heat treatment, perhaps many people are not too understand, then we are here through the furnace temperature tester to understand the basic knowledge of metal heat treatment.
1. Heat treatment of beryllium bronze
Beryllium bronze is a widely used deposition hardening alloy. After solid solution and aging treatment, the strength can reach 1250-1500MPa (1250-1500kg). Its heat treatment is characterized by: solid solution treatment has outstanding plasticity, can be cold working deformation. But then after aging treatment, but has excellent elastic limit, together with the hardness, strength has also been improved.
(1)Solution treatment of beryllium bronze
General solid solution treatment of heating temperature between 780-820 ℃, used as an elastic component of the material, choose 760-780 ℃, the main thing is to prevent coarse grain size affects the strength. Solid solution treatment furnace temperature uniformity should be strictly controlled at ± 5 ℃. Holding time can generally be 1 hour / 25mm accounting, beryllium bronze in the air or oxidizing atmosphere for solid solution heating treatment, the appearance will constitute an oxide film. Although the mechanical function after aging strengthening has little effect, it will affect the service life of the mold when it is cold processed. In order to prevent oxidation should be in a vacuum furnace or ammonia decomposition, inert gas, restorative atmosphere (such as hydrogen, carbon monoxide, etc.) in the heating, and then obtain bright heat treatment effect. In addition, also pay attention to minimize the handling time (when this quench water), otherwise it will affect the mechanical function after aging. Thin material shall not exceed 3 seconds, general parts do not exceed 5 seconds. Quenching medium generally choose water (no heating request), of course, the shape of complex parts in order to prevent deformation can also choose oil.
(2) Beryllium bronze aging treatment
Beryllium bronze aging temperature and Be content, containing Be less than 2.1% of the alloy are suitable for aging treatment. For Be greater than 1.7% of the alloy, the best aging temperature of 300-330 ℃, holding time 1-3 hours (according to the shape and thickness of the parts). Be less than 0.5% of the high conductivity electrode alloy, because the solubility point increases, the best aging temperature of 450-480 ℃, holding time 1-3 hours. These years also developed a dual-level and multi-level aging, that is, first in the high temperature short time aging, and then in the low temperature long time insulation aging, so the advantage of doing so is the function of the progress but the deformation is reduced. In order to improve the scale accuracy of beryllium bronze after aging, can choose the fixture clamping for aging, and sometimes also can choose two segregated aging treatment.
(3) Beryllium bronze stress relief treatment
Beryllium bronze de-stressing annealing temperature of 150-200 ℃, holding time of 1-1.5 hours, can be used to eliminate the residual stress due to metal cutting, straightening, cold forming, etc., to stabilize the parts in the long-term use of the shape and scale accuracy.
2. Heat treatment stress and its effects
Heat treatment residual force refers to the workpiece by heat treatment after the final residual stress, the shape of the workpiece, scale and function have a very important impact. When it exceeds the yield strength of the material, it will lead to the deformation of the workpiece, beyond the limit of the strength of the material will make the workpiece cracking, which is its harmful side, should be reduced and eliminated. But under certain conditions to manipulate the stress so that the reasonable distribution, you can improve the mechanical function and service life of the parts, harmful to favorable. Analysis of steel in the heat treatment process stress distribution and change rules, so that the reasonable distribution of product quality has far-reaching practical significance. For example, for the surface layer of the reasonable distribution of residual compressive stress on the service life of the parts of the question has led to a wide range of attention.
Ⅰ. the heat treatment of steel stress
Workpiece in the heating and cooling process, because the surface layer and the heart of the cooling speed and time of inconsistency, constituting a temperature difference, will lead to volume expansion and shortening of uneven and the onset of stress, that is, thermal stress. Under the effect of thermal stress, because the surface layer of the beginning of the temperature is lower than the heart, the shortening is also greater than the heart and the heart of the tensile, when the end of the cooling, because the heart of the final cooling volume shortening can not be free to carry out the heart of the surface layer of the pressurized heart of the tensile. That is, under the effect of thermal stress finally make the workpiece surface pressure and the heart of the tensile. This phenomenon is affected by the cooling rate, material composition and heat treatment technology and other factors. When the cooling rate is faster, the higher the carbon content and alloy composition, the cooling process in the thermal stress effect of the onset of uneven plastic deformation is greater, and ultimately constitute the residual stress is greater. On the other hand, steel in the heat treatment process because of the change in the arrangement of austenite to martensite change, due to the increase in specific volume will be accompanied by the volume of the workpiece expansion, the workpiece parts of the successive phase transition, constituting the volume of growth is inconsistent and the onset of the arrangement of stress. Arrangement stress change the final result is the surface layer by tensile stress, the heart by compressive stress, just the opposite of thermal stress. Arrangement of the magnitude of stress and workpiece in the martensite phase transition area of the cooling rate, shape, chemical composition of the material and other factors.
Practice has proved that any workpiece in the heat treatment process, as long as there is a phase change, thermal stress and arrangement stress will occur. Only thermal stress in the arrangement of the change has now been attacked, and the arrangement of stress is in the arrangement of the change in the process of seizure, in all the cooling process, thermal stress and arrangement of the results of the effect of the stress is the workpiece in the practical existence of the stress. The results of these two stresses are very complex, subject to the influence of many factors, such as composition, shape, heat treatment technology. In terms of its development process as long as two types, namely, thermal stress and arrangement of stress, the effect of the opposite direction of the two cancel, the effect of the direction of the two together with each other. Whether they cancel each other or each other, the two stresses should have a dominant element, thermal stresses dominate the results of the effect is the heart of the workpiece tensile, the exterior is pressurized. Arrangement stress dominates the results of the effect is the workpiece heart pressure exterior tensile.
II. Influence of heat treatment stress on quenching cracks
Existing in the quenching of different parts of the elements can lead to stress concentration (including metallurgical defects, including), the onset of quenching cracks have a promotional effect, but as long as the tensile stress field (especially in the maximum tensile stress) will be manifested, if in the compressive stress field and does not promote the effect of cracking.
Quenching cooling rate is a quenching quality and resolution of the primary elements of the residual stress, but also a quenching cracks can be assigned to the primary and even the decisive influence of the elements. In order to achieve the intent of quenching, it is generally necessary to accelerate the cooling rate of the part at high temperatures and to exceed the critical quench cooling rate of the steel in order to obtain a martensitic arrangement. In terms of residual stress, do so because it can increase the thermal stress value to offset the effect of the arrangement of stress, so it can reduce the tensile stress on the surface of the workpiece to achieve the intention of pressing the longitudinal crack. The effect will increase with the acceleration of high temperature cooling rate. Moreover, in the case of quenching, the larger the cross-section size of the workpiece, although the actual cooling rate is slower, but the risk of cracking is greater. This is all because of the thermal stress of such steel with the scale of the increase in the practice of the cooling rate slows down, the thermal stress decreases, the arrangement of stress with the increase in scale and increase, and ultimately constitute the arrangement of stress-based tensile stress effect on the exterior of the workpiece effect characteristics of the composition. And the slower the cooling stress the smaller the traditional concept is very different. For this type of steel, quenching under normal conditions in the high hardenability steel can only constitute a longitudinal crack. Prevent quenching cracking of the solid principle is to try to minimize the unequal timing of martensite changes inside and outside the section. Only the implementation of slow cooling in the martensite change zone is not enough to prevent the formation of longitudinal cracks. Generally can only occur in the non-hardenability pieces of the arc crack, although the whole rapid cooling for the necessary conditions, but its real composition, but not in the rapid cooling (including martensite change zone) itself, but the local orientation of the quenched parts (by a number of structure resolution), in the high-temperature critical temperature zone cooling rate slowed down significantly, and therefore not hardened due to. Attacked in the large non-hardened parts of the transverse and longitudinal split, by the thermal stress as the main component of the residual tensile stress effect in the base of the quenched parts, and in the quenched parts at the end of the quenched base of the cross-section, the primary composition of cracks and from the inside to the outside of the expansion and the formation of the. In order to prevent such cracks from occurring, water – oil quenching technology is often used. In this technology, the implementation of high temperature section of the rapid cooling, the intention is only to ensure that the outer layer of metal to get martensite arrangement, and from the point of view of the internal stresses, this time the fast cooling is harmful. Secondly, the intention of cooling the late slow cooling, the primary is not to reduce the expansion rate of martensite phase transition and arrangement of the stress value, but to minimize the temperature difference between the cross-section and the cross-section of the base part of the metal shortening speed, and then reach the stress value and ultimately press the intention of quenching and cracking.
III. Effect of residual compressive stress on the workpiece
Carburizing exterior strengthening is widely used as a factor to improve the fatigue strength of the workpiece. On the one hand, because it can be useful to increase the strength and hardness of the workpiece appearance, and improve the wear resistance of the workpiece, on the other hand, carburizing can be useful to improve the stress distribution of the workpiece, and obtain a larger residual compressive stress in the outer surface layer of the workpiece, and improve the fatigue strength of the workpiece. If isothermal quenching is carried out after carburizing, the residual compressive stress in the surface layer will be increased, and the fatigue strength will be further improved. Some people on the 35SiMn2M0V steel carburizing after isothermal quenching and carburizing after quenching and low temperature tempering of the residual stress has been tested for its
3. Heat treatment technology
Residual stress value (kg/mm2) after carburizing 880-900 degrees salt bath heating, 260 degrees isothermal 40 minutes -65
After carburizing 880-900 degrees salt bath heating quenching, 260 degrees isothermal 90 minutes-18
After carburizing 880-900 degrees salt bath heating, 260 degrees Celsius isothermal 40 minutes, 260 degrees Celsius tempering 90 minutes -38
Test results can be seen isothermal quenching than the general quenching and low temperature tempering technology has a higher residual compressive stress on the exterior. Even after isothermal quenching and low temperature tempering, the residual compressive stress on the exterior is also higher than after quenching and low temperature tempering. Thus, it can be concluded that the carburization after isothermal quenching than the general carburization quenching low temperature tempering to obtain a higher exterior residual compressive stress, from the exterior surface residual compressive stress on fatigue.
More Staiyu main products:
Coating machine、High speed labeling machine、Automatic soldering machine。
