Optical microscopy is the main tool for assessing the grain structure of steels. Usually the structure, seen through a light microscope, called microstructure. The mechanical properties of any steel are highly dependent on its microstructure..
View of grains of steel in a light microscope
The grain structure of iron and steel is revealed by polishing its surface to a mirror finish, etching in acid solution and examination under a light microscope (cm. Pure iron grain structure).
As a result of etching, atoms are removed from the polished surface, and the rate of this removal depends on the orientation of the grain crystal and the type of grain, eg, ferrita or austenite. The contrast of a given grain under a microscope depends on the degree of smoothness of its surface after etching..
As it shown on the picture 1 grains, which are the same phase, such as ferrite, austenite, cementite, will etch evenly over the entire surface. Therefore, after etching, the original mirror surface will remain smooth inside each individual grain..
How a light microscope works
A light microscope uses reflected light to form its images. A beam of light is directed from top to bottom onto the surface of the steel (light arrows in the picture 1). The image is formed by photographic film (photoplate, digital device) or an eye from the light, which is reflected along the same direction (black arrows).
Very high proportion of light for smooth surfaces, came to sample, reflect back and create a bright (white) picture. Therefore, single-phase ferrite grains, austenite and cementite will appear white. Because they appear white, they are often impossible to distinguish under a microscope without additional information..
Perlite structure under a light microscope
Consider now perlite. Conventional etchants, which are used for steels, are nital (nitric acid in alcohol) and picral (picrylic acid in alcohol). These etchants etch ferrite pearlite plates much faster., than cementite plates. Therefore, after etching, the cementite plates protrude from the ferrite plates.. Cementite plates are very thin, and they scatter the light falling on them well (picture 2). This results in a dark gray image.. For this reason, perlite appears in shades of gray to black under a light microscope..
However, this is not always the case.. If the distance between the cementite plates is large enough, they look like dark lines with white ferrite plates in between. The optical microscope can only "see" distances up to approximately 0,2 μm at a maximum magnification of about 1000x. therefore, when the inter-plate distance in pearlite becomes less 0,2 μm, then the optical microscope shows pearlite grains in the form of dark gray variegated images, as shown in the picture 3.
Interlamellar spacing in pearlite
The distance between the cementite plates in pearlite depends on, how quickly the sample was cooled from the austenitic region through the temperature A1 - the faster the cooling, the less this distance. Usually, to achieve this cooling, the sample is simply removed from the oven and allowed to cool in air.
Cooling rate, understandably, depends on sample size. Even for rather large samples, air cooling gives an inter-plate distance less than 0,2 μm. Therefore, perlite in the samples, which were air cooled, almost always look like the picture 3.
Picral etchant, which was used for the sample in the figure 3, dissolves ferrite more uniformly by changing grain crystal orientation, what does nital etchant do. Therefore picral gives a more uniform gray color for each grain and is the preferred etchant for perlite..
Source: John D. Verhoeven, Steel Metallurgy for Non-metallurgist, 2007