Under the working conditions of high energy impact, the application range of high manganese steel and ultra-high manganese steel castings is broad. Many foundries lack the necessary knowledge to produce such steel castings. Here is a brief description of the specific operation for the reference of producers.
1 Chemical composition
According to the national standard, high manganese steel is divided into 5 grades, the main difference is the carbon content, which ranges from 0.75% to 1.45%. High impact and low carbon content. The manganese content is between 11.0%-14.0%, generally not less than 13%. There is no national standard for ultra-high manganese steel, but the manganese content should be greater than 18%. The level of silicon content has a great influence on the impact toughness, so the lower limit should be taken, preferably no more than 0.5%. Low phosphorus and low sulfur are the most basic requirements. Since the high manganese content naturally plays a role in desulfurization, phosphorus reduction is the most important thing. Try to keep phosphorus below 0.07%. Chromium improves wear resistance, generally around 2.0%.
2 Charges
The furnace material is determined by its chemical composition. The main charge is high-quality carbon steel (or steel ingot), high-carbon ferromanganese, medium-carbon ferromanganese, high-carbon ferrochrome, and high-manganese steel return charge. What is especially reminded here is that people think that as long as the chemical composition is suitable, more recycled materials can be used. This person is harmful. The reason why some factories have poor product quality is because of this. Not only high-manganese steel, and ultra-high manganese steel, but also any metal castings must not use too much-recycled material, and the recycled material should not exceed 25%. So, what about the surplus of recycled materials? As long as the waste is minimized, there will be no surplus of recycled materials.
3 Smelting
Here we focus on the order of feeding, whether it is smelted in an intermediate frequency furnace or an electric arc furnace, the carbon steel is always smelted first, and all kinds of ferromanganese and other precious alloy materials must be divided into multiple times, each time a small amount is put into the furnace, and the precious elements are at the end. Added to reduce burning loss. The material block should be as small as possible, preferably 50-80mm. After melting, when the furnace temperature reaches 1580-1600°C, deoxidation, dehydrogenation, and denitrification are required. Aluminum wire, Si-Ca alloy or SiC, and other materials can be used. The deoxidizer must be pressed deep into the furnace. At this time, the metal liquid surface is covered with a covering agent to isolate the outside air. It is also necessary to calm down for a while so that oxides and inclusions have enough time to float up. However, many companies only sprinkle aluminum wire or even aluminum chips on the surface of the metal liquid without covering it. Wouldn’t it be a waste? During this period, use medium carbon ferromanganese in time to adjust the content of manganese and carbon.
Before the molten steel comes out of the furnace, it is very necessary to bake the ladle to above 400°C. Modification treatment with V-Fe, Ti-Fe, rare earth, and other trace elements during the furnace is a necessary means to refine the primary crystallization, which is crucial to the product performance.
4 Charge and molding materials
To extend the furnace order, one should distinguish the properties of the steel type and the furnace lining. Manganese steel is alkaline, and the lining of the furnace is of course made of magnesium. Ramping the furnace lining will take turns to change positions again and again. The addition of lining material should not be too thick, about 80 cm each time is appropriate, and it should be baked at low temperature for a long time after pounding. To improve production efficiency, the author recommends using a shaped crucible (Shenyang Lide Factory and Hengfeng Factory both sell finished products), which can be put into production in less than one hour from dismantling the furnace. . Of course, the length of the furnace order has a lot to do with the operator. Many operators, like shot-put athletes, throw the charge into the furnace from three or four meters away. Place the furnace charge along the furnace mouth to preheat, and then slowly put the furnace charge along the furnace wall with a clip.
Modeling materials and coatings should also be consistent with the properties of the metal liquid. If you want to obtain a group of crystallization and refinement, it is correct to use chromite sand with large heat storage, especially for lost foam production plants, which will overcome the shortcoming of slow heat dissipation.
5 Casting Process Design
Manganese steel is characterized by large solidification shrinkage and poor heat dissipation. Therefore, in the process design, the casting shrinkage rate should be 2.5%-2.7%. The longer the casting, the higher the upper limit should be. The concession of molding sand and sand core must be good. The gating system is open. A plurality of scattered ingates are introduced from the thin wall of the casting and form a flat and wide horn shape, and the cross-sectional area near the casting is larger than that connected with the runner so that the molten metal can be injected into the mold quickly and smoothly, to prevent excessive temperature differences throughout the mold. The diameter of the riser should be larger than the diameter of the hot joint, close to the hot joint, and the height is 2.5-3.0 times the diameter. The hot riser or even the pouring riser must be used in one so that sufficient high-temperature molten metal can not fill the space of the casting when it solidifies and shrinks. . It is also correct to place the sprue and riser at a high place (the sandbox has a slope of 5-8°). When pouring, pour as low temperature and fast as possible. Once solidified, loosen the sandbox in time. Smart designers are always good at using chilled iron, including inner chilled iron and outer chilled iron, which not only refines the primary crystallization, eliminates shrinkage cavity and porosity, but also improves the yield of the process. Of course, the appropriate dosage and specification should be considered. The inner cooling iron should be clean and fusible, and the dosage should be less. The three-dimensional size of the outer chill is a function of 0.6-0.7 times the three-dimensional size of the cooling material. Too small will not work, too large will cause the casting to crack. Castings should be kept warm in the mold for a long time until the temperature is lower than 200°C before opening the box.
6 heat treatment
Heat treatment cracking is caused by the rapid temperature rise in the low-temperature stage. Therefore, the correct operation is below 350°C, the heating rate is <80°C/h, below 750°C, <100°C/h, and there are different periods of heat preservation. When the temperature reaches >750°C, the inside of the casting is in a plastic state, and the temperature can be raised rapidly. When it reaches 1050°C, determine the holding time according to the thickness of the casting, and then rise to above 1100°C. Leave room for the furnace to cool down and then enter the water as soon as possible. At high temperatures, the temperature rise is too slow, the holding time is too short, and the time interval between being out of the furnace and entering the water is too long (should not be >0.5min), all of which will affect the quality of the casting. The entering water temperature should be <30°C, after quenching, the water temperature should be <50°C, and the amount of water should not be less than 8 times the weight of the casting. Cold water enters from the bottom of the pool and warm water exits from the top of the pool. The casting must move continuously in three directions in the pool.
7 Cutting and welding
Due to the poor thermal conductivity of manganese steel, great care should be taken when cutting the riser. It is best to put the casting in water, and the part to be cut is exposed to the water. A certain amount of stubble is left during cutting, and it will be ground off after heat treatment.
In many factories, welding and welding repairs have become inevitable. Austenite-based manganese-nickel electrode (D256 or D266 type) is selected, the specification is slender, φ3.2mm×350mm, and the outer coating is alkaline. When operating, adopt the operation method of small current, weak arc, small welding bead, and multiple welding layers, and always keep a low temperature and less heat. Beat while welding to relieve stress. Important castings must be inspected.
What producers need to consider is not only to reduce production costs, but more importantly, to produce high-quality products to the maximum extent without waste products, and then to maximize their market share. This seems to be slow and costly, but in fact, it is fast and economical. This concept is not only recognized but more importantly, it must be achieved.
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