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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, with 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 the liquid phase

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

2. Graphite precipitation from the austenite

   • $\gamma$ → $G_{II}$

3. Graphite generation from eutectic reaction

   • $\gamma$ → $\gamma + G$

Graphitization process during heating#

$Fe_3C$ → $3Fe + C$

Two stages of graphitization of cast iron:

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

Formation conditions of cast iron structure#

According to the degree of graphitization in the two stages, cast iron has different structures.

Common grades and performance characteristics of cast iron#

Generally speaking, 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.

• Austempered ductile 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" represents "gray iron", and "×××" represents the minimum tensile strength value.

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

Performance characteristics of gray cast iron#

1. Low mechanical properties

   • The flake graphite has a significant effect on the division of the steel matrix, resulting in stress concentration at the tips.

2. Good wear resistance and vibration damping

   • The presence of graphite is beneficial for lubrication, oil storage, and absorption of vibration energy, and the vibration damping is better than that of carbon steel.

3. Good processability

   • Low melting point, good fluidity in the liquid state, easy to cast, especially for complex castings; easy to break chips during cutting (better cutting performance than steel).

Inoculation treatment#

Adding inoculants (modifier): ferrosilicon alloy, ferrosilicon calcium alloy, with more crystallization nuclei and smaller and more uniform graphite flake size.

Application#

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

Ductile iron#

Change the form of graphite to improve the mechanical properties. After adding spheroidizing agents and inoculants before casting, spheroidizing treatment and inoculation treatment are performed.

Spheroidizing agents: magnesium, rare earth, rare earth magnesium.

Grades of ductile iron#

QT×××-××. "QT" represents "nodular iron", "×××" represents the minimum tensile strength value, and "××" represents the minimum elongation.

• Example: QT500-05
  • QT is the pinyin prefix for "nodular iron"
  • 500 represents that the minimum tensile strength of the ductile iron is not less than 500MPa
  • 05 represents 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 division effect of spheroidal graphite on the steel matrix is minimized, the continuity of the steel matrix is good, and the stress concentration is weakened.

2. Poor vibration damping

   • The vibration damping is not as good as that of gray cast iron.

3. Poor castability

Application#

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

Compacted graphite iron#

Obtained by heat treatment and inoculation treatment of molten iron.

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" represents "compacted iron", and "×××" 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 absorption than ductile iron.
   • The head of the worm-like graphite is blunt and round, which significantly reduces the division 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 similar 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" represents "black malleable iron", which is called blackheart malleable iron and has a ferrite matrix.
2. KTZ×××-××. "KTZ" represents "pearlitic malleable iron", which is called pearlitic malleable iron and has a pearlite 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 heat treatment of cast iron:

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, not 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 flake graphite, which has a significant effect on the division of the matrix, 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 effect on the division of the matrix, so heat treatment can significantly improve its mechanical properties.

Aging treatment#

Purpose: Release casting stress.

Process:

• Artificial aging: heat the casting to 500-560°C, hold for a certain period, and then cool in the furnace.

• Natural aging: place the casting outdoors for 6-18 months to allow the stress to naturally release.

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

Annealing to eliminate white iron#

Purpose: Eliminate white iron structure.

The surface or thin-walled parts of the casting will have a white iron structure due to too fast cooling during the casting process, which has high hardness and brings difficulties to machining and 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 the casting.

Process:

• 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, furnace cooling to 600°C and then air cooling.
   - Suitable castings: castings with white iron structure.
   - Structure after annealing: $F+G_{spherical}$

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

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

Normalizing#

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

Process: Heating to 850-900°C, furnace cooling.

• Suitable castings: 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: $M_{tempered}+A'+G_{spherical}$. Can be used for manufacturing bearings.
2. Quenching (860-900°C) → Medium-temperature tempering (500-600°C)
   - Structure: $S_{tempered}+G_{spherical}$. Can be used for manufacturing shaft parts.

Austempering#

Purpose: Obtain $B_{lower}$ to make ductile iron have good comprehensive mechanical properties.

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

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

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