Influence of holding time of HFC134a nitrogen in new material and new process sealing smelting furnace on protection of AZ91D magnesium alloy Nie Shuhong, Xiong Shoumei (Tsinghua-Dongyang Magnesium-Aluminum Alloy Forming Technology Research and Development Center, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, with very little dosage. No matter Without surface agitation, the AZ91D magnesium alloy was well protected at 2 h, 5 h or 10 h, respectively, and no evaporate was observed. All surface films were densely packed with a surface film thickness of about 15 pm. It is relatively uniform. As the holding time increases, the C content in the surface film gradually decreases to zero, and the oxygen content gradually increases. The furnace atmosphere has no observable corrosion on the inner wall of the melting furnace.
In the air, liquid magnesium and its alloys are easily oxidized and burned, so casting and smelting of magnesium and its alloys need to be carried out in a flux or a protective atmosphere. More than 20 years ago, the use of SF6 as a shielding gas in the smelting of magnesium and magnesium alloys was once the biggest advancement in the magnesium industry because it eliminated most of the problems previously used with SO2 and flux smelting. But by 1990, the high greenhouse effect of SF6 and its analogues (which was 24,000 times that of CO2 and could persist in the atmosphere for 3,200 years forced the magnesium industry users to find technically, environmentally and economically acceptable alternatives, At the same time take measures to reduce the amount of gas used.
In today's magnesium industry, the most commonly used melting furnace is an open melting furnace that continuously discharges shielding gas into the protective gas. Some researchers have proposed a protective atmosphere in a sealed melting furnace. The relevant atmosphere and process equipment still need to be done. a lot of work. Therefore, it is of great significance to study the effects of different types and concentrations of protective gases on the protective effect of magnesium alloys in smelting furnaces under different temperatures, different holding times, different air mixing amounts and different smelting furnace gas leakage rates.
Research and testing have been recognized by the refrigeration industry. It is characterized by an ozone depletion capacity coefficient (ODP) of zero, a global warming effect (GWP) value of 0.26, non-flammability, and toxicity.
In this paper, the effect of holding time of 0.01% vol. HFC134a nitrogen in sealed smelting furnace on the protection of AZ91D magnesium alloy was studied. The results of previous studies on the effects of different HFC134a concentrations on AZ91D magnesium alloy protection indicate that the fund project: Tsinghua-Dongyang Magnesium-Molybdenum Alloy Forming Technology Research and Development Center research project, postdoctoral fund-funded project (200403531S).
Received deadline: 2005-10-10 received the first draft, 2005-11-21 received the revised draft.
It has good protection effect and is economical. Therefore, the experiment is carried out with 0.01% vol. HFC134a nitrogen atmosphere.
1 Test conditions and methods 1.1 Test equipment test using the smelting furnace schematic view 圄 1. Furnace pressure S 6Pa, pressure rise rate S 0.67Pa / h. The internal volume of the furnace is 20L. With fresh melt exposure device, sampling device , sample rapid cooling device. The top of the furnace and the wall of the furnace are provided with light-transmitting holes and observation windows. The D07-7B/21 mass flow meter was used to control the flow rate of each gas introduced into the furnace. Using KSY-12-16S thyristor temperature control instrument temperature control, temperature control accuracy is less than 3C. The Chengdu Ruibao ZDZ-52T resistance vacuum gauge is used to control the residual air volume in the furnace.
The smelting furnace and the small crucible in the furnace are ordinary low carbon steel used for melting the bulk magnesium alloy for testing.
1.2 Test method 1.2.1 Control of furnace atmosphere First, the furnace is evacuated until the vacuum gauge displays 6Pa, then the vacuum pump interface valve â–¡ is closed, and the D07-7B/21 mass flow meter is used to charge a predetermined proportion of various gases. All gases are merged into the furnace at the outlet of the flow meter. When the pressure in the furnace is 1 atmosphere, the inflation port is closed, the flow meter is turned off, the gas is stopped, and the melting process is started. The vacuum gauge shows that the lower the pressure in the furnace, the less residual air in the furnace.
1.2.2 Preparation of the sample The cut magnesium alloy of the appropriate size of the block AZ91D (see Table 1) is placed in a small crucible in the furnace, heated to a set temperature in a predetermined furnace atmosphere, and kept warm to a preset The time to observe the protective effect through the observation hole. For the test results with good protection effect, part of the test was re-tested. After the pre-set time, the plug at the bottom of the small crucible was pulled out, and the surface film formed on the surface of the magnesium alloy was repeatedly broken, and the protective effect was observed through the observation hole. Finally, the small crucible was cooled to below 100 C in the furnace atmosphere. The small pieces were cut on the obtained sample to analyze and analyze the surface morphology and composition of the surface film. Before the test, the mass of the block sample AZ91D magnesium alloy was 140. Table 1 Preparation of the AZ91D magnesium alloy component cross-section test sample: The cut small sample section was polished with SiC sandpaper of different particle sizes and polished with a diamond abrasive paste having a particle size of 025 Mm.
123 surface section morphology observation, composition distribution detection analysis SM-6301 field emission scanning electron microscope was used to observe the surface film surface, section morphology observation and composition distribution analysis.
2 Test results and analysis 2.1 The effect of different holding time and surface agitation on the protective effect In the smelting furnace filled with 001% vol. HFC134a nitrogen at one time, the surface was stirred for 2h, 5h and 10h under the condition of surface agitation. The effect of AZ91D magnesium alloy protection (see Table 2. It can be seen from Table 2 that all AZ91D magnesium alloy samples are protected, no evaporation is observed in the furnace, regardless of surface agitation, regardless of the holding time of 2h, 5h or 10h. The product, and the surface of the AZ91D magnesium alloy sample was clean and metallic, and 圄2 was the surface morphology of the AZ91D magnesium alloy which was stirred twice on the surface.
It can be seen from 圄2 that when the temperature is 760C, the surface of the stirring zone does not change after the surface is stirred twice, and the surface is clean and metallic in the same manner as the non-stirring zone.
Table 2AZ91D magnesium alloy in 0.01 °% HFC134a nitrogen in different holding time protection effect temperature / C holding time / h protection no surface agitation protection effect surface agitating protection 22 different holding time AZ91D magnesium alloy surface film surface section morphology component analysis h, a "Ci cross-section shows that the surface film thickness is about 1.5ym, and it is relatively uniform, that is, there is no significant change in thickness at different regions. 圄3a2c2 shows no significant change in surface morphology. The surface morphology in a2-c2 corresponds accordingly. The regional component analysis (arc) showed that the O content increased significantly with the increase of holding time; the C content was zero when the temperature was kept for 5h and the temperature was kept for 10h. The surface composition analysis of the sample at 760C for 2h showed that the average C content was 25%, C is The reaction product of magnesium solution and HFC134a. Therefore, with the increase of holding time, the content of O in the surface film gradually increases, and the C formed by the magnesium solution and HFC134a gradually leaves the surface at the initial stage of surface film formation, and its destination is: C+0 because HFC134a The reaction with magnesium may occur as follows: at the temperature, C and trace oxygen in the atmosphere may react as follows: check the chemical manual for pure substances, C2O, CO, CO2 at the test temperature. The logarithm of the equilibrium constant of the reaction with O2 is shown in Table 3. Calculate the equilibrium constants of C2O, CO, and CO2 at the test temperature as shown in Table 3. As can be seen from Table 3, the equilibrium constant of C2O is small, AG>0; CO The equilibrium constant of CO2 is very large, AG<0. Assuming that the partial pressure of O2 in the melting furnace is 6 Pa, the equilibrium CO pressure of the C2O, CO and CO2 reaction equilibrium constant Ka, logKf and A temperature 827C at different melting temperatures in Table 3 is 6.50. XL07Pa, the equilibrium CO2 pressure is 374XL014Pa. Therefore, even if the atmosphere contains a trace amount of 02, C in the surface film is easily oxidized, and the oxidation product of C should exist in the form of CO and CO 2 .
2.3 Corrosion of protective gas against the inner wall of the melting furnace The results of six tests in the paper show that the furnace atmosphere has no visible corrosion to the inner wall of the melting furnace. After the end of each test, the inner wall of the melting furnace was observed to be very clean. The inner wall of the furnace was free of visible corrosion observed by the naked eye, and there was no F corrosion observed by the naked eye, and there was no corrosion peeling. It is beneficial to improve the service life of the melting furnace and the small crucible in the furnace and reduce the pollution of magnesium liquid.
24 Sealed Melting Furnace Protection Gas Savings Estimation Generally, the melting of magnesium alloys needs to be carried out at temperatures higher than the melting point of 50-100C or higher. A large number of test results show that the higher the melting temperature, the more the shielding gas is consumed. The smelting and casting of the AZ91D magnesium alloy is usually carried out below 760C. Therefore, in the case of smelting AZ91D magnesium alloy, the proportion of protective gas consumption will be lower than in the text.
Assume that the shielding gas used in the open furnace is the same as in the text, that is, 001% HFC134a chlorine gas (the concentration of HFC134a is higher in the actual use atmosphere).
In the sealed smelting furnace, since it is not necessary to continuously charge the gas, the gas in the furnace has sufficient time to be uniformly mixed by itself, and the use concentration of the oxidizing gas HFC134a is remarkably reduced. Generally, in the open furnace test, the gas flow rate for protecting the same area is 0.66 L/min. When the temperature in the furnace reaches 300 C, the protective gas is introduced, and when the temperature in the furnace is cooled to less than 300 C, the protective gas is stopped.
Assume that the protection gas inlet time is 1h during the cooling temperature rise, the gas is heated, the heat preservation and cooling need a protective gas of 396+39.6, and the internal volume is 20L. The whole process saves the shielding gas 435.6-gas content is 48%. Further test The results have shown that the design of a reasonable melting furnace, the sealing requirements of the melting furnace is not high, therefore, the sealing smelting furnace design technology deserves further improvement. The selection of the corresponding protective atmosphere and the use of process parameters deserve further study.
3 Conclusions (1HFC134a is suitable as a protective gas for magnesium alloys in a sealed smelting furnace, and nitrogen is a good diluent gas. The amount of shielding gas is small compared to conventional melting furnaces.
(2) Regardless of surface agitation, the AZ91D magnesium alloy was well protected at 2 h, 5 h or 10 h, respectively, and no evaporate was observed.
(3) The surface morphology of all the surface films is dense cell mass, the surface film thickness is about 15Mm, and it is relatively uniform. With the increase of holding time, the C content in the surface film gradually decreases to zero, and the oxygen content gradually increases.
(4 The atmosphere in the furnace has no visible corrosion to the inner wall of the melting furnace.
(5) The research results can provide a test basis for the development of new magnesium alloy melting furnaces, reducing the emission of protective gases, and the formulation of protective gas selection and protection processes.
(Finish)
Metal Trash Can,Metal Bucket Trash Can,Trash Can With Lid,Galvanized Trash Can With Lid
xiacun factory , https://www.xiacunmetal.com