BIZOMAT MAKEOS · Materials
Grain Size Acceptance (Field Readings, Average ASTM Grain Size, Spread, Specification)
From several microscope field readings, what is the average ASTM grain size number, how much do the fields vary, and does the material meet the grain size specification?
Acceptance of a material for grain size from up to five fields: the average of the readings (grains per square inch at 100×, grains per mm² at 1× or mean lineal intercept), the ASTM grain size number G from the average and from each field, the equivalent counts and intercept length (as the grain size tool), the spread between the coarsest and finest field, against the specified G range and your largest acceptable spread.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Measurement | choice | What was measured in each field. |
| Field 1 reading (0 = unused) | Reading in field 1. | |
| Field 2 reading (0 = unused) | Reading in field 2. | |
| Field 3 reading (0 = unused) | Reading in field 3. | |
| Field 4 reading (0 = unused) | Reading in field 4. | |
| Field 5 reading (0 = unused) | Reading in field 5. | |
| Specification | ||
| Coarsest grain size allowed (lowest G) | G | Lowest grain size number the specification allows (coarsest grains). |
| Finest grain size allowed (highest G) | G | Highest grain size number the specification allows (finest grains). |
| Largest spread between fields | G | Largest difference in G between fields you accept before treating the structure as mixed. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Measurement
- What it is
- What each field reading is: a count of grains per square inch on an image at 100× magnification, a count of grains per mm² of actual surface, or the mean lineal intercept length.
- Units and format
- Choose one of 3 options from the list: Grains per square inch at 100×; Grains per mm² at 1×; Mean lineal intercept length (µm).
- Example
- The form opens with “Grains per square inch at 100×” selected as a worked example. Choose the option that matches your case.
- Get it right
- All five readings must be of the same kind; the mean intercept is entered in micrometres.
What this means: Field 1 reading (0 = unused)
- What it is
- The reading in the first microscope field, of the kind chosen under Measurement. A count of grains or an intercept length, depending on the method.
- Units and format
- Enter a plain number with no unit. The form accepts 0 or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 64 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Readings are averaged before G is worked out; 0 leaves a field out.
What this means: Field 2 reading (0 = unused)
- What it is
- The reading in the second microscope field, of the same kind as field 1.
- Units and format
- Enter a plain number with no unit. The form accepts 0 or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Choose fields at random across the specimen rather than picking typical-looking areas; 0 leaves the field out.
What this means: Field 3 reading (0 = unused)
- What it is
- The reading in the third microscope field, of the same kind as field 1.
- Units and format
- Enter a plain number with no unit. The form accepts 0 or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 to leave the field out of the average and the spread.
What this means: Field 4 reading (0 = unused)
- What it is
- The reading in the fourth microscope field, of the same kind as field 1.
- Units and format
- Enter a plain number with no unit. The form accepts 0 or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 58 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 to leave the field out of the average and the spread.
What this means: Field 5 reading (0 = unused)
- What it is
- The reading in the fifth microscope field, of the same kind as field 1.
- Units and format
- Enter a plain number with no unit. The form accepts 0 or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 66 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 to leave the field out of the average and the spread.
Specification
What this means: Coarsest grain size allowed (lowest G)
- What it is
- The lowest ASTM grain size number the specification allows. A lower G means coarser grains, so this is the coarsest structure accepted.
- Units and format
- Enter a number in G: G. The form accepts -5 G to 20 G. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 G as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Do not swap the limits: the coarse limit is the smaller G number.
What this means: Finest grain size allowed (highest G)
- What it is
- The highest ASTM grain size number the specification allows. A higher G means finer grains, so this is the finest structure accepted.
- Units and format
- Enter a number in G: G. The form accepts -5 G to 20 G. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 G as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take both limits from the material specification or purchase order.
What this means: Largest spread between fields
- What it is
- The largest difference in grain size number between the coarsest and finest field that you accept before treating the structure as mixed (duplex).
- Units and format
- Enter a number in G: G. The form accepts 0 G to 20 G. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 G as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Set it from your specification or quality plan; exceeding it gives a warning, not a rejection.
Method and formulas
- G = 1 + log₂ n₁₀₀; G = 3.3219 log N_A − 2.954; G = −6.6439 log (ℓ̄ in mm) − 3.288 (as the grain size tool).
- Readings are averaged, then G is found from the average; each field’s G gives the spread.
- Accept when the average G lies within the specified range.
- Example: the average reading gives G 6.991, with fields from G 7.1293 to 6.858.
Limitations
- Equiaxed, single-phase grains; elongated or duplex structures need the full standard’s methods.
- Five fields give only a rough average; the standard sets field counts for a given precision.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Accept bar or forgings against a grain size requirement
- Average several fields for a test report
- Spot a duplex (mixed) grain structure
Who it is for
- Metallurgists
- Quality engineers
- Lab technicians
What you get
- Average reading and grain size number
- Each field’s G and the spread
- Against the specification
Interpreting the result
- Each step of one in G halves or doubles the number of grains per unit area.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 369–380. Topic reference; no source tables or recommended settings are reproduced.
- ASTM E112 Standard Test Methods for Determining Average Grain Size
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
The steel and its transformation
In Induction Hardening by the Numbers
What it covers: What happens inside the steel: austenite, martensite and carbon, why fast heating needs a higher temperature, starting structure, prior austenite grain size, and a bar supplied annealed.
Method explained here · BIZOMAT original book
Defects: gas, oxides and grain
In Metal Casting by the Numbers
What it covers: Recognising and preventing the defects feeding cannot cure: hydrogen gas porosity and the reduced-pressure test, oxide films, telling gas from shrinkage, and measuring grain refinement.
Method explained here · BIZOMAT original book
Grain size acceptance and spread
In Microstructures by the Numbers
What it covers: Turning several field readings into one grain size result, averaging correctly, judging field-to-field spread, recognising duplex structures and writing a defensible acceptance decision.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.