SAND CASTING EQUIPMENT.

The quality requirements of steel castings for railway vehicles are very strict and cannot have any defects. In addition to testing chemical composition, mechanical properties and metallographic structure, PT, MT and UT non-destructive testing are also required. The guide post is one of the cast steel parts of railway wheels. Different specifications, sizes and weights are slightly different. The mass is 14-18 kg. The grade is ZG230-450. The casting is prone to surface micro-crack defects at the transition between large and small outer diameters

1. Introduction to the original production situation
In the initial production, the modified furan resin (cast steel resin) sand process is used. The amount of resin added accounts for 1.1% of the amount of molding sand, and the amount of sulfonic acid curing agent added accounts for about 50% of the amount of resin. The measured 24h properties were tensile strength of 1.2 MPa, shear strength of 0.6 MPa, gas production of 12.5 mL/g, and ignition loss of 2.6%. The intermediate frequency electric furnace is used for smelting, the smelting temperature and release temperature are 1 660 ℃, and the molten steel pouring temperature is 1 580 ℃. Ferromanganese diffusion pre-deoxidation, and then ferrosilicon diffusion pre-deoxidation, aluminum block or aluminum wire bag is used for deoxidation and degassing. The amount of aluminum added is 0.12%. The heat treatment process is normalizing at 880 ℃ × 1 h + tempering at 600 ℃ × 2 h.
Other steel castings produced by this process have surface micro-cracks, which can meet the requirements of use after welding repair, but the defect rate of surface micro-crack defects of guide post castings is high, the welding repair workload is large, and the performance of the product is affected.

In order to determine whether cracks are caused by heat treatment, the flaw detection process was changed before heat treatment to prove that cracks were formed after casting and had nothing to do with heat treatment. The mechanical properties of the castings after heat treatment are all qualified. Then change the pouring temperature for the test, increase or decrease the pouring temperature by 15 ℃, the probability of micro-crack defects on the surface of the first box of each package and the castings poured at high temperature is slightly higher, but there is no significant change in the incidence of crack defects in the comprehensive statistics, and each package of steel It is impossible to control the pouring temperature of the first box and the last box, and the actual production cannot achieve exactly the same pouring temperature for each piece.

In order to solve the surface crack defect of the guide pillar casting, it was produced by the coated sand process. The high temperature resistant coated sand for cast steel was used, no coating was applied, and the core was poured. The other process parameters and chemical composition control did not change. As a result, the surface roughness of the castings The Ra value has been reduced, and the crack defect has decreased by 5%, but it is still high and cannot reach the ideal target, and needs to be improved.

2. Analysis of crack generation mechanism
2.1 Crack location and generation mechanism
The cracks are slightly grid-shaped, with oxidation color in the cracks. The cracks only appear on the surface of the casting, and there are no cracks inside. According to the crack location and crack shape, it is confirmed that the crack is a hot crack.

The hot crack occurs in the temperature range of the feeding boundary and the solidus boundary in the mushy zone. During the solidification of the steel casting, if there is still a liquid film between the dendrites, at this time, the casting solidifies and shrinks and is affected by the casting mold. hindered by a tensile direction stress; under the action of the tensile stress, the liquid film between the dendrites is elongated. If its tensile amount is greater than its bearing capacity, it will be pulled off at the liquid film to form cracks. The position where the guide post cracks is not only the position where the shrinkage of the casting is blocked, but also the position where the riser cannot be fed, so the crack occurs here.

2.2 Influence of residual aluminum content
Comparative analysis of the chemical composition spectral analysis results of the castings shows that the residual aluminum content at the crack is between 0.05% and 0.075%, indicating that the residual aluminum content is relatively high.
The optimal residual aluminum content of steel castings is in the range of 0.035% to 0.055%; the data points out that the optimal residual aluminum content of steel castings is about 0.021%. Too high residual aluminum content will easily cause crack defects in the castings; according to the introduction of the data, aluminum Aluminum nitride will precipitate on the primary austenite grain boundary to embrittle the grain boundary. During the primary crystallization of molten steel to form austenite, aluminum nitride will precipitate in the form of a coarse film at the grain boundaries between the austenite grains, resulting in grain boundary segregation defects and weakening the bonding strength of grain boundaries. As a result, the plasticity and toughness of the steel are greatly reduced, and the grain boundary of the steel shows a weak strength area during cooling and shrinkage, and the grain boundary is easy to become a crack source; under the action of shrinkage stress, the crack expands and extends, forming macro cracks, which will eventually reduce the cast steel. mechanical properties of the parts.

In the past, a single deoxidizer, aluminum, was used to smelt molten steel, and the deoxidation and degassing effect was poor. Compared with cast iron, the yield of steel castings is lower, and there are more return materials for pouring risers. The return materials are used repeatedly for a long time, and there is a problem of accumulation of aluminum, resulting in the excessive amount of residual aluminum in the castings, which is the main cause of micro-cracks on the surface of the castings. one of the reasons.

2.3 Influence of molding process
2.3.1 Influence of surface roughness of sand mold (core)
Comparing the resin sand molding and the coated sand core molding process, the probability of crack defects in the castings of the two processes is different.

The surface roughness of the resin sand mold (core) is large, and the surface roughness of the formed casting is also large. Ra≤100, the incidence of micro-crack defects on the surface of the casting is high; while the surface roughness of the coated sand mold (core) is low, and the surface of the formed casting is rough. The degree of hardness is also low, Ra≤50, and the incidence of micro-crack defects on the surface of the casting is low. This shows that the surface roughness of the sand mold (core) has a direct impact on the surface cracks of the casting.

The optimal residual aluminum content range of investment casting steel is 0.03% to 0.065%. The reason for the higher upper limit of residual aluminum content here is that the surface roughness of the investment mold is small, the surface of the casting is smooth, and cracks are not easy to occur, so the residual aluminum content The upper limit can be higher.

2.3.2 Influence of sulfur content in sand mold (core)
The resin sand mold (core) not only has high surface roughness, but also contains a certain amount of sulfur.
Although the modified resin for casting steel is used, compared with the coated sand, the resin sand curing agent contains sulfur. After pouring, under the action of high-temperature molten steel, the sulfur in the curing agent will decompose and penetrate into the surface layer of the casting; the surface layer of the casting The sulfur content is high, forming iron sulfide and manganese sulfide slag inclusions. Iron sulfide is concentrated at the grain boundary, which reduces the bonding strength at the grain boundary. When the casting cools and shrinks, under the action of shrinkage stress, microcracks occur at the grain boundary; during the cooling process of the casting, the microcracks continue to expand to form macroscopic cracks.

The damage of iron sulfide and manganese sulfide slag inclusions on the surface of castings is similar to that of aluminum nitride slag inclusions, especially iron sulfide inclusions are easy to cause hot cracking defects in steel. Sulfide is not only the source of cracks, but also the least labor-saving route for crack propagation. The more curing agent is added when mixing sand in low temperature weather, the more serious the phenomenon of sulphurization of the sand mold to the surface of the casting, the higher the sulfur content on the surface of the casting, and the easier it is to produce crack defects. The resin used in the coated sand does not contain sulfur, and there is no phenomenon of increasing sulfur on the surface of the casting, so there are fewer cracks on the surface of the casting when the coated sand is used to produce the guide post of the steel casting.

2.4 Influence of molten steel purity
According to the introduction, the purity of molten steel smelting has a direct impact on the surface cracks of castings;

The molten steel smelted in the intermediate frequency electric furnace is not refined by the refining furnace, and its purity is not as good as that of the molten steel refined by the refining furnace. The impurities in the molten steel, one is from the slag formed by the etching of the furnace lining and the lining, and the other is the impurities in the charge such as the metallurgical reaction products rusted in the molten steel, and the impurities generated by the oxidation of the molten steel.

Comprehensive analysis shows that the surface crack of the guide column is located at the junction of the two walls of the large cylindrical surface and the small cylindrical surface, which is the shrinkage stress concentration area.

3. Productivity test to eliminate cracks
3.1 Reduce the amount of residual aluminum in castings
In order to reduce the accumulation of aluminum, the amount of returned charge should be reduced. Use aluminum + silicon calcium alloy + rare earth silicon iron alloy composite deoxidizer to deoxidize. Adding rare earth alloys to the deperoxidized molten steel can not only refine the grains, improve the tensile strength and yield strength of the steel castings, but also reduce the tendency of hot cracking during the solidification process of the castings. In normal smelting of molten steel, ferromanganese and ferrosilicon are used for pre-deoxidation before being released from the furnace, and then composite deoxidizer is added. The amount of aluminum added is controlled within 0.1% to reduce the amount of residual aluminum in castings. % range.

3.2 Improve the purity of molten steel
The returned material is shot blasted to remove the sticky sand on the surface; during the molten steel smelting process, the liquid surface is tightly covered to prevent oxidation. Fluorite is added to make slag and slag is repeatedly removed before the furnace is released. When the molten steel is released, the molten steel purifying agent is added with the flow. After the furnace is released, the slag is fully sloughed again, covered tightly, and argon gas is blown at the bottom of the package.

3.3 Use cast steel coated sand mold (core) to form the core
Since the resin used in the coated sand for cast steel is alkaline, there is no problem of sulphurizing the sand mold (core) to the surface of the casting, which avoids the surface crack defect of the casting caused by high sulfur.

3.4 Reduce the surface roughness of the core and casting
After making the mold (core), apply a layer of water-based cast steel coating to the surface of the coated sand mold (core) as soon as possible while it is hot, and use the mold (core) to dry the paint by self-heating to reduce the surface roughness value of the sand mold (core). If the paint cannot be dried sufficiently, it can be dried manually. Compared with alcohol-based coatings, water-based coatings and coated sand have good wettability, which can reduce the surface roughness of sand molds (cores), thereby reducing the surface roughness of castings and helping to avoid crack defects on the surface of castings.

3.5 Reduce casting shrinkage stress as much as possible
In order to reduce the shrinkage stress, the pouring temperature is strictly controlled not to be too high, and the sand mold is opened within 10-15 minutes after pouring, and the hindering effect of the sand mold on the shrinkage of the casting is relieved before the plasticity of the casting is transformed.
After taking the above measures, the surface crack defects of the guide post steel castings were reduced to less than 3%, the surface roughness of the castings reached Ra≤50, and the quality was significantly improved.

4 Conclusion
4.1 The main reasons for cracks on the surface of guide posts of railway steel castings are high residual aluminum content in castings, poor surface roughness of castings, impure molten steel, and resin sand curing agent sulphurizing the surface of castings.
4.2 The reasonable range of residual aluminum in the guide post of railway steel castings is related to the type of sand mold, the surface roughness of the casting, and the purity of the molten steel; in the basic sand mold (core) and the casting, the surface roughness is good, the purity of the molten steel is high, and the casting is not In the case of cracks, a higher amount of residual aluminum is allowed.
4.3 The use of composite deoxidizer can reduce the residual aluminum content of castings; the intermediate frequency electric furnace melting molten steel can be treated by adding fluorite slag in the furnace, adding purifying agent during tapping, and blowing argon in the molten steel ladle after tapping to improve the purity of molten steel The coating sand mold (core) can effectively reduce the surface roughness value of the casting by using the water-based refractory coating on the surface; after pouring, unpack the sand in time to reduce the shrinkage stress of the casting. These measures can effectively eliminate the micro-cracks on the surface of the guide column of the railway steel casting .

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