Metallographic Analysis

What Is Metallographic Analysis?

Metallographic analysis is the microscopic examination of a sample after it is cut, mounted, ground and polished, and etched with a suitable reagent. The material's heat-treatment history, grain structure, phase distribution and defects become visible in this examination.

Purpose of Metallographic Analysis

The purpose is to see the structural cause behind the numbers given by mechanical tests. Unexpected hardness or brittleness can often only be explained by examining the microstructure.

Which Products Is Metallographic Analysis Applied To?

Heat-treated steel parts
Welded joints
Castings and forgings
Carburised and nitrided surfaces
Failure analysis samples

In Which Sectors Is Metallographic Analysis Used?

MetalAutomotiveAerospaceEnergyMachinery Manufacturing

Standards Related to Metallographic Analysis

ASTM E3 (preparation of metallographic specimens)
ASTM E112 (determining average grain size)
ISO 643 (ferritic or austenitic grain size in steels)
ISO 3887 (determination of decarburisation depth in steels)
ISO 17639 (macroscopic and microscopic examination of welds)

Metallographic Analysis Conditions

Kesme: Cooling cutting that prevents heating; the structure should not change
Preparation: Baking, gradual sanding and polishing
Etching: Etchant and time suitable for the material
Examination: At appropriate magnifications in the light microscope; SEM if necessary

Equipment for Metallographic Analysis

Cooled cutting machine
Mounting press
Grinding and polishing machine
Etching setup and fume hood
Light microscope and image analysis system
Microhardness tester (supplementary)

Sample Requirements for Metallographic Analysis

The sample must be taken so as to represent the batch or production run.
Sample description, material type, and production direction if any, must be stated.
The standard to be applied and any customer specification must be provided together with the sample.
The test may be destructive; the required number of samples must be confirmed in advance.
Surfaces that will be tested must be protected during transport.

Duration of Metallographic Analysis

A sample is generally completed within one to two working days including preparation and examination.

Metallographic Analysis Procedure

1The section to be examined is identified and the sample is cut with cooling.
2The sample is mounted.
3Stepwise grinding and polishing are performed.
4It is etched with a suitable reagent and washed.
5It is examined under the microscope at different magnifications.
6Grain size, phase fraction, or layer depth is measured.
7Images are reported with scale.

Metallographic Analysis Acceptance Criteria

The method standard defines how the test is performed, not what the result must be. The acceptance limit comes from the material or product standard, the customer specification, or the relevant regulation; this page therefore does not give a numerical limit.

Evaluation of Results

Assessment relies on comparing the observed structure with the structure defined in the material standard or specification. Quantities such as grain size and decarburisation depth are given numerically; the acceptance limit comes from the product standard.

Metallographic Analysis Report

Sample description, material/product information, and batch number if any
Sample preparation method and conditioning conditions
Standard and test conditions applied
Instrument used and calibration status
Measurement results and units
Measurement uncertainty (if requested)
A statement that the result applies only to the specimen submitted for testing

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Harmonised standards applied according to the product family
EU 765/2008 (market surveillance and accreditation framework)
Relevant product regulations and customer specifications

Metal and Metallurgy Testing — What Does This Testing Field Cover?

The section below describes the field that the Metallographic Analysis test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Metal and metallurgical testing determines the load a metal or a welded joint can carry, how it fails and what its internal structure looks like. The output of this family is often not about the product itself but about the identity of the MATERIAL: is the steel really of the declared grade, does the weld seam deliver the strength the design assumes, does the casting contain an unacceptable internal defect?

The practical importance of the field comes from the fact that a metal part rarely fails gradually. A plastic part deforms, ages, cracks and usually gives warning. A weld seam or a fastener, by contrast, often breaks without warning — especially where fatigue and brittle fracture are involved. That is why in fields such as pressure equipment, lifting gear, structural steel and pipelines, material testing is a requirement of REGULATION rather than of design.

The family brings together three groups of methods, and each answers a different question. Mechanical tests quantify how the material behaves under load. Non-destructive examination looks for defects without damaging the part. Metallography and chemical analysis explain why the material behaves the way it does — through grain structure, phase distribution, inclusion content and alloy composition.

The acceptance criterion for these tests almost never comes from the test standard, and this is the key to understanding the field. The test standard says how the measurement is to be made; the acceptable values come from the product standard, the material grade or the fabrication code. ISO 6892-1 tells you how to run a tensile test; EN 10025 tells you what yield strength that steel must deliver. When reading a report, the product standard it was assessed against is essential.

Sampling is an inseparable part of the result in this field. Two specimens taken from the same heat may give different values depending on the direction and location they were taken from; in rolled products, specimens parallel and perpendicular to the rolling direction produce different results. Product standards therefore also govern sampling location and orientation, and the report must document this.

Method groups in this field

Mechanical tests
Tensile, compression, bending, notch impact (Charpy), hardness (Brinell, Vickers, Rockwell), fatigue, creep and fracture toughness. These give the behaviour of the material under load in numerical form.
Non-destructive examination
Radiography, ultrasonic examination, penetrant, magnetic particle and visual examination. It looks for surface and internal defects without damaging the part and is an indispensable step in welded fabrication.
Metallography
Specimen preparation, etching and microscopic examination. Grain size, phase distribution, inclusion content, case depth and the weld heat-affected zone are assessed.
Chemical analysis
Alloy composition is verified by optical emission spectrometry and wet chemistry; confirmation of the material grade runs through this step.
Corrosion behaviour
Salt spray, pitting and crevice corrosion and intergranular corrosion tests. In stainless steels these can indicate the quality of the heat treatment.

Commonly confused in this field

A material certificate (such as EN 10204 3.1) is not a test report; it is the manufacturer's declaration. In critical applications independent verification is carried out, and the two must not be confused.
ASTM and ISO tensile tests measure the same quantities but use different specimen geometries; the results are not treated as directly equivalent, and whichever the specification calls for is the one applied.
Conversion tables between hardness and strength are APPROXIMATE. Calculating yield strength from hardness is not sufficient evidence for material acceptance.
In non-destructive examination, "no defect found" does not mean there is no defect; defects below the detection limit of the method may be present. The report must state the method applied and the sensitivity level.
A weld seam passing does not show that the welder or the procedure is qualified; these are separate processes.

Which decisions these results support

Verifying the material certificate and accepting incoming material
Approving welded fabrication; welder and procedure qualification
Manufacturing control of pressure equipment and structural components
Failure analysis and determining the root cause of field damage
Verifying heat treatment and surface treatment processes

Related Tests

Frequently Asked Questions About Metallographic Analysis

Does sample preparation affect the result?

It directly affects it. A cut that heats the sample, an over-pressed grinding step, or the wrong etchant can change or hide the structure. Preparation is therefore done per the standard and stated in the report.

What is done if a light microscope is not enough?

A scanning electron microscope (SEM) is used for very fine structures and fracture surfaces; EDS is used alongside it if elemental distribution is needed.