The quality of silicon carbide for casting is centered on "meeting the main content standard, controllable impurities, and suitable size." Key indicators and their definitions are as follows:
| Indicator Type | Core Indicator | Definition and Function | Industry Recommended Limits |
| Key Content Indicators | SiC (Silicon Carbide) Content | The core indicator determining deoxidation, silicon enrichment, and carbon enrichment efficiency | High-end castings ≥97%, ordinary castings ≥88%, economical castings ≥70% |
| Key Auxiliary Indicators | Free Carbon (F.C) Content | Auxiliary carbon addition; excessive content can easily cause graphite floating defects. | ≤0.3% (high-end castings), ≤4.0% (ordinary castings) |
| Harmful Impurity Index | Fe₂O₃ (Iron Oxide) Content | Introducing iron impurities affects the purity and corrosion resistance of castings. | ≤0.6% (High-end castings), ≤1.5% (Ordinary castings) |
| Limiting Impurity Parameters | Al₂O₃ (Alumina) Content | Forms hard and brittle inclusions, reducing the machinability of castings | ≤3.0% (Overall Limit) |
| Moisture Content | H₂O (Moisture) Content | To prevent porosity and pinhole defects in castings | ≤0.5% |
| Physical Properties | Size Distribution | Affects Dissolution Rate and Reaction Uniformity | Lumps 1-50mm, Powder 100-240 mesh |
Specific Impacts of Core Indicators on Casting Effect
(1) SiC Content: The Core Guarantee of Casting Function
Deoxidation Efficiency Correlation:
SiC≥97% (High-end grade): With an addition of 0.5%-1.0%, the oxygen content in the molten iron decreases from 60-80ppm to 20-30ppm, achieving a deoxidation efficiency of 70%, and a scrap rate of only 0.3% for oxide inclusions;
SiC=88% (Ordinary grade): With the same addition amount, the oxygen content decreases to 30-40ppm, achieving a deoxidation efficiency of 55%-60%, suitable for ordinary carbon steel and alloy castings;
SiC=70% (Economic type): Requires an increase of 20%-30% in the addition amount to achieve the same deoxidation effect, suitable for castings with lower quality requirements.
Silicon and carbon enrichment effects:
SiC≥90%: After addition, the silicon content of molten iron increases by 0.3%-0.5%, and the carbon content increases by 0.1%-0.2%, requiring no additional carbon enrichment agent, simplifying the process;
SiC＜80%: The silicon and carbon enrichment effects are unstable, requiring supplementation with ferrosilicon and graphite powder, increasing production costs.
(2) Impurity content: The "hidden risk" to casting quality
Free carbon (F.C):
Compliant state (≤0.3%): Aids in carbon enrichment without producing graphite floating, resulting in a uniform casting structure;
Excessive state (＞4.0%): Easily forms graphite accumulation on the casting surface, increasing the defect rate from 0.5% to 2.8%, unsuitable for precision castings.
Fe₂O₃ and Al₂O₃:
Fe₂O₃ > 1.5%: Excessive iron impurities lead to uneven hardness in castings and a 40% increase in tool wear;
Al₂O₃ > 3.0%: Al₂O₃ inclusions (hardness above HV1800) are formed, reducing the impact toughness of castings by 30% and making them prone to fracture under stress.
(3) Size distribution: Key to adapting reaction efficiency to application scenarios
Blocking silicon carbide (1-50mm):
1-10mm: Suitable for medium-frequency furnaces and cupola furnaces, dissolution time 3-5 minutes, uniform reaction, silicon recovery rate 75%-85%;
10-50mm: Suitable for deoxidation in large ladles, requires thorough stirring after addition to avoid incomplete local reactions. Silicon
carbide powder (100-240 mesh):
100-180 mesh: Used as an additive in coatings and core sands to improve coating wear resistance (reducing wear by 50%) and core permeability;
220-240 mesh: Suitable for precision casting, it can be evenly dispersed in molding sand, reducing sand adhesion defects on the casting surface.
(4) Moisture content: A key cause of porosity defects.
Moisture content > 0.5%: Decomposes at high temperatures to produce H₂, increasing the hydrogen content in molten iron from 2-3 ppm to 8-10 ppm, and raising the porosity defect rate in castings from 0.2% to 1.5%. It needs to be dried (100-120℃, 2 hours) before use.
Classification and Application of Silicon Carbide for Casting
(1) Classification by Quality Grade
| Quality Grade | Core Indicator Requirements (SiC/F.C/Fe₂O₃) | Suitable Scenarios | Typical Applications |
| High-end grade | ≥97%/≤0.3%/≤0.6% | Precision castings, high-end alloy castings | Engine blocks, machine tool beds |
| Standard Grade | ≥88%/≤4.0%/≤1.5% | Ordinary carbon steel castings, machine parts | Agricultural machinery parts, building hardware |
| Economy type | ≥70%/≤5.0%/≤3.0% | Low-requirement castings, recycled cast iron | Counterweights, simple structural components |
(2) Classification by Morphology
| Morphology | Size Range | Core Advantages | Compatible Processes |
| Blocky silicon carbide | 1-5mm, 5-10mm, 10-50mm | Moderate dissolution rate, stable reaction | Induction furnace melting, ladle deoxidation |
| Silicon carbide powder | 100-180 mesh, 220-240 mesh | Large specific surface area, uniformly dispersed | Coating additives, core sand modification |
| Silicon carbide lumps (briquettes) | 10-30mm | Good formability, low dust generation | Large-scale deoxidation in casting ladles |
Selection and Usage Control Points of Silicon Carbide for Casting
(1) Selection Logic: Matching indicators according to casting requirements
| Casting Type | Recommended Quality Grade | Key Performance Indicator Requirements | Recommended Dosage |
| Precision alloy castings | High-end grade | SiC≥97%, F.C≤0.3%, 1-10mm | 0.5%-0.8% |
| Ordinary mechanical castings | Ordinary grade | SiC≥88%, F.C≤4.0%, 1-5mm | 0.8%-1.2% |
| Low-requirement castings | Economical | SiC≥70%, 10-50mm | 1.2%-1.5% |
| Coatings / Core Sand | Powder Grade | SiC ≥ 98.5%, 100-240 mesh | Coating Addition Rate: 5%-8% |
(2) Usage Precautions
Incoming Inspection:
Sampling and testing of SiC content, impurities, and moisture in each batch. Spectrometers are used to measure composition, and drying methods are used to measure moisture content to ensure compliance with standards.
Addition Timing:
Block silicon carbide is added in the later stages of smelting (molten iron temperature 1450-1500℃). Powdered silicon carbide is mixed evenly during coating preparation.
Storage and Protection:
Store in a dry and ventilated environment. Blocked silicon carbide should not be stored for more than 6 months. Powdered silicon carbide must be sealed to prevent moisture absorption and clumping.
Synergistic Use:
When added in combination with calcium silicon alloys and ferromanganese, it can improve desulfurization (desulfurization rate up to 60%) and reduce hot brittleness defects in castings.
Industry Trends: Upgrade Directions for Silicon Carbide in Casting
High Purity: The increasing demand for high-end products with SiC ≥ 99% in precision casting is driving further reductions in impurity content (F.C ≤ 0.1%, Fe₂O₃ ≤ 0.2%);
Customization: Developing silicon carbide with specific sizes and compositions for different casting processes (medium-frequency furnace, cupola furnace, precision casting);
Greening: Adopting environmentally friendly production processes to reduce dust pollution while improving resource utilization, aligning with the carbon neutrality trend in the casting industry.
