Compare white and nodular cast irons with respect to (a) composition and heat treatment (b) microstructure (c) mechanical characteristics.

Cast irons are ferrous alloys that are highly versatile due to their varying composition which influences their properties significantly. Starting with white cast iron, which is defined by its unmistakable hard and highly wear-resistant nature, its carbon content ranges from 1.8 to 3.6%. This is combined with silicon, usually around 0.5 to 1.2%, while the rest is primarily iron. The unique structure of white cast iron comes from its cooling process during manufacturing, which leads to carbon being trapped in the form of cementite.

In contrast, nodular or ductile cast iron contains slightly higher carbon content, typically between 3.5 to 4%. The silicon content is also higher at 2 to 3%, and it has trace amounts of magnesium or cerium, which serve as a nodularizing agent. This combination along with controlled cooling creates a microstructure containing spheroidal graphite nodules, which are responsible for the material's improved ductility and toughness. Understanding these compositional differences is essential as they lay the groundwork for the distinct mechanical properties and applications of each cast iron type.

Heat treatment plays a pivotal role in achieving the desired properties in both white and nodular cast irons. For white cast iron, the process usually involves austenitizing the material at temperatures ranging from 800°C to 950°C. This step is followed by a rapid quenching in a medium like water or oil, which traps the carbon in the form of hard cementite.

On the other hand, nodular cast iron's heat treatment process requires austenitization as well, but typically at slightly higher temperatures of 900°C to 950°C. The key difference lies in the controlled cooling phase instead of rapid quenching. This deliberate process enables the formation of the characteristic graphite nodules, thus providing nodular cast iron with its notable toughness and malleability. The specific heat treatment not only refines the grain structure but also enhances the material's response to further working and its longitudinal performance in practical applications.

Diving deep into the microstructure of cast irons allows us to understand why they behave so differently. White cast iron presents a microstructure that's scattered with hard cementite alongside a pearlite matrix. This assembly leads to a material that's highly resistant to wear yet tends to be brittle, curbing its use in applications requiring toughness.

With nodular cast iron, the microstructure is characterized by spheroidal graphite nodules which are embedded in a matrix of pearlite, ferrite, or a combination of both. These nodules act like 'crack-stoppers', significantly enhancing the material's ability to withstand tensile stresses and impacts without fracturing. They provide flexibility and resistance, making nodular cast iron the go-to for major engineering challenges, where its mechanical advantages can be fully utilized.

The mechanical properties of cast irons are a direct consequence of their composition and microstructure. White cast iron demonstrates a high Brinell hardness number, typically between 450-750 HB, which underscores its superb abrasion resistance. This makes it ideal for grinding balls or mill liners where such properties are invaluable. However, its brittleness parallels its strength, leading to a material with poor impact resistance and low ductility.

Conversely, nodular cast iron exhibits a higher range of tensile strength, about 400-600 MPa, and an elongation percentage that ranges between 6-12%. These properties suggest a material with a balance of strength and flexibility. It's not just strong; it can also bend without breaking—a crucial trait for components in the automotive industry or in the construction of pipelines. Appreciating the mechanical differences between these cast irons helps predict their behavior in real-world applications and drives innovation in material engineering.