Comparative Analysis of Cutting Fluids for Stainless Steel, Aluminum, and Cast Iron

1. Composition and Key Differences

Cutting Fluids are formulated differently based on the material being machined due to varying thermal conductivity, hardness, and chemical reactivity. For stainless steel, Cutting Fluids typically contain 10-20% sulfurized or chlorinated extreme pressure (EP) additives to counteract work hardening (austenitic grades have a work hardening rate of 1.8-2.5 GPa). Aluminum-specific fluids use non-staining sulfur (≤0.5%) and pH buffers (8.5-9.5) to prevent staining, as aluminum alloys like 6061 have thermal conductivity of 167 W/m·K versus just 15 W/m·K for 304 stainless. Cast iron fluids often employ graphite (2-5μm particle size) as a solid lubricant since its 3-4% free graphite content enables self-lubrication.

2. Performance Characteristics

Stainless steel fluids demonstrate 30-50% higher heat absorption capacity (specific heat ~2.1 J/g°C) compared to aluminum fluids (~1.8 J/g°C), critical when machining at cutting speeds of 60-120 m/min for 316L stainless versus 300-500 m/min for aluminum 7075. EP additives in stainless formulations provide seizure resistance up to 1,200°C, whereas cast iron fluids focus on chip evacuation (flake graphite particles >50μm require 20% higher flow rates). Aluminum fluids incorporate 0.1-0.3% fatty alcohols to reduce smearing during finishing operations (< Ra 0.8μm).

3. Application Scenarios

• Stainless Steel: Used in CNC turning of duplex steels (2205) where cutting forces reach 2,500 N, requiring fluids with minimum film thickness >0.5μm. Common in valve manufacturing and nuclear components.

• Aluminum: Essential for high-speed milling (15,000+ RPM) of aerospace alloys (e.g., 2024-T351), where tool-chip interface temperatures exceed 300°C. Dominates in automotive cylinder head production.

• Cast Iron: Preferred for honing of engine blocks (GG25) where fluid must carry away 30-40% iron fines by volume. Widely adopted in brake disc machining.

  
  

4. Maintenance Protocols

Stainless Steel Fluids: Monitor chloride content weekly (keep <500 ppm) using silver nitrate titration. Maintain concentration of 8-12% via refractometry (nD correction factor 1.0). Centrifuge systems should remove sub-10μm tramp oil every 200 operating hours.

Aluminum Fluids: Conduct pH testing daily (8.5-9.5 range) with colorimetric strips. Use magnetic separators to remove ferrous contamination (>95% efficiency at 0.5T field strength). Bactericide dosage should not exceed 0.15% isothiazolinone concentration.

Cast Iron Fluids: Implement settling tanks with 30-minute residence time for graphite sedimentation. Check specific gravity (1.05-1.10) weekly using hydrometers. Filter systems should handle particle loads up to 5 g/L with 25μm rating.

5. Equipment Compatibility

Stainless steel fluids demand EPDM or fluorocarbon seals due to sulfur reactivity, whereas aluminum systems use nitrile seals (70 durometer). Cast iron applications require hardened steel pumps (HRC 55+) to withstand abrasive graphite. Modern minimum quantity lubrication (MQL) systems deliver fluids at 50-200 mL/hour for aluminum, but require 3-5x higher flow for stainless steel.

6. Environmental Considerations

Stainless steel fluids generate COD levels of 15,000-25,000 mg/L requiring activated sludge treatment. Aluminum fluids exhibit BOD5 of 8,000-12,000 mg/L needing pH adjustment before discharge. Cast iron fluids show suspended solids >2,000 mg/L mandating dissolved air flotation (DAF) systems.