Treatment for severe porosity in cylindrical tubular sand cores during the production of ductile iron parts using green sand molding technology

Today we will analyze a case of porosity defects occurring at the coated sand core of a ductile iron component. The material is GGG40, produced using a vertical production line.

Due to the solid area in the lower part of the sand core of the casting, it is difficult to exhaust the gas inside the round tube of the casting. Therefore, the gas generated by the sand core is "trapped" in the final solidification area (hot spot) inside the casting during the solidification process of the molten iron, and cannot be discharged smoothly. The following are detailed reasons for the formation and systematic solutions:

Core cause analysis: The peak gas emission of the sand core does not match the timing of the solidification of the molten iron. When the coated sand core encounters high-temperature molten iron, the resin binder will quickly burn and decompose, producing a large amount of gas. If these gases cannot be discharged smoothly, they will invade the molten iron and form pores on the final solidified surface.

Solution:

1. Optimize the sand core itself (most importantly!): reduce the gas generation of the sand core: check the brand and model of the coated sand you are using. It is recommended to switch to low emission gas coated sand, which usually uses low emission gas resin and curing agent. Improve the breathability of the sand core: Communicate with the coated sand supplier to appropriately reduce the strength requirements of the sand core. Excessive strength means that there is a large amount of resin added and a high gas generation. The lower the strength, the better, while meeting the requirements for styling and casting. Check if the compactness of the sand core is too high. When making the core, the sand injection pressure should not be too high to prevent the sand core from becoming overly dense. Ensure smooth exhaust of sand cores: When making sand cores, exhaust channels must be made! For this small core with a diameter of 3cm, several small exhaust holes can be pierced in the center of the sand core with a ventilation needle, or pre embedded wax threads can be used to melt and form exhaust channels during pouring. Check the fit clearance of the sand core head to ensure that the exhaust channel at the core head is smooth and unobstructed, allowing gas to escape smoothly through the core head and into the mold or sand exhaust system.

2. Solidification sequence and gas trap: Ductile iron has a paste like solidification characteristic, and the interior remains in liquid state for a long time after the shell is formed. The wall thickness of the casting is uniform, but the central area of the inner wall is the final solidification zone. The gas that cannot be discharged forms high pressure inside the mold cavity, and at the weak moment when the surface of the molten iron crust or begins to solidify (usually the inner wall of the middle and upper parts), it invades the metal that is still in liquid state. Due to the expansion and solidification pressure of graphite, these gases are eventually "locked" in the final solidification area, forming subcutaneous pores or invasive pores.

3. The chemical properties of molten iron exacerbate the situation: excessive residual magnesium (Mg) content can increase the surface tension of the molten iron, making it more difficult for invading bubbles to float and escape. Iron liquid oxidation (high oxygen content) or incomplete furnace charge (rust, oil stains) will increase the tendency of self precipitated pores, forming severe pores together with invasive gases.

2、 Systematic solutions should be investigated and tested in order from primary to secondary:

1. Sand core optimization (the most direct and effective measure) to reduce gas generation: Immediately contact the coated sand supplier and switch to low gas coated sand. This material is specifically designed to solve such problems by using special resins and additives to reduce gas generation and delay peak gas generation. Ensure that the exhaust of the sand core is absolutely unobstructed (of utmost importance!): For sand cores with a diameter of 30mm, an exhaust system must be installed during the core making process. Best method: Use pre embedded exhaust wax wire. One or more wax wires are embedded during the core making process, and the wax wires melt during casting, forming a perfect exhaust channel. Simple method: Insert a vent hole through (or near) the center of the sand core or with a ventilation needle. Ensure that these channels are connected to the core head. Optimize core design: Check the position of the core in the mold to ensure that the gap between the core and the sand mold cannot be completely sealed after the sand core is placed, which is the final channel for gas to escape outside the mold. If necessary, the gap between the core heads can be increased or specialized exhaust slots can be made.

2. Process optimization (adjusting the interaction between molten iron and sand core) to increase pouring temperature: This is the fastest and most effective temporary measure on site. Appropriately increasing the pouring temperature (such as 1380 ° C → 1400-1420 ° C) can prolong the time for the molten iron to remain liquid and give more time for the gas to be discharged. Make the surface of the sand core sinter faster to form a "vitrified" hard shell, preventing the deep resin from continuing to emit gas. Attention: Excessive temperature may cause other problems (such as sand sticking), and an equilibrium point needs to be found. Accelerate pouring speed: shorten the filling time while avoiding turbulence. The rapidly established metal static pressure can better suppress gas invasion and complete exhaust before the lower solidification. Ensure smooth pouring: Adopt a bottom pouring system to avoid direct flushing of the sand core by molten iron, reduce turbulence and curling. 3. Smelting and iron liquid control (to eliminate self problems and avoid adding insult to injury) strictly control the residual magnesium content: excessive Mg residue is the "catalyst" for pores. Ensure that the residual Mg content after spheroidization treatment is controlled within the lower limit required by the process (such as 0.03% -0.04%), and should not be too high. Use clean furnace materials: Eliminate scrap steel and recycled materials with severe rust and oil stains, and prevent their decomposition to produce [H], [O], and CO gases. Thoroughly remove slag: Before spheroidization treatment and pouring, the slag must be thoroughly removed to prevent the slag from being rolled into the mold cavity.

Summary and action priority recommendations

1. First priority (immediate inspection): Check if the sand core has an exhaust duct! If not, this is the problem that must be solved first. Try burying wax threads or tying ventilation holes.

2. Second priority (rapid testing): Increase the pouring temperature by 20-30 ° C and observe the improvement of porosity. If the effect is significant, it strongly points to the problem of gas generation in the sand core.

3. Third priority (contacting suppliers): Request low emission gas coated sand samples for comparative testing, which is often the key to solving the problem.

4. Fourth priority (detection and recording): Check the residual Mg content in the iron liquid after spheroidization to ensure that it is within a reasonable low range.