Microstructure

The microstructure of a material determines its properties. The understanding and modification of microstructure is, in many respects, the foundation of materials analysis and engineering. Like all matter, metals are composed of atoms. These atoms combine in small clusters which are called crystals. Groups of crystals combine to form grains. The size, shape, orientation and combination of the grains with other compositional elements in metals make up their microstructure.

These factors also govern their physical properties such as tensile strength, fatigue strength, hardness, brittleness, corrosion resistance, machinability, weldability and many other critical characteristics. These characteristics can be modified and selectively optimized by a variety of processes including heat treating, alloying, cold working and others.

A classic example of microstructure modification to dramatically enhance a materials properties is the conversion of gray iron to ductile iron. Gray iron is the most common form of cast iron. It is inexpensive, reasonably strong and hard. But it is also very brittle. If bent or stretched, it easily breaks. This lack of “give”, defined as low ductility and quantified by the amount a material will elongate or stretch before breaking, required the substitution of more expensive steel in components subjected to bending or stretching (tensile) loads in service.

microstructure

microstructure

The microstructure of gray iron (above left) consists of laminations of carbide and iron called Pearlite (A), relatively pure iron called Ferrite (B), and a high level of carbon in the form of graphite flakes (C).  In the late 1940’s a new form of cast iron was developed called Ductile Iron. A measured amount of magnesium is added to the molten iron minutes before it is poured. This produces the microstructure shown above at the right, which also contains Pearlite (D) and Ferrite (E), but with the graphite converted from flakes to spheres (F).

This microstructural change dramatically improves Ductile Irons ability to accept bending and tensile loads compared to Gray Iron. While Gray Iron will elongate only 0.06% before breaking, Ductile Iron will elongate 18%.