Smelting and Alloy Control (Prevention of Intergranular Corrosion)
Strict chemical composition control: Austenitic stainless steel is highly sensitive to the content of chromium (Cr) and nickel (Ni). During the casting process, chromium is prone to burnout due to high temperatures, requiring precise adjustment before pouring.
Silicon content control: A moderate increase in silicon content can improve the fluidity of molten steel, which is crucial for impellers with complex shapes. However, excessive levels may lead to hot brittleness.
Deoxidation and Refining: Austenitic stainless steel is highly prone to gas absorption, so it is essential to ensure the furnace lining is dry and use high-quality refining agents to prevent honeycomb-like gas pores from forming inside the castings.
2. Special Challenges in Casting Processes
The linear shrinkage is significant: the linear shrinkage rate of austenitic steel (approximately 2.0%~2.5%) is much greater than that of general carbon steel.
Consequences: If sufficient shrinkage allowance is not left, the casting dimensions will inevitably exceed the tolerance; and cracks are highly prone to occur during the cooling process.
Countermeasure: During model fabrication, the shrinkage coefficient must be magnified. Additionally, process ribs should be added at the corners of the casting and removed after shakeout to prevent cracking.
Poor fluidity (difficult filling): Austenitic stainless steel has high viscosity and inferior fluidity compared to carbon steel.
Countermeasure:
Increase the pouring temperature: typically 20-50°C higher than that of carbon steel.
The pouring speed must be fast: adhere to the "fast pouring" principle to prevent mid-process cold shuts.
Fine exhaust system: The sand mold should have good permeability. It is recommended to use resin sand technology and avoid ordinary clay sand, as austenitic steel is highly prone to pinholes in wet sand.
3. Heat Treatment (the "soul" of austenitic castings)
Austenitic castings must undergo solution treatment after casting and cannot be used directly
Solution treatment (1050°C - 1150°C):
Objective: To eliminate casting stress, ensure uniform distribution of alloying elements, and most importantly, dissolve carbides by solid-solutionizing carbon into the austenite matrix, thereby achieving optimal corrosion resistance and mechanical properties.
Cooling: Rapid water quenching is essential. If the cooling rate is too slow, carbides may precipitate, causing the stainless steel to lose its corrosion resistance and even leading to severe intergranular corrosion.
4. Common "crash" points
Sand adhesion and roughness: Austenitic steel melt exhibits strong chemical erosion on sand molds at high temperatures.
Solution: The coating must be made of high-quality zircon powder or magnesia-based coating. Ordinary quartz powder coatings are prone to surface sticking due to the "fayalite" reaction when casting stainless steel, making it impossible to clean effectively.
Hot cracking: Austenitic steel exhibits extremely low strength and severe shrinkage during the late stage of solidification.
Suggestion: In sand mold design, it is essential to control the "yieldability" of the sand (make the sand mold softer) to prevent the casting from being "locked" by the sand mold during contraction, which could lead to cracking.