Thinking Small

These are examples of the reliability and performance issues that can arise from defects in a metal’s microstructure. Indeed, there are several microstructures that have proven to be reliable and effective in the past. To grasp metallurgy, one must be able to think on a microscopic scale. Measured in millimeters or less. Shorter than one micron. Occasionally on the atomic level as well.

Understanding microscopic processes and structures conceptually

Metals’ characteristics, performance, and dependability are profoundly impacted by microscopic structures and processes. Understanding the behavior of metals and the impact of mechanical processing and heat treating on metal characteristics is greatly aided by having a mental picture of these structures and processes.

In my opinion, here is where most people stall out. The metallurgical jargon is confusing. When I initially started studying metallurgy, I found it really challenging. When I took more classes about metals and read more about them, though, I began to develop a microscopic perspective and understand the tiny structures and processes that determine metal qualities.

To understand how metallurgy can be used to develop better, cheaper components and swiftly fix metal failures and quality concerns, one needs only a basic understanding of the field.

Structure of grains

The left image depicts the cold-rolled metal’s elongated grains. Because of the orientation of the grains, the metal is strongest in the rolling direction and weakest perpendicular to the metal’s plane. The annealed metal with grains on the right was produced by cold rolling. To get rid of the cold-worked grains, annealing creates and grows new grains within the existing ones.

The annealing temperature and time, as well as the quantity of cold working done beforehand, determine the final grain size. Metals’ strengths, malleabilities, toughnesses, and creep strengths are all influenced by their grain sizes.

The strength is also affected by the microscopic structure (dislocations). That’s a topic for a different article, though.

Phases

One or more phases can be formed in a metal when the components contained in the metal come together. Each phase has a unique arrangement of atoms, consists of a distinct combination of atoms or compounds, and is chemically distinct from any other phase in terms of its exact composition or compositional range. A compound has a fixed number of atoms in a given configuration. A mixture can have a variety of different compositions.

There are two typical configurations of metal atoms.

It is the composition of a metal that determines the phases it is capable of forming. The phases that can occur in a metal are determined by the metal’s chemical make-up and the processing it has undergone.

When many phases may occur in a metal, those phases each exhibit unique characteristics. It is possible to vary a metal’s characteristics by changing the phases present and the relative amounts of those phases. The composition and processing history (mechanical working and thermal history) of a metal determine the types of phases it contains and the amounts of each.

Carbon steel, like the one depicted on the left, contains two distinct phases: the lighter ferrite and the darker cementite (dark-colored plates). The iron in ferrite is combined with a trace quantity of carbon. Cementite is an iron-carbon combination with a 3:1 atomic ratio. As compared to cementite’s hardness, strength, and brittleness, ferrite’s low strength and high ductility shines. The combination of ferrite and cementite results in a hybrid material.

The proportions of ferrite and cementite in a steel determine the material’s characteristics. Steel’s strength and other qualities can be altered by adjusting the cementite plates’ spacing.

Atoms in motion

At high enough temperatures, the atoms in a metal will move around, hopping from one location to the next. Diffusion describes this atomic shuffle.

The microscopic architectures of grains, the phases present, and the amounts of those phases all vary as atoms move around. The content, temperature, and duration of exposure to heat determine the nature of the reactions that take place.

The goal of any heat treatment is to alter the microstructure of the metal so that desired characteristics can be achieved. So, for any given alloy, it is necessary to select and manage the appropriate temperature and duration at temperature in order to achieve the desired microstructure changes.

Not everyone needs to be a metals specialist. It’s challenging enough to become an expert in a single area of engineering. However, non-metallurgists who want to be aware of how metals engineering can be applied to design components and solve problems should have a basic understanding of microscopic structures, their effects on metal characteristics as well as microscopic structures as a result of mechanical and thermal processing. Having this background knowledge can also help you have fruitful conversations with your company’s suppliers, metallurgists, and lab technicians.