Almost all metals are made up of grains. The number and size of the grains within a metal depends on what happens during its manufacturing process and greatly affect that metal’s resulting strength, durability, and ductility. 

Experts conduct grain size analysis to determine the size of the grains within a metal. This helps them understand how the metal will react to force, heat, and other factors it may encounter during use and, therefore, determine what the metal should and should not be used for. 

To help you better understand why grain size analysis is so important in contemporary metallurgical work, this is a breakdown of what metallic grains are, how they affect metals, and what grain size analysis helps us learn about them.

What are metallic grains?

Most metals, including aluminum and steel, are polycrystalline in nature, meaning they are composed of differing crystallites. When a metal is created, metallic atoms naturally join together into predetermined structural arrangements with a specific orientation and pattern, known as a crystal structure.  

Within a metal, a “grain” is an individual crystal with atoms having the same arrangement and orientation.  The atoms in the metal have well-defined structures, such as face center cubic (FCC) or body center cubic (BCC). For example, the most commonly used stainless steel, austenitic stainless steel, is made up of the FCC structure. This means the atoms that make up the stainless steel are iron atoms that can be represented by a cube shape with atoms at each corner position and at the faces of the cube.

While this face-centered cubic structure repeats throughout the metal, the orientation of the atoms inside each grain does not. This is because small crystallites form during the solidification of the metal after casting and these crystallites form grains that are maintained throughout the processing of the metal. A metal’s grain structure is made up of a vast number of these repeating grains.  The size and orientation of the crystals (grains) in the final product are affected by the thermomechanical processing of the metal during the forming of the part.

grain structure of metal

What is grain size? 

Grain size refers to the size of the crystallites (grains) within the metal.  Grain size may be measured in the final project after all of the thermomechanical processing is complete and the grains are “locked in” prior to the service of the part or component. In this case, the grain size is important for determining the performance of the metallic part. Grains may also be measured intermittently throughout the thermomechanical processing of the metal to determine the evolution of the grain structure during the manufacturing process.  

Grain size is destructively measured by preparing well-polished and etched metallographic specimens. After etching, grain boundaries become highlighted when observed under a microscope. A Materials Engineer can then either measure the actual grain size or determine the ASTM Grain Number. ASTM E112 provides a specification for measuring grain size. It’s essential to accurately determine the average grain size, as individual grains may vary slightly in size. Additionally, the distribution of grain sizes can be measured to assess the variability of this metallurgical characteristic.

Why does grain size matter?

A grain boundary is a “transition region” where the metal’s atoms are not perfectly aligned. The atoms within a metal’s grain boundaries are less efficiently packed than they are within grains and have higher bonding energy. 

When exposed to stress, dislocations travel along atomic planes and lead to plastic deformation of a metal.  This is an extremely important feature unique primarily to metals and their alloys, allowing metals to be formed into complex shapes and to be used safely in service. When dislocations reach a grain boundary, however, that grain boundary can prevent it from continuing to pass to the next grain.

Grain size can be engineered to provide the best product performance in service.  For example, finer grain sizes are desirable when high cycle (low stress) fatigue of metals is expected or when better formability may be required. For improving corrosion resistance, tests like ASTM B117 have shown that corrosion of many metal alloys occurs along grain boundaries.

On the other hand, in some circumstances, such as high stress-high temperature applications where creep becomes important, having a larger grain size is often desirable.  Or if a metal is expected to undergo low cycle (high stress) fatigue, a larger grain size is often more desirable.  

What is grain size analysis and why is it so important?

Grain size analysis is the process of determining how many grains exist within a given area. By figuring this out, metallurgical experts can evaluate how suitable a given piece of metal is for an application. 

Grain size analysis has many applications within metal materials testing. For example, Secat uses grain size analysis for billet homogenization evaluation, heat treatment evaluation, rolling sheet evaluation, and more.

By knowing how to measure grain size and how to leverage the grain size to optimize the performance of a metal part in service, engineers can design better, lighter and cheaper products. This makes grain size analysis critical for all manners of sensitive metallurgical analysis. Simply put, if you want to make sure your metal product is the best it can be, you should know what your grain size is.

If you think you need grain size analysis, the experts at Secat can help. We use precise metallographic and microscopic techniques and follow industry standard procedures to provide the most accurate grain size measurements.