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Selection and heat treatment equipment heat treatment commonly caused by defects

2016-06-22 17:08:51
A commonly usedFurnace heat treatment furnaces Choice

    Gasifier Model

    Furnace should be determined on the basis of different types of process requirements and the workpiece

    1. Not for the batch production of stereotypes, unequal size of the workpiece, the type more, on process requires versatility, versatility, and can choose a box furnace.

    2. When heated long axis and a long screw, pipes and other parts, it can be used in deep furnace.

    3. Small quantities of carburizing parts, the choice of pit gas carburizing furnace.

    4. For large quantities of cars, tractors and other parts of the production gear carburizing optional continuous production line or box-type multi-purpose furnace.

    5. When the heating of the blank sheet metal stamping mass production, the best selection of rolling furnace, roller hearth furnace.

    6. Stereotypes of bulk parts, the choice of the pusher or belt type resistance furnace (pusher furnace or cast belt furnace) production

    7. Small mechanical parts such as: screws, nuts, etc. can be used vibrating mesh belt furnace or hearth furnace.

    8. Ball and Roller heat treatment can be used within the spiral rotary tube furnace.

    9. Non-ferrous metal billet pusher furnace can be used in high-volume production, while the non-ferrous metal materials and small parts available for air circulation oven.

    Second, the defect and heating control

   (A), overheating

    We know that overheating of the heating during the heat treatment most likely to lead coarse austenite grains, the mechanical properties of the parts is reduced.

    1. General overheating: heating temperature is too high or too long holding time at a high temperature, causing the austenite grain coarsening called overheating. Becomes coarse austenite grains will result in reduced strength and toughness of steel, brittle transition temperature, increasing the quenching Shaped cracking tendency. The cause of overheating furnace temperature meter is out of control or mixing (often do not understand the processes occurring). Organizations can overheat after annealing, normalizing or tempering times, under normal circumstances, re-austenite grains of fine Of.

    2. Genetic fracture: There are steel tissue overheating, re-heating after quenching, although make austenite grain refinement, but sometimes still appear coarse granular fracture. Genetic Theory dispute arising fracture more, but it had generally believed that the heating temperature is too So high that MnS like debris dissolved in austenite and enriched in the crystal interface, while cooling these inclusions will precipitate along the grain interface, easy along the coarse austenite grain boundary fracture upon impact.

    3. Genetic thick Organization: Coarse martensite, bainite, when steel Wilcoxon re-organization of the austenite to slow to a conventional heating quenching temperature, or even much lower, its austenite grain coarse grains is still a phenomenon known as hereditary organization. To eliminate hereditary thick tissue, can be used repeatedly or intermediate annealing temperature tempering.

   (B), burnt phenomenon

    Heating temperature is too high, not only causing austenite grain coarsening, grain boundary and the emergence of local oxidation or melting, leading to grain boundary weakening, called burnt. After burning performance over the serious deterioration of the steel to form cracks during quenching. Burnt tissue can not be restored, can only be scrapped. Therefore work to avoid over-burning occurs.

   (III) oxidation and decarbonization

    When steel is heated, the surface layer of the carbon with the medium (or atmosphere) oxygen, hydrogen, carbon dioxide and water vapor react, reducing the surface carbon concentration after decarburization known, from carbon steel quench surface hardness, fatigue strength and corrosion grind down Low, and surface residual tensile stress is easy to form a surface network cracks. When heated, the surface of the steel and iron and alloy elements and media (or atmosphere) of oxygen, carbon dioxide, water vapor and other phenomena occur reaction of the oxide film Called oxidation. High temperature (generally above 570 degrees) after oxidation of the workpiece dimensional accuracy and surface brightness deterioration of the oxide film having poor hardenability of steel prone to quenching soft spots. In order to reduce oxidation and decarbonization measures to prevent are: Surface coating, stainless steel foil packet sealed heated salt bath heating, heating using a protective atmosphere (inert gas such as purified, the furnace carbon potential control), flame combustion furnace (the furnace gas that was reduced).

   (Iv), hydrogen embrittlement

    Ductility and toughness decrease phenomenon when heated hydrogen rich atmosphere of high strength steel called hydrogen embrittlement. Workpiece occurs by the addition of hydrogen embrittlement process (such as tempering, aging, etc.) can eliminate hydrogen embrittlement, vacuum, low hydrogen atmosphere or an inert gas Heating the atmosphere to avoid hydrogen embrittlement. Like now after the continuous heat treatment furnace quenching and tempering time when treatment can both drive during tempering oxygen treatment, according with the current situation in the use of statistics and continuous controlled atmosphere heat treatment furnace process Products are generally not appear embrittlement phenomenon.

    Of course, everything has its two sides, the actual work was to exploit this phenomenon to human services (such as alloy pulverization treatment).

    three,Heat Treatment Stress and Its Influence

    Refers to the final heat treatment residual force after surviving artifacts stress by heat treatment, the shape, size and performance of the work has a very important influence. When it exceeds the yield strength of the material, it caused deformation of the workpiece, more than material When will make the ultimate strength of the material of the workpiece cracking, which is its harmful side, it should be reduced and eliminated. However, under certain conditions, to make a reasonable distribution of stress control, you can improve performance and service life of mechanical parts, becomes harmful to favorable. Analysis of steel distribution and variation of stress during heat treatment, so that a reasonable distribution to improve the quality of products has far-reaching real? Meaning. Such as fair distribution of residual stress on the surface of the part of the life of Movies Sound problems have attracted widespread attention.

   (one),Heat treatment of steel stress

    Workpiece in the heating and cooling process, due to inconsistent surface and center cooling rate and time, the formation temperature, will lead to volume expansion and contraction of the uneven stress that the heat stress. Under the influence of thermal stress due to Start the surface temperature is lower than core section, leaving the central portion is greater than the contraction also central portion tension, when the end of the cooling, since the cooling core section final volume shrinkage can not freely leaving the center portion of the pressure receiving surface tension. That is, in the final action of heat stress The workpiece surface pressure and heart Ministry tension. This phenomenon is cooled velocity, material composition and heat treatment and other factors. When cooled, the faster, the higher the carbon content and alloy composition, the cooling process under thermal stress Uneven plastic deformation caused by the greater residual stress in the final form of the greater. On the other hand during the heat treatment of steel due to changes in the organization that is when the austenite to martensite, because the work will be accompanied by increased hematocrit volume expansion Swelling, each part of the work has a phase change, resulting in inconsistent volume grew produce tissue stress. The end result is that the stress changes the surface tensile stress, the heart portion compressive stress, and thermal stress exactly the opposite. Size and organizational stress Workpiece factors martensitic transformation zone cooling rate, shape, chemical composition of the material.

    Practice has proved that any of the workpiece during the heat treatment, as long as there is a phase change, thermal stress and stress will occur. Thermal stress just before the organizational changes have been produced, and the stress is produced in the process of organizational transformation Students throughout the cooling process, the results of the thermal stress and the stress of the combined effect is actually present in the work stress. The results of these two combined effects of stress is very complex, being affected by many factors, such as Points, shape, heat treatment processes. Only two types, namely thermal stress and stress, the role of the opposite direction when the two offset its development process, the role of the same direction both mutual superposition. Whether or cancel each other out Mutual superposition, two stress should be a dominant factor in the role of thermal stress results when the dominant portion of the workpiece heart tension, surface pressure. Organizational stress results when the dominant center portion of the workpiece surface by pressure Pull.

   (two),Effect of Heat Stress on quenching cracks

    Present on different parts of the quenching member can cause stress concentration factors (including metallurgical defects included), produce quenching cracks have promoted, but only tensile stress field (especially at maximum tensile stress) will show out if the venue does not promote stress cracking effect.

    Quench speed is an important factor in a decision can affect the quality of quenching and residual stress, it can also impart a significant and even decisive influence on the quenching cracks factor. In order to achieve the purpose of quenching, parts usually must be accelerated In the high temperature section of the cooling rate, and it exceeds the critical cooling rate of quenching of steel to get martensite. On the residual stress is concerned, this can be offset by an increase due to the stress of thermal tissue stress value, it can reduce the work Tensile stress on the surface and inhibit longitudinal purposes. The effect will vary with temperature to accelerate the cooling rate increases. Moreover, in the case of energy quenching, the larger the cross-sectional dimensions of the workpiece, although the actual cooling rate more slowly, cracking But instead, the greater the risk. All this is due to thermal stress such steel with the size of the actual increase in the cooling rate slows, reducing the thermal stress, the stress increases with increasing the size of the final form of stress-based organization should pull Force in the role of the characteristics of the surface caused. And cooling the slower the smaller the traditional concept of stress differ. For this type of steel, under normal conditions of high hardenability hardened steel member can only form diastema. Avoid quench cracking Reliable principle is to try to minimize the unequal sectional inside and outside the martensite transformation. Only the implementation of martensitic transformation zone slow cooling is not sufficient to prevent the formation of longitudinal cracking. Under normal circumstances can only be produced in a non-arc hardenability member of Crack, although the overall rapid cooling of the necessary conditions for the formation, but its real causes, not in the rapid cooling (including district martensite) per se, but rather quenching LOCAL position (determined by the geometry), high Temperature and cooling rate of the critical temperature region significantly slowed down, so there is no due hardened. Produced in large non-hardenability member of the cross-sectional and longitudinal split is determined by the residual tensile stress of heat stress as the main ingredient in hardened member centers, while in Quench hardened end cross-section at the center, resulting in first formed by the outward expansion of internal cracks. In order to avoid such cracks, tend to use water - oil double quenching process. Rapid implementation of the cold temperature within the segment in this process But the aim is to ensure that only the outer layer of the metal to be martensite, and from the perspective of internal stress, then rapidly cooled harm than good. Secondly, the purpose of cooling the late slow cooling, not primarily to reduce the martensitic transformation of the expansion speed And stress the value of the organization, but rather to minimize the temperature difference between the cross-section and cross-section of the central portion of the metal contraction velocity, so as to achieve reduced stress and ultimate purpose of suppressing quench cracking.

   (Iii) the impact of residual compressive stress on the workpiece

    Carburizing surface hardening to improve the fatigue strength of the workpiece as a method of application of a very wide range of reasons. Partly because it can effectively increase the strength and hardness of the surface of the workpiece, improve the wear resistance of the workpiece, the other is to have carburizing Improve the work efficiency of the stress distribution in the surface layer to obtain a larger residual compressive stress and improve the fatigue strength of the workpiece. If after isothermal carburizing quenching will increase the surface residual compressive stress, fatigue strength to get into Step increase. It was after 35SiMn2MoV steel isothermal carburizing quenching after carburizing and quenching and tempering residual stress was tested the results are shown in Table 1.

     From Table 1, the test results can be seen austempering than conventional quenching? Tempering process has a higher surface residual compressive stress. And so even if low temperature tempering after quenching, the surface residual compressive stress, than after quenching low back High fire. So you can come to this conclusion, namely carburizing surface residual compressive stress than usual after isothermal quenching carburizing quenching and tempering obtained higher residual compressive stress of the surface layer from the beneficial effects of fatigue resistance point of view Look, isothermal carburizing quenching process is an effective way to improve the fatigue strength of carburized parts. Why carburizing process to obtain the surface residual compressive stress? Why can carburizing austempering greater surface residual compressive stress? The main There are two reasons: One reason is that the surface carbon martensite hematocrit greater than low carbon martensitic core part of the specific volume, surface large volume expansion after quenching, and low carbon martensitic core section volume expansion is small, restricted free expansion surface, causing the table Layer compression core section tension stress state. The other more important reason is the high carbon supercooled austenite to martensite transformation start temperature (Ms), lower than the central portion of the carbon content of supercooled austenite to martensite transformation start temperature Degree (Ms) is low. This means that in the quenching process is often central portion first produced martensite transformation unit volume expansion caused by heart and get strengthened, but also the end of the cooling surface corresponding to its martensitic transformation start point (Ms), so Still in the supercooled austenitic state, with good ductility, does not mind the volume expansion unit martensitic transformation from a serious deterrent effects. With declining quenching temperature so that the surface temperature drops HERE (Ms) of points Under the surface to produce martensitic transformation, causing the surface of the volume expansion. But the heart unit at this time already transformed to martensite and strengthen, so central portion of the surface of the volume expansion will play a significant role in suppression, so that the surface residual compressive should get force. And in the isothermal carburizing quenching, when the temperature of the isothermal layer in the Martensitic start temperature (Ms) above, the core part of martensite start temperature (Ms) point below the appropriate temperature isothermal hardening than even Continued cooling characteristics of quenching better ensure the order of this transformation (which is to ensure the surface of the martensitic transformation is only produced in the cooling process after the isothermal). Of course, after carburizing austempering isothermal temperature and time of isothermal surface residue Have a great impact on the size of stress. Some of the surface residual stress after 40 minutes at 260 ℃ and 320 ℃ isothermal after 35SiMn2MoV carburizing steel specimens have been tested, and the results are shown in Table 2. From Table 2 Known in temperature than 260 ℃ and other actions in the surface residual stress 320 ℃ isothermal higher surface residual stress than doubled steel table 2.35SiMn2MoV different isothermal temperatures.

    four,Temper brittleness

    Quenching and tempering the steel, the higher the tempering temperature, the strength generally, lowering the hardness and ductility, and toughness mention high. But in some tempering temperature range, the impact toughness of the steel is not only not improved, but significantly reduced, this phenomenon is called brittle temper brittleness. Thus, the general is not 250-350 degrees tempering, which is due to To temper brittleness occurs when hardened steel is tempered at this temperature range. This is called tempering brittleness temper brittleness or first class temper brittleness. The first class temper brittleness once produced can not be eliminated, so the production is generally not Tempering this temperature range.

    Containing chromium, manganese, chromium - alloy after quenching nickel and other elements in brittle temperature (400 ~ 500 ℃) region tempering, or by higher temperatures gradually cooled back through the brittle brittle transition temperature region generated by the said second temper brittleness, also known as high-temperature tempering brittleness. This brittleness can be eliminated by rapid cooling temperatures again above the embrittlement after tempering.

    Cause tempering brittleness, is not yet very clear. It is generally considered to be due to intermittent carbide flakes along martensite sheet or strip martensite interfaces caused by precipitation. Such hard and brittle carbide sheet binding between martensite weak, reducing the strength of martensite at the grain boundaries, thus making the impact toughness decreased.