Rockwell hardness testing

In Rockwell hardness testing, the hardness of a material is determined by pressing an indenter against the material be tested with a predefined force. The hardness of the material can then be determined by measuring the indentation depth.

Development

The hardness test is named for its eponymous inventors, Stanley P. Rockwell and Hugh M. Rockwell. Working in 1914 as metallurgists at New Departure Manufacturing Co., they were looking for a quick and simple way to test the hardness of bearing races.

While a number of methods for testing material hardness were available in the early twentieth century, these methods were either very time-consuming (Brinell hardness test) or very complicated to apply – or simply not suited for materials at the harder end of the scale.

The Rockwells took nearly 5 years to develop the first test unit, which was patented in February 1919. In the same year, Stanley P. Rockwell moved to another employer, where he patented another, improved test unit in September 1919.

The true potential of the device was recognised by Charles H. Wilson. He developed it further, making a number of important modifications, including the introduction of a diamond cone as a second indenter and increasing the maximum test load from 100 kg to 150 kg.

Wilson also organised the production and distribution of the tester, which led to the rapid adoption of the Rockwell testing method.

Test materials

A great many test methods have been developed for Rockwell hardness testing, which can be adapted to a wide variety of materials by selecting the right indenter and the right level of force to apply to the test subject. Before the test begins, the prescribed indenter must first be selected for the material to be tested. For Rockwell testing, a total of three indenters are available:

  • Ball indenter with a 1/16-inch diameter (≈1.6 mm)
  • Ball indenter with an 1/8-inch diameter (≈3.2 mm)
  • Diamond cone with sides angled at 120° and a ball tip of 0.2 mm

The test load to be applied must also be chosen to suit the material to be tested. To simplify hardness testing, several standardised test methods have been developed, in which both the test load and the test indenter to be used can simply be read off from a table.

Regular Rockwell

The following test methods are suitable for a wide range of steels, cast irons, aluminium and magnesium alloys, and for bronze and copper.

Hardness test Indenter Preload Additional load Total load Materials
HRA Cone 98,07 N 490,30 N 588,37 N hardened / stress relieved steel
HRB Ball 1/16″ 98,07 N 882,60 N 980,67 N structural steel / non-ferrous metal
HRC Cone 98,07 N 1373,00 N 1471,07 N ardened / stress relieved steel
HRD Cone 98,07 N 882,60 N 980,67 N surface hardened materials
HRE Cone1/8″ 98,07 N 882,60 N 980,67 N Cast iron, aluminum, magnesium alloys, bearing metals
HRF Ball 1/16″ 98,07 N 490,30 N 588,37 N thin sheets from 0.6 mm and annealed copper alloys
HRG Ball 1/16″ 98,07 N 1373,00 N 1471,07 N bronze and copper
HRH Cone 1/8″ 98,07 N 490,30 N 588,37 N aluminum, zinc and lead
HRK Ball 1/8″ 98,07 N 1373,00 N 1471,07 N bearing and non-ferrous metals

Superficial Rockwell

This test method features a very small indentation depth, making it suitable for use with case-hardened materials as well as very thin sheet metals.

Hardness test Indenter Preload Additional load Total load Materials
HR15N Kegel 29,42 N 117,70 N 147,12 N Materials with thin case hardening, otherwise like HRA
HR30N Cone 29,42 N 264,80 N 294,22 N Materials with thin case hardening, otherwise like HRD
HR45N Cone 29,42 N 411,90 N 441,32 N Materials with thin case hardening, otherwise like HRC
HR15T Ball 1/16″ 29,42 N 117,70 N 147,12 N Thin sheets, otherwise like HRF
HR30T Ball 1/16″ 29,42 N 264,80 N 294,22 N Thin sheets, otherwise like HRB
HR45T Ball 1/16″ 29,42 N 411,90 N 441,32 N Thin sheets, otherwise like HRG

Performing a hardness test

Once preparations are complete, the actual test can now be performed.

First, the material sample is brought against the indenter until the minor load (preload) is achieved. Since the metric used is indentation depth, the depth measurement instrument must be zeroed once the minor load is achieved.

The indenter is then pressed into the material using the major (additional) load. The duration (dwell time) for load application depends on the material’s propensity for elastic recovery. The correct dwell time for the load can also be found by consulting the tables. In most cases, application of the test load is maintained for 2 to 8 seconds.

After this time, the major load is removed until the value of the minor load is achieved again. This approach eliminates any inaccuracies that may be traceable back to the plastic deformation of the apparatus. The hardness can now be read off the Rockwell tester’s dial gauge.

Advantages

  • The measurement procedure is quick and very easy to perform.
  • Hardness can be read off directly.
  • Also usable for very hard materials.
  • Minimal damage to material samples.
  • Automated testing is possible.

Drawbacks

  • Rates of error increase for thin materials in particular due to the low indentation depth.
  • Large number of discrete test methods and variables.
  • Not suitable for testing softer materials.

Examples

Material Description Rockwell-Hardness
1.0037 S235JR 55 HRC
Hardox 400 40 HRC
Hardox 450 44,5 HRC
Hardox 500 49 HRC
Hardox 600 55 HRC
1.4878 X8CrNiTi 18 10 95 HRB
1.4301 X5CrNi 18 10 90 HRB
3.3535 AlMg 3 49 HRB
3.3547 AlMg 4,5 Mn 53 HRB
1.4828 X15CrNiSi 20 12 85 HRB
1.4571 X6CrNiMoTi 17 12 2 80 HRB
CW009A Cu OFE 40 HRB