When cast iron components operate at high temperatures, they may undergo an irreversible dimensional expansion known as growth. This phenomenon not only reduces the strength of the cast iron but can also damage other components in contact with it. Cast iron growth is most severe in a CO/CO₂ atmosphere, followed by air. Minor growth may also occur in vacuum or hydrogen atmospheres.
Causes of Cast Iron Growth
1. Internal Oxidation
Oxygen infiltrates the interior of the metal, resulting in internal oxidation. Since oxides occupy a larger volume than the original metal, this leads to irreversible volumetric expansion of the casting. The pathways for oxygen ingress include micro-cracks at the metal-graphite boundaries in the oxide film, micropores in the metal, and voids left by the burnout of graphite. Therefore, internal oxidation is the main cause of growth. This effect is particularly intense during repeated heating and cooling, especially when passing through phase transformation temperatures, where phase transformation stress leads to the formation of micro-cracks between graphite and metal, accelerating internal oxidation.
2. Decomposition of Cementite
At high temperatures, cementite decomposes into graphite, increasing the volume.
3. Cyclic Phase Transformations
During heating, graphite dissolves into austenite; upon cooling, it precipitates again—but not in the original location. Each heating–cooling cycle leaves behind voids, causing a net increase in volume. Additionally, phase transformation stress contributes to increased cast iron growth.
4. Carbon Deposition from Atmosphere
Cast iron components working in CO/CO₂ atmospheres experience particularly severe growth. This is mainly due to the reaction 2CO → CO₂ + C, where carbon deposits onto existing graphite, increasing the overall volume. This also leads to micro-cracks in the matrix, facilitating deeper oxygen penetration and thus further oxidation.
Measures to Prevent Growth
Based on the causes of cast iron growth, the following measures can be taken to prevent or reduce it:
1. Add Alloying Elements like Si, Cr, or Al to Improve Oxidation Resistance
Since internal oxidation is the primary cause of growth, improving the oxidation resistance of cast iron is a key measure to prevent growth.
2. Add Alloying Elements to Stabilize Pearlite and Raise Its Decomposition Temperature
Adding small amounts of chromium, manganese, or trace amounts of tin or antimony can stabilize pearlite and raise the decomposition temperature of cementite, thereby improving resistance to growth. However, for originally ferritic cast iron, adding these elements increases pearlite content, which may be counterproductive.
3. Use Alloying Elements to Raise or Lower the Eutectoid Transformation Point
Elements like silicon, chromium, and aluminum can raise the eutectoid transformation point. Conversely, sufficient amounts of nickel and manganese can lower it below room temperature, preventing phase transformations within the operating temperature range and enhancing growth resistance.
The effect of silicon on the eutectoid transformation point of ductile iron
| w(Si)(%) | 3.57 | 3.98 | 4.25 | 4.44 | 4.88 | 5.26 | 5.55 | 5.96 |
| Ac1 /℃ | 840 | 860 | 875 | 878 | 900 | 915 | 930 | 940 |
The effect of chromium on the eutectoid transformation point of cast iron
| w(Cr)(%) | 0.80 | 1.26 | 3.90 | 14.4 | 22.10 | 26.48 | 34.70 |
| w(C)(%) | 2.03 | 2.04 | 2.07 | 2.63 | 2.67 | 2.82 | 2.71 |
| Ac1 /℃ | 810 | 830 | 850 | 890 | 900 | 950 | 960 |
Eutectoid transformation point of Al-Si ductile iron
| w(AI)(%) | 4.24 | 5.54 | 3.98 |
| w(Si)(%) | 3.79 | 3.63 | 3.82 |
| w(Mo)(%) | — | — | 0.27 |
| Ac1/℃ | 930 | 972 | 940 |
| Ar1/℃ | 900 | — | 909 |
4. Reduce Pearlite and Free Cementite Content
Through inoculation treatment, alloying, and heat treatment, the content of pearlite and cementite can be reduced or eliminated, thereby minimizing growth caused by their decomposition.
5. Reduce Graphite Content and Improve Graphite Morphology
Lowering the graphite content and modifying its morphology—such as transforming it into spheroidal (nodular) form—can reduce oxygen infiltration and internal oxidation, thus improving cast iron’s resistance to growth.
Post time: Jul-10-2025