Shell casting is the use of coated sand as raw material, the mold is heated to a certain temperature, through sand shooting, insulation to make coated sand solidification, molding, forming a certain thickness of the shell, the upper and lower shell bonded together with binder, forming a complete cavity for casting molding castings. Shell casting has the characteristics of less investment in equipment, high production efficiency, short cycle, low manufacturing cost, less dust in the production site, low noise, low pollution to the environment, high surface finish of castings, stable size and process performance, and has been widely used in automobile, motorcycle, construction machinery and other industries.
Since the introduction of shell casting process, the stable and efficient production of shell cast iron has been achieved. However, orange peel and sticky sand on the surface of castings are found to be particularly serious in the production of steel castings, and the surface quality is poor. The proportion of orange peel and sticky sand in the defective products is as high as 50%, which seriously reduces the cleaning efficiency and product quality of castings.
1.1 Brief introduction of original production process
Production of part of the cast steel products, using coated sand shell casting process, one type of two pieces, two layers of stacked boxes, using double station reverse sand shooting mechanism shell.
1.2 Proportion and location of defects
The location and number of defects were analyzed, and the orange peel and sand sticking defects were especially obvious in the inner gate and upper surface of the casting.
2 defect and cause analysis
2.1 Defect formation mechanism
Orange peel refers to the flake or tumor formed on the casting surface when metal and molding sand are mixed on the casting surface. In casting, shell surface due to continuous scouring of high temperature metal liquid, resulting in shell surface local collapse, collapse sand and molten steel together into the cavity in the casting surface formed protruding scar, namely the formation of orange peel, scar and other defects, cast iron products are less common in cast steel products. Sand sticking is a defect on the surface of the casting. It is difficult to remove the gritty burr or compound formed by molding sand and metal oxide adhered to the surface of the casting, resulting in rough casting surface, which usually increases the workload of casting cleaning, reduces the finishing efficiency and affects the appearance of the product.
2.2 Cause Analysis
Combined with the formation mechanism of sticky sand and orange peel, it can be judged that the formation reasons of sticky sand and orange peel on the surface of shell cast steel are as follows:
(1) During the pouring process, the temperature of molten steel is high, and the casting shell near the gate is heated for a long time. Because the coated sand shell is easy to collapse and heated for a long time, the sand shell at this part is overheated, and the collapse of the sand shell on the surface of the cavity causes the phenomenon of sticking sand and orange peel on the surface of the casting;
(2) The curing layer of the sand shell is thin and the strength of the sand shell is low. When the pouring temperature is high or the flushing time of molten steel is long and the flushing strength is large, the surface of the sand shell is easy to break and break, leading to the “infiltration” of molten iron into the inside of the sand shell, or the broken sand particles and molten steel solidify together to form the defect of sand sticking;
(3) The refractoriness of the coated sand is low. When the molten steel enters the cavity, the surface of the cavity of the sand shell has begun to collapse before the solidification of the molten steel, leading to the “infiltration” of molten iron into the interior of the sand shell, or the broken sand particles solidify with the molten steel to form sticky sand;
(4) The impact force of the sprue is large, and the sprue part of the scouring time is the longest, the sprue is directly connected with the inner gate, when the high temperature molten steel rushed into the sprue directly into the cavity, due to the turbulent flow of molten steel, lead to the root of the gate sand shell surface collapse, floating sand with liquid iron into the cavity.
3. Process optimization test and analysis
3.1 Lower the pouring temperature
The coated sand used for casting steel is quartz refractory material. The casting temperature is too high or local overheating, which is easy to collapse, crack, sand flushing and other phenomena, resulting in sand sticking, orange peel and other casting defects. In the process of shell mold production, in order to reduce the manufacturing cost, shell mold casting process generally does not use refractory coating, directly after casting. The area near the inner gate of the casting is used as the water inlet. The molten steel temperature is high, and the part of the sand shell overheats for a long time. The surface of the sand shell breaks up, and the high temperature molten steel continues to scour, resulting in sticky sand and orange peel. On the premise of not affecting the product quality, the pouring temperature should be appropriately reduced, and the shell type casting is cold shell casting. The casting temperature should not be too low to prevent cold isolation. Therefore, reducing the casting temperature can improve the surface quality to a certain extent, but can not completely solve the problems of orange peel and sticky sand on the surface.
3.2 Improve the solidified layer thickness of sand shell
The curing layer of sand shell is thin and the strength of sand shell is low. When the pouring temperature is high or the flushing time of molten steel is long and the flushing strength is large, the surface of sand shell is easy to break and collapse, leading to the “infiltration” of molten iron into the interior of sand shell, or the broken sand particles solidify with molten steel to form sticky sand and orange peel. The shell layer is too thin, the strength of the sand shell is reduced, and there is a risk of overheat breaking and sand washing in the pouring process. Because this part is directly impacted by molten steel, the strength of the sand shell here is directly related to the surface quality of the casting. In the continuous production process, the mold is cooled quickly, resulting in the phenomenon of sand generation and immature sand shell. If the bottom of the sprue is too thick, the crust time will lead to the overburning of other parts of the sand shell, and the strength of the sand shell will be reduced. After optimization, the sand shell will be completely solidified in the continuous production without sand generation and skin and bone.
3.3 Improve the refractoriness of coated sand
The coated sand has low refractoriness. When molten steel enters the cavity, the surface of the cavity of the sand shell has begun to collapse before the solidification of molten steel, leading to the “infiltration” of molten iron into the interior of the sand shell, or the broken sand particles solidify with molten steel to form sticky sand. After adjusting the composition of coated sand, the small batch verification showed that the orange peel phenomenon on the casting surface was basically eliminated, but the sticky sand phenomenon still existed, and the defect of sticky sand on the product surface could not be solved completely.
3.4 Optimize the gating system design
Pouring system has a great influence on obtaining high-quality castings. In the process of mold filling, the sand shell near the gate breaks in advance, resulting in molten iron “infiltrating” into the inside of the sand shell or broken sand particles solidifying with molten steel, thus forming defects such as sticky sand and orange peel near the gate and in large plane. Reducing the impact force of molten steel on the surface of sand shell and increasing the buffering capacity of pouring system can also improve the phenomenon of sticky sand and orange peel on the surface of products. The steady flow casting system is considered to replace the original casting system, which makes the molten steel entering the cavity stable and reduces the scouring strength of the mold shell. The shape of the spate adopts a flat trapezoid, which can reduce the turbulent flow phenomenon of liquid iron scour the sand shell. The length of the sprue should be as short as possible to reduce the defects such as cold isolation and flow lines due to the cooling of molten steel.
Post time: Oct-14-2021