First, the protective atmosphere for heat treatment The ignition point of magnesium alloy in air is above 400°C. Of course, the difficulty of combustion is also related to the size and shape of the material itself. Therefore, when the solution treatment temperature exceeds 400°C, a protective atmosphere must be used to prevent oxidation and combustion of the casting surface. The types and selection of protective atmospheres are shown in Table 13-7. Certain noble gases such as helium and helium may also be used as a protective atmosphere for the heat treatment of magnesium alloys. However, due to their high price and impracticality, the plants are mostly based on pyrite.
Second, the quality control of heat treatment
1 heat treatment furnace. The solid solution and aging treatment of magnesium alloy castings should be carried out in a furnace with circulating air flow. Electric heating is generally used. During the solution treatment, the furnace contains protective gas ((C02' SO: or SF6 Portugal), so the furnace should be kept sealed and with inlet gas for introducing protective gas. The furnace must be equipped with enough thermocouples for continuous and complete Measure the temperature of the furnace.The heat source must be well shielded to avoid local overheating due to radiation.
2 casting furnace temperature. Mg-AI-Zn alloys ZM5 and ZM10 should be treated in a furnace close to 260°C in solution, and then slowly raised to the required temperature to prevent the eutectic compound from melting and forming pores. The time required to raise the solution temperature from 260°C depends on the load and the chemical composition, size, mass, and section thickness of the casting. Typical 2h is the typical time.
The aging treatment can be installed at the processing temperature.
3 heat treatment temperature control. The temperature of the magnesium alloy must be strictly controlled during the heat treatment, and the temperature in the heat treatment furnace is required to be uniform and accurate, and the sealability of the furnace is good. The maximum allowable deviation of solution temperature is ± 5 °C 4 casting deformation control. Magnesium alloy castings have a large drop in rigidity at the solution heat treatment temperature, which may be warped due to the release of casting stress, and may deviate from the original shape or size of the casting due to its own weight and falling or other reasons. Warpage is largely influenced by the size, shape, and cross-sectional thickness of the casting. Therefore, according to these factors on the accuracy of the size control, using the support method or select the appropriate placement on the heat treatment rack to reduce or eliminate distortion. Sometimes special fixtures are needed to keep the correct shape of the casting. The precautions taken cannot prevent the thermal cycle around the casting.
The use of fixtures and other measures can reduce the warpage of the castings, but some castings still need to be corrected after the solution treatment. The correction is usually after the solution treatment and before the aging treatment.
5 Quenching medium. The quenching of cast magnesium alloys after solution treatment is generally carried out in still air. Artificial air flow is used only for castings with large loads and dense or thick sections. Some ZM6 alloy castings requiring improved mechanical properties can be tested for 60-95°C 6 heat treatment defects and elimination measures. The common defects of heat treatment of magnesium alloy castings and elimination measures are shown in Table 13-8 7 Post-weld heat treatment. The purpose of the post-weld heat treatment of magnesium alloy castings is to eliminate stress and restore the casting or weld zone with altered mechanical properties.
General magnesium alloy castings can be repaired in any heat treatment state, but ZM5 and ZM10 alloys should be solution treated before welding to avoid abnormal grain growth in the weld zone. Mg-Al series that do not require solution treatment after welding Alloy castings should be treated for stress relief at 260°C for lh to avoid stress corrosion and cracking. The welding electrode alloy composition for repair welding must be in accordance with the composition of the casting.
8 magnesium alloy heat treatment safety technology. Combustion of magnesium and its alloys starts when molten metal appears, and with the release of a large amount of heat, the metal is further melted and ignited. The heat treatment of magnesium alloys is the most dangerous when it comes to heat treatment, but the degree of difficulty in combustion is also related to the size and shape of the material itself. Magnesium alloys with fine particles and dust can easily burn or explode. The fines produced during machining and sawing are large, and the degree of fire hazard is also lower than that of powder. However, once the chips are heated above the ignition point, they will burn easily. Therefore, magnesium alloys must first be cleaned of dust, burrs, debris, oil or other impurities and kept dry before the furnace is installed. Prior to heat treatment in the furnace, the workpiece should be blown sand or chemically treated with chromate.
The heat treatment furnace itself must also prevent such contamination, and the scale should be descaled and thoroughly dried in the furnace. Furnace installation work should be carried out with caution. Only one type of alloy casting can be installed in the furnace. The heating temperature must be strictly in accordance with the recommended heat treatment specifications.
Because the temperature of magnesium alloy solution treatment approaches its solidus, any part of the furnace exceeding this temperature may cause ignition. To reduce this risk of fire, the furnace should maintain the recommended protective atmosphere content. When using sulfur dioxide as a protective atmosphere, the content should not be too high, because the volume fraction of sulfur dioxide exceeds 1.0%, it is unfavorable to the heater resistance wire.
During operation, strict compliance with the safe operating rules listed in Table 13-9 generally prevents the risk of burning of the magnesium alloy casting in the heat treatment furnace.
If the equipment is not operating normally, carelessness or mistakes, it may cause the burning of the casting in the furnace. The characteristics of the combustion of the castings in the furnace are that white smoke leaks from the furnace and the temperature of the furnace rises. At this point, the furnace should be immediately powered off and, if necessary, the other furnaces should be de-energized. The fire is not high and the temperature of the furnace slowly rises. (450-50090 hours, the furnace door is allowed to open, the casting is quickly removed from the furnace, and the combustion source is eliminated with dry sand or flux etc. If the casting is burning violently in the furnace, When the temperature of the furnace rises sharply and it is impossible to remove the casting from the furnace, the furnace door should not be opened but all the imperfections of the furnace should be blocked, so that the combustion of the casting in the furnace will be extinguished by the lack of air. Only at 250-300 °C can the lid be opened and the castings removed. Individually burnt castings can be extinguished with extinguishing agents.
Do not use water to extinguish the fire in any case. General types of foam, dry powder or carbon dioxide and other types of fire extinguishing agents are also indispensable for the combustion of magnesium. The use of these extinguishing agents only accelerates the burning of magnesium and may cause explosions. Burned magnesium can decompose sulfur dioxide, reduce carbon dioxide and continue to burn and emit a lot of heat. Therefore, magnesium combustion can only be extinguished by the method of covering air and materials that do not react with the burning magnesium. The magnesium alloy heat treatment site should be equipped with the following commonly used extinguishing agents: graphite powder, dry sand, and unoxidized dry cast iron. Chips or melting fused Qi I1. In addition to extinguishing fire with normal safety equipment, fire extinguishers should wear colored glasses when extinguishing the magnesium fire to prevent the magnesium from burning and causing intense white light to damage the eyes.
9 heat treatment effect assessment. The effect of heat treatment on magnesium alloy castings can be evaluated in terms of hardness, tensile properties and microstructure. The hardness of castings is usually measured by Brinell hardness tester, and the measured value is only used as a reference for studying the heat treatment effect of materials.
Since the strength performance index corresponding to the hardness is too scattered, it cannot be used to calculate the strength.
The tensile test can more accurately assess the heat treatment effect of magnesium alloys, and special samples are used. Generally, the unprocessed samples are cast separately. The specimens observed by the microstructure need to be cut from the castings to evaluate the residual amount of the bulk compounds in the alloy, precipitates, grain size, and pore fusion.
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