Roles of Various Elements in Casting of High Chromium Cast Iron Cr15, Cr20, and Cr26

High chromium cast iron (HCCI) is widely used in industries such as mining, cement, metallurgy, and power generation due to its excellent wear resistance, corrosion resistance, and high-temperature stability. Among the common grades, Cr15, Cr20, and Cr26 are the most representative, with their performance largely determined by the composition and proportion of alloying elements. This article systematically explains the role of each element in the casting process, microstructure formation, and service performance of Cr15, Cr20, and Cr26 high chromium cast iron, providing practical guidance for casting process design and material selection.

1. Carbon (C): The Core Element Determining Wear Resistance

Carbon is the most critical element in high chromium cast iron, with a general content range of 2.0%–3.3% for Cr15, Cr20, and Cr26 (basically consistent among the three grades). Its core role is to form hard carbides, which are the primary source of the material’s wear resistance.

In Cr15 high chromium cast iron, the carbon content is usually 2.4%–3.0%, resulting in a carbide volume fraction of approximately 25%–30%. For Cr20, the carbon content ranges from 2.3%–3.1%, with carbides accounting for 30%–35%. Cr26, with a carbon content of 2.2%–3.0%, has the highest carbide volume fraction (35%–40%) due to its higher chromium content.

The influence law of carbon is clear: as the carbon content increases, the number of carbides increases, which significantly improves the hardness and wear resistance of the material. However, when the carbon content exceeds 3.3%, it will lead to the formation of network or coarse carbides, which sharply reduces the toughness of the cast iron and makes it prone to brittle fracture. It is crucial to match carbon with chromium: the Cr/C ratio must be greater than 4 (especially for Cr26, the Cr/C ratio should be greater than 7) to ensure that the main carbide type is M₇C₃ (instead of brittle M₃C), thus balancing wear resistance and toughness.

2. Chromium (Cr): The Key Element Differentiating Grades

Chromium is the defining element of high chromium cast iron, and its content directly distinguishes Cr15, Cr20, and Cr26 grades. Its core functions include determining carbide type and quantity, improving corrosion resistance, and enhancing high-temperature stability.

Cr15 high chromium cast iron contains 11%–18% chromium. It mainly forms M₇C₃ carbides with a small amount of M₂₃C₆, offering moderate wear and corrosion resistance but better toughness compared to higher chromium grades. Cr20 (18%–23% chromium) has a higher and more stable proportion of M₇C₃ carbides, resulting in significantly better wear and corrosion resistance than Cr15, achieving an optimal balance between performance and cost.

Cr26 high chromium cast iron (23%–30% chromium) has the highest volume fraction of M₇C₃ carbides (≥35%), making it superior in high-stress wear resistance, corrosion resistance, and high-temperature oxidation resistance. However, when chromium content exceeds 25%, it is prone to the formation of brittle phases such as M₆C and M₂₃C₆, which reduces toughness and increases casting difficulty.

A common feature of chromium in all three grades is that it dissolves in the matrix to form a Cr₂O₃ passive film, which effectively improves the material’s corrosion resistance and oxidation resistance.

3. Silicon (Si): A Auxiliary Element for Deoxidation and Refinement

Silicon is added as an auxiliary element in high chromium cast iron, with a strictly controlled content of ≤1.2% for all three grades (Cr15, Cr20, Cr26). Its main roles are as follows:

• Deoxidation: Silicon can effectively reduce the oxidation loss of chromium, manganese, and other alloying elements during the casting process, ensuring the stability of the alloy composition.
• Carbide refinement: It reduces the solid-liquid two-phase region during solidification, making carbides finer and more dispersed, thereby improving the uniformity of the microstructure.
• Solid solution strengthening: Silicon dissolves in the matrix to improve the strength and elastic limit of the material.

It should be noted that when the silicon content exceeds 2%, it is prone to graphite precipitation, which will significantly reduce the hardness and wear resistance of the cast iron. Therefore, strict control of silicon content (≤1.2%) is essential in casting.

4. Manganese (Mn): Improving Hardenability and Microstructure Uniformity

Manganese is usually added in the range of 0.5%–1.0% (maximum ≤2.0%) for Cr15, Cr20, and Cr26 high chromium cast iron. Its main functions include:

• Stabilizing austenite and lowering the Ms point, which reduces the formation of pearlite and improves the hardenability of the material.
• Solid solution strengthening and dendrite refinement, making the microstructure more uniform and improving the overall performance.
• Promoting the precipitation of secondary carbides during heat treatment, further improving the hardness and wear resistance of the material.

Excessive manganese (more than 1.5%) will lead to an excessive amount of retained austenite, resulting in unstable hardness and dimensional changes of the cast parts. Therefore, reasonable control of manganese content is crucial.

5. Molybdenum (Mo): Enhancing Hardenability and Toughness

Molybdenum is an important alloying element for strengthening and toughening high chromium cast iron, with a common content range of 0.5%–1.5% for Cr15 and Cr20, and 1.0%–2.0% for Cr26. Its core roles are:

• Strongly improving hardenability, ensuring that even large-section cast parts can obtain a full-section martensite or bainite structure, avoiding the formation of soft pearlite.
• Refining grains and inhibiting the formation of network carbides, thereby improving the toughness and crack resistance of the material.
• Achieving solid solution and precipitation strengthening, increasing the matrix hardness to HRC 50–60, which can effectively support carbides and reduce carbide spalling during service.
• Improving high-temperature stability, enhancing resistance to temper softening and red hardness (at 500–600℃), making the material suitable for high-temperature working conditions.

For Cr26 high chromium cast iron, the higher molybdenum content (1.0%–2.0%) is used to compensate for the decreased hardenability and toughness caused by the high chromium content.

6. Nickel (Ni): Stabilizing Austenite and Improving Toughness

Nickel is usually added in the range of 0.5%–1.5% for Cr15 and Cr20, and 0.8%–1.8% for Cr26. Its main functions are:

• Acting as a strong austenite stabilizer, expanding the γ-phase region, improving hardenability, and inhibiting the formation of pearlite.
• Improving low-temperature toughness and reducing the cold brittle transition temperature, making the material suitable for low-temperature working environments.
• Synergizing with molybdenum: molybdenum improves hardenability, while nickel stabilizes austenite, resulting in a uniform structure and high toughness for thick and large cast parts.

Excessive nickel will lead to an excessive amount of retained austenite, resulting in low hardness of the material. Therefore, the nickel content should be controlled within a reasonable range.

7. Copper (Cu): Auxiliary Strengthening and Corrosion Resistance

Copper is an auxiliary alloying element with a content of ≤2.0% in high chromium cast iron. Its main roles are:

• Solid solution strengthening the matrix, improving the strength and hardness of the material.
• Stabilizing austenite and assisting in improving hardenability (weaker than nickel).
• Improving corrosion resistance, especially in dilute acids and atmospheric corrosion environments.
• Slightly improving the machinability of the material.

8. Sulfur (S) and Phosphorus (P): Strictly Controlled Harmful Elements

Sulfur and phosphorus are harmful impurities in high chromium cast iron, and their contents must be strictly controlled: sulfur ≤0.06% and phosphorus ≤0.10% for Cr15, Cr20, and Cr26.

Sulfur forms low-melting inclusions such as MnS, which cause grain boundary embrittlement, hot cracking, and reduced impact toughness. Phosphorus forms brittle compounds such as Fe₃P, which increase low-temperature brittleness and cold cracking tendency during casting. Strict control of sulfur and phosphorus content is essential to ensure the casting reliability and service performance of high chromium cast iron.

9. Comparison of Element Design for Cr15, Cr20, and Cr26

10. Conclusion and Casting Key Points

The performance of Cr15, Cr20, and Cr26 high chromium cast iron is jointly determined by the interaction of various alloying elements. Carbon and chromium are the core elements determining carbide quantity, type, and wear resistance: the higher the chromium content, the better the wear resistance, but the lower the toughness and the higher the casting difficulty. Molybdenum and nickel form a key strengthening-toughening combination: molybdenum improves hardenability and grain refinement, while nickel stabilizes austenite and enhances toughness.

Silicon and manganese should be controlled at low levels to ensure deoxidation and strengthening, while avoiding graphite precipitation and excessive retained austenite. Sulfur and phosphorus must be strictly controlled to prevent hot cracking, cold brittleness, and grain boundary embrittlement. In terms of material selection: Cr15 is cost-effective with good toughness, suitable for general wear parts; Cr20 achieves the best balance between wear resistance and toughness, serving as the mainstream general grade; Cr26 offers extreme wear resistance, corrosion resistance, and high-temperature performance, but at the cost of higher brittleness, casting difficulty, and cost.

By reasonably designing the alloy composition and optimizing the casting process, the performance potential of Cr15, Cr20, and Cr26 high chromium cast iron can be fully exerted, meeting the requirements of different industrial working conditions.