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Cast iron is an iron-carbon alloy with a carbon content greater than 2.11%.

Cast iron is one of the earliest metal materials used by humans. It has the characteristics of simple production methods, low cost, and excellent performance, and is still widely used today. Disadvantages: hard and brittle, difficult to process, can only be cast.

Typical parts: machine tool bed, cylinder body and liner, camshaft, crankshaft.

Categories of cast iron:

1. White cast iron: cast iron in which carbon is mainly present in the form of cementite ($Fe_3C$).

   • Hard and brittle, rarely used as parts, used as raw materials (pig iron).

2. Gray cast iron: cast iron in which carbon is mainly present in the form of graphite (G).

3. Mottled cast iron: cast iron between white cast iron and gray cast iron.

Graphitization of cast iron#

$Fe-C$ and $Fe-Fe_3C$ dual phase diagram#

Graphitization process of cast iron#

Graphitization process during cooling (crystallization)#

1. Graphite precipitation from liquid phase

   • $L$ → $G_I$
   • $L$ → $\gamma + G$ (eutectic reaction)

2. Graphite precipitation from austenite

   • $\gamma$ → $G_{II}$

3. Graphite formation from eutectoid reaction

   • $\gamma$ → $\gamma + G$

Graphitization process during heating#

$Fe_3C$ → $3Fe + C$

Two stages of graphitization of cast iron:

• First stage of graphitization: graphitization process above the P'S'K' line.
• Second stage of graphitization: graphitization process below the P'S'K' line.

Formation conditions of cast iron structure#

Depending on the degree of graphitization in the two stages, cast iron has different structures.

Common grades and performance characteristics of cast iron#

In general, white cast iron and mottled cast iron are not widely used, while gray cast iron is more commonly used.

Categories of gray cast iron#

According to the form of graphite, gray cast iron can be divided into four categories.

• Gray cast iron: graphite is in the form of flakes.

• Ductile iron: graphite is in the form of spheres.

• Compacted graphite iron: graphite is in the form of worm-like structures.

• Malleable iron: graphite is in the form of clusters.

The state of graphite is mainly influenced by the chemical composition of cast iron and the process.

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Structure of gray cast iron#

Structural characteristics: different forms of graphite are distributed on the steel matrix.

• F+G steel matrix (ferrite) and graphite

• F+P+G steel matrix (ferrite+pearlite) and graphite

• P+G steel matrix (pearlite) and graphite

Performance of gray cast iron#

Mainly depends on the form, size, and quantity of graphite.

Gray cast iron#

The cheapest and most widely used, accounting for more than 80%.

Grades of gray cast iron#

HT×××. "HT" stands for "gray iron", and "×××" represents the minimum tensile strength value.

• Example: H200
  • HT is the pinyin prefix for "gray iron"
  • 200 indicates that the minimum tensile strength of the gray cast iron is not less than 200 MPa

Performance characteristics of gray cast iron#

1. Low mechanical properties

   • The flake graphite has a large dividing effect on the steel matrix, resulting in stress concentration at the tips.

2. Good wear resistance and vibration damping

   • The presence of graphite facilitates lubrication, oil storage, and absorption of vibration energy, and has better vibration damping than carbon steel.

3. Good processability

   • Low melting point, good liquid flowability, easy casting, especially for complex castings; easy chip breaking during cutting (better cutting performance than steel).

Inoculation treatment#

Adding inoculants (modifier): ferrosilicon alloy, ferrocalcium alloy, to increase the number of nucleation cores and make the graphite flakes smaller and more uniform.

Applications#

Machine tool beds, bases, engine cylinder bodies, etc.

Ductile iron#

Changing the form of graphite to improve the mechanical properties. Adding nodulizers and inoculants before casting, and then undergoing nodularization and inoculation treatment.

Nodulizers: magnesium, rare earth, rare earth magnesium.

Grades of ductile iron#

QT×××-××. "QT" stands for "ductile iron", "×××" represents the minimum tensile strength value, and "××" represents the minimum elongation.

• Example: QT500-05
  • QT is the pinyin prefix for "ductile iron"
  • 500 indicates that the minimum tensile strength of the ductile iron is not less than 500 MPa
  • 05 indicates that the minimum elongation of the ductile iron is not less than 5%

Performance characteristics of ductile iron (compared to gray cast iron)#

1. High mechanical properties

   • Ductile iron has high tensile strength and bending fatigue limit, good plasticity and toughness.
   • The dividing effect of spherical graphite on the steel matrix is minimized, the continuity of the steel matrix is good, and stress concentration is reduced.

2. Poor vibration damping

   • Vibration damping is not as good as gray cast iron.

3. Poor castability

Applications#

Under certain conditions, it can replace cast steel and forged steel, and is used for parts with complex stress, large loads, and high wear resistance requirements, such as crankshafts, camshafts, valve bodies, and automotive rear axle housings.

Compacted graphite iron#

Obtained by treating the molten iron with modifiers and inoculants.

Modifiers (compacting agents): rare earth silicon iron magnesium alloy, rare earth silicon iron magnesium alloy, rare earth calcium iron alloy, etc.

Grades of compacted graphite iron#

RuT×××. "RuT" stands for "compacted iron", "×××" represents the minimum tensile strength value.

Performance characteristics of compacted graphite iron#

1. Mechanical properties are between gray cast iron and ductile iron

   • Higher strength and toughness than gray cast iron, but not as good as ductile iron; better wear resistance; better vibration damping than ductile iron.
   • The head of worm-like graphite is blunt and round, which significantly reduces the dividing effect on the matrix compared to gray cast iron.

2. Processability is between ductile iron and gray cast iron

   • Better castability than ductile iron, similar to gray cast iron.

3. Thermal conductivity is close to gray cast iron

   • High temperature strength and thermal fatigue performance are much better than gray cast iron.
   • Suitable for manufacturing parts that withstand alternating thermal loads.

Malleable iron#

Malleable iron is obtained by:

1. Obtaining white cast iron;

2. Graphitizing white cast iron.

   • Malleable iron is obtained by graphitizing white cast iron during heating.

   • Gray cast iron, ductile iron, and compacted graphite iron are obtained by graphite precipitation during cooling.

Grades of malleable iron#

1. KTH×××-××. "KTH" stands for "black malleable iron", which refers to blackheart malleable iron with ferrite as the matrix.
2. KTZ×××-××. "KTZ" stands for "pearlitic malleable iron", which refers to pearlite malleable iron with pearlite as the matrix.

Performance characteristics of malleable iron#

The performance of malleable iron is between gray cast iron and ductile iron, with better corrosion resistance, but lower production efficiency.

Heat treatment of cast iron#

Purpose of cast iron heat treatment:

1. Change the structure of the steel matrix to improve the performance of cast iron;

2. Eliminate casting stress.

Special note:

1. Heat treatment can only change the structure of the steel matrix and cannot change the form and distribution of graphite.

   • Flake or spherical graphite will not change into other shapes through heat treatment.

   • The size of graphite will not increase or decrease through heat treatment.

   • The distribution of graphite will not change through heat treatment.

2. Gray cast iron is not suitable for strengthening heat treatment (such as quenching).

   • Gray cast iron has a large dividing effect on the matrix due to the flake graphite, and even with strengthening heat treatment, significant effects are difficult to achieve.

3. Ductile iron is suitable for various heat treatments similar to steel.

   • Ductile iron has spherical graphite, which has a small dividing effect on the matrix, so heat treatment can significantly improve its mechanical properties.

Aging treatment#

Purpose: release casting stress.

Processes:

• Artificial aging: heating the casting to 500-560°C, holding it for a certain period, and then air cooling.

• Natural aging: placing the casting outdoors for 6-18 months to allow the stress to be naturally released.

Artificial aging is the most commonly used method in current production.

Elimination of white spot annealing#

Purpose: eliminate white spot structure.

During the casting process, white spot structure may appear on the surface or thin walls of the casting due to rapid cooling, which has high hardness and makes it difficult for cutting processing. It must be eliminated.

Process:

• Heating temperature: 880-900°C.
• Holding time: 1-2 hours.
• Cooling method: slow cooling to 400-500°C and then air cooling after holding.

Surface heat treatment#

Purpose: improve the surface hardness, wear resistance, and corrosion resistance of castings.

Processes:

• Induction heating surface quenching

• Laser heating surface quenching

• Nitriding, metal infiltration

Heat treatment of ductile iron#

Annealing#

Purpose: improve the toughness of ductile iron castings.

Process:

1. Heating to 880-900°C, holding, and then air cooling to 600°C.
   - Suitable for castings with white spot structure.
   - Structure after annealing: $F+G_{spherical}$

2. Heating to 700-760°C, holding, and then air cooling to 600°C.

   • Suitable for castings with $F+P+G_{spherical}$ structure.
   • Structure after annealing: $F+G_{spherical}$

Normalizing#

Purpose: transform the matrix into fine pearlite structure to improve strength, hardness, and wear resistance.

Process: heating to 850-900°C, holding, and then air cooling.

• Suitable for castings with $F+P+G_{spherical}$ structure.
• Structure after normalizing: $P+G_{spherical}$

Quenching and tempering#

Purpose: improve the mechanical properties of ductile iron.

1. Quenching (860-900°C) → Low-temperature tempering (250-350°C)
   - Structure: $M_{tempered}+A'+G_{spherical}$. Suitable for manufacturing bearings.
2. Quenching (860-900°C) → Medium-temperature tempering (500-600°C)
   - Structure: $S_{tempered}+G_{spherical}$. Suitable for manufacturing shaft parts.

Austempering#

Purpose: obtain $B_{lower}$ to give ductile iron good comprehensive mechanical properties.

Process: heating to 830-870°C, holding, and then quenching in a molten salt at 280-350°C.

Structure: $B_{lower}+A'+G_{spherical}$

This article is also updated on xLog by Mix Space
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