How to deal with cold insulation defects in thin-walled small parts of ductile iron?

The occurrence of cold shut and insufficient pouring defects in thin-walled small parts of ductile iron is indeed a common problem in production. Thin walled components dissipate heat quickly, and ductile iron itself has poorer fluidity than gray iron, making it easier to solidify before the mold cavity is filled with molten iron. Solving this problem requires system optimization from multiple aspects.

Core idea: Improve the fluidity of molten iron, accelerate the filling speed, delay the cooling of the mold cavity, and improve exhaust. The following are specific measures that can be taken:

1. Optimize the composition and treatment of molten iron: Increase carbon equivalent (CE): While ensuring the spheroidization grade and mechanical properties (especially elongation), appropriately increase the carbon equivalent (carbon+1/3 silicon). This is the most effective way to improve liquidity. Thin walled ductile iron parts allow for higher CE values (usually 4.3-4.7%), which can be attempted to approach the upper limit or slightly exceed (performance needs to be verified). Prioritize increasing carbon content, followed by considering silicon. Strictly control the sulfur content of the original molten iron: low sulfur is the foundation for good spheroidization. High sulfur will consume spheroidizing agents, produce more slag, and reduce fluidity. The target original molten iron S is less than 0.02%. Optimization of spheroidization incubation process: Adequate incubation: Using efficient inoculants (such as silicon barium strontium calcium), multiple pregnancies are carried out (in package incubation+flow incubation+in mold incubation). Breeding with flow is crucial for improving liquidity and preventing decline. Control the amount of spheroidizing agent added: Ensure good spheroidization (spheroidization level ≥ 3), excessive spheroidizing agent will increase slag and oxides. The residual Mg should be controlled at 0.03-0.05%, and the residual RE should not be too high. Raising the pouring temperature: This is a crucial measure for thin-walled components. Properly increasing the pouring temperature can significantly increase the fluidity of molten iron and prolong the filling time. The target temperature range usually needs to be ≥ 1400 ° C, and even 1420-1450 ° C can be attempted (specific needs to be determined based on the casting structure, weight, and pouring system design tests). But it is necessary to balance the risks of shrinkage, slag inclusion, and sand sticking caused by high temperatures. Ensure the purity of molten iron: Strengthen slag removal and blocking operations, keep the ladle nozzle clean, and if necessary, use a teapot ladle or add a filter screen (inside the sprue cup, at the bottom of the sprue or transverse sprue) to reduce the entry of slag and oxides into the mold cavity and hinder flow.

2. Optimize the design of the pouring system: This is the core link to solve the problem of insufficient cold isolation pouring. Open pouring system: adopting an open system with ∑ A straight>∑ A horizontal>∑ A inside, which is conducive to rapid filling. Increase the cross-sectional area of the sprue: For thin-walled parts, a larger total cross-sectional area of the sprue is required than conventional calculations to inject molten iron into the mold cavity at an extremely fast speed and fill it before solidification. It may be necessary to increase the number or width of sprues. Shorten the process and disperse the introduction: The sprues should be evenly distributed near the thin-walled parts of the casting as much as possible to shorten the distance of molten iron flow. Avoid long-distance flow of molten iron in the mold cavity. For complex thin-walled components, multiple sprues may be required. Reduce the flow rate of the sprue: Although rapid filling is required, excessive flow rate can cause spraying, curling, and the formation of secondary oxide slag, which can actually exacerbate cold insulation. By increasing the cross-sectional area of the sprue, the flow rate can be reduced while ensuring the flow rate. Increase the height of the sprue/use sprue cups: raise the metal indenter and increase the filling power. Consider a stepped pouring system: for thin-walled components with higher heights, use stepped runners to introduce molten iron layer by layer from the bottom, middle, or even top, shortening the flow distance of each layer of molten iron. Using a "wide, thin, and flat" sprue is beneficial for the iron to enter the mold cavity horizontally, steadily, and dispersedly, covering a larger area.

3. Strengthen exhaust: Fully set up exhaust holes/risers: at the highest point of the mold cavity, the last filling area of the molten iron (usually the part where cold separation is easy to occur), and deep in the core, set up a sufficient number and size of exhaust holes or overflow risers (which also serve as exhaust and slag collection). Ensure that the gas inside the mold cavity can be quickly expelled to avoid "gas blockage" hindering the filling of molten iron. Check the air permeability of the molding sand: Ensure that the molding sand (especially the surface sand) has sufficient air permeability. The moisture content of green sand should not be too high. Reasonably compact to avoid local tightness affecting exhaust.

4. Optimize pouring operation: Fast pouring: The pouring worker must concentrate their efforts to achieve high flow and fast pouring, complete the pouring in the shortest possible time, and ensure that the molten iron has sufficient heat and kinetic energy to fill the mold cavity. Long pouring time is one of the main reasons for cold insulation of thin-walled parts. Continuous pouring: The pouring process must be continuous and cannot be interrupted. Flow interruption can easily form a cold barrier at the point of interruption. Pouring timing: After the spheroidization incubation treatment is completed, it should be poured as soon as possible (usually within 8-10 minutes) before the incubation decay to ensure good incubation effect and fluidity.

5. Other considerations: Check the weight of the molten iron to ensure sufficient pouring weight, taking into account the requirements of the sprue system. Reduce the number of sand cores/optimize core exhaust: Complex sand cores can hinder flow and exhaust. Optimize the core design to ensure smooth exhaust (such as setting exhaust ducts, using exhaust ropes/wax wires, and using breathable core sand). Strength and compactness of molding sand: Ensure that the molding sand has sufficient strength to resist the erosion of molten iron and prevent sand from blocking the sprue or cavity. But the compactness should be uniform to avoid local hardness affecting shrinkage or breathability.