Carbon Steel and Alloy Steel: Properties, Types, and Applications

Carbon Steel and Alloy Steel: Properties, Types, and Applications

Steel is one of the most widely used materials in engineering, construction, and power plants. Its properties vary depending on the percentage of carbon and the presence of alloying elements. Understanding the difference between carbon steel and alloy steel is essential for industries like boiler manufacturing, pipelines, mechanical engineering, and energy systems.

🔹 Carbon Steel

Carbon steel is classified based on the percentage of carbon present in the composition.  

- Low Carbon Steel (Mild Steel) – Contains less than 0.3% carbon. It is ductile, easy to weld, and widely used in pipes, tubes, and fittings.  

- Medium Carbon Steel – Contains 0.3% to 0.45% carbon. It offers a balance of strength and ductility, making it suitable for mechanical parts.  

- High Carbon Steel – Contains 0.45% to 0.75% carbon. It is harder and stronger but less ductile, often used in applications requiring wear resistance.  

Carbon steel also contains:  

- Less than 1.65% manganese  

- Less than 0.6% copper  

- Small amounts of silicon, sulphur, and phosphorus  

- According to the American Iron and Steel Institute (AISI), steel is considered carbon steel when no minimum content of alloying elements like chromium, nickel, molybdenum, titanium, or vanadium is specified.  

Key Properties of Carbon Steel

- Increasing carbon content makes steel harder and stronger.  

- Higher carbon reduces ductility and weldability.  

- Widely used in boiler tubes, pipelines, and structural applications.  

🔹 Alloy Steel

Alloy steel is defined as steel containing:  

- More than 1.65% manganese  

- More than 0.6% copper  

- Or a guaranteed minimum amount of other metals like chromium, molybdenum, nickel, vanadium, tungsten, silicon, titanium, or niobium.  

Alloy steels are designed to provide superior thermal, mechanical, and chemical properties compared to carbon steel. They are especially important in boiler superheaters, where high temperatures and erosion from dust particles demand stronger materials.  

Common Alloying Elements and Their Benefits

- Chromium – Improves wear resistance, corrosion resistance, and hardness. Forms a protective passive film against oxidation.  

- Molybdenum – Enhances strength at high temperatures, improves heat resistance, and reduces brittleness.  

- Manganese – Improves ductility and counteracts harmful effects of sulphur.  

- Nickel – Increases toughness, lowers eutectoid temperature, and strengthens steel by refining pearlite.  

- Vanadium – Acts as an oxide scavenger, forms carbides, and improves mechanical properties after heat treatment.  

- Tungsten – Refines grain size, enhances heat resistance, and reduces decarburisation.  

- Silicon – Provides oxidation resistance and acts as a general-purpose deoxidiser.  

- Titanium & Niobium – Prevent intergranular corrosion and stabilize steel structure.  

📌 Applications in Boilers and Power Plants

- Carbon Steel – Used in economizers, evaporators, and low-temperature tubes (up to 420 °C).  

- Alloy Steel – Preferred for high-temperature superheater tubes and pipes, where strength and resistance to oxidation are critical.