1.Introductionof Machine High Mn Steel Castings
High Mn steel is a kind of high anti-impact wear materials,which is suitable for making heavy impact working condition,such as Excavator shovels and teeth,big crusher parts.
In 1882, the first high manganese steel with austenite structure was obtained. In 1883, Mr.R.A. Hadfield, an Englishman, achieved a patent for high manganese steel.
2.ClassificationOf Mn Steel
High manganese steel can be divided into two categories according to different purposes:
2.1Abrasive High Mn Steel
This kind of Mn steel contains 10% ~ 15% manganese, and its carbon content is high, generally 0.90% ~ 1.50%, most of which is above 1.0%. The chemical composition is (%): Si:0.30 ～ 1.0 S ≤ 0.05 P ≤ 0.10.
Because of high abrasion,it is commonly used to make shovel teeth of excavators, concaves and mantles of cone crushers, Jaw plates of jaw crushers, lining plates of SAG & AG mills, railway fork points, blow bars, hammers, etc.
The micro structure of the high manganese steel with the above composition is usually composed of Austenite, carbide and Pearlite, and sometimes contains a small amount of phosphorus eutectic. When the amount of carbides is large, they often appear as network on the grain boundary. Therefore, as cast high manganese steel is brittle and can not be used, so it needs water quenching treatment at the range between 1050 and1100 ℃.As the as cast structure is eliminated by heat preservation, and the single-phase Austenite structure is obtained, and then water quenching is carried out to keep the structure at normal temperature. After heat treatment, the strength, plasticity and toughness of steel are greatly improved, so this heat treatment method is often called water toughening. The mechanical properties after heat treatment are: σ b615-1275mpa σ 0.2340-470mpa ζ 15% – 85% ψ 15% – 45% Impact Value: ak 196-294j / cm2 Hardness: HB 180-300.
After solution treatment, there will be a small amount of undissolved carbides in the high manganese steel, which can still be used when the quantity is less and meets the inspection standard.
When the high manganese steel with austenite structure is subjected to impact load, plastic deformation occurs on the metal surface. As a result of deformation strengthening, there is obvious work hardening phenomenon in the deformation layer, and the surface hardness is greatly improved. It can reach HB300-400 at low impact load and HB500-800 at high impact load. With the different impact load, the depth of surface hardening layer can reach 10-20 mm. The hardened layer with high hardness can resist the impact abrasive wear. High manganese steel has excellent anti-wear performance under the condition of strong impact abrasive wear, so it is often used in mining, building materials, thermal power and other mechanical equipment to make wear-resistant parts. Under the condition of low impact, because the work hardening effect is not obvious, high manganese steel can not play its work hardening characteristics of materials.
ZGMn13-1 (C:1.10%～1.50%) is used for low impact parts;
ZGMn13-2 (C:1.00%～1.40%) is used for common parts,
ZGMn13-3 (C:0.90%～1.30%) is used for complex parts,
ZGMn13-4 (C:0.90% ～1.20%) is used for high impact parts. The manganese content of the above four grades of steel is 11.0% – 14.0%.
|Standard Specification for Austenitic Manganese Steel|
|Reference Standard ASTM A128||Nominal Chemical Composition (W%)||Eqv. Standard GB5680|
|C||Mn||Cr||Mo||Ni||Si (Max.)||S (Max.)||P(Max.)|
|A||1.05-1.35||≥11.0||–||–||–||1.00||0.06||0.05||GB 5680 ZGMn13-1 AS2074/H1A|
|B-1||0.90-1.05||11.5-14.0||–||–||–||1.00||0.06||0.05||GB 5680 ZGMn13-2|
|B-2||1.05-1.20||11.5-14.0||–||–||–||1.00||0.06||0.05||GB 5680 ZGMn13-2,3 AS2074/H1A|
|B-3||1.12-1.28||11.5-14.0||–||–||–||1.00||0.06||0.05||GB 5680 ZGMn13-2,3 AS2074/H1A|
|B-4||1.20-1.35||11.5-14.0||–||–||–||1.00||0.06||0.05||GB 5680 ZGMn13-2,3 AS2074/H1A|
|C||1.05-1.35||11.5-14.0||1.50-2.50||–||–||1.00||0.06||0.05||GB 5680 ZGMn13-4 AS2074/H1B|
|E-1||0.70-1.30||11.5-14.0||–||0.90-1.20||–||1.00||0.06||0.05||GB 5680 ZGMn13-5 AS2074/H1C|
|E-2||1.05-1.45||11.5-14.0||–||1.80-2.10||–||1.00||0.06||0.05||GB 5680 ZGMn13-5|
In the cold deformation process under impact load, due to the increase of dislocation density, dislocation delivery, dislocation accumulation and interaction between dislocation and solute atoms, the steel is strengthened. This is an important reason for work hardening.Another important reason is the low stacking fault energy of high manganese Austenite.When deformation occurs,stacking fault appears, which creates conditions for the formation of ε Martensite and deformation twin. High density
dislocations, dislocation packing and entanglement can be seen in the deformation hardening layer of conventional high manganese steel. The appearance of ε Martensite and deformation twin makes it difficult for steel to deform, especially the latter. All of the above factors make the hardened layer of high manganese steel be strengthened to a high degree, and the hardness is greatly improved. High manganese steel is castable. The melting point of steel is low (about 1400 ℃), the temperature interval between liquid and solid phase is small (about 50 ℃) and the thermal conductivity of steel is low, so the liquid steel has good fluidity and is easy to be cast. The linear expansion coefficient of high manganese steel is 1.5 times of that of pure iron and 2 times of that of carbon steel. Therefore, the volume shrinkage and linear shrinkage of high manganese steel are large during casting, and stress and crack are easy to appear.
In order to improve the properties of high manganese steel, a lot of researches have been carried out, such as alloying, microalloying, carbon manganese content adjustment and precipitation strengthening treatment, and they have been applied in production practice. The appearance of metastable Austenitic Manganese steel can reduce the content of carbon and manganese in the steel and increase the speed of deformation strengthening. It can be applied to the working conditions of high and medium low impact load, which is a new development of high Manganese steel.
2.2 Non Magnetic Steel High Mn Steel Sand Castings
This kind of steel contains more than 17% manganese and less than 1.0% carbon. It is often used to make retaining rings in the motor industry. The density of this kind of steel is 7.87-7.98g/cm3. Due to the high content of carbon and manganese, the thermal conductivity of steel is poor. The thermal conductivity is 12.979w / (m ·℃), about 1 / 3 of carbon steel. The permeability μ of steel is 1.003-1.03 (H / M) because it is austenitic and non-magnetic.
3.Cast Process of High Mn Steel Sand Castings
3.1 Chemical Composition High Mn Steel Sand Castings
Under the working condition of high energy impact, the application range of high manganese steel and ultra-high manganese steel castings is wide.
According to the national standard, high manganese steel is divided into five grades, the main difference is the carbon content, the range is 0.75% – 1.45%. High impact and low carbon content. The manganese content is between 11.0% and 14.0%, generally not less than 13%. There is no national standard for ultrahigh manganese steel, but the manganese content should be more than 18%. The silicon content has a great influence on the impact toughness, so the lower limit should be taken to be no more than 0.5%. Low phosphorus and low sulfur are the most basic requirements. Since high manganese content naturally plays a role in desulfurization, phosphorus reduction is the most important thing. Try to make phosphorus lower than 0.07%. Chromium improves wear resistance, generally around 2.0%.
3.2 Charging Materials for Melting Furnaces High Mn Steel Sand Castings
The material in the furnace is determined by the chemical composition. The main charge is high quality carbon steel (or ingot), high carbon ferromanganese, medium carbon ferromanganese, high carbon ferrochrome and high manganese steel scrap materials.The recycle material shall not exceed 25%.
3.3 Smelting High Mn Steel Sand Castings
The size of the block should be as small as possible, and 50-80mm is suitable. After melting, when the furnace temperature reaches 1580-1600 ℃, deoxidization, dehydrogenation and denitrification are required. Aluminum wire, Si Ca alloy or SiC can also be used. The deoxidizer must be pressed to the depth of the furnace. At this time, the metal level shall be tightly covered with covering agent to cut off the external air. It also needs to be sedated for a period of time, so that oxides and inclusions can float up in sufficient time. However, many foundries only put aluminum wire or even aluminum chips, sprinkle on the metal surface, and do not cover, it is a waste! During this period, the content of manganese and carbon should be adjusted with medium carbon ferromanganese in time. Chinese style
It is necessary to bake the ladle above 400℃ before the molten steel is discharged from the furnace. It is necessary to use V-Fe, Ti Fe, rare earth and other microelements to modify during the process of discharging, which is a necessary means to refine the primary crystallization, and it is of great importance to the product performance.
3.3 Charge and Molding Material High Mn Steel Sand Castings
Manganese steel is alkaline, and the lining is of course made of magnesium. The ramming of furnace lining shall be carried out repeatedly. The lining material should not be too thick. It is better to add 80 cm each time. After ramming, it should be baked at low temperature for a long time. For example, to improve the production efficiency, the author suggests to use the forming crucible (both Shenyang Lide factory and Hengfeng factory sell the finished products), which can be put into production in less than 1 hour from the furnace dismantling and ramming, and at the same time, the forming crucible is of great benefit to the furnace penetration prevention. Of course, the length of the furnace order is closely related to the operator. Many operators, like shot putters, put the charge into the furnace from a distance of three or four meters, which is not safe and damages the furnace order. They should preheat the charge by the furnace mouth, then slowly preheat the charge along the furnace mouth with clamps, and then slowly put the charge along the furnace wall with clamps.
Molding materials and coatings shall also be consistent with the properties of liquid metal, or ZTE materials (such as chromite sand, brown corundum, etc.) shall be used. If we want to obtain a crystal refining group, it is correct to use chromite ore with large heat storage capacity, especially in the EPC plant, which will overcome the disadvantage of slow heat dissipation.
3.4 Foundry Process Design High Mn Steel Sand Castings
The characteristics of manganese steel are large solidification shrinkage and poor heat dissipation. Therefore, the casting shrinkage is 2.5% – 2.7% in the process design, and the larger the casting, the higher the upper limit. The yield of molding sand and sand core must be better. The gating system is open type. A plurality of scattered internal sprues are introduced from the thin wall of the casting, which are in the shape of flat and wide trumpet. The cross-sectional area near the casting is larger than that associated with the cross sprue, so that the molten metal can be injected into the mold quickly and stably to prevent the excessive temperature difference in the whole mold. The riser diameter should be larger than the hot joint diameter, close to the hot joint, and the height is 2.5-3.0 times of the diameter. It is necessary to adopt the combination of the hot riser and even the pouring riser to make enough high temperature liquid metal to be insufficient for the vacancy of the casting during solidification shrinkage.
Pouring shall be carried out as soon as possible at low temperature. Once solidified, loosen the sand box in time. Smart designers are always good at using cold iron, including internal cold iron and external cold iron. It not only refines the primary crystallization, eliminates shrinkage cavity and porosity, but also improves the process yield. Of course, appropriate dosage and specification should be considered. The internal cooling iron should be clean and fusible, and the amount should be less. The three-dimensional dimension of external cooling iron is 0.6-0.7 times of that of cooling substance. Too small does not work and too large causes the casting to crack. The casting shall be kept warm for a long time in the mold until it is lower than 200 ℃ before unpacking.
3.5 Heat Treatment High Mn Steel Sand Castings
Therefore, the correct operation is below 350 ℃, the heating rate is less than 80 ℃ / h, below 750 ℃, and less than 100 ℃ / h, and there are different periods of heat preservation. When the temperature is more than 750 ℃, the casting is in a plastic state and can be heated rapidly. When the temperature reaches 1050 ℃, the holding time is determined according to the thickness of the casting, and then it rises to more than 1100 ℃. Leave room for cooling out of the furnace and put water in as soon as possible. When the temperature is high, the temperature rise is too slow, the holding time is too short, and the time interval from the furnace outlet to the water inlet is too long (should not be more than 0.5min), which all affect the casting quality. The water inlet temperature shall be less than 30 ℃, after quenching, the water temperature shall be less than 50 ℃, and the water volume shall not be less than 8 times of the weight of the casting. Cold water enters from the bottom of the pool and warm water flows out from the top of the pool. The casting should move continuously in three directions in the pool.
3.6 Cutting and Welding High Mn Steel Sand Castings
It is better to put the casting in water and expose the cut part outside the water. leave a certain amount of stubble during cutting, and grind it off after heat treatment. In many factories, welding and repair are inevitable. The austenitic manganese nickel electrode (D256 or d266 type) is selected. The specification is long and thin, φ 3.2mm × 350mm, and the outer coating is alkaline. The operation method of small current, weak current arc, small weld bead with multiple welding layers and low temperature and less heat is adopted. Welding and striking at the same time to eliminate stress. Important castings must be inspected.
3.7 Delivery of High Mn Steel Sand Castings to Australia
On Nov.3,we finished all the inspection of Mn13Cr2 Bar Castings.
We machined all the surfaces of the Mn Steel parts to Ra 6.3.Due to machining allowance is left too big for sand castings and high work hardening feature, the Mn steel castings are very difficult to be machined.We tried many different milling cutters and found a better solution to machine hundreds of castings in a high efficient and economic way to complete the job.
Eternal Bliss Alloy Casting & Forging Co, Ltd.
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