Characteristics and prevention measures of subcutaneous porosity in gray cast iron parts

The subcutaneous pores of gray cast iron parts have the following characteristics: distribution location: usually located 1-3mm below the surface of the casting, mostly at the opposite end of the gate, the bottom of the pouring position, and other parts. Appearance: Small in size, with a diameter of generally 1-3mm and a length of 4-6mm, it is spherical, pinhole shaped or oblong, often densely distributed, and in severe cases, forms a honeycomb shape. Characteristics of pore wall: The pore wall is smooth and shiny, partially covered with graphite film, appearing silver white, and a few pore walls with open cavities are oxidized in color. Timing of occurrence: The pores will only be exposed after heat treatment, shot blasting cleaning, removal of oxide scale, or mechanical processing.

The following is a detailed breakdown of the main gas sources in subcutaneous pores:

Direct gas: The gas in subcutaneous pores is mainly H ₂ and N ₂. CO is an important participating gas, but more importantly, it serves as a product of the reaction to create conditions for the invasion of other gases. Formation mechanism core: The presence of oxide film (FeO) on the surface of molten iron is a key prerequisite for inducing subcutaneous pore chemical reactions (especially FeO+C → Fe+CO). Without an oxide film, the reaction is difficult to initiate, and the tendency of subcutaneous pores is greatly reduced. Mold contribution: The moisture content of molding sand (producing H ₂) and the nitrogen content of resin (producing N ₂) are the main sources of mold gas. The wet coating and organic matter decomposition are also important factors. Internal factors of molten iron: high hydrogen and nitrogen content in molten iron, as well as excessive oxidation of molten iron (FeO), are inherent causes. Solidification conditions: Subcutaneous pores occur in the early stage of solidification (paste like zone), and gas accumulates at the front of solidification and is captured by the growing dendrites. The cooling rate and solidification method of castings also affect the formation and size of pores. Simply put, the pores under the gray cast iron sheet are small pores formed by the chemical reaction (especially the CO production reaction) between the surface oxidation of molten iron (FeO) and the gas source provided by the mold (mainly H ₂ O and nitrogen-containing organic compounds) at the high-temperature interface, resulting in the aggregation, invasion, and capture of hydrogen, nitrogen (sometimes CO) at the solidification front. **The key to prevention is to control the degree of iron oxidation, reduce the moisture/resin nitrogen content of the molding sand, and ensure the drying of the coating.

What are the measures to solve the porosity under the gray cast iron sheet?

Systematic and targeted measures need to be taken to solve the defects of gas pores (pinholes) under gray cast iron sheets, with the core being "reducing gas sources, suppressing interface reactions, promoting gas discharge, and optimizing solidification environment". The following are specific and actionable solutions, classified by key control steps:

1、 Cut off the source of gas (fundamental solution) 1 Strictly control the molding sand system (especially green sand and resin sand) to reduce the moisture content of the molding sand (key to green sand): strictly control the effective bentonite content to avoid excessive water addition in pursuit of strength. Strengthen the cooling of old sand to ensure that the temperature of recycled sand is less than 50 ° C (hot sand is the root cause of moisture migration and failure). Optimize the sand mixing process to ensure even distribution of moisture. Target moisture: Adjust according to the sand system and casting wall thickness, usually controlled within the range of 3.0% -4.2% (lower limit for thin-walled parts, slightly higher for thick walled parts, but other measures need to be taken). Reduce the nitrogen content of resin sand (key to resin sand): choose low nitrogen or nitrogen free resin and curing agent. For gray cast iron, it is recommended that the total nitrogen content of the resin be<3%, and for important or sensitive parts be<1.5%. Strictly control the amount of resin and curing agent added to avoid excess. Strengthen the regeneration of old sand, remove micro powder and ineffective binders (micro powder adsorbing nitrides). Reduce organic gas emissions: Control the amount of additives such as coal powder and starch added. Select bentonite and additives with low volatile matter and low gas generation. Ensure thorough drying of the coating: Water based coatings must be thoroughly dried after spraying, with priority given to baking in a drying room (150-250 ° C for 1-2 hours) to avoid relying solely on air drying or surface drying. Control the thickness of the coating layer, especially at the corners and grooves of the sand core. Choose low gas emission coatings. 2. Purify molten iron and reduce dissolved gas content. Dry and clean furnace materials: pig iron, scrap steel, and recycled materials must be rust free, oil-free, and dry. Severely corroded materials require shot blasting or preheating (>300 ° C). Avoid using furnace materials containing excessive organic matter (such as waste motor rotor enameled wire) or high nitrogen alloys. Strict control of auxiliary materials: Carbonizers, inoculants, and spheroidizers must have low sulfur, low nitrogen, low volatile matter, and low moisture content. Preheat to 200-300 ° C or above before use (especially for inoculants). The covering agent must be dry. Optimize smelting operation: Fully preheat/bake the furnace lining (especially after new lining or shutdown). Ensure sufficient overheating temperature of molten iron (1500-1550 ° C) and appropriate holding time (5-10 minutes) to promote the upward escape of dissolved gases (H ₂, N ₂). Avoid excessive oxidation. In the later stage of smelting, it can be briefly allowed to stand and remove gas. Inert gas (Ar) purification can be carried out if conditions permit. Control the atmosphere inside the furnace to prevent humid air from entering (cover the furnace mouth and maintain a slight positive pressure). Control processing: Spheroidization/incubation treatment uses teapot bags, Tundish covers, etc. to reduce curling air. Pregnancy is carried out by following the flow, reducing the local supercooling and gas release caused by excessive one-time addition.

2、 Inhibiting harmful reactions at the interface between molten iron and mold (key breakthrough) 1 Prevent surface oxidation of molten iron (eliminate FeO) and strictly control the oxidizability of molten iron: avoid excessive stirring and exposure to air. In the later stage of smelting, a small amount of aluminum (0.01-0.03%) or rare earths can be added for deoxidation, but extreme caution is required (excessive aluminum can cause abnormal structure, and rare earths increase the tendency to shrink). The optimal amount needs to be determined through experimentation. Clean up the slag in a timely manner. Optimize pouring temperature: Increase the pouring temperature appropriately (generally>1380 ° C, adjusted according to wall thickness). High temperature molten iron has good fluidity and slow solidification, which is conducive to gas flotation and decomposition of interfacial reactants, while reducing the tendency for oxide film formation. But avoid excessive heat that may cause sand mold sintering. Strengthen pouring process protection: Bake and dry the ladle, and use a covering agent to protect the surface of the molten iron. Adopting bottom pouring system or high flow stable filling to reduce the oxidation of iron water stream. 2. Weaken the "FeO+C → Fe+CO" reaction to control the effective carbon content in the molding sand: Ensure that an appropriate amount of coal powder is added (usually the effective coal powder content in the green molding sand is 3-5%) to form a reducing atmosphere at the interface, but avoid excessive gas generation. An appropriate amount of iron oxide powder (Fe ₂ O3) or high manganese steel shot can be added to resin sand to consume some carbon or change the reaction pathway (to be tested). Quickly establish a reducing atmosphere: Ensure that the mold cavity is quickly filled with high-temperature molten iron after pouring, allowing organic matter on the surface of the molding sand to rapidly pyrolyze and form a dense and bright carbon film, isolating the molten iron from the sand mold.

Solving subcutaneous pores is a systematic engineering that requires multiple approaches. *When problems arise, a detailed analysis of the causes should be conducted based on the characteristics of the pores (location, size, distribution, color) combined with on-site data (molding sand parameters, pouring temperature, resin type, furnace charge situation). Priority should be given to trying the most likely cause (such as checking nitrogen content and exhaust for resin sand parts first, and checking moisture and permeability for green sand parts first) to avoid blind adjustments. Continuous process monitoring and strict process discipline are key to preventing recurrence.