Minimum Thickness for Hardness Testing Tool Steel

A hardness test presses an indenter into the part, and the material around the indentation is deformed with it. If the part is too thin for the load, the reading is no longer the hardness of the material. This page carries the published thickness limits for each Rockwell and Brinell loading.

The rule behind the limits

The table follows a thickness of at least ten times the depth of the indentation for diamond indenters and fifteen times for ball indenters. The published values were established on low carbon steel and on hardened and tempered strip steel, so they are a guide that has been checked against test data rather than a theoretical limit.

One Rockwell number equals 0.002 mm of depth on the standard scales and 0.001 mm on the superficial scales. A 60 HRC reading therefore means the indenter travelled 0.08 mm into the part between the minor load and the major load, and the limits below are that arithmetic worked through for each scale.

Material thinner than the last row of the table should be tested by a microindentation method. For a Vickers indenter the workpiece has to be at least 1.5 times the indentation diagonal thick, and for a Knoop indenter at least half of the long diagonal.

After the first test, examine the face directly below the indentation. A bulge or a mark on the underside is the anvil effect, which means the part is too thin for that load and the next lighter scale has to be used. Inspect the anvil as well, because a marred anvil surface pulls the following readings low.

Stacking thin specimens to build thickness is not allowed, because the pieces slip against each other. Where more than one scale still qualifies, use the heavier load, because the larger indentation covers more of the material and returns a value that is more representative of the part.

Minimum hardness for Rockwell testing at each thickness

Read the row for the part thickness. If the part is harder than the value in that column, the scale and load cannot be used safely and the next lighter scale has to be chosen.

Source: ASM Handbook, Vol. 8, minimum work metal hardness values for testing various thicknesses of metals with standard and superficial Rockwell hardness testers. (a) means no minimum hardness for metal of equal or greater thickness. The source table also carries the ball indenter columns for the B, F, G, 15T, 30T and 45T scales, which apply to soft metals such as copper and aluminium alloys and are not reproduced here.

Worked example for a thin hardened strip

The case that decides which scale a hardened thin section can be tested on is a 0.36 mm (0.014 in.) tool steel strip at 63 HRC, which is too thin for the C scale.

A 63 HRC value needs about 0.71 mm (0.028 in.) of thickness for a valid C scale test, so the strip cannot be tested on that scale. Converted to the superficial scales, 63 HRC is about 69.9 HR45N, 80.1 HR30N and 91.4 HR15N.

The 45N scale is out, because the table asks for at least 74 HR45N at that thickness. The 30N scale asks for at least 80 HR30N and the 15N scale for at least 76 HR15N, so either one can be used.

Between the two, 30N is the better choice. One point of hardness on the 30N scale equals about half a point on the 15N scale in the hard steel range, so the 30N scale separates small differences better.

Minimum thickness for Brinell testing

The Brinell ball leaves a larger and deeper impression than the Rockwell indenter, so the thickness limit arrives sooner. The table gives the minimum hardness that can be tested safely at two specimen thicknesses with a 10 mm ball.

Source: ASM Handbook, Vol. 8, minimum thickness requirements for Brinell hardness tests. A specimen has to be at least ten times the depth of the impression thick, which is 3.2 mm (0.125 in.) for a reading of 300 HB at 3000 kgf. Material above 375 HB, about 40 HRC, moves to the Rockwell C scale or to Vickers.

Where the limits matter in tool steel

Knife stock, slitter knives, shim dies and saw blades arrive hardened and thin. The C scale is usually out on the last grinding allowance, and the superficial N scales are the method that stays valid.

The same thickness logic applies to a nitrided or induction hardened layer. A load that reaches through the case reads the core hardness, and the value comes back low.

The reading has to be taken on a ground flat, away from the sheared or lasered edge, and from material below the decarburised layer. Surface preparation that heats or cold works the surface changes the value that comes back.

Before quoting a hardness value

The limits on this page assume a clean, flat, parallel specimen. A radius, a burr, a sheared edge or a marred anvil all move the result, and the smallest of those effects is larger than the tolerance of the test itself.

The values are published reference data. Confirm the acceptance criteria, the scale and the test location against the standard that governs your own product, and keep the reading inside the valid band of the scale you choose.

Related reference pages on this site: Hardness test correction factors for round bar, Hardness testing methods, Tool steel hardness conversion calculator, Tool steel quality verification, Tool steel supply.

Source: ASM Handbook, Volume 8, Mechanical Testing and Evaluation (ASM International), article Macroindentation Hardness Testing by Edward L. Tobolski, Wilson Instruments Division, Instron Corporation and Andrew Fee, Consultant, and article Selection and Industrial Applications of Hardness Tests by Andrew Fee, Consultant. Values are reproduced for reference only. Aobo Steel supplies tool steel in the annealed condition.