What Is Grain Size?
Grain size determination is a measurement in which the average grain size on a prepared section, etched to reveal grain boundaries, is determined by comparison, intercept or planimetric methods and expressed as a standard grain-size number.
Purpose of Grain Size
The purpose is to verify that the grain size is within the range required by the product standard; grain size directly affects strength and toughness.
Which Products Is Grain Size Applied To?
- Steel and stainless steel
- Heat-treated parts
- Aluminium and copper alloys
- Weld heat-affected zone
In Which Sectors Is Grain Size Used?
Standards Related to Grain Size
- ASTM E112
- ISO 643
- ASTM E1382
Grain Size Conditions
- Method
- Comparison, intercept (Heyn) or planimetric (Jeffries)
- Etching
- Etchant that makes grain boundaries clearly visible
- Magnification
- Defined in the standard; chosen according to grain size
- Number of fields
- A representative number of fields, as defined in the standard, is measured
Equipment for Grain Size
- Metallurgical microscope
- Image-analysis software
- Calibrated measurement scale
- Specimen-preparation line
Sample Requirements for Grain Size
- The section must be prepared metallographically
- Etching must fully reveal the grain boundaries
- Measurement fields must be chosen representatively
- Section direction must be reported
Duration of Grain Size
Typically 1 day, including specimen preparation.
Grain Size Procedure
- 1The section is prepared and etched to reveal grain boundaries.
- 2The appropriate magnification is selected.
- 3Measurement is performed on the defined number of fields.
- 4The average grain diameter is calculated.
- 5The result is given as a standard grain-size number.
Grain Size Acceptance Criteria
The acceptance criterion is the grain-size-number range given in the product standard.
Evaluation of Results
A single average is misleading in duplex-grain structures; the standard requires separate reporting in this case.
Grain Size Report
- Standard and method applied
- Etchant and magnification
- Number of fields measured
- Average grain-size number
- Note on structural homogeneity
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 authorisationsRelated Regulations
- Product standards
Metal and Metallurgy Testing — What Does This Testing Field Cover?
The section below describes the field that the Grain Size 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 Grain Size
According to which standard is Grain Size made?
Grain Size is executed in scope ASTM E112, ISO 643 and ASTM E1382. The limit values and acceptance criteria to be applied are defined in the product standard; The version valid for your product is confirmed in writing at the offer stage.
Which products does Grain Size apply to?
Mainly applied to Steel and stainless steel, Heat-treated parts, Aluminium and copper alloys and Weld heat-affected zone. Whether your product is included in the scope is determined by examining its technical specifications and intended use.
Under what conditions is Grain Size done?
The determining parameters are: Method — Comparison, intercept (Heyn) or planimetric (Jeffries), Etching — Etchant that makes grain boundaries clearly visible and Magnification — Defined in the standard; chosen according to grain size. The full terms are in the "Test Conditions" section on the page and vary depending on the product standard.
What kind of sample is needed for Grain Size?
The section must be prepared metallographically, Etching must fully reveal the grain boundaries and Measurement fields must be chosen representatively. The number of samples is determined according to the product standard and the scope to be reported.
How long does Grain Size take?
Typically 1 day, including specimen preparation. This duration belongs to the experiment itself; sample preparation, queuing and reporting are also evaluated. A realistic schedule for your product is given in the offer.
What is the Grain Size result called "appropriate"?
The acceptance criterion is the grain-size-number range given in the product standard.