High Manganese Steel Castings
Casting process (self-hardening sand) traditional process
1) Sand craft
High-manganese molten steel is alkaline, and the production of sand with quartz sand is easy to stick. Corresponding measures must be taken to solve it.
- Water glass quartz sand: quartz sand: 40/70 mesh 100%; water glass 5%-7%; bentonite 4%-5%; sodium carbonate 0.2%-0.4%.
- Diesel: 2.0% (improve the fluidity of sand and prevent sand sticking). Use carbon dioxide or liquefied gas to dry and harden; brush magnesia powder quick-drying paint to prevent high manganese steel from sticking to sand.
- Forsterite sand: Forsterite has a refractoriness of 1910°C, Mohs hardness of 6-7, a smaller expansion coefficient than quartz sand, uniform expansion, no phase change, no free SiO2, and no combination with Fe and Mn. Oxide reaction, with strong corrosion resistance to metal oxides, is an ideal surface sand for high manganese steel.
- Limestone sand: Limestone sand came out in 1970, also called 70 sand. The advantages are good dispersibility, easy sand removal, smooth casting surface, and elimination of occupational silicosis. The disadvantage is that it has a large amount of gas and low sand strength. Therefore, the four principles of “hardness, lightness, transparency, and cleanness” are required for modeling and core making, namely: ①The sand mold and core should be tight, measured with a type A surface hardness tester, and the hardness value is greater than 50: ②The mold should be polished and scraped. Flat, make the edges and corners clear; ③ Open more air passages, and pass through; ④ The mold must be hardened well, if there is looseness, sand must be scrapped; ⑤ The mold cavity must be clean and clean before pouring and closing the box.
- For small and medium-sized thin-walled parts with a wall thickness of less than 40mm, clay sand can also be used.
- Some small and medium-sized factories only use 2% to 3% sodium silicate sand, hardened with carbon dioxide gas, and use No. 4 river sand for the back sand to produce crusher jaws under 200 kg without sand sticking.
1) Preparation of paint
(1) Quick-drying paint: First dissolve the rosin in anhydrous alcohol to form a rosin alcohol solution, and then mix in the magnesia powder clay. It is recommended to use 200-320 mesh fine magnesia powder.
(2) Water-based magnesia paint: mix the magnesia powder and clay according to the ratio, dry blend in a sand mixer for 10 minutes, add water and asphalt for 4 hours and 20 minutes, and then add water in batches and mix for 1 hour and 30 minutes. Roll it into a paste, put it in an iron bucket and cover it with a small amount of clean water. If the room temperature exceeds 24 degrees and the paint exceeds 24 hours, it is prone to hydration, and it will be remixed for 3 hours.
Design of pouring system for high manganese steel castings (traditional casting)
(1) Shrinkage rate of high manganese steel wire:
2.4%～3.2% for small parts, 2.0%～2.5% for large parts. Under sand casting conditions, the general casting linear shrinkage rate (scale reduction) is selected from 2.5% to 2.7%, and different castings have different linear shrinkage rates in different directions in different directions. The body shrinkage of high manganese steel is higher than that of any steel, about 30% higher than that of carbon steel, and the design of the pouring riser should be paid attention to.
(2) Riser design requirements:
① there must be enough molten metal for feeding; ② the molten metal in the riser should solidify later than the casting; ③ the feeder channel for the riser should be unobstructed.
(3) Easy-to-cut riser:
An easy-to-cut piece made of refractory material or sodium silicate hardened by carbon dioxide is placed at the neck of the riser. It can be purchased or self-made.
(4) Heat preservation riser:
In order to improve the feeding efficiency, heat preservation risers should be widely used.
(5) No riser casting:
suitable for non-thick large castings with uniform wall thickness, castings without hot joints, such as mill liners, small jaw crusher jaws, etc., but the horizontal gate must be enlarged, and the inner pouring The number of ports should be large, the shape should be flat, and more vents should be opened.
(6) Cold iron:
①Inner cold iron is not used, but outer cold iron is widely used, and it can be used in conjunction with the riser to obtain dense castings; ②Put forming outer cold iron at the inner corners of the casting to eliminate the problem caused by poor feeding Cracks; ③Because of the poor thermal conductivity of high manganese steel, sand-coated outer chilled iron should be used to avoid columnar crystals due to the serious temperature gradient in the mold after pouring. The chilled iron is improperly used and the pouring temperature is high, even if the crystal is crossed; ④ The thickness of the outer chilled iron is generally 0.6 to 0.8 times the thickness of the casting wall.
(7) Pouring system:
The design and calculation of the gating system are based on the type of the ladle. The bottom of the stopper rod is used to pour the molten steel to be clean, and there is no requirement for slag retention in the gating system. The gating system is designed as an open type, so that the molten steel can be poured into the mold smoothly; with sub-ladle pouring, the gating system must have the slag retention capacity and is designed to be half Closed type or closed type, sub-pouring for less than 200kg, bottom casting for more than 200kg, the pouring system is semi-closed design: ΣF straight: ΣF horizontal: ΣF inner=1: (0.8～0.9): (1.1～1.2) . The horizontal runner is the blocking surface, and the inner gate is larger than the horizontal gate to ensure the stable pouring of molten steel.
Steel tapping and pouring temperature
(1) The molten steel must have a certain degree of superheat to ensure uniform composition and eliminate heredity. When the degree of superheat is high, the degree of supercooling is large and the crystal grains are refined, but excessively high promotes defects such as gas inhalation and oxidation of the molten steel.
(2) The pouring temperature affects both the grain size and the morphology of the crystal grains. The higher the pouring temperature, the coarse grains and the low impact toughness; when the pouring temperature decreases, the proportion of equiaxed crystal regions increases, while the proportion of columnar crystal regions decreases. The larger the wall thickness of the main body of the casting, the lower the pouring temperature should be. The formula is: T=1485-0.3δ, T- pouring temperature (℃), δ- main body wall thickness of casting (mm). Due to the uneven thickness of the casting, the final casting temperature must be determined based on experience.
Lost Foam Process
1. White mold
Lost foam casting is a method of replacing traditional models with foam plastic molds and replacing them with molten metal at high temperatures, which is convenient and quick. Different from other casting processes, the factors that affect castings are also different. Since the model is an indispensable consumable in the lost foam casting process, each casting consumes one mold, so the mold is the key factor for the success or failure of the lost foam casting. The model is foam. The white mold is made, and the quality of the white mold is the top priority of the finished casting.
In the production of high manganese steel, whether it is foam molding or cutting assembly, the choice of foam cannot be ignored. There are three main points in foam selection:
1) The white mold foamed pellets should be uniform.
2) Quality control is 16～18kg/m3.
3) After the second curing before making the mold, it will reach the saturated state.
2. Preparation of paint
1) Selection of additives
Adopting Guilin No. 5 burn-resistant and high-temperature reinforced type or Guilin No. 6 one-material pass, these two kinds of coatings have good heat resistance, high strength, erosion resistance under high temperature conditions, and are fully suitable for hollow shell vibration pouring.
In order to ensure that the prepared coating slurry has a good coating and uniformity for the entire pattern, three hang coatings are generally used. The thickness of the coating is 2 to 3 mm. Large thick-walled castings can be thickened as needed, generally not exceeding 4 mm.
The specific preparation is as follows:
10% Guilin No. 5 + 70% Forsterite + 30% bauxite per 10kg No. 5 + 100kg composite aggregate
Guilin No. 6+5% Magnesium Powder (250 mesh)
Both of the above two preparation methods can be used for vibrating pouring of the hollow shell, and the effect is very good.
The entire process flow and the setting of the gating system are 70% similar to traditional sand casting, which will be described in detail below.
3. Vibration + empty shell pouring
The prerequisite for vibrating pouring is the air flow of the vacuum system, which is generally equipped with 45KW, and the air pumping volume is more than 25 cubic meters per minute. If the pumping volume is too small, it cannot be vibrated casting.
When the molten metal is out of the furnace, the slagging time will burn out the white mold. Generally, the empty shell is poured into open casting and it is easy to burn out. When the blank mold is burned, the air pressure drops to 0.03MP2～0.05MP2, which will maintain the entire cavity of the mold. It can be poured without worrying about collapse of the box, and the whole process is stable.
Start the vibrating table after the vibration and pouring are stable. The vibrating frequency is 80～120HZ for thick parts and 150～180HZ for small parts. When the hollow shell is vibrated casting, the principle of high temperature filling is very different from that of general casting. The molten metal enters the cavity and fluctuates 5000 to 8000 times per minute under vibration. The crystal grains that have not yet formed in the molten metal are repeatedly pulled up and down, and the grains in the liquid and paste state are further refined. At the same time, the impurities in the molten metal will rise further.
Due to the continuous fluctuation of the molten metal under vibration, the black alleys that seemed invisible and uncontrollable in the original solid pouring have become visible and controllable empty shells, turning the complication of solid pouring into the simplification of empty shell pouring. In addition, vibration is applied to effectively control the coarse grain size and shrinkage porosity of the casting.
Vibration casting under negative pressure has strict requirements on coatings and vacuum system, which has a larger air extraction volume than real mold casting. The size and air extraction volume of the castings are also different. The dense, high-strength castings and physical equipment are different. An inseparable stake.
The Impact of Vibration Pouring on Castings
Vibration pouring refers to the process of vibration filling and crystallization of molten metal and casting molding. Vibration refers to the variable movement of a certain frequency. This movement has variable speed and acceleration to form a vibration impact force acting on the molten metal and make it It is deformed. The compression of a uniform liquid requires a great deal of pressure, but the crystalline molten metal is formed by liquid, gas, and solid, and is not a uniform liquid. The solution is periodically compressed and stretched when it is vibrated. The degassing, crystallization, increased nucleation rate of metals, the refinement of crystal grains and the uniform distribution of impurities, the reduction of solution viscosity and feeding are all extremely beneficial.
In summary, the following points are summarized:
① Remarkably refine the crystal grains of metal crystals, and compact the structure of the casting.
② Eliminate the inhomogeneity of the chemical composition of the casting structure.
③Greatly reduce the gas content of the metal, eliminate or greatly reduce the pore defects of the casting.
④Conducive to the feeding process and the removal of metal impurities, reducing shrinkage porosity of castings and improving compactness.
⑤ It is beneficial to reduce the viscosity of molten metal, improve fluidity and filling, and obtain high-quality castings with clear contours.
⑥ It is beneficial to eliminate the gaps and cracks between the metal liquid crystals, and plays a very important role in improving the mechanical properties of the castings.
⑦Improve the metal crystallization conditions and the temperature of the molten metal crystallization process, and accelerate the solidification process of crystallization.
⑧Comprehensive optimization and strengthening of the mechanical properties of castings, tensile strength elongation, yield strength, wear resistance and other comprehensive mechanical strength.
All in all, in the production of high manganese steel, hollow shell + vibration casting is an indispensable part. Under the conditions of relatively mature technology and technology, product quality will be improved qualitatively.
V method process
In the casting of high manganese steel, a series of castings made of Mn13, Mn18 and 65Mn were cast by the V method many years ago. The manifestations are mostly coarse grains, short wear resistance time, and poor strength, which cannot reach the level of sodium silicate sand. state. The fundamental reason is still in the exploratory stage. The V method casting has good surface finish and beautiful appearance. The V method casting high manganese process controls its coarse grains.
I personally think:
1. If conditions permit, use more chilled iron to accelerate the solidification of the molten metal and refine the crystal grains.
2. Properly grasp the time of unpacking, try to get out of the box as early as possible to contact the outside air, speed up the cooling of the solid casting, and also effectively prevent the coarse grains and improve the wear resistance.
3. High temperature discharge, low temperature pouring, shorten pouring time.
4. After pouring is completed, uncover a part of the sealing film on the top of the upper box to let the cold air flow into the accelerated solidification process.
The above points have also been verified in the production process, and we will share with you for better process methods in the future.
Recently, the author used the V method to produce a large jaw plate in a factory. Its weight is 6780kg. The casting process, products, and internal and external quality are good, which surpasses the traditional self-hardening (water glass) stone process. After detection, the internal non-shrinkage porosity, metallographic structure, and grain chemical element ratio have all reached the product quality requirements.
The production method is as follows:
1. Paint ratio
①300 mesh zircon powder (content above 65%) 30% + 300 mesh forster 20% + Guilin No. 6 (alcohol-based strong porcelain) 50%
②Guilin No. 6 (alcohol base) 60% + zircon powder 40%
2. Suction film
The low-carbon casting film developed by Mr. Liu Dehan of Wuhan Hengde is selected as the casting film. It has short baking time, high elongation, and good sealing effect. It is the top product of the casting jaw plate.
High temperature tapping and low temperature pouring. The tapping temperature is generally 1520℃. The pouring temperature is controlled between 1440 and 1480, preferably not more than 1480℃. The pouring speed is basically in line with traditional casting.
The V method casting process is basically based on some large-scale high-manganese steel castings with wall thickness, such as the rolling mortar wall and crushing wall of a large-scale cone crusher. The large-scale jaw casting process design requires a larger amount of external chilled iron to strengthen the mold.
Although the cooling rate is cold iron, the temperature of the cold iron should be kept at 25～30℃, otherwise, the probability of cracks in the casting will increase.
When the high-manganese steel molten metal suddenly encounters cooling stimulation below 20℃ at high temperature, it solidifies rapidly, and instantaneous grain refinement will occur. During the refinement process, the gap between the grains and the grains will increase and the gap will increase. Cracks are generated after violent shrinkage when connected together. This kind of crack is very common in the V method. It is rare in the traditional casting. It is called hot crack in the V method. The temperature of the crack phase is generally between 1200 and 1300 ℃.
In the solid state (just solidified), cold cracking is generally about 6500 in the V method. The reason is that phosphorus exists in the form of phosphorus eutectic in steel. The binary phosphorus eutectic is (Fe•Mn)3P+Y, and the melting point is 1005. ℃, the ternary phosphorus eutectic is (Fe•Mn)3C+Y, and the melting point is 950℃. Due to the low melting point, it finally solidifies and exists between the dendrites.
In the case of thermal cracking, it is still liquid and has no strength, which promotes the initiation of interdendritic cracks. When P>0.06, phosphorus has serious thermal cracking on high manganese steel. When the phosphorus increases from 0.06 to 0.09, the thermal cracking is serious. At that time, it is not seen that once the casting temperature drops below 650℃, cracks will occur. The risk factor of hot cracking and cold cracking in V method casting is several times or even ten times that of traditional casting. Times.
All in all, V method casting not only has a certain residual temperature of the external cooling iron to prevent cracks, but also the control of the sulfur and phosphorus content. With the development of technology, the process space is also broad. The following is the description of smelting and heat treatment. The three processes are the first to be smelted. The common points are very similar and no more will be written.
High Manganese Steel Smelting Process Code
1. Scope of application
This process code is suitable for smelting and pouring high-manganese steel, ultra-high manganese steel hammers, lining plates and tooth plates and other high-impact wear-resistant castings in intermediate frequency furnaces (alkaline furnace linings).
(1) The operator must first wear labor protection products before entering the job; (2) Prepare all kinds of tools: sample cup, sampling tool, slag rake, ramming rod; (3) slagging agent, set Slag agents, modifiers, deoxidizers, and alloys with adjustment components must be baked and dried; (4) Check the state of the ladle and furnace body, and all should be in good condition; (5) The weighing device should meet the requirements for use; (6) Check the thermometer Work in good condition; (7) Check the intermediate frequency furnace equipment to keep it in normal operation.
3. Chemical composition:
4. Raw materials
(1) Waste castings of the same grade, pouring riser, and “cast residue” of the same grade shall be used for the reheating material and the “casting residue” of the same grade shall be stored strictly according to the grade, and no sticky sand is allowed. If there is sticky sand, the necessary technical treatment should be done After entering the furnace, the maximum lumpiness does not exceed 300mm.
(2) Scrap steel must be clean carbon steel. The chemical composition requirements are P<0.03%, S<0.03%, Si<0.4%, and the steel material should be kept dry. Before use, take representative steel scrap for laboratory test and re-examination, and calculate it as ingredients in accordance with.
(3) Ferroalloys Each batch of ferroalloys entering the factory must be accompanied by a certificate of conformity provided by the manufacturer, and re-inspected by the laboratory, and then put into storage after being qualified; each batch of ferroalloys must be stacked separately according to different grades and batches for clear identification.
(4) Slagging agent The slagging agent should be CaO and CaF2 particle size ≤ 20mm, and it must be kept dry; it must be sealed and stored in a thick plastic bag without damage; transfer to a special sealed container before use, and seal it immediately after use.
(5) Deoxidizer: ① Calcium silicon (Ca-Si) alloy, Ca content 28-31, Si content 60, particle size, block 3-8mm, powder 70 mesh -1mm; ②Pure aluminum, according to GB1196-83 national standard The middle level requires procurement, shape, aluminum wire or aluminum block.
(6) Modifier rare earth ferrosilicon alloy 1# (containing 20%-22% Re), purchased according to the FeSiRe21 standard in the GB4137-84 national standard, with a particle size of 0.5-5mm, and must be baked and dried before use.
(1) Regulations for high manganese steel and ultra high manganese steel ingredients: the added amount of reheating charge (this steel type) shall not exceed 30% of the total molten steel; the remaining new charge 70% shall not be used for serious sticky sand, oily pollution, and serious mud sand. Charge. It must be used after technical treatment if necessary.
(2) The operator must clearly grasp the chemical composition of the charge: recharge, scrap, various ferroalloys, etc., and must not use charge that has not been analyzed by laboratory analysis.
(3) It is necessary to master the burning loss and control recovery technology of each element in the smelting process.
(4) According to the above requirements, careful calculation of ingredients should be carried out, and the operational level of chemical composition control should be continuously improved in combination with the results of the pre-furnace test.
(1) The charging operation, whether cold or hot, should be gently put into the furnace to prevent impact damage to the furnace lining.
(2) Loading sequence: ① Add 2% (percentage of molten steel weight) to the bottom of the bottom slag, particle size ≤20mm, bottom slag CaO80%, CaF220%; ②Scrap-recycle material ferro-molybdenum-high carbon ferrochrome-high carbon Manganese steel (minus the balance required for pre-deoxidation), added with the furnace; ③Electrolytic manganese-added during alloying; ④ Ferro-titanium-added after deoxidation before tapping; ⑤ Modifier-added to the ladle after tapping .
(3) The principle of charging: first load small materials or materials with low melting point on the bottom slag to form a molten pool as soon as possible, and place large pieces of recycled materials in the middle or upper part of the crucible, which is conducive to smooth melting. The charging should be “down tight” “Upper pine”, size matching, compact, prevents bridging, and is easy to poke the material.
(1) About 60% of the power is supplied within 5 to 8 minutes after the start of power transmission. After the current shock becomes stable, the power is increased and quickly melted. During the melting process, in order to prevent the “bridging” of the steel material, the material should be continuously pierced, and the material should be added one after another until it is completely melted.
(2) The bottom slag is melted, and a proper slag layer is formed on the surface of the molten steel, which can reduce heat loss, protect the molten steel surface, reduce metal oxidation and gas absorption, and can also partially remove P and S; observe at any time, adjust the slag with CaF The consistency is normal.
(3) During the entire smelting process, the molten steel surface is not allowed to be exposed to the air. If it is exposed, some alkaline slag should be added to cover it in time.
(4) After the charge is cleared, at 1400°C, use a slag collector to remove the molten slag, regenerate alkaline slag, and increase the temperature.
(5) Pre-deoxidation is performed when the temperature of molten steel rises to 1540℃. For pre-deoxidation, add 0.2% of high-carbon ferromanganese, and then add 0.1% of Ca-Si block (percentage of molten steel weight). Increase carbon ferromanganese by 0.1%, then add ferrosilicon 0.05%-0.1%, and finally add Ca-Si block 0.1%); the order of addition: first Mn, then Si, then Ca-Si to reduce inclusions and purify molten steel.
(6) After pre-deoxidation, the molten steel sample can be taken and sent for chemical analysis. Before sampling, stir the molten steel and probe the furnace bottom to confirm that all kinds of furnace materials (especially high melting point iron alloys) are all cleared, and then sample and send for analysis (full analysis); Sampling part for chemical analysis: inserted into the molten steel at a depth of about 150mm (notes for chemical analysis sampling are the same below).
3) Refining-deoxidation, heating, alloying
(1) Heating and deoxidizing: During the heating process, the viscosity of the alkaline slag is continuously adjusted, the fluidity and melting point of the slag are observed, and a little Ca-Si powder is added to the surface of the slag (the amount of addition is 0.1%—0.2%), Or other Fe-Si powders and AL powders can diffuse and deoxidize molten steel (a small amount of CaF2 or scrap refractory bricks can also be used to adjust the viscosity of the slag).
(2) Alloying: After the temperature rises, 1540℃ can be alloyed and electrolytic manganese can be added. It should be added in stages and batches. The amount of each addition matches the melting speed. It should not be easy to “sink down” due to excessive addition. , Resulting in a deviation in composition control, the alloy added should not be in contact with the slag liquid phase, otherwise the recovery rate will be affected.
(3) When adding electrolytic manganese, only add 85%-90% of the rough calculation amount in advance, and then calculate after the first test result is obtained, and decide whether to add or how much to add.
(4) After the electrolytic manganese is added, stir the molten steel to make it uniform; take samples and make final adjustments based on the results. Electrolytic manganese must be added within 8 minutes before tapping. If the temperature of the molten steel has reached the tapping temperature, the power must be reduced. After the alloying elements are added, the temperature will be adjusted for tapping. Once the ferro-titanium is deoxidized, add it 2 minutes before tapping.
(5) When the temperature of the molten steel reaches about 1500℃, continue to perform diffusion deoxidation on the slag surface (and decide whether to change the slag according to the needs) to make the color of the slag lighter and white. After adjusting the alloy elements, the temperature is appropriate. Final deoxidation, insert AL0.10% (percentage of molten steel weight), continue to power on for 2 minutes to tap.
4) Steel tapping
(1) Requirements before tapping: qualified chemical composition, good deoxidation (see deoxidation for sampling), tapping temperature 1520-1550℃. Use a temperature measuring gun to measure the temperature. If it is not easy to measure the temperature in front of the furnace, you can use the method of counting seconds in front of the furnace.
(2) Clean the ladle in advance and bake it to red ≥700℃.
(3) Steel tapping after power failure.
(4) The steel slag is mixed, and the baked 0.2% Ca-Si and 0.3% rare earth ferrosilicon alloy are placed in the ladle (the added amount is the percentage of the weight of the molten steel); Ca-Si contains 28%～31% of Ca and 60% of Si , Particle size 3-8mm; rare earth ferrosilicon alloy No. 1, containing 20%-22% Re, particle size 0.5-5.0mm; all are modified by the impulse method.
(5) Molten steel is sedated for 4 to 8 minutes after deterioration (best control is 5 to 6 minutes).
(6) After sedation, take a sample of the finished product for chemical analysis. Check the deoxidation of molten steel again and observe whether the deoxidation is good. When the deoxidation is good, the round cup sample should shrink concave shape, otherwise, it needs to be inserted into AL (aluminum) deoxidation once, adding 0.15% to 0.25% (weight percentage of molten steel).
(7) Slag blocking pouring is performed on the molten steel surface during pouring. The smelting method of high manganese steel castings:
Use scrap steel, ferromanganese, and ferrochrome as raw materials, smelt with an intermediate frequency induction electric furnace, first add scrap steel, add ferrochrome after melting, add ferromanganese after all melting, and stop when the temperature rises to 1580～1600℃ Furnace, the out-of-furnace temperature is between 1520 and 1560°C, the aluminum is placed in the ladle to deoxidize by 0.05% to 0.10%, and then pouring is carried out, and the casting temperature is between 1410 and 1450°C.
(1) Master the sedation of molten steel for 4-7 minutes (preferably 5-6 minutes) and start pouring. If the sedation in the ladle exceeds 10 minutes, add aluminum 0.1% deoxidation.
(2) Pouring temperature: The pouring temperature of the big hammer head and the large liner (high manganese steel) is 1395-1415°C; the casting temperature of the castings produced by the lost foam is 1430-1450°C.
(3) Pouring speed: slow-fast-slow, the pouring speed is fast in the middle of filling, and it should be slower in the later period.
(4) Pay attention to the measures of internal slag blocking.
(5) Fill the riser and sand bleed air in time, and ignite.
6) Unpacking time of high manganese steel castings
After pouring high manganese steel castings, the box should be loosened at the right time to avoid hindering the shrinkage of the castings. High-manganese steel castings have poor thermal conductivity, large shrinkage, low as-cast strength, and large casting stress, so the sand out of the box takes longer than carbon steel. 4-12 hours for small and medium items, 14-30 hours for large items.
7) Cutting of pouring riser for high manganese steel castings and welding repair of castings
(1) Cutting: The pouring riser of high manganese steel castings is cut by a movable suspended grinder; many units are used to cutting with oxyacetylene, and it is best to cut quickly after heat treatment to reduce casting cracks and prevent carbides from re-precipitating; When cutting, the casting should be immersed in water to expose only the part that needs to be cut; if it cannot be immersed in water, the cut part should be cooled in time with flowing water. The surface to be cut should be polished 3 to 5 mm to remove the microcracks generated during cutting and the metal layer whose structure and performance deteriorated in the heat-affected zone.
(2) Welding repair: high-manganese steel can be repaired by welding. In order to make the metal at the part after repairing still get austenite in the structure, the electrode with high Mn/C ratio and containing molybdenum is used: AC and DC welding can be used. The coating contains high manganese and yellow smoke during welding. Pay attention to environmental protection. Before welding repairs, clean up the place to be welded and grind away all micro cracks; the diameter of the electrode should be small to keep the current as small as possible. Use short arc welding to ensure penetration and reduce the width and depth of the heat-affected zone.
(3) In DC welding, the welding rod should be connected to the positive electrode of the power source, and the welding piece must be effectively grounded. Reverse connection castings are severely heated and difficult to penetrate; for AC welding, the no-load voltage should not be less than 70 volts.
The high-manganese steel has been water toughened before welding repair, and cannot be preheated before welding repair; in order to reduce the heat-affected zone, intermittent segment welding repairs are used, each section is 50-60 mm long, and the heat of welding repairs is dispersed. Hammer the weld metal immediately for each welding section, so that the compressive stress generated by the hammering and the tensile stress generated by the cooling after welding cancel each other out.
Heat treatment of high manganese steel castings
(1) Basic requirements for heat treatment
A. Furnace temperature and heating rate:
Due to the poor thermal conductivity of high manganese steel, large thermal expansion coefficient, and large thickness of the hammer head, it is easy to crack due to high stress during heating. Therefore, the temperature of the furnace should be low, generally entering the furnace at 150°C. After the temperature is uniformed for one hour, the temperature is increased to 650-700°C for 1.5 hours at a rate of 80-100°C/1 hour, which can eliminate the casting stress, reduce the temperature difference between the inside and outside of the thick wall part, and make the metal enter the flexible state from the elastic state; It can be 120-150°C/h after it is in a new state, or it can be heated to a solid solution temperature of 1080°C with the furnace.
B. Solution temperature and holding time:
The temperature of solution treatment is determined based on the full decomposition of carbides, and the decomposition products-carbon and alloying elements are dissolved in austenite and diffused in the austenite to obtain the composition as much as possible Uniformly alloyed austenite. Generally, the carbide (Fe.Mn) 3C can be completely decomposed by heating to 1000°C. In order to accelerate the decomposition, dissolution and diffusion, and promote the homogenization of the composition, the solid solution temperature is set at 1050~1100°C. The austenite grains begin to grow when the temperature exceeds 1050°C, and the austenite grains become coarser when the temperature exceeds 1150°C, and the superheated structure appears. As ZGMnl3Cr2VTiRE contains a variety of alloying elements, the formed special carbides are not easy to decompose, dissolve and diffuse, and the solid solution temperature can be increased to 1080-1120°C. The holding time is determined according to the casting thickness, chemical composition, solid solution temperature and other factors. The empirical data is that the holding temperature is 1 hour per 25 mm thickness, and the holding time is extended by 1.5 hours when the alloy elements are added.
C. Water tough cooling:
The purpose of water cooling is to obtain supercooled austenite, that is, to keep the high-temperature austenite structure to room temperature. According to the phase transformation analysis, carbides will begin to precipitate from the austenite at 960°C. Therefore, the austenitic manganese steel liner must be filled with water above 960°C. The temperature will drop by 70-90°C in 30 seconds. The water entering the lining board should be controlled within 30 seconds to ensure that the water entering the lining board is above 960°C. The amount of water should be 10-12 times that of the liner. The water temperature should be below 30°C. After treatment, the water temperature should be less than 60°C to ensure the cooling rate of 30°C per second for the solid solution treatment of austenitic manganese steel.
(2) Specific heat treatment process
A. Heating rate: starting from 150°C at a rate of 80-100°C per hour, rising to 680°C for 1.5 hours, and then at a rate of 120-150°C per hour (or heating with the furnace) to 1080°C.
B. Quenching temperature: Keep the temperature at 1080°C for 3-4 hours, and calculate the heat preservation time for every 25 mm of the casting thickness for one hour. Properly extend the heat preservation time according to the furnace installation.
C. Quenching medium: It should be 20-30℃ flowing water. The casting should be filled with water within 35-40 seconds. The temperature of the casting must be greater than 960℃ when entering the water, otherwise carbides are easy to precipitate and crack the casting.
The temperature rise rate of 80-100℃ per hour means that the temperature is continuously increased and kept for one hour. The temperature control meter of the electric furnace will display the temperature for 15-20 minutes. The temperature must be kept for one hour and then the temperature will be increased; the inside and outside of the casting can reach this temperature; 680℃ keeps the temperature 1.5 hours to fully carry out the eutectoid transformation; the holding time of the quenching temperature at 1080℃ should be determined according to the thickness of the casting and the condition of the furnace, generally 3-4 hours. Baked.
High manganese steel castings are treated with solid solution water toughness, heated to 1050-1100℃, holding for sufficient time to dissolve the carbides in the as-cast structure, and obtain a single-phase austenite structure with uniform composition; then quickly quench into water Cooling to obtain a supercooled austenite solid solution structure. The solution-treated high-manganese steel castings are artificially aged at a certain temperature, so that carbides are dispersed and precipitated on the Austenite matrix to make their shape, size and distribution reasonable, without reducing toughness, but also improving wear resistance.
(3) Selection of heat treatment parameters for high manganese steel
A. Enter the furnace temperature
Due to the poor thermal conductivity of high manganese steel, the large thermal expansion coefficient, there are a large number of network carbides in the as-cast structure, which are easy to crack due to internal stress when heated. If the structure of the casting is complex, the wall thickness is very different, the casting has a large casting stress, and the thermal stress casting stress superimposes during heating, which is easy to cause cracks in the casting, so low temperature furnace is required. The temperature of the furnace should be less than 240℃ for complex parts, and less than 400℃ for simple parts. The heating rate of thick-walled parts and complex parts is less than 50°C/hour, and the general parts are 80-100°C/hour. In order to eliminate the casting stress, keep the temperature at 650-700℃ for 1-2 hours, so that the metal will change from the elastic state to the cozy state. Afterwards, the heating rate can be 100-120°C/hour, or even faster.
B. Quenching (solution treatment) temperature and holding time
In order to fully decompose carbides, the decomposition products, carbon and alloying elements are dissolved into austenite. At the same time, carbon and alloying elements diffuse in the austenite to obtain the alloy austenite as uniform as possible, and the general melting temperature is 1050-1100°C. The austenite grains begin to grow when the temperature exceeds 1050°C, and grow up significantly when the temperature reaches 1120°C, and the grains grow when the temperature is greater than 1150°C Coarse, overheated tissue. When elements such as Cr, Mo, V, Ti, etc. are added, their binding force with carbon is not easy to decompose, especially TiC is not easy to decompose when heated at 1100°C; Melting temperature.
Holding time: In order to fully dissolve the carbides, diffuse the alloying elements, and obtain a uniform austenite structure, the holding time should be determined according to the chemical composition of the solid solution temperature and the structural characteristics of the casting. Generally, the wall thickness is 25 mm for heat preservation for 1 hour. Adding MoV, Ti and other elements will extend the heat preservation time.
The purpose of cooling is to obtain supercooled austenite, that is, to keep the high-temperature austenite structure to room temperature. According to the phase transformation analysis, carbides will precipitate from the austenite at 960°C. To ensure that water enters before 960°C, open the furnace door to The water entering time should be less than 30 seconds, and the temperature will be reduced by 60-80% at this time. The heat treatment furnace is designed as a trolley turning type to ensure that the water enters in 30 seconds.
The water volume of the pool should be 8-10 times that of the casting. For example, 5t, the casting pool must have 40-50t of water, and ensure that all castings are immersed in the water, there must be gaps when the castings are installed in the furnace, and the gaps should be punched after the water enters so that each casting can be quenched; the pool must have circulating water, High-pressure water is sprayed from the Quartet, or the casting is swayed in the water with a hanging basket, so as to destroy the vapor film on the surface of the casting and quickly quench the casting. The water temperature should be less than 30°C, and the water temperature after the casting is less than 60°C, it becomes martensite after entering the water, and the volume expands during the phase transformation, which will cause the casting to crack under tensile stress at high temperature-oxidation.
C. Clean up
Before the high manganese steel casting enters the water, the sticky sand must be cleaned, and the flash burrs must be polished off. Because the sticky sand will reduce the water cooling rate by 30℃/sec, the quality of heat treatment will be reduced; the drape seam is thinner, and the decarburization will be serious after heat treatment. The surface carbon content of the decarburized layer of high manganese steel can be reduced to ω(C)0.1%~ 0.2%, the depth is several millimeters.
(4) New heat treatment process for high manganese steel
A, grain refinement heat treatment
Heat the as-cast high manganese steel to 550°C and keep it for 3 hours. About 40% of the austenite is transformed into pearlite. Then it is heated to 650°C for 1 to 2 hours, and then heated to 1050°C. But the temperature should not exceed 1050℃, otherwise the crystal grains will grow up easily. Because part of the austenite transforms into pearlite when heated to 550℃, pearlite transforms into austenite when it exceeds 650℃, which is austenite recrystallization, which forms several nuclei on an austenite grain boundary. After the crystal grains increase, the purpose of refinement is achieved.
B. Dispersion strengthening heat treatment
The carbide strengthening phase is dispersed and precipitated on the austenite matrix, which is dispersion strengthening. The structure of the high manganese steel after water toughening and solid solution treatment is a single austenite, and the austenite can be dispersed and precipitated by heating and aging. The initial precipitation temperature is 125°C, but the number of precipitation is very small. As the temperature increases and the time increases, the process of precipitation and precipitation of carbides in austenite accelerates, and the number of precipitation increases. When the temperature is further increased, carbides will accumulate and grow up. When the temperature exceeds 400°C, the carbides appear in a continuous network distribution of grain boundaries, and intragranular needle-like carbides appear, as if the as-cast structure has been restored. Therefore, the aging temperature is less than 350°C, the impact toughness remains unchanged, and the impact toughness drops sharply above 400°C.
C. Two temperature aging tests are carried
out on high manganese steel containing special carbide forming elements Mo, V, Ti. The aging temperature is 350 ℃ for 8 hours and still maintains high impact toughness. The aging time is extended to 12 hours and the impact toughness is slightly reduced. When the aging temperature is 450℃ for 4 hours, the impact toughness will be reduced. When the aging time exceeds 8 hours, the impact toughness will drop sharply. Therefore, the aging temperature for ordinary high manganese steel must be less than 400℃, and the time should not exceed 4 hours; for the formation of special carbides The aging temperature of high manganese steel with elements Mo, V and Ti must be less than 450°C, and the time should not exceed 4 hours, in order to achieve the purpose of dispersion strengthening, increasing yield strength, and improving wear resistance without reducing the impact toughness.
Heat treatment method:
①Air-cooled normalizing (or annealing) at a normal heating rate to 950℃, cooling to below 300℃, and then heating to 1050℃ for water toughening and solid solution treatment of high manganese steel, which can homogenize and refine austenite , Improve overall performance and wear resistance.
②As-cast water toughening treatment of high manganese steel. Since the 1980s, some small companies have used small and simple castings with 30-40 mm wall thickness 12-17 because they lack heat treatment equipment, or to save time and energy consumption. Minutes picking the sand into the water, 28-38 minutes for the wall thickness of 50-60 mm, you can get the basic austenite structure, but it cannot be guaranteed to enter the water before 960℃, so there will be compounds and other structures precipitated, which makes the impact toughness somewhat Reduced, the hardness is improved, and the wear resistance is improved for the working conditions with small impact load. But it is absolutely not feasible for large and complex parts, and it does not conform to the common sense of heat treatment.
Development trend of dynamic and static jaw plate production technology
(1) Casting process
The movable jaw plate and the static jaw plate adopt sand casting: sodium silicate quartz sand is hardened with carbon dioxide, resin sand, etc., advanced technology should be used on the tooling. The use of V method to produce movable and static jaws, with accurate dimensions, good surface quality, and high production efficiency, is one of the development directions for small and medium-sized movable and static jaws in a certain batch in the future; at the same time, the pouring riser should be designed reasonably , Add external cooling iron, add suspending agent, add internal cooling iron, add cemented carbide column, etc. to the mold to refine the grain and improve the wear resistance.
Actively promoting the numerical simulation technology of casting CAE and reasonably determining the casting process is one of the development directions of the casting process in the future.
(2) Smelting process
Strictly control the chemical composition, the manganese/carbon ratio should be greater than 10, the carbon content should be 0.95% to 1.2%, the silicon content should be less than 0.6%, and the phosphorus content should be less than 0.045%.
Using refining techniques, such as blowing argon at the bottom of the furnace, blowing argon inside the bag, adding composite modifiers, nano modifiers, etc., deoxidize and degas, and remove slag. Pure molten steel makes nitrogen, hydrogen, and oxygen metal non-metallic slag extremely low, which is the development direction of the smelting process. At the same time, the steel tapping temperature must be strictly controlled at 1480-1550°C and the pouring temperature of 1390-1430°C to ensure grain refinement. The grain size should be greater than 2 to improve the wear resistance of castings.
(3) Heat treatment process
It is necessary to strictly implement a reasonable heat treatment process, to have advanced and convenient heat treatment tooling equipment, and to install the furnace upright; due to the large thickness and complexity of the large movable jaw plate and static jaw plate castings, the material is made of alloy austenitic manganese steel with strengthening elements. Therefore, the heating rate of the heat treatment should be low. First, keep it at 150℃ for 1 hour, then increase it to 650℃ at 60-100℃/h, keep it for ～3 hours, then heat it up with the furnace to 1080-1100℃ and keep it for 4-5 hours ( The casting is insulated for 1 hour for every 25 mm of thickness, and an additional hour is added). The water temperature is controlled at 20-30℃, the movable jaw and the static jaw must be filled in within 45 seconds, and the water must be erected, and the water must be swayed up and down in the water to break the steam film and improve the cooling effect.
The heat treatment furnace adopts a long bell-jar type electric trolley furnace, which is sealed, environmentally friendly, has good energy-saving effects, less decarburization of castings, and convenient use. It is a direction for the development of heat treatment of movable and static jaw plates.
Eternal Bliss Alloy Casting & Forging Co, Ltd.
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