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microstructure analysis and microstructure investigation

Metallography – important for quality

The quality and properties of metals depend, among other things, on the alloying elements they contain, such as Cr, Ni, Mn, Si, etc., and their proportions, as well as on the shape, size, distribution, homogeneity and orientation of the crystals (grains) they contain. The properties of metals can be strongly influenced not only by the alloying elements, but also by heat treatment or mechanical processing. For modern quality control, visual documentation - quantitative and qualitative microstructure analysis or microstructure examination - with a high-quality light microscope (metal microscope) at 50 to 500 times (up to 1000 times) magnification is indispensable.

Microscopes (and macro microscopes) are offered by a large number of manufacturers and dealers. Electronic cameras (e.g. 5 MB to 50 MB) with software are recommended for image acquisition. Some of this software includes image analysis (automatic determination of grain boundaries, structure components). If you talk to experienced metallographers, you will hear: "There is no software that can perform a truly reproducible structure analysis. The accuracy is only around 80%). But what is the use of an image analysis that is only "maybe" correct.

One of the foundations of qualitative analysis is stereology (Greek stereos = solid, physical). This involves the spatial interpretation (inferences) of sections through bodies (here metallographic sample preparation) to determine the total volume of the structure to be assessed from a two-dimensional microscope image. The spatial content is determined by measuring the structure geometries using mathematical functions. Stereology is a statistical approximation method for the quantitative determination of areas or volume fractions.

The following applies to the analysis: AA =LL= PP = VV (A area proportion | L length proportion | P point proportion | V volume proportion)

Metal microscope: Metallography requires 500x magnification (option 1000x) — metal microscope: Metallography requires 500x magnification (option 1000x)

recordings segmentation

analysis using area measurement methods

analysis using line intersection methods

analysis using point counting methods

grain size determination / grain size analysis

In addition to determining the grain size, determining the ratio of the elements is of great importance for assessing the material properties. The quantitative microscopic analysis (determination of the structural parameters) of these different phases - also called phase analysis - in materials makes it possible to make statements about the material properties (the expected mechanical properties and durability). Quantitative microscopy (stereology) is an old science and has been used industrially since around 1850.

In addition to determining phase proportions, determining the ferrite or austenite grain size analysis is an essential part of the microstructure assessment. Different evaluation methods are used to analyze the phase proportions. Many other parameters (such as grain size, grain boundaries / lamella spacing) can be determined from the parameters determined in this way. The basic methods for qualitative and quantitative analysis are:

planimetry (area analysis)
the line intersection method (line analysis)
the point counting method (point analysis)

These three evaluation methods can be used to quantitatively determine the structural properties of materials. By determining and classifying the structural components, statements can be made about the mechanical, chemical and, for example, the electrical properties of materials. The structural components can be determined both manually and automatically.

Determination of grain size distribution using area measurement method

Determination of the grain size distribution using the line intersection method (alternative method)

Determination of the average grain size using the circular measuring method

Determination of the mean grain size using the line intersection method

Basic procedure for determining the microstructure

Processing step	Description of the processing step
sample preparation	A sample is metallographically prepared by cutting, grinding and polishing until it has a mirror-like shiny surface
etching	To distinguish the metal components (grains / crystals) a grain boundary etching followed
Image creation	Microscopic image of the structure at 50 – 1000x magnification (standard 400x)
image feed	The viewed image is loaded into an image analysis software
grayscale processing	Adjustment / optimization work for image improvement
threshold determination	Selection of the microstructure to be analyzed, setting the sensitivity threshold
binary processing	Grain size classification (histogram classes), cleaning and exclusion of structural components that do not correspond to any class
measurement	Field measurement and object measurement in the structure
Statistical processing	Evaluation of the mathematical components, class distribution according to Gauss
image archiving	image storage including measurement data
result output / distribution	The images / reports are printed, sent (PDF), data export to ERP systems

1. The planimetry method (area analysis)

The graphite content can be automatically evaluated in a casting sample.

The spheroidal graphite content was outlined in red for evaluation

a.) Automatically through image analysis software:

Complex but easy-to-use image analysis software is used to determine:

area fraction volume fraction (Ratio of the surface area of the structures / grains to the overall image)
specific line length (total length ratio of the structures / grains to the overall image),
number of particles per unit area and size distribution of the structures / grains in relation to the overall picture)

The grain size is calculated automatically using the surface area in this process. Automatic evaluation using ZEISS ZEN or Axiovision software requires that the grain boundaries on the metallographically prepared and etched sample are mostly closed. The surface areas are shown in color using image analysis. The detection thresholds can be optimized with a simple mouse click. The process allows both the surface areas and the grain boundaries to be shown in different colors. The automated analysis provides a clear histogram based on the number and distribution of grains of different sizes.

b.) Manual evaluation:

For certain evaluations that cannot be automated, such as grain size determination, pore analysis or graphite evaluation, standardized standard tables are also available (in the image analysis software). The material tester can make an assignment by means of a simple but fairly reliable visual image comparison between the structure to be assessed and pre-made comparison images (so-called standard tables). For this purpose, the known standard tables are displayed in the software and the user can assign the structure to be examined to a table within a short time (manual image comparison) and carry out a qualification of the material.

Grain size determination after image cleaning

The grain size can be determined automatically using ZEISS ZEN software

Grain size determination – picture guide series according to ASTM

In manual microstructure analysis, the grain sizes are determined by manual assignment to a comparison table

2. Line intersection method

In the line cutting method, lines with a defined distance are plotted on the microscopic image. A mark is made at each point where the line crosses a grain boundary. The number of cuts is related to the measuring distance and thus provides information about the quantitative number of grains (crystals) in the structure. The following are determined:

Linear component: Ratio of the total length of the cut chord through a structural component to the length of the measuring line
Number of cuts / number of points: Ratio of the number of intersection points of the intersected grain boundaries to the total measuring line length

In addition to the simple line (several lines in one image), other geometries (multiple circles, etc.) are also available.

The grain size can be determined using the line intersection method

3. The point counting method (point analysis) according to ASMT E562-02

Picture: pore analysis: Total score: 96, hit points pores: 15, pore percentage: 15,6%

The microscopic image is covered with a grid of dots. The dots (hits) that hit a part of the structure are then counted (the ratio of hits to the total number of dots gives the area or volume share of the structural components).

In this point counting method, the phase proportions are determined using a microscopic micrograph with a dot pattern superimposed on it. The dot grid makes it possible to determine the percentage area of phases in microscopic images by counting the hit points.