Light microscopy of carbon steels

Steel structure

Known, that steel has the widest variety of properties and characteristics. This is because, that steels are iron-carbon alloys, which are rich in types and types of microstructures. These diverse microstructures are formed at different carbon contents, as well as various modes of heat and thermomechanical treatment.

To describe these microstructures it is necessary, At first, identify the various phases and phase mixtures that exist, and, Secondly, determine their proportions and distribution. There are currently a large number of methods for determining these various parameters.. Each of these methods has its own specific purpose and is used to fully describe the microstructure..

Light microscopy in metallurgy

The workhorse of metallurgy - an unpretentious powerful heavy draft - no doubt, is light microscopy. by the way, the term "light microscopy" is preferred, than the often used term "optical microscopy". This is fairly fixed in GOST 28489-90 “Light microscopy. Terms and Definitions", so quite logically it agrees, eg, with the term "electron microscopy". Because the optics itself, as we know, maybe "light", and "electronic", and even "x-ray". AND, if I'm really strictly speaking, then that instrument, which is used in metallography, it would be more correct to call even so: Reflected light microscope.

Light microscope, anyway, all metallographs are used or, how often do we say, metallurgists. Really, it can be found in almost any laboratory in almost any industrial, and even more so a metallurgical - small and large - enterprise.

The meaning of the term "microscopy" naturally follows from its name, namely - "observation and description of structures with a microscope". but, it means in itself only a goal. This goal is, to understand:

  • how these structures were formed;
  • what factors governed the formation of structures and
  • how these structures relate to the physical and mechanical properties of steel.

Phase identification in steel

Usually, light micrographs of carbon steel clearly show the distribution of characteristic phases, however, these phases cannot be 100 % to identify. Such identification is carried out on the basis of more complex methods., such as electron or X-ray diffraction, chemical analysis, electronic microanalysis, as well as the data of the state diagram of the studied alloy - steel.

In most cases, phase identification is based on many years of metallography experience and does not require additional research.. In those cases, when the classical interpretation of the phases is not so obvious or when doubts arise, additional research methods are used for their reliable identification, such as electron or X-ray microscopy, chemical analysis, as well as state diagrams of the considered types of steels.

Features of ordinary carbon steels

Let's decide on that, what steels do we call "carbon steels". The term "steel" usually means an iron-based alloy, containing carbon up to 2 %. However alloys, which contain very little carbon, eg, less 0,1 %, it would be better to just call it brands of iron, how is it done with technically pure aluminum in the aluminum industry.

Common carbon steels are commonly referred to as steels, which contain, in addition to carbon, only impurities of all other chemical elements. but, this does not apply to silicon and aluminum, which "came" to the steel during its deoxidation, as well as manganese and cerium, which are introduced into steel to counteract the harmful effects of phosphorus. The content of silicon and manganese in these steels can reach 0,60 % for silicon and 1,65 % for manganese and this is considered acceptable.

Industrial carbon steels always contain small, but noticeable amounts of oxygen and nitrogen, depending on the method of steelmaking. Certain other elements are specially added to steel to impart special properties.. for instance, phosphorus is added to low carbon steels to improve their machining on metal cutting machines.

Steel sample for light microscope

Examining metals under a light microscope requires:

  • so that the sample is cut,
  • so that the sample surface is prepared in such a way, so that it reflects light well,
  • that this surface is properly etched to reveal the structure of interest to us.

A critical requirement for surface preparation is, so that the surface prepared for microscopy is identical to the corresponding section of the product before cutting out the sample. For this, At first, mechanical methods of cutting samples should not affect the structure of this surface. Secondly, for etching the surface, used and unambiguous etchants and methods of their application should be used.

From a flat picture to a three-dimensional structure

When studying a plane section under a light microscope, it is necessary to constantly remember, that this section may not be strictly flat. Besides, according to the results of microscopic examination of this two-dimensional flat "picture", they usually try to represent the three-dimensional morphology of the metal structure. At the same time, annoying mistakes often occur.. for instance, when describing the phases, which have an elongated shape in a given section, use the term "needle", although in a different plane of cut this structure may look very different.