Abstract
For a long time, pitting has been a major defect in the surface quality of precision castings. This issue is commonly observed on the surfaces of 200-series stainless steel, 400-series stainless steel, carbon steel, and low-alloy steel. After shot peening or sandblasting, gray-black spots, pits, and even dents appear on the casting surface. These defects are difficult to repair in post-processing. Minor defects can be polished with grinding machines, but severe ones can easily lead to scrap, increasing production costs and affecting delivery deadlines. This paper analyzes the causes of pitting in detail and provides prevention measures for reference.
1. Defect Characteristics
The casting surface exhibits numerous点状凹坑, often occurring on castings made of 200-series and 400-series martensitic stainless steel, carbon steel, and low-alloy steel. (Image of typical defects on 200-series and 400-series stainless steel can be inserted here)
2. Causes and Prevention Measures
| Cause | Prevention Measures |
|---|---|
| Unreasonable Gating System Design | – Reasonably design the gating system. Deviate from the hot spot when placing the inner gate to reduce overheating and improve process yield. – Ensure complete wax removal during the design of inner gates and tree assembly processes. Consider adding wax removal channels, process ribs, or secondary wax removal and cleaning for high-quality castings. |
| Excessive Metal Oxides in the Surface Refractory Material | – Select appropriate shell materials, such as zircon sand, and strictly control the impurity content, especially Fe2O3, to be less than 0.05%. – Use high-grade refractory materials with low impurity content for shell surface coatings and sands, such as ceramic-grade zircon sand powder. – Regularly inspect incoming raw materials and revalidate stored materials. Do not use unqualified surface materials. |
| Incomplete Firing of the Mold Shell | – Ensure complete firing of the mold shell at temperatures ranging from 950 to 1,200 °C for 0.5 to 1 hour to eliminate free water, crystal water, and inorganic salts. – Cover the cast steel with a special insulating agent immediately after pouring to prevent secondary oxidation of the molten steel. |
| Incomplete Deoxidation in Melting | – Use dry and clean charging materials, and control the number of times or amount of recharging materials used to avoid increasing oxides in the molten metal. – Adopt complete deoxidation by adding ferromanganese first, followed by ferrosilicon and then calcium-silicon for deoxidation. After power is cut off for static settlement for 2 minutes, add aluminum or composite deoxidizer for final deoxidation before pouring. – Develop and strictly follow reasonable melting and pouring processes, such as ensuring sufficient settlement time before tapping to allow oxides to float up fully and cleaning slags thoroughly. |
3. Prevention Measures for Carbon Steel and Low-Alloy Steel Castings
| Measure | Details |
|---|---|
| Improving Melting Quality | – Use dry and clean charging materials and control the reuse times or amount of recharge materials. – Adopt complete deoxidation and strictly control the amount of aluminum added during final deoxidation. – Develop and strictly follow reasonable melting and pouring processes. |
| Selecting Reasonable Shell Firing Process | – Commonly used shell firing processes: firing temperature of 950 to 1,050 °C for water glass shells and 1,050 to 1,200 °C for silica sol shells, with durations of 0.5 to 2 hours and 30 minutes, respectively. |
| Using Neutral or Alkaline Binders | – Select reasonable binders based on casting material. Consider using neutral or alkaline binders when the previous measures are ineffective. |
| Selecting Reasonable Pouring Process | – Strictly control the temperatures of the molten metal and shell during pouring to prevent excessive interaction between the molten metal and shell surface material. |
4. Pouring and Melting Processes as Key Focus
- The primary factor affecting pitting is the quality of the molten steel (i.e., completeness of deoxidation and slag removal).
- Follow the melting process strictly: adding bottom slag, covering during melting, pre-deoxidation after melting is complete, power-off static settlement and slag removal, and final deoxidization before pouring.
- Choose deoxidizers that can fully deoxidize the molten steel and produce oxides with low melting points that are easy to aggregate and float.
- Ensure the steel material is clean and avoid using excessive recharge materials that increase native inclusions. Minimize the exposure time of the molten steel surface to prevent oxidation of Cr, Fe, and Si elements.
- Prevent secondary oxidation of the casting surface by adding sawdust or wood charcoal to the insulating material during pouring of large castings and immediately covering the mold with waste wax blocks after pouring.
5. Conclusion
- Pitting is the aggregation of complex oxide inclusions of Fe, Cr, Si, and Al in the molten steel on the casting surface.
- The primary measures to prevent pitting include complete deoxidation during melting to make deoxidation products easy to float, preventing secondary oxidation of the casting surface during mold cooling, strictly following the shell firing process, and purchasing graphite sand for use in the shell’s transition layer.