Editor's introduction: The structure and technical requirements of the column castings of large-scale vertical lathes are introduced, the original production process and the location and characteristics of cracks in the castings are elaborated, and the cracks existing in the columns of large-scale vertical lathes are analyzed. Measures have been taken to optimize the structure, shape, size and level of the core head of the end face, optimize the round part of the casting, adjust the chemical composition, ensure the uniform wall thickness of the upper surface of the casting, and adjust the unpacking and pit process. The production results showed that none of the 16 column castings of the same type and type verified by the production had any crack defects, and the crack defects were effectively solved.
As one of the important functional components of the machine tool load-bearing, the column is required to have no crack defects, but its structure and shape are complex, and the wall thickness difference is large, which brings greater difficulty to the production of column blank castings, at the same time, the large vertical lathes produced by the author company CH5125B series of column end face structure is more special, it is "M" shape, its corner is unique, the wall thickness is uneven, and the flange and the end, the sharp angle mutation and other places are easy to produce cracks in the casting process, and the crack tendency is much greater than that of other types of columns. Small cracks can be generated, and penetrating cracks at the end will be formed, which will lead to the scrapping of column castings. The causes and countermeasures of crack defects in large vertical lathes CH5125B series column blank castings are explored, so as to prevent and solve the occurrence of such defects and ensure the quality of column castings.
1. Introduction to CH5125B column castings
The CH5125B column produced by the author is the main load-bearing component of the large vertical lathe, the cross-sectional structure of the casting is shown in Figure 1, the length, width and height of the maximum size of the casting are 4.55 m, 2.96 m and 1.56 m respectively, the gross weight is 14.9 t, the average wall thickness is 25 mm, and the wall thickness at the guide rail is 100~130 mm. The material grade is HT250, technical requirements: type A graphite, tensile strength ≥ 250 MPa. As can be seen from Figure 1, its unique "M" shaped structure, with abrupt "sharp corners" and poor uniformity of casting wall thickness, makes the casting structure complex.
Figure 1 CH5125B cross-section of a column casting
2. CH5125B production of column castings
2.1 CH5125B column casting process
The casting process adopts furan resin sand, pit modeling, the casting guide rail is at the bottom of the sand mold, and the side bottom injection pouring method enters the molten iron, and the closed pouring system is adopted, the sprue ratio is ∑S straight: ∑ S horizontal: ∑ S = 1.22:1.15:1, the cross-sectional area of the straight sprue is 307 cm2, the cross-sectional area of the cross sprue is 288 cm2, and the cross-sectional area of the inner sprue is 250.8 cm2.
The smelting process uses 20 T medium frequency induction furnace to smelt molten iron, and the raw materials are scrap steel, Z18 pig iron, return material, 93% amorphous carburizer, 99% crystal carburizer, 90% silicon carbide, 75% ferrosilicon, and 65% ferromanganese. The charge ratio is melted by 40% scrap + 15% pig iron + 45% return material, and the specific chemical composition control range is shown in Table 1.
Table 1 Chemical composition of CH5125B columns (mass fraction, %)
The inoculant was a long-acting barium silica inoculant with a dosage of 0.3%~0.5%, a particle size of 5~15 mm, a pouring temperature of 1 310~1 350 °C, and a cooling of 180 h after pouring.
2.2 Problems with CH5125B columns
After the first round of casting trial production test, the two blank castings both had serious penetrating crack defects at a certain end, resulting in the scrapping of the castings, and cracks were produced at the windows of the left and right core heads, and they were not in a fixed position, and the cracks were random, resulting in large economic losses. The crack picture is shown in Figure 2.
Fig.2. Cracks penetrate through the flanges of CH5125B columns
2.3 Analysis of the causes of cracks in CH5125B columns
As can be seen from Figure 2, the cracks are cracked from the fillet of the core of the end near the middle wall to the upper parting surface. Observing the macroscopic performance of the crack of the CH5125B column, it was found that the shape was straight and the inner wall of the crack showed obvious torn metallic luster, and the existence of the square core hole in the upper half of the casting made the casting form a stress frame similar to the "day" shape, resulting in tensile stress around the end of the casting.
However, due to the fast cooling of the thin walls at both ends, the solidification can be completed quickly, the middle wall thickness is cooled slowly, and its solidification is still in the elastic temperature range, and the solidification speed of the thick-walled parts is slow compared to the thin-walled parts, and the tensile stress formed by the shrinkage resistance will crack the low-strength parts in the plastic state at the high stage, that is, the casting stress exceeds the mechanical strength limit of the alloy, which leads to the generation of cracks. The crack also extends from the weak point of the right and lower corners of the left core head or the left and lower corners of the right core head and the fillet to the direction of strong tensile stress. Combined with the cold cracks described in the literature: the typical characteristics of straight or regular curves in shape, passing through crystals, clean fractures with metallic color or light oxidation color, and the actual macroscopic manifestations of CH5125B column cracks, it is comprehensively judged that the cracks of CH5125B columns belong to typical cold cracks.
The main reason for the occurrence of cold cracks is related to the casting structure, fillet size, alloy composition, pouring system, unpacking time, etc., because the unique "M" shape structure of the CH5125B column is unreasonable and cannot be changed, therefore, it is necessary to start from the casting process and smelting process to solve the problem of cold cracks in castings.
3. CH5125B measures and effects to prevent cracks in columns
3.1 Preventive Measures
(1) Optimize the structure, shape, size and level of the end face core
Combined with the above analysis, the core size and fillet of the square hole were optimized for the CH5125B column in the casting structure, and the method of increasing the radius of the core fillet and reducing the size of the end face core was adopted, and the method of increasing the cross stiffener at the reduced core head was adopted to improve the crack resistance at this position. The reduction of the core head and the integration of the casting, as well as the addition of tension ribs, can balance the stress distribution in the slow solidification of the middle wall thickness in the later stage, so as to effectively reduce the probability of cracks at this position, and the improved core head is shown in Figure 3.
Fig.3 The structure and shape of the core
(2) Optimize the fillet of the casting
Optimize the fillet structure and shape at 1 places on both sides of the casting, the inner side of which is the fillet shape, as shown in the solid line of Fig. 4 (a), now the fillet shape is obliquely pulled into the dotted line as shown in Fig. 4 (a), and the cross-sectional shape of the optimized casting is shown in Fig. 4 (b), and the solidification time here is appropriately slowed down to balance the stress, so as to effectively reduce the probability of cracks at this position.
Fig.4. Schematic diagram of the modification of the inner fillet on the surface of the casting section
(3) Adjust the chemical composition
For gray cast iron, although Mn has a good effect of stabilizing pearlite, increasing hardness and improving strength, for crackable parts, high Mn is more likely to lead to carbide production and white mouth tendency. That is, when the Mn content is too high, carbides will be formed on the boundary between the matrix and the eutectic group, which reduces the tensile strength of the casting, and the reduction of tensile strength means the increase of cracking tendency, so it is necessary to reduce the Mn content to 0.9%, and at the same time appropriately increase the Sn alloy of 0.02%, and stabilize the pearlite content while reducing the Mn content, so as to ensure the tensile strength of the casting and reduce the probability of cracking in the casting.
(4) Ensure that the wall thickness of the upper surface of the casting is uniform
An important cause of cracks in column castings is the uneven wall thickness of adjacent positions. In order to avoid the above situation, in addition to requiring the operator to core in strict accordance with the process size, it is also necessary to ensure that the wall thickness of the column casting is relatively uniform, so as to reduce the stress distribution and reduce the probability of cracks in the casting.
(5) Adjust the unpacking and pit lifting process
The CH5125B column belongs to the thin-walled thick guide rail casting, the heat dissipation is fast in some positions, and the heat in some positions is large and the heat dissipation is slow, resulting in uneven temperature and large stress, so the insulation is unpacked according to 12 h/t, and it is placed for one day after unpacking, and the two ends of the cross sprue and the two ends that need to be lifted are dug up on the second day, and the two ends of the lifting are loosened after the temperature drops on the third day, and the pit operation is carried out again on the fourth day, with a total of 278 h of cooling, and the stress of the casting can be appropriately reduced by increasing the cooling time. This also reduces the probability of cracks in the casting.
3.2 Improvements
After taking the above measures, the 16 pieces of column castings of the same type and type produced in the follow-up did not crack the phenomenon, and the metallographic structure and mechanical properties of the castings met the technical requirements. The photo of the improved casting is shown in Figure 5.
Fig.5. Condition of the improved casting
4 Conclusion
After optimizing the structure, shape, size and level of the end face core, optimizing the part circle of the casting, adjusting the chemical composition, ensuring the uniform wall thickness on the upper surface of the casting, and adjusting the unpacking and pit lifting process, the column castings of the same type and the same type produced by the author company have no cracks, and the problem of crack defects has been effectively solved.
Author's Affiliation: Wuhan Heavy Machine Tool Group Co., Ltd. Wuhan Wuzhong Casting and Forging Co., Ltd
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