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Difference between SG iron, cast iron, and high chrome cast iron – grades, properties, microstructure, and applications

 

Introduction

When engineers search for cast iron grades, SG iron properties, or the difference between SG iron and cast iron, they are usually looking for one thing — the right material for their application.

As a foundry manufacturing SG iron and cast iron castings, Shanthala Spherocast Private Limited frequently receives queries such as:

Which is better, SG iron or cast iron?

What is the difference between FG 260 and SG 500/7?

Can SG iron replace cast steel?

What heat treatment is required for SG iron?

 

What is cast iron?

 

Cast iron is an iron–carbon alloy containing 2.5% to 4.0% carbon, where carbon is present mainly in the form of graphite flakes.

 

These graphite flakes provide:

Excellent castability

Good machinability

High vibration damping

Lower tensile strength compared to SG iron

 

Cast iron is widely used where compressive strength, rigidity, and cost efficiency are more important than ductility.

 

What is SG iron (ductile iron)?

 

SG iron, also known as ductile iron or nodular iron, is a modified form of cast iron in which graphite is present as spheroidal graphite instead of flakes.

 

This spheroidal graphite structure is achieved by adding magnesium or cerium during molten metal treatment.

 

As a result, SG iron offers:

High tensile strength

Excellent ductility

Better fatigue resistance

Higher impact strength

Properties approaching cast steel

 

 

Key difference between cast iron and SG iron

 

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Grey cast iron grades and properties (IS 210)

 

FG 200 – 200 MPa

FG 220 – 220 MPa

FG 260 – 260 MPa

FG 300 – 300 MPa

 

Typical applications include engine blocks, flywheels, gearbox housings, machine tool beds, and brake components.

 

SG iron grades and properties (IS 1865 / ASTM A536)

 

SG 400/15 – 400 MPa, 15% elongation

SG 450/10 – 450 MPa, 10% elongation

SG 500/7 – 500 MPa, 7% elongation

SG 600/3 – 600 MPa, 3% elongation

SG 700/2 – 700 MPa, 2% elongation

 

Microstructure comparison – why it matters

 

Cast iron microstructure:

Flake graphite acts as stress concentrators

Crack propagation is easier

Lower fatigue and impact resistance

 

SG iron microstructure:

Spheroidal graphite arrests crack propagation

Uniform load distribution

Higher strength and ductility

 

Heat treatment requirements

 

Cast iron heat treatment:

Stress relieving

Annealing for machinability

Normalizing for strength improvement

 

SG iron heat treatment:

Annealing for improved ductility

Normalizing for increased strength

Austempering to produce ADI

Stress relieving for dimensional stability

 

ADI grades can achieve tensile strengths up to 1400 MPa.

 

Mechanical properties comparison

 

FG 260 vs SG 500/7:

Tensile strength: ~260 MPa vs ~500 MPa

Elongation: <1% vs 7%

Impact strength: Low vs medium to high

Fatigue resistance: Low vs high

Pressure tightness: Poor vs excellent

 

Typical applications – where to use what

 

Use cast iron when:

Vibration damping is critical

Loads are static

Cost sensitivity is high

Complex shapes are required

 

Use SG iron when:

Strength and ductility are required

Components are dynamically loaded

Pressure containment is needed

Weight reduction is important

 

Can SG iron replace cast steel?

 

Yes. In many applications SG iron provides comparable strength, better castability, lower cost, and superior machinability.

 

High chrome cast iron – wear resistant alloy cast irons

 

In addition to grey and SG iron, Shanthala Spherocast Private Limited manufactures high chrome cast iron castings for severe abrasion environments.

 

High chrome grades offered:

28% chromium

30% chromium

35% chromium

 

These alloys provide:

Exceptional abrasion resistance

High hardness (550–700 BHN)

Martensitic matrix with chromium carbides

 

Typical applications include crushers, mill liners, chutes, hoppers, and pump impellers.

 

Why customers choose Shanthala Spherocast Private Limited

 

Controlled chemical composition

Verified microstructure and nodularity

Mechanical property validation

Green sand and no-bake molding capability

Support in grade selection and casting development

 

Final thoughts

 

Choosing between cast iron, SG iron, and high chrome cast iron is not about which material is better, but which material is right for the application.

 

Understanding grades, microstructure, and mechanical behavior ensures longer component life, reduced failures, and optimized cost.

 

For technical support, grade selection, or casting development, contact Shanthala Spherocast Private Limited.