ASTM Grain Size Chart for Tool Steel

What the ASTM grain size number measures

The ASTM grain size number is a count rather than a measurement. A micrograph is taken at 100 magnification, the grains that fall inside a square inch of it are counted, and the count is converted to a grain size number. A count of n grains per square inch at 100x gives a number G through n equals two raised to the power G minus one, which rearranges to G equals one plus the logarithm of n divided by the logarithm of two. Doubling the grain count raises the number by exactly one step, so every step on the scale halves the number of grains in that square inch. Working along the table the other way, one step shrinks the average grain diameter by a factor of about 1.41, which is the square root of two that the doubling of the count fixes.

The boundaries between the grains only show up after the sample is polished and etched, which is why a grain size reading is quoted beside the etch and the magnification it was taken at. The count is not affected by the alloy, so the same scale is used for a plain carbon bar, a stainless strip and a tool steel block.

Grain size number and average grain diameter

ASTM number against average diameter and band

Source, ASTM grain size numbers, Table 8.1 on printed page 67 of the source, which is PDF page 71 of the copy consulted. Values were read from the rendered page image.

Average diameter is given in microns, where one micron is 0.001 mm. The relative size column is the source’s own grouping of the scale into bands, each band three numbers wide except the first, which covers minus one and zero.

Which band a size falls in

The right hand column of the table is the source’s own way of grouping the scale rather than a separate measurement. Very coarse covers minus one and zero and turns up only in large ingots or in steel held near its melting point for a long time. Coarse covers one to three, medium covers four to six and fine covers seven to nine, and those are the ranges a normal austenitize and quench lands in. Very fine covers ten to twelve and ultrafine covers thirteen to fifteen, and both need something done to the steel that holds the grains back. A reading of eight on a mill certificate and a reading of eight on a die block are the same eight.

How austenitizing sets the grain size

Grain size is set while the steel is austenitic, so the austenitizing step decides it. When steel is heated into the austenite range the new austenite grains are at their smallest immediately after the pearlite has been consumed and before grain growth has had time to run. Grain growth is driven by the surface tension in the grain boundaries and it works by small grains shrinking away while larger ones take their place, so the average size only travels in one direction once the steel is hot. There are two ways to arrive at the smallest starting size. A faster heating rate makes the austenite nucleate in more places, and a finer starting structure gives finer austenite because the new grains nucleate on the old boundaries. A lower austenitizing temperature inside the range and a shorter hold both leave less time and less driving force for growth.

Cycling the steel through the transformation is the other lever. Each pass heats into the austenite range, holds briefly and quenches, and each pass starts from a structure finer than the one before it, so the grain size number climbs with every cycle. Repeating the cycle several times is the standard way of reaching the very fine and ultrafine numbers without changing the grade.

Why the number matters in tool steel work

A finer austenite grain makes the martensite finer too, because the lath and plate size in the martensite follows the prior austenite. Toughness is the property that tracks that, which is the practical reason a grain size reading matters on a tool steel that will be used in a shock application. Two heats of the same grade, quenched to the same hardness, can sit two or three numbers apart on this scale and behave differently in service. The number is also the easiest thing to check against what was ordered, so it belongs on the purchase order beside the hardness and the delivery condition, and it belongs on the certificate that comes back with the steel. A reading taken on a small test coupon applies to that coupon, and the heat treatment given to the coupon has to match the one given to the part before the two can be compared.

Related reference data

The temperatures that set the number are on the tool steel annealing and normalizing temperature chart and the tool steel forging temperature guide, and the full hardening and tempering sequence by grade is on the tool steel heat treatment guide. The grades we hold and ship include D2 tool steel and H13 tool steel, and their heat treatment steps are set out on the D2 heat treatment guide. How the microstructure that comes out of that treatment behaves is covered on the tool steel impact toughness chart.

Reference data compiled from Metallurgy of Steel for Bladesmiths and Others Who Heat Treat and Forge Steel (John D. Verhoeven). Table 8.1 is printed on page 67 of the book, which is PDF page 71 of the copy consulted. The table is a published reference rather than a specification of ours.