The metallographic analysis of the microstructure is an important method in the evaluation of metallic components. The sample is examined in the light microscope after a complex preparation. The formation of structural constituents is described and interpreted. That allows finding correlations between manufacturing processes, heat treatment, phase formation, anomalies and mechanical properties.
Un- and low-alloyed steels are iron-carbon alloys with an alloy content below 5 wt.%. They are used, for example, as constuction steels for mechanical engineering.
By a metallographic analysis, ferrite and perlite fractions can be determined, perlite structures can be classified and impurities like manganese sulfides can be evaluated. Line-shaped manganese sulfides are shown exemplary in the left picture. Furthermore, we can check the heat treatment conditioning of your components or semifinished products. Various steel properties be set by annealing, hardening and tempering conditions. These states can be evaluated metallographically in order to draw conclusions on the manufacturing process and possible heat treatment failures. The right picture shows a normalized microstructure of an S235 (formerly St37).
Normen: DIN 50602
Austenitic steels are used in almost every technical field and are characterized by their resistance to corrosion. In practice, however, repeatedly damage by corrosion occure. Depending on the quality , manufacturing or processing status , the austenitic stainless steels can form corrosion-prone phases. And that although the chemical composition answers the specification. By metallographic examination with special color etching these phases can be detected and evaluated in the light microscope .
The cast iron are iron materials, which have min. 2.06% C and an increased silicon share. Unlike steel, these can not be further edited by forging operations.
Gray cast iron is forming graphite in different shapes, such as lamellar or globular. This formation of graphite shapes and sizes are classified according to DIN EN ISO 945-1 using the metallographic micrograph. The graphite formation has a direct influence on the properties of the material.
The term white cast iron is because this material does not contain graphite and therefore the fracture surface appears "white". Technically, this material is used in the form of chilled cast iron, for example for camshafts.
One of the most common causes of corrosion on cast iron is spongiosis. Here, the structural constituents ferrite and pearlite are selectively dissolved while forming a sponge iron, while the graphite retains and maintains the component as porous scaffold.
The cut examination of CFRP serves for the classification of diverse properties (dry spots, waviness, porosity, etc.). By using light microscopes and high-definition scanners, as well as a professional measuring software evaluation in the µm level are no problem.
Normen: DIN EN ISO 12345; DIN 1235, AV 1245
Aluminum and its alloys can be clearly classified and evaluated by their specific microstructure .Due to the already visible unetched microstructure aluminum materials can be clearly distinguished in wrought and cast alloys . Decisive for the technical characteristics are e.g. the formation of intermetallic phases (aluminides), the forming state of the eutectic (finishing), the dendrite arm spacing or the porosity.
Normen: VDG P220
Titanium materials show special characteristics. Due to their low density combined with outstanding mechanical properties and a good biocompatibility, they are used in aircraft construction, in the automotive sector and in medical technology.
Titanium alloys can be divided in three different classes, which can be differentiated metallographically. Depending on their composition, the alloys can be differentiated in α-, (α+β)- and β-alloys. Depending on their heat treatment and processing, different microstructures are formed, which affect the corrosion resistance as well as the mechanical properties. The left image shows an (α + β) alloy after an annealing step just below the α-β transformation temperature and a subsequent rapid cooling to form an acicular α-phase.
Normen: ASTM E112, DIN 17851, DIN EN 3114, ASTM F1854
There are several possibilities to classify the numerous rolling bearing damages. One method is the rough distinction between wear and fatigue damage. The causes of wear damage range from faulty installation, to incorrect designs and lubricant problems. In the case of damage, we can detect microstructural defects by a metallographic analysis. Since all microstructural changes have different reasons, you can use the information obtained by metallography to prevent them in future and extend the life span of your bearings, gears and other machine parts!
The conventional microstructural changes due to fatigue include formation of dark etching regions (DER), white etching bands (WEB) and so-called butterflies (left picture). These structural changes occur in the range of the maximum shear stress (Hertzian pressure). DERs and WEBs are named after their appearance after etching of the metallographic specimen and are significantly softer compared to its surrounding material. Butterflies develop on impurities in the steel, with the "wings" being represented by a white etching area (WEA) and are significantly harder than the matrix.
Other, unconventional changes in the microstructure due to fatigue are white etching cracks (WEC) (sometimes called white structured flakings (WSF)). WECs are a common cause of premature failure of bearings in wind turbines. WECs are very thin cracks which grow radially or tangentially in the rolling bearing rings. White etching areas (WEA) develop at the flanks of these cracks. The causes of the formation of WEC have not been clarified, yet. The hydrogen hypothesis is the most probable cause.
According to this theory, the yield strength is locally reduced by the intrusion of hydrogen into the material and the mentioned structural changes occur. The diffusion of atomic hydrogen is probably favored by a combination of material defects, high loads and chemical lubricant problems. The WEAs at the WECs can be characterized as carbon-supersaturated, nanocrystalline ferritic structures which have a higher hardness than the surrounding matrix.
(Pictures in cooperation with AEB-Kompetenzcenter)
Pure copper is mainly used in electrical engineering due to its high electrical conductivity. Well-known copper alloys are brass and bronze which can be differentiated in wrought or cast alloys. They are widely used where corrosion resistance combined with a good mechanical processability is required.
A metallographic analysis can visualize characteristics like cuprous oxide (Cu2O) in copper materials. Thus, impurities like oxygen can be detected (left picture). Heat treatment conditioning and the production process (right image) of copper components can be classified and checked, too.
Usually, Nickel and its alloys are used when high corrosion and/or heat resistance is required. Nickel based superalloys, which can be used for gas or aircraft turbines, are one example of these alloys. Their cuboid γ‘ structure can be visualised and assessed by an etching process (left picture).
Nickel materials can also show different microstructures which we are able to interpret and evaluate using light microscopy.