Complete Workflow for Determining Average Grain Size of Metals

Applicable Samples and Objectives

This workflow applies to grain size determination of carbon steels, alloy steels, stainless steels, non-ferrous metals and their wrought products, and is commonly used to evaluate grain condition after forging, rolling and heat treatment. The objective is to characterize grain size quantitatively as a grain size number (or mean intercept), providing a unified scale for structure-property analysis, incoming inspection and process comparison.

Common methods include the comparison method, the planimetric (area) method and the intercept method: the comparison method is fast and suitable for batch inspection; the planimetric method balances efficiency and accuracy; the intercept method is statistically the strongest and is preferred for arbitration and precise rating.

Complete Workflow

1. Sampling and Cutting

   Sample at representative locations. For wrought materials, prepare both longitudinal (parallel to the working direction) and transverse sections; for equiaxed products either section may be used. Sample with a precision cut-off machine under ample coolant to prevent localized structural change from cutting heat.

2. Mounting

   Hot or cold mount small samples and thin sheets; stack and clamp multiple thin sheets to ensure adequate examination area; use low-shrinkage cold mounting resin when edges must be evaluated.

3. Grinding and Polishing

   Grind stepwise with SiC papers from P240 to P1200~P2000, rotating 90 degrees between steps; polish with 3 μm and 1 μm diamond to a mirror finish, with an optional final 0.05 μm colloidal silica polish. Scratches and relief from insufficient polishing will be misread as grain boundaries.

4. Etching to Reveal Grain Boundaries

   Select the etchant by material: 4% nital for carbon and low-alloy steels, saturated picric acid solution for prior-austenite boundaries, aqua-regia-type reagents or electrolytic etching for stainless steels, and Keller's reagent for aluminum alloys. Etch until boundaries are clear and grain interiors clean; avoid over-etching.

5. Metallographic Microscope Examination

   Observe at 100× (or a suitable magnification giving sufficient grains per field); first confirm structural uniformity at low magnification, then select the rating magnification. Images must be sharply focused with continuous, complete grain boundaries.

6. Method Selection and Implementation

   Comparison method: match the field or image against standard rating charts and read the closest number. Planimetric method: count grains within a known area and convert to a grain size number using the standard table. Intercept method: place test lines (straight or circular) of known total length in the field, count intersections P between the lines and grain boundaries, calculate the mean intercept l̄ = L/(M·P), where L is total test-line length and M the magnification, then obtain the grain size number from G = (-6.644·lg l̄) - 3.288 (l̄ in mm). Generally measure at least 3~5 representative fields and average.

7. Data Processing and Determination

   Calculate the mean and scatter of the measurements and round the grain size number as specified in the standard; for non-uniform structures, rate separately and note the field locations.

8. Recording and Reporting

   The report shall state material condition, sampling direction, etchant, method, magnification, number of fields, results and representative micrographs.

Key Control Points

• Sampling direction must match the evaluation purpose, and the report must state longitudinal or transverse.
• Grain-boundary revelation is critical: over-etching widens boundaries while under-etching causes missed counts; boundaries should be fine and continuous.
• For prior-austenite grain size, the specimen must not undergo any subsequent treatment that alters the original boundaries; picric-acid-based reagents are preferred.
• Test lines for the intercept method must be placed randomly, avoiding bias toward coarse or fine regions.
• Counting rules for grains at the field edge (counted or not) must be consistent throughout.
• Choose a magnification giving a statistically adequate number of grains per field; lower the magnification for very coarse grains.

Common Deviations and Remedies

• Broken or unclear grain boundaries: re-polish and use fresh etchant, or switch to electrolytic etching.
• More than one grain size number difference between fields of the same sample: indicates mixed grain structure; add fields, rate separately and state the range.
• Large discrepancy between comparison and intercept results: the intercept method prevails; use comparison only for rapid screening.
• Scratches or pits misjudged as boundaries: pre-check polish quality under bright field before etching.
• Difficulty rating duplex or banded structures: rate the matrix phase separately per the standard and add explanatory notes.

Reference Standards

• GB/T 6394 Determination of estimating the average grain size of metal
• ASTM E112 Standard Test Methods for Determining Average Grain Size
• GB/T 13298 Metallographic examination methods for metals

The above is a methodological example for technical communication and process discussion, not actual measured data from a specific customer; the actual test plan, sampling locations and determination rules are subject to sample evaluation and the procedure confirmed by both parties.