Microscopy

Grain size determination methods

When observing a suitably prepared metal sample under a microscope, it is possible to see that the structure is made up of a multitude of grains of assorted sizes. The size of these grains is important for many properties of the metal, such as hardness, strength, elongation, deep-drawing ability, etc.

The 'grain size' of a metal can be varied using different methods. Grain size measurements can be used to determine whether the process has been carried out properly or whether the required grain size has been achieved for the intended application. The grain structure visible on the metallographic section, which can be recognized by the grain boundaries, initially does not say anything about the actual grain size, since the section surface only represents a flat section through the actual grains. It cannot be determined whether this section was made at the 'equator' or the 'pole' of the grain.

It has therefore become established to define the grain size of a metal with the help of the cut surfaces visible in the micrograph. The indication of an average size, either with the average grain diameter or the average surface area of the crystallites is sufficient. For higher accuracy requirements, grain statistics should be determined. 

Testing of ferrite and austenite grain size according to DIN EN ISO 643 is generally carried out microscopically at a magnification of 100:1, typically with the comparison method. For equi-axial grains that do not show any elongation, this method is sufficiently accurate and is the quickest to execute. For elongated grains, or for greater accuracy, either the line intersection method or the counting method should be used. Twin lines in grains are not considered as grain boundaries, but as part of the crystal structure within the surrounding grain boundaries. The samples for metallographic evaluation are ground, polished and etched according to their material and condition.

Layer thickness measurement

The layer thickness measurement of compound layers after nitriding is standardised in DIN 30902 'Light microscopic determination of the thickness and porosity of compound layers of nitrided and nitrocarburised workpieces'.After suitable grinding, the compound layer is preferably measured at a magnification of 1000:1. It must be ensured that measurements are taken only where:

• There is no gap between the mounting compound and the surface of the part to be examined
• The compound layer does not show any breakouts or cracks produced during the grinding process
• The area is sufficiently far from an edge.

The layer thickness is to be measured at a minimum of ten points over a length of approximately 5 mm. The result is to be given as an arithmetic mean value rounded and without decimal places as follows:

• Thickness of the compound layer: CLT = 16 µm‘CLT’ stands for ‘Compound Layer Thickness’.

If the thickness of the porous part of the compound layer is given, the CLT must be provided with index p:

• Thickness of the porous part of the compound layer: CLTp = 4 µm

If the thickness of the pore-free part of the compound layer is given, the CLT must be provided with index np:

• Thickness of the pore-free part of the compound layer: CLTnp = 12 µm

Modern image processing software allows for automatic layer thickness measurement. This analysis method is available for QATM hardness testers.