Nodular/Ductile Cast Iron GGG-40 vs GGG-60: Insert Selection by Hardness

Ductile cast iron — also known as nodular iron, spheroidal graphite iron, or SG iron — is a family of cast irons where the graphite exists as spherical nodules rather than flakes. This nodule morphology gives ductile iron dramatically higher toughness and ductility than grey cast iron while maintaining good castability and vibration damping. GGG-40 (EN-GJS-400-15) and GGG-60 (EN-GJS-600-3) represent the low-strength/high-ductility and high-strength/low-ductility ends of the ductile iron spectrum. Their different matrix structures — ferritic for GGG-40, pearlitic for GGG-60 — create significantly different machining behaviors that demand different tooling strategies.

Material Properties Comparison

The fundamental machining difference is driven by the matrix structure. GGG-40’s ferritic matrix is soft, ductile, and tends to produce built-up edge on cutting tools — similar to machining low-carbon steel. GGG-60’s pearlitic matrix is harder, more abrasive, and produces higher cutting forces — similar to machining medium-carbon steel.

Machining Characteristics: GGG-40 (Ferritic)

GGG-40’s ferritic matrix creates several machining challenges:

• Built-up edge (BUE): The soft, ductile ferrite tends to weld onto the cutting edge, particularly at cutting speeds of 60-150 m/min. This BUE changes the cutting geometry and produces poor surface finish.
• Long chips: While the graphite nodules help break chips, the ferritic matrix produces longer, more continuous chips than pearlitic grades.
• Surface smearing: The soft surface can smear rather than shear cleanly, leaving a burnished appearance with poor dimensional accuracy.

Machining Characteristics: GGG-60 (Pearlitic)

GGG-60’s pearlitic matrix creates different challenges:

• Abrasive wear: The alternating ferrite/cementite lamellae in pearlite are abrasive to carbide cutting edges, causing progressive flank wear.
• Higher cutting forces: The harder matrix requires 30-50% more cutting force than GGG-40.
• Hard spots: Some GGG-60 castings contain small amounts of cementite or bainite that create localized hard zones (300+ HB).
• Shorter chips: The pearlitic matrix produces more segmented chips, which is favorable for chip control.

Insert Grade Selection by Material and Operation

GGG-40 (Ferritic Ductile Iron)

For GGG-40, select grades with sharp edges and polished rake faces to combat BUE. PVD-coated grades generally outperform CVD in ferritic ductile iron because they maintain a sharper edge.

GGG-60 (Pearlitic Ductile Iron)

For GGG-60, CVD-coated grades with thick Al2O3 layers provide the best wear resistance against the abrasive pearlitic matrix. The thick coating protects the substrate from the micro-abrasion caused by cementite lamellae.

Cutting Parameters: Turning GGG-40

For GGG-40, maintain cutting speeds above 150 m/min to avoid the BUE formation zone (60-150 m/min). If you must run below 150 m/min, use a PVD-coated grade with a polished rake face.

Cutting Parameters: Turning GGG-60

Milling Parameters

Insert Geometry Recommendations

Drilling Ductile Iron

Both GGG-40 and GGG-60 machine well with solid carbide drills. Key parameters:

Surface Finish Comparison

GGG-60 typically produces better surface finish than GGG-40 because its pearlitic matrix shears more cleanly, while GGG-40’s ferritic matrix tends to smear. However, the graphite nodules in both materials can leave small pits on the machined surface (typically 20-50 μm diameter), which limits the achievable finish compared to steel.

Common Challenges and Solutions

Dross and Sand Inclusions: Both grades can contain non-metallic inclusions from the casting process. These cause micro-chipping on insert edges. Solution: use tougher PVD grades (PC3545) for the first roughing pass, which cuts through the casting skin.

Varying Hardness in a Single Casting: Ductile iron castings often have hardness variation of 30-50 HB between thick and thin sections. Solution: use a versatile grade like Korloy NC3220 or Iscar IC808 that handles the hardness range without chipping or excessive wear.

Austempered Ductile Iron (ADI): Some components are austempered after casting, reaching 300-400 HB. This requires CBN or ceramic tooling — treat it like hardened steel.

Summary

GGG-40 and GGG-60 require different approaches: GGG-40’s ferritic matrix demands sharp, BUE-resistant PVD grades at higher speeds, while GGG-60’s pearlitic matrix benefits from wear-resistant CVD grades with thicker coatings. Use Korloy PC3545 for GGG-40 finishing and interrupted cuts, and Korloy PC8110 for GGG-60 roughing and continuous cuts. Machine dry whenever possible, and maintain cutting speeds above the BUE formation zone for ferritic grades.

hooguu.com is an authorized Korloy distributor and stocks the complete range of carbide inserts for ductile iron machining — from ferritic GGG-40 to pearlitic GGG-60 and austempered grades. Our technical team can recommend the optimal grade and geometry for your specific casting conditions and production volumes.