ASTM A536 Grade 120-90-02 Ductile Iron: Properties, Applications, and Specifications

ASTM A536 Grade 120-90-02 ductile iron is a high-strength, wear-resistant material achieved through quench and temper heat treatment. With a tensile strength of at least 827 MPa and yield strength of 621 MPa, it is ideal for applications requiring exceptional mechanical performance. Its microstructure, characterized by spheroidal graphite, provides a balance of strength and ductility, making it suitable for demanding industrial applications.

1. Chemical Composition and Microstructure

ASTM A536 Grade 120-90-02 ductile iron exhibits a spheroidal graphite microstructure, which enhances its mechanical properties compared to traditional gray iron. The typical chemical composition includes:

• Carbon (C): 3.65–3.90%

• Silicon (Si): 1.70–1.90%

• Manganese (Mn): 0.30–0.50%

• Phosphorus (P): <0.06%

• Sulfur (S): <0.03%

• Magnesium (Mg): 0.035–0.050%

The presence of magnesium is crucial for the formation of spheroidal graphite, which enhances ductility and toughness. This composition ensures the material meets the stringent mechanical requirements specified for Grade 120-90-02.

2. Mechanical and Physical Properties

Grade 120-90-02 ductile iron exhibits exceptional mechanical properties, making it suitable for high-stress applications. Key properties include:

• Tensile Strength: ≥ 827 MPa (120,000 psi)

• Yield Strength: ≥ 621 MPa (90,000 psi)

• Elongation: ≥ 2%

• Hardness: 280–360 HB (Brinell Hardness)

• Density: 6.64–7.20 g/cm³

• Thermal Conductivity: 0.074 cal/cm·sec·°C

• Specific Heat: 0.12 Btu/lb·°F

• Melting Temperature: ~2120°F

These properties are achieved through a quench and temper heat treatment process, which enhances strength and wear resistance while maintaining adequate ductility.

3. Heat Treatment and Manufacturing Process

The production of Grade 120-90-02 ductile iron involves precise control over the heat treatment process to achieve the desired mechanical properties. The typical steps include:

1. Melting and Alloying: Base iron is melted and alloying elements are added to achieve the desired chemical composition.

2. Inoculation: Magnesium is introduced to promote the formation of spheroidal graphite.

3. Casting: The molten iron is poured into molds to form the desired shapes.

4. Quenching: Castings are rapidly cooled to transform the microstructure into martensite, increasing hardness and strength.

5. Tempering: Quenched castings are reheated to a specific temperature to relieve internal stresses and improve toughness.

This heat treatment sequence ensures the material achieves the high strength and wear resistance characteristic of Grade 120-90-02.

4. Applications in Various Industries

Due to its superior mechanical properties, Grade 120-90-02 ductile iron is utilized in various demanding applications across multiple industries:

• Automotive: Gears, crankshafts, and suspension components.

• Agriculture: Tillage equipment, plowshares, and tractor parts.

• Mining: Crusher components, wear plates, and conveyor parts.

• Construction: Heavy-duty brackets, frames, and structural supports.

• Oil and Gas: Pump housings, valve bodies, and drilling equipment.

Its combination of high strength, wear resistance, and adequate ductility makes it ideal for components subjected to high loads and abrasive conditions.

5. Comparison with Other Ductile Iron Grades

Grade 120-90-02 is one of several ductile iron grades, each with distinct properties tailored to specific applications. A comparison with other common grades is as follows:

Grade 120-90-02 offers the highest strength and wear resistance among these grades, making it suitable for the most demanding applications.

6. Frequently Asked Questions (FAQs)

Q1: What distinguishes Grade 120-90-02 from other ductile iron grades?

A1: Grade 120-90-02 ductile iron is characterized by its high tensile and yield strengths, achieved through a quench and temper heat treatment process. This grade exhibits superior wear resistance and hardness compared to other ductile iron grades, making it suitable for applications involving high stress and abrasive conditions. However, it has lower ductility, which may limit its use in applications requiring significant deformation.

Q2: Can Grade 120-90-02 ductile iron be welded?

A2: Welding Grade 120-90-02 ductile iron is challenging due to its high hardness and potential for cracking. If welding is necessary, preheating the material and using appropriate filler materials designed for cast iron are essential. Post-weld heat treatment may also be required to relieve residual stresses and restore mechanical properties. Consulting with a welding engineer experienced in cast iron is recommended for critical applications.

Q3: How does the heat treatment process affect the properties of Grade 120-90-02?

A3: The quench and temper heat treatment process transforms the microstructure of Grade 120-90-02 ductile iron, enhancing its mechanical properties. Quenching rapidly cools the material, forming a martensitic structure that increases hardness and strength. Tempering then reheats the material to a specific temperature, relieving internal stresses and improving toughness. This process results in a balance of high strength and adequate ductility.

Q4: What are the machining considerations for Grade 120-90-02 ductile iron?

A4: Due to its high hardness, machining Grade 120-90-02 ductile iron requires the use of carbide or ceramic cutting tools. Proper selection of cutting speeds, feeds, and coolant application is crucial to minimize tool wear and achieve desired surface finishes. Pre-machining heat treatment can also be considered to reduce hardness and improve machinability.

Q5: Is Grade 120-90-02 suitable for high-temperature applications?

A5: Grade 120-90-02 ductile iron has a melting temperature of approximately 2120°F (1160°C). While it can withstand elevated temperatures, prolonged exposure to high temperatures may lead to microstructural changes that affect mechanical properties. For applications involving sustained high temperatures, materials specifically designed for thermal stability should be considered.

Q6: How does Grade 120-90-02 compare to steel in terms of performance and cost?

A6: Grade 120-90-02 ductile iron offers comparable strength and wear resistance to certain steel grades but at a lower cost due to its castability and lower material density. However, steel generally provides better ductility and toughness, making it more suitable for applications involving significant deformation or impact loads. The choice between ductile iron and steel depends on the specific requirements of the application, including mechanical performance, cost, and manufacturing considerations.

References:

1. ASTM A536 – Standard Specification for Ductile Iron Castings

2. Ductile Iron – Wikipedia

3. NIOSH Metalworking and Materials Safety

4. Mechanical Properties and Heat Treatment of Ductile Iron | AZoM