The hottest iron mold sand coated casting and its

2022-08-13
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Sand coated iron mold casting and its application

1 introduction

sand coated iron mold casting is a casting process in which a thin layer of molding sand is coated on the inner cavity of a metal mold (called an iron mold) to form a mold. Because the sand layer is relatively thin (4 ~ 8mm), it is economically reasonable to use expensive high-quality molding materials. The result is that the casting quality is greatly improved and the waste products are significantly reduced; Due to the good rigidity of sand coated iron mold, the dimensional accuracy and compactness of castings are significantly improved

Germany, the former Soviet Union and other countries began to apply sand coated iron mold casting to casting production around the 1960s, mainly for the production of ductile iron crankshafts, brake hubs, brake discs, cylinder liners, bomb shells, tank tracks, motor bases and other more than 30 kinds of castings. The applied research on sand coated casting of iron mold in China began in the early 1970s. From 1979, Zhejiang electromechanical design and Research Institute and Yongkang tractor factory cooperated to apply this process to the batch casting production of S195 crankshaft blank for the first time. At the same time, the performance evaluation of ductile iron crankshaft produced by this process was completed, and the fatigue strength (fatigue limit stress σ- 1), breaking strength (threshold value Δ The comparison of KTH and fracture toughness K1C) and service life (comparison of 10000h bench durability test) have been compared with sand casting crankshaft, which are better than sand casting. In the following 10 years, the process has been continuously improved in application. By the early 1990s, seven enterprises have applied the process, especially the iron mold sand coated casting process of single cylinder crankshaft and four cylinder crankshaft, which has achieved great success. The representative enterprises in this period are Yongkang tractor factory, Shangyu power machine factory, Wangdu crankshaft and connecting rod factory, Wanbei Crankshaft factory, Jinhua internal combustion engine parts factory, Changzhou Diesel Engine Factory, etc. In 1991, the State Development Planning Commission approved sand coated iron mold casting as a key new technology promotion project of the national "Eighth Five Year Plan", and took Zhejiang electromechanical design and Research Institute as the technical support unit of the project, which played a great role in promoting the development of sand coated iron mold casting technology in China. After undertaking the promotion project, our institute basically solved a series of problems of iron mold sand coated casting for mass production in the next 5 ~ 6 years

mainly:

① the sand covering molding machine is designed and finalized, which solves the problem of refitting and substituting the core shooting machine for a long time

② the standard sand coated iron mold casting production line has been finalized and standardized, so that the original relatively simple sand coated iron mold casting production line has been improved and applied in Shanghai ductile iron plant and other enterprises

③ the application of sand coated iron mold casting is extended to some castings with relatively difficult casting process, such as six cylinder crankshaft and three cylinder crankshaft

④ the plastic coated sand is introduced into the iron mold sand coated casting production, which greatly improves the molding quality of sand coated

⑤ the design of iron mold sand coated casting process is further standardized, and the design level is also greatly improved. The computer simulation software of iron mold sand coated casting process and the computer-aided design software of iron mold sand coated casting process are developed

at present, nearly 100 enterprises across the country have applied the iron mold sand coated casting process to produce more than 30 kinds of castings, such as nodular iron crankshaft, camshaft, balance shaft, pressure resistant valve body, cylinder liner, wear-resistant gear plate, etc., and it is estimated that the annual output of castings is 10 × About 104t. Typical enterprises include Shanghai Automobile Foundry ductile iron plant, Shenyang first crankshaft plant, Guangxi baikuang group, Yixing machinery plant, Shandong Jiuyang group, Zhejiang Shuguang crankshaft plant, Benxi Tianyuan crankshaft plant, Baoding film machinery plant, Shanxi Lucheng crankshaft plant, Hebei Xinji crankshaft plant, etc. However, these enterprises have different ways of introducing this process: some entrust our institute to design or construct, and some imitate and develop it by themselves. Therefore, their mastery of sand coated iron mold casting process is far from satisfactory. Taking the scrap rate of sand coated iron mold casting as an example, many enterprises that have a good grasp can stabilize at about 3%, and have achieved very good economic benefits. However, the scrap rate of iron mold sand coated casting in a few enterprises is as high as about 20%, which greatly offsets the economic benefits of this process. The reason is that these enterprises have not fully mastered the design and production essentials of the process, as well as neglect of production management

iron mold sand coating casting process design and actual production are mainly solved:

① iron mold wall thickness and sand coating thickness and their coordination to meet the different requirements of solidification and cooling for castings with different wall thickness and different materials

② convenient and economical sand coating molding method to meet the requirements of different castings for surface quality and dimensional accuracy

③ process parameters. Such as the determination of gating system, sand shooting system, exhaust system, etc

④ realization of mass production. For example, the design finalization of production line and sanding host and auxiliary machine

⑤ formulation of process procedures, such as pouring, cooling, unpacking and other procedures, as well as adjustment of casting composition

2 heat exchange characteristics of sand coated iron mold

after the liquid metal is poured into the sand coated iron mold, "casting sand coating iron mold" is an unstable heat exchange system. In order to simplify the problem, it is assumed that the casting is semi finite; It is assumed that the temperature field of each component in the system is distributed according to the straight-line law. Figure 1 shows a part of the system. Obviously, the same specific heat flow Q passes through each component in the system:

Figure 1 temperature distribution of casting sand coating iron mold

makes two heat transfer criteria representing the heat exchange intensity between casting and sand coating, iron mold and sand coating respectively. K1 is the ratio of the thermal resistance of the casting to the thermal resistance of the sand coating; K2 is the ratio of thermal resistance of iron mold to that of sand coating. Considering the combination of K1 and K2, with the change of the thickness of the sand coating, there are three different heat transfer situations between the "castings - sand coating - iron mold" that may actually occur:

① when k ≤ 1 and K2 ≤ 1, the sand coating is within the normal thickness, and the cooling rate of the casting increases with the decrease of the thickness of the sand coating

② when the thickness of the sand coating exceeds a certain thickness, the iron mold has no effect on the cooling of the casting, which is equivalent to ordinary sand mold casting or resin sand casting. Because the thermal conductivity of the sand layer is much smaller than that of the iron mold, the casting cools slowly

③ when k ≥ 1 and K2 ≥ 1, the thickness of sand coating is too thin, which is equivalent to metal mold casting

the above heat exchange characteristics have been confirmed by experiments when the crankshaft (CT Ц- 14) When the thickness of sand coating layer of iron mold sand coating casting gradually changes from 4 to 32mm, the amount of carburization in the crankshaft structure continues to decrease, and the amount of pearlite and ferrite continue to increase. When the thickness of the sand coating is less than 4mm, the cooling strength of the casting is similar to that of the metal mold (thick coating); When the sand layer is larger than 32mm, its cooling strength is equivalent to that of ordinary resin sand casting

when iron mold sand coated casting is used in the production of various castings, it is through experiments or empirical analogy to determine different sand coating thickness and iron mold thickness to control the solidification rate of castings. For example, in the sand coated casting process design of 490Q nodular iron crankshaft iron mold, the thickness of sand coating is 5 ~ 8mm, and the wall thickness of iron mold is 20 ~ 30mm, which produces high-quality as cast nodular iron without riser. The main reasons are:

① the sand coating effectively regulates the cooling rate of the casting, on the one hand, the casting is not prone to white mouth, on the other hand, the cooling rate is greater than that of sand casting. As shown in Figure 2, when molten iron is poured into the sand coated iron mold, the casting temperature drops to about 930 ℃ after 8 minutes, while it takes 24 minutes for the sand mold to fall to the same temperature, and the cooling rate is increased by about 3 times, which results in a significant improvement in the mechanical properties of the casting

② the iron mold is non yielding, but a very thin sand coating can appropriately reduce the shrinkage resistance of the mold; The rigidity of the iron mold effectively makes use of the graphitization expansion of ductile iron in the solidification process, and realizes riser free casting; Due to the thin sand layer, the mold cavity is not easy to deform, and the casting accuracy is greatly improved than that of sand mold

1-iron mold sanding 2-sand mold

Figure 2 cooling curve of nodular iron after pouring

3 cooling rate of iron mold sanding castings

the factors affecting the cooling rate of iron mold sanding castings include casting wall thickness, casting material, pouring temperature, thickness of sanding layer, material of sanding layer, iron mold thickness, iron mold material and mold temperature. Here, only the effects of casting wall thickness (BC), sand layer thickness (BM) and iron mold thickness (BI) are discussed

3.1 effect of BC, BM and Bi on the cooling of castings

Figure 3 shows the effect of different casting wall thickness (10mm, 20mm, 40mm and 80mm respectively), different sand coating thickness (4mm and 32mm respectively) and different iron mold wall thickness (32mm and 8mm respectively) on the cooling rate of iron mold sand coated castings under the following experimental conditions: chemical composition of castings 3.52% C, 2.46% Si, 0.80% Mn, 0.18% P, 0.031% s, The chemical composition of the sand layer is: quartz sand 90%, clay 8%, pulverized coal 2%, moisture 3%

Fig. 3 Effect of casting wall thickness, sand coating thickness and iron mold wall thickness on cooling rate

it can be seen from Fig. 3: ① casting wall thickness, sand coating thickness and iron mold wall thickness jointly affect the cooling rate of castings. Therefore, in actual production, the appropriate thickness of iron mold and sand coating should be selected according to different casting wall thickness to obtain the required cooling rate. ② Castings with different thickness can obtain the same cooling rate by selecting the appropriate thickness of sand coating and iron mold wall thickness. For example, in Figure 3, zone I represents the thickness of 10mm and 20mm, zone II represents 20mm and 40mm, and zone III represents the overlap between the cooling ranges of 40mm and 80mm castings. ③ Although BM and Bi can be changed to obtain the same cooling rate for castings with different thicknesses, not all castings with any thickness can obtain the same cooling rate. Under the experimental conditions, castings with a thickness of 10mm and a thickness of 40mm cannot obtain the same cooling rate (there is no overlap in the curve)

3.2 selection of sand cover thickness (BM) and iron mold wall thickness (BI)

BM and Bi are generally determined based on experience or experiments. Here is a chart method. Figure 4 is a graph used to determine the application scope of sand coated iron mold casting, which is applicable to the conditions that the thickness (BC) of the casting is from 10 to 80mm and the unpacking temperature is 600 ℃. The ordinate is the cooling time. The abscissa of the curve on the right side of the figure is marked with the thickness of the sand coating, which can be checked from the time it takes for the known casting to cool to 600 ℃ and the thickness of various castings. Moreover, if one of the required casting wall thickness (10, 20, 40, 80mm) is known, it is very convenient to determine the thickness of the sand coating and the thickness of the iron mold. Find the corresponding BC (such as bc=20mm) on the abscissa of the left half curve and draw a horizontal line. If these two lines intersect in the curve range with a section line, it indicates that this kind of casting is suitable for iron mold sand coating casting. Extend this horizontal line to the right, and it will extend into the area bc=20mm. In this area, lead a vertical line downward to obtain the required thickness of sand cover. However, this vertical line should be drawn to the right as far as possible, so as to obtain the minimum thickness of sand coating and iron mold. If the thickness of sand cover required to be determined is not within this range, it can be found from the adjacent curve range in a similar way

Figure 4 Relationship curve of iron mold thickness, sand coating thickness, casting wall thickness and casting cooling rate

if the wall thickness of the casting is uneven, first see whether the casting can be cast with iron mold sand coating, and then determine the sand coating according to each wall thickness

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