Producing 410 stainless steel parts with silica sol, weighing 205 grams, with surface oxidation spots defects: causes and solutions

When using zircon powder/sand as the surface layer, oxidation points and spots appear in the production of 410 stainless steel parts (especially small parts weighing around 200 grams). How should we investigate the causes and develop solutions. Let's analyze the core conclusions one by one: this "point and spot" oxidation is usually not caused by a single factor, but rather the result of a violent reaction between highly active steel liquid and locally contaminated shell interface. The root cause of the problem mainly lies in the "shell quality" and "steel liquid shell interface reaction".

1、 The main reasons for the formation of oxidation spots/spots are analyzed, combined with the characteristics of "zircon powder/sand surface layer" and "point oxidation". The main reasons are ranked in order of possibility as follows:

1. Surface layer contamination of the shell (primary suspect) Zirconia material itself: Poor quality or damp Zirconia powder/sand may contain impurities such as iron oxide (Fe ₂ O3) and titanium oxide (TiO ₂). At high temperatures, these impurities will react chemically with elements such as chromium (Cr) and aluminum (Al) in stainless steel, leaving localized reaction marks (i.e. oxidation marks) on the surface of the casting. Pollution during operation: In the shell making workshop, rust, dust, and organic matter (such as glove fibers and grease) may be mixed in during the surface coating or sanding process. These pollutants will form "weak points" with low melting points or high activity locally after shell calcination. Stability of silica sol: if the silica sol has local gel or pollution, it will affect the uniformity of the coating, resulting in insufficient local strength or impurity enrichment.

2. Insufficient shell roasting and residual moisture (key reason): Moisture residue is one of the most common reasons for the formation of "oxidation points". If the roasting temperature of the shell is insufficient (<900 ℃) or the insulation time is not enough, there will be residual crystal water or chemical water in the deep layers of the shell (especially thick and large shells). When high-temperature molten steel is injected, the water evaporates instantly, and the vapor pressure is extremely high, breaking through the solidified thin shell at the front of the molten steel, exposing the fresh molten steel inside and undergoing oxidation reaction with water vapor: Fe+H ₂ O → FeO+H ₂, forming point like pits and oxide scales. Organic carbon residue: Incomplete roasting can lead to carbonization of organic compounds in silica sol and mold release agents instead of complete combustion, forming localized carbon rich areas. When the molten steel comes into contact with this area, carbon will reduce SiO ₂ in the shell, producing CO gas, which will also damage the surface of the molten steel and cause local oxidation and carburizing.

3. Insufficient melting and pouring protection (fundamental reason) incomplete deoxidation: Chromium in 410 stainless steel is prone to oxidation. If the final deoxidation (usually using aluminum) is insufficient, the dissolved oxygen content in the molten steel will be high, and it will tend to aggregate on the surface or combine with the shell reactants at the end of solidification, forming point like oxides. Insufficient casting protection flow: Even with argon gas protection, if the airflow is too weak, unevenly dispersed, or disturbed, air will still be drawn into the casting stream and sprue cup, causing steel droplets to splash and oxidize and enter the mold cavity with the stream, forming dispersed oxidation points.

4. Mismatch of process parameters (triggering factor) Mismatch between shell temperature and pouring temperature: The preheating temperature of the shell is too low (such as<600 ℃), while the pouring temperature of the molten steel is too high. The temperature difference between the two is too large, which will intensify the interface gas explosion and thermal shock, and induce point reactions. Overheating of molten steel: Excessive melting temperature (such as exceeding 1650 ℃) will intensify the chemical reactivity between the molten steel and the shell.

2、 Systematic solution (from emergency to root cause) Step 1: On site emergency investigation and handling (immediate execution)

1. Check the shell baking furnace: calibrate the temperature measuring instrument. Ensure that the roasting temperature is ≥ 950 ℃ and the holding time is ≥ 2 hours (depending on the increase in shell thickness), and check the circulation of the furnace atmosphere to ensure that the exhaust gas can be discharged.

2. Check the raw materials: Take a new batch of high-purity (chemically pure or first grade) zircon powder/sand for comparative testing. Pay special attention to its iron (Fe) and titanium (Ti) content.

3. Check the shell making environment: Clean the shell making workshop, ensure that the surface coating is isolated from the sanding area, and prevent rust dust pollution. Check the silica sol for particles or gel.

4. Strengthen casting protection: temporarily increase the strength of argon gas protection to ensure that the pouring cup is completely covered by argon gas during casting.

Step 2: Short term process optimization (within 1-2 weeks)

1. Optimize the roasting process: implement "step heating roasting": increase the insulation time in the 400-600 ℃ stage to allow organic matter to fully decompose and evaporate; Maintain sufficient insulation above 900 ℃ to expel chemical water. For important components, pour immediately after baking or store in a high-temperature oven (>200 ℃) to prevent moisture absorption.

2. Strengthening melt treatment: Strict final deoxidation: Before tapping, insert aluminum wire into the deep part of the molten steel for final deoxidation, and control the residual aluminum content at 0.02% -0.08%. Appropriately reduce the pouring temperature: On the premise of ensuring complete filling, reduce the pouring temperature from superheat (such as 1550 ℃) by 10-20 ℃ to reduce thermal reactions.

3. Adjust the temperature of the mold shell: shorten the interval between the mold shell being taken out of the furnace and pouring to the shortest possible time, ensuring that the temperature inside the mold shell is between 800-900 ℃. High temperature shells can reduce interface temperature differences and ensure smooth solidification of molten steel.

Step 3: Long term systematic control (fundamental solution)

1. Shell material and process upgrade: Surface layer material replacement test: If the problem persists, consider replacing the surface layer material with more inert fused alumina (Al ₂ O3) or "white corundum". Although the cost is higher, the reactivity with high chromium steel is lower. Introduction of surface layer sintering process: After completing the surface layer and second layer shell making, an additional low-temperature (800 ℃) sintering is added to densify the surface layer and eliminate some gas emitting substances in advance.

2. Upgrading the melting and pouring system: implementing argon protection melting: using argon gas to cover or blow during induction furnace melting. Using vacuum or protective atmosphere casting: For high demand products, investing in vacuum induction furnace melting casting or argon filled casting boxes is the most thorough solution.

3. Establish process monitoring points: Raw material inspection: Conduct impurity content sampling for each batch of zircon powder. Record of shell roasting: Establish temperature time curve monitoring for each roasting furnace. Casting defect map: Take photos and archive the location and morphology of oxidation points, analyze the correlation with the tree position, and trace the source of pollution.

Summarize the recommended troubleshooting process for the problem of "oxidation points/spots on the surface layer of zircon powder sand in a 205 gram casting". It is recommended to prioritize troubleshooting as follows:

1. Primary suspicion: Is the shell roasting sufficient? Conduct comparative experiments by increasing the roasting temperature and holding time.

2. Secondary suspicion: Is the zircon material pure? Replace a batch of known high-purity materials for comparative testing.

3. Simultaneously check: Is the pouring protection truly effective? Check the airflow status at the argon pipeline, flow meter, and sprue cup.

4. Final optimization: Adjust the matching of process parameters, mainly the shell temperature and pouring temperature. Through the above systematic investigation and optimization, especially ensuring absolute dryness and cleanliness of the shell and strengthening interface protection, the oxidation points and spots on the surface of 410 stainless steel precision castings can be effectively eliminated.