GGG-60 and GGG-70: High-Strength Ductile Iron Turning

GGG-60 and GGG-70: High-Strength Ductile Iron Turning and Tooling Guide

GGG-60 (EN-GJS-600-3) and GGG-70 (EN-GJS-700-2) are pearlitic-ferritic ductile iron grades specified for heavy-duty components including crankshafts, camshafts, hydraulic cylinders, gear housings, and wind turbine main shafts. Their combination of high tensile strength (600-700 MPa), good elongation (2-3%), and excellent damping capacity makes them a cost-effective alternative to forged steel in many applications. However, their abrasive graphite nodules and hard pearlite matrix demand insert grades and cutting parameters specifically optimized for cast iron machining.

Material Properties

GGG-60 (EN-GJS-600-3):

• Microstructure: 60-70% pearlite, 30-40% ferrite, with spheroidal graphite nodules
• Hardness: 200-250 HB
• UTS: 600 MPa minimum, yield: 370 MPa minimum, elongation: 3% minimum
• Nodule count: 150-300 nodules/mm2 (depending on section size and inoculation)

GGG-70 (EN-GJS-700-2):

• Microstructure: 80-95% pearlite, 5-20% ferrite, spheroidal graphite
• Hardness: 225-280 HB
• UTS: 700 MPa minimum, yield: 420 MPa minimum, elongation: 2% minimum
• Higher pearlite content increases both strength and abrasiveness

Machinability: GGG-60 rates at approximately 55-70% of B1112; GGG-70 at 45-60%. The primary challenge is abrasive wear from graphite nodules and pearlite lamellae, not cutting forces (which are moderate compared to steel).

Insert Grade Selection

ISO Application Group: K (Cast Iron)

Roughing – GGG-60:

• Sandvik GC3215 (CVD Al2O3/TiCN, K10-K20 grade). CNMG 120412 with wiper geometry for high material removal rates. The thick Al2O3 coating provides excellent abrasion resistance against graphite and pearlite.
• Kennametal K060 (CVD coated, silicon nitride-reinforced). CNMG 120412.
• Iscar IC808 (CVD TiCN/Al2O3, medium-tough substrate). Proven performer on ductile iron crankshafts.

Roughing – GGG-70:

• Sandvik GC3210 (harder CVD grade for higher pearlite content) or GC3215 for interrupted cuts.
• Kennametal K050 (harder substrate, CVD coating) for continuous cuts on shafts and cylinders.
• Seco CBN300 (PCBN) for high-speed roughing of hardened surface layers or chilled casting surfaces. Vc up to 600 m/min with ap below 1.0mm.

Finishing – Both Grades:

• Sandvik GC3205 (fine-grain CVD, K05 equivalent). DNMG 150408 with polished rake and wiper flat.
• Mitsubishi UC5115 (CVD Al2O3, fine-grain substrate). Excellent surface finish on ductile iron.
• For mirror finish requirements: PCBN Sumitomo BN500 or Kyocera KBN250 at Vc 300-500 m/min.

Interrupted Cuts (keyways, splines, cross-holes):

• Use tougher grades: Sandvik GC3225 (toughest K-group carbide) or Kennametal K090.
• Reduce speeds by 20-30% and increase edge hone to 0.03-0.05mm to prevent chipping.

Cutting Parameters

Turning – GGG-60:

• Vc: 180-300 m/min (CVD carbide roughing), 250-400 m/min (finishing)
• fn: 0.20-0.40 mm/rev roughing, 0.10-0.20 mm/rev finishing
• ap: 2.0-6.0 mm roughing, 0.3-1.5 mm finishing
• Coolant: Dry machining preferred for roughing (graphite acts as solid lubricant). Flood coolant for finishing to improve surface quality.
• Tool life: 60-120 minutes per edge at 220 m/min roughing

Turning – GGG-70:

• Vc: 150-260 m/min (CVD carbide roughing), 200-350 m/min (finishing)
• fn: 0.18-0.35 mm/rev roughing, 0.08-0.18 mm/rev finishing
• ap: 2.0-5.0 mm roughing, 0.3-1.2 mm finishing
• Tool life: 40-90 minutes per edge at 200 m/min roughing

The Dry vs Wet Machining Decision

Ductile iron is one of the few materials where dry machining often outperforms flood coolant in roughing operations:

• Dry roughing advantages: Graphite nodules provide inherent lubrication at the chip-tool interface. Thermal cycling from flood coolant causes micro-cracking in the CVD coating (thermal shock), reducing tool life by 20-40%. Dry cutting maintains a stable insert temperature that preserves the coating.
• When to use coolant: Finishing operations benefit from flood coolant for surface finish improvement and dimensional stability. Through-tool coolant is recommended for drilling and deep boring.
• Dust management: Dry machining produces fine graphite dust that requires extraction systems rated for combustible dust. Ensure machine enclosures are sealed and dust collection meets ATEX or NFPA standards.

Milling Ductile Iron

• Face milling: Indexable cutters with SEKN 1203 or APKT 1604 inserts, Sandvik GC3215 or Kennametal K060 grade
• Vc: 150-280 m/min, fz: 0.15-0.30 mm/tooth
• Positive-rake cutter bodies reduce cutting forces and improve surface finish on the relatively brittle pearlite matrix
• End milling: 4-flute solid carbide, TiAlN coating, 12-25mm diameter. Vc: 100-200 m/min, fz: 0.04-0.10 mm/tooth.

Drilling

• Indexable insert drills (Sandvik CoroDrill 880 or Kennametal KSEM) for holes above 15mm diameter
• Solid carbide drills for holes below 15mm: 135 deg point, TiAlN coating
• Vc: 80-150 m/min, fn: 0.08-0.20 mm/rev (depending on diameter)
• Through-tool coolant recommended for all drilling to evacuate graphite-containing chips

GGG-70 Specific Considerations

The higher pearlite content in GGG-70 creates specific challenges:

1. Chilled surfaces: Thin sections of GGG-70 castings may contain chilled (white iron) surfaces with hardness exceeding 400 HB. These areas contain cementite (Fe3C) and are extremely abrasive. Use PCBN inserts or reduce carbide speeds to 60-80 m/min when encountering chilled zones.
2. Pearlite abrasiveness: The lamellar pearlite structure acts like a file against the cutting edge. Expect 25-40% shorter tool life on GGG-70 versus GGG-60 at equivalent parameters.
3. Surface integrity: GGG-70 components often serve in fatigue-critical applications (crankshafts, main shafts). Ensure turning does not create tensile residual stresses at the surface. Use finishing parameters with fn below 0.15 mm/rev and fresh inserts (VB below 0.2mm) to produce compressive surface stress.

Wind Turbine and Heavy Machinery Applications

Large GGG-60 and GGG-70 components for wind turbines (main shafts, planetary carriers, hub castings) require specialized approaches:

• Heavy-duty horizontal boring mills with 40+ kW spindle power and rigid workholding
• Program roughing from the inside out (bore first, then OD) to maintain workpiece rigidity
• Use anti-vibration boring bars (Sandvik Silent Tools or Kennametal AFT) for deep bore operations exceeding 4:1 L/D ratio
• Inspect for casting defects (shrinkage porosity, sand inclusions) before final finishing passes. Hard inclusions can fracture insert edges catastrophically.

Cost Optimization

GGG-60 and GGG-70 machining is generally economical compared to steel due to lower cutting forces and the ability to dry machine. Key cost factors:

• Insert cost per part is 40-60% lower than alloy steel programs due to longer tool life and lower insert grades
• Cycle times are 20-30% shorter than equivalent steel parts at equivalent stock removal volumes
• Eliminate coolant costs through dry roughing (savings of $0.50-2.00 per part in coolant consumption and disposal)
• Graphite dust filtration systems represent the primary additional equipment cost versus steel machining