In high‑temperature service, the chemical composition of heat‑resistant cast steel directly determines its strength, oxidation resistance, and longevity. Among all the alloying elements, carbon-though the most basic-plays a dual‑role: its content level exerts a decisive influence on material performance. However, contrary to what many newcomers might assume, the carbon content of heat‑resistant cast steel is not a fixed value; rather, it is precisely controlled within a relatively wide range, tailored to specific grades, service conditions, and performance requirements.
Generally speaking, the carbon content of commonly used heat‑resistant cast steels falls mainly in the range of 0.20% to 0.75%. Different grades under various standards have their own optimal carbon windows.
1. The Dual Role of Carbon: Strengthening Agent and Potential Risk Source
To understand why carbon content must be "finely tuned," one must first recognise the two‑fold nature of carbon in heat‑resistant cast steel:
Positive aspect – the foundation of high‑temperature strength
Carbon is one of the most effective solid‑solution strengthening elements in steel. In heat‑resistant steels, part of the carbon combines with strong carbide‑forming elements such as chromium, molybdenum, and tungsten to precipitate stable alloy carbides dispersed throughout the matrix. These hard particles effectively pin grain boundaries and dislocations, significantly enhancing the material's creep resistance and tensile strength at elevated temperatures. In essence, sufficient carbon is the key to preventing plastic deformation under high‑temperature loads.
Negative aspect – risks to toughness and processability
When carbon content becomes too high, its detrimental effects emerge. First, excessive carbides may precipitate continuously along grain boundaries, forming a brittle network that severely impairs impact toughness and thermal‑fatigue resistance, increasing the risk of cracking under rapid heating and cooling cycles. Second, high carbon content significantly degrades weldability, making the heat‑affected zone more prone to hardening and cold cracking. Moreover, excessive carbon can tie up large amounts of chromium in carbides, reducing the dissolved chromium in the matrix and potentially compromising oxidation resistance.
Therefore, for every well‑established heat‑resistant cast steel grade, the carbon content represents a carefully optimised balance, designed to achieve the best combination of strength, toughness, oxidation resistance, and processability for specific service conditions.
2. Common Heat‑Resistant Cast Steel Grades and Their Carbon Ranges
To illustrate the design rationale behind carbon content more intuitively, here are the carbon ranges of several typical grades from the Chinese national standard Heat‑Resistant Steel Castings (GB/T 8492):
ZG40Cr9Si2 (0.35% – 0.50% C)
This is a commonly used low‑carbon heat‑resistant steel, with a service temperature up to about 800 °C. Its moderate carbon content ensures adequate high‑temperature strength and good economy, making it suitable for moderately loaded heat‑resistant components.
ZG30Cr18Mn12Si2N (0.26% – 0.36% C)
This grade incorporates nitrogen for partial strengthening, allowing the carbon content to be kept relatively low. This approach secures high‑temperature strength while improving toughness and weldability, making it suitable for applications with both elevated temperatures and some impact loading.
ZG40Cr25Ni20 (0.35% – 0.45% C)
This is a widely used austenitic heat‑resistant steel, often referred to as "25‑20" type stainless steel. Its carbon content is set at a medium level to achieve good high‑temperature creep‑rupture strength through carbide strengthening, while maintaining excellent oxidation and corrosion resistance. It is extensively used for furnace hearth plates, radiant tubes, and other critical parts.
ZG45Ni35Cr26 (0.35% – 0.75% C)
With very high nickel and chromium contents, this grade belongs to the high‑alloy austenitic heat‑resistant steels. Its broad carbon range allows adjustment according to specific performance needs: the higher end is chosen for applications demanding maximum high‑temperature strength, while the lower end favours toughness.
ZG30Ni35Cr15 (0.20% – 0.35% C)
The high nickel content gives this material outstanding thermal‑fatigue resistance, while the lower carbon level ensures excellent toughness and resistance to carburisation, making it ideal for environments with severe temperature fluctuations.
3. Carbon Content in International Standards
In addition to the Chinese standard, other international standards are frequently encountered in global trade or imported equipment:
ASTM standards (USA)
For example, the classic HK40 (ASTM A297) has a carbon content of 0.35% – 0.45%, similar to that of ZG40Cr25Ni20, and is widely used in high‑temperature, high‑pressure petrochemical furnace tubes. The HP series (e.g., HP‑Nb) typically has slightly higher carbon, in the range of 0.35% – 0.50%, and is further strengthened by additions of niobium, titanium, or other elements.
JIS standards (Japan)
For instance, SCH13 (equivalent to ZG40Cr25Ni20) also has carbon in the range of 0.20% – 0.50%, while ferritic grades like SCH2 have lower carbon, usually below 0.20%.
4. Selection Implications: Tailoring Carbon Content to Service Conditions
In practical material selection, carbon content is a crucial screening criterion. The following guidelines can help with preliminary decisions:
For applications dominated by high‑temperature strength and creep resistance (e.g., load‑bearing parts at high temperatures):
Prefer grades with medium‑to‑high carbon content (e.g., 0.35% – 0.45%) to ensure sufficient carbide strengthening phases.
For applications involving thermal cycling, thermal shock, or alternating loads (e.g., furnace rails, fixtures):
Choose grades with lower carbon content (e.g., ≤ 0.35%) to avoid brittle fracture associated with higher carbon levels.
When welding is required or when complex castings are involved:
Carbon content must be strictly controlled; generally, grades with carbon below 0.35% are preferred to ensure weld joint quality and casting crack resistance.
5. Concluding Remarks
In summary, the carbon content of heat‑resistant cast steel is by no means an arbitrary figure, but a precisely engineered core process parameter. It is far from being simply "the more the better" or "the less the better"; rather, it represents a delicate balance among strength, toughness, oxidation resistance, and manufacturability. Understanding this balance is the essential first step in the scientific selection of heat‑resistant cast steel materials for high‑temperature applications.
