ROCKWELL

Theory of the Rockwell hardness test

INTRODUCTION

SCOPE

The Rockwell hardness test is an empirical indentation hardness test that can provide useful information about metallic materials. This information may correlate to tensile strength, wear resistance, ductility, and other physical characteristics of metallic materials, and may be useful in quality control and selection of materials.

PRINCIPLE OF THE ROCKWELL HARDNESS TEST

An indentation hardness test using a verified machine to force a diamond spheroconical indenter or tungsten carbide (or steel) ball indenter, under specified conditions, into the surface of the material under test, and to measure the difference in depth of the indentation as the force on the indenter is increased from a specified preliminary test force to a specified total test force and then returned to the preliminary test force. The test is divided into three steps of force application and removal:

  1. The indenter is brought into contact with the test specimen, and the preliminary test force is applied. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.
  2. The force on the indenter is increased at a controlled rate by the additional test force to achieve the total test force. The total test force is held for a specified dwell time.
  3. The additional test force is removed, returning to the preliminary test force. After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the final and baseline indentation depths while under the preliminary test force. The preliminary test force is removed and the indenter is removed from the test specimen.

CALCULATION OF THE ROCKWELL HARDNESS NUMBER

During a Rockwell test, the force on the indenter is increased from a preliminary test force to a total test force, and then returned to the preliminary test force. The difference in the two indentation depth measurements, while under the preliminary test force, is measured in mm. From the value in mm, the Rockwell hardness number is derived. The Rockwell hardness number is an arbitrary number, which, by method of calculation, results in a higher number for harder material.

CLASSIFICATIONS OF THE ROCKWELL TEST

The Rockwell hardness test and the Rockwell superficial hardness test. The significant difference between the two test classifications is in the test forces that are used. For the Rockwell hardness test, the preliminary test force is 10 kgf (98 N) and the total test forces are 60 kgf (589 N), 100 kgf (981 N), and 150 kgf (1471 N). For the Rockwell superficial hardness test, the preliminary test force is 3 kgf (29 N) and the total test forces are 15 kgf (147 N), 30 kgf (294 N), and 45 kgf (441 N).

REPRESENTATION OF ROCKWELL HARDNESS VALUE

Rockwell hardness values shall not be designated by a number alone because it is necessary to indicate which indenter and forces have been employed in making the test. The hardness number is followed by the symbol HR and the scale designation. Examples: 64 HRC = Rockwell hardness number of 64 on Rockwell C scale, 81 HR30N = Rockwell superficial hardness number of 81 on the Rockwell 30N.

ROCKWELL HRC HARDNESS TEST

INDENTER HRC

Diamond cone with an angle of 120° at the summit and ending with a radius of 0,2 mm. Mirror polished.

DEFINITION

An initial force called “pre-load” of 98,1 N ( Fo) is applied; in this phase the indent depth reader must be zeroed. This phase is necessary in order to settle the test piece, after which a supplementary force is applied (F1) until a full load (F) of 588,6 or 981 or 1471,5 N is applied. Once the time required for the depth indicator to stop has passed, the supplementary load (F1) is removed leaving only the initial load (Fo).

The Rockwell C Hardness Test

ROCKWELL HRB HARDNESS TEST

INDENTER HRB

Tungsten carbide W ball with a hardness not less than 850 HV and a diameter of 1/16” (1,5875mm).

DEFINITION

As “definition of the diamond cone test” and taking as reference zero of the scale 130 units. This will allow you to have 30 more points to allow tests on very soft materials where the sphere will penetrate a lot. Read the number indicated by the reader and subtract from 130 and consider it as hardness in Rockwell units. In fact, the value 130 in the Rockwell sphere scale corresponds to maximum hardness.

ROCKWELL AND SUPERFICIAL ROCKWELL MINIMUM THICKNESS

WHAT IS THE SUPERFICIAL ROCKWELL HARDNESS TEST?

Superficial Rockwell hardness testing follows the same procedures as the standard Rockwell Hardness test. The main difference is in the minor test forces applied in each stage. Superficial scales HRN (diamond indenter) and HRTW (ball indenter) have a preliminary test force of 3 kgf (29 N) and total test forces of 15 kgf (147 N), 30 kgf (294 N), and 45 kgf (441 N).

The Superficial Rockwell hardness test is a more surface sensitive hardness measurement scale.  This hardness scale is suitable for samples with hardness gradients at the surface, coatings, layers, to test small areas and for thin components.  Superficial Rockwell hardness scales HRN and HRTW are used to test metals and HRW, HRX, and HRY for nonmetallic materials and coatings.

MINIMUM THICKNESS MEASURABLE

It is recommended that the thickness should exceed 10 times the depth of indentation. In general, no deformation should be visible on the back of the test piece after the test, although not all such marking is indicative of a bad test.
Special consideration should be made when testing parts were case-hardened by processes such as carburizing, carbonitriding, nitriding, induction, etc

ROCKWELL AND SUPERFICIAL ROCKWELL SCALES

ScaleIndenterMain Load (N)Pre Load (N)Application
HRBBall 1/16"98198,1Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
HRCDiamond cone 120°1471,598,1Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, and other materials harder than HRB100.
HRADiamond cone 120°588,698,1Cemented carbides, thin steel, and shallow case-hardened steel.
HRDDiamond cone 120°98198,1Thin steel and medium case hardened steel, and pearlitic malleable iron.
HREBall 1/8"98198,1Cast iron, aluminum and magnesium alloys, bearing metals.
HRFBall 1/16"588,698,1Annealed copper alloys, thin soft sheet metals.
HRGBall 1/16"1471,598,1Malleable irons, copper-nickel-zinc and cupro-nickel alloys. Upper limit G92 to avoid possible flattening of ball.
HRHBall 1/8"588,698,1Aluminum, zinc, lead.
HRKBall 1/8"1471,598,1Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that does red not give anvil effect.
HR 45 NDiamond cone 120°441,429,43Thin components and layers
HR 30 NDiamond cone 120°294,329,43Thin components and layers
HR 15 NDiamond cone 120°147,129,43Thin components and layers
HR 45 TBall 1/16"441,429,43Thin components and layers
HR 30 TBall 1/16"294,329,43Thin components and layers
HR 15 TBall 1/16"147,129,43Thin components and layers

MAXIMUM ALLOWABLE REPEATABILITY AND ERROR

ScaleHardness rangeMaximum Repeatability R
(HR units)
Maximum Error E
(HR units)
HRA< 702.0± 1.0
>= 70 and < 801.5± 1.0
>= 801.0± 0.5
HRB< 602.0± 2.5
>= 60 and < 881.5± 2.5
>= 881.5± 1.0
HRC< 352.0± 1.0
>= 35 and < 601.5± 1.0
>= 601.0± 0.5
HR 15 N< 782.0± 1.0
>= 78 and < 901.5± 1.0
>= 901.0± 0.7
HR 30 N< 552.0± 1.0
>= 55 and < 771.5± 1.0
>= 771.0± 0.7
HR 45 N< 372.0± 1.0
>= 37 and < 661.5± 1.0
>= 661.0± 0.7
HR 15 T< 812.0± 1.5
>= 81 and < 871.5± 1.0
>= 871.5± 1.0
HR 30 T< 572.0± 1.5
>= 57 and < 701.5± 1.0
>= 701.5± 1.0
HR 45 T< 332.0± 1.5
>= 33 and < 531.5± 1.0
>= 531.5± 1.0

TERMINOLOGY & TIPS

Calibration

Determination of the values of the significant parameters by comparison with values indicated by a reference instrument or by a set of reference standards.

Indenters

The standard Rockwell indenters are either diamond spheroconical indenters or tungsten carbide balls of 1.588 mm (1⁄16 in.), 3.175 mm (1⁄8 in.), 6.350 mm (1⁄4 in.), or 12.70 mm (1⁄2 in.) in diameter. Dust, dirt, or other foreign materials shall not be allowed to accumulate on the indenter, as this will affect the test results.

Portable Rockwell hardness testing machine

A Rockwell hardness testing machine that is designed to be transported, carried, set up, and operated by the users, and that measures Rockwell hardness by the Rockwell indentation hardness test principle.

Rockwell hardness machine

A machine capable of performing a Rockwell hardness test and/or a Rockwell superficial hardness test and displaying the resulting Rockwell hardness number.

Specimen Support

A specimen support or “anvil” shall be used that is suitable for supporting the specimen to be tested. The seating and supporting surfaces of all anvils shall be clean and smooth and shall be free from pits, deep scratches, and foreign material. If the anvil is damaged from any cause, it shall be repaired or replaced. Common specimen support anvils should have a minimum hardness of 58 HRC.
Flat pieces should be tested on a flat anvil that has a smooth, flat bearing surface whose plane is perpendicular to the axis of the indenter.
Small diameter cylindrical pieces shall be tested with a hard V-grooved anvil with the axis of the V-groove directly under the indenter, or on hard, parallel, twin cylinders properly positioned and clamped in their base.
Special anvils or fixtures, including clamping fixtures, may be required for testing pieces or parts that cannot be supported by standard anvils.

Standardization

To bring in conformance to a known standard through verification or calibration.

Test Piece

For best results, both the test surface and the bottom surface of the test piece should be smooth, even and free from oxide scale, foreign matter, and lubricants. An exception is made for certain materials such as reactive metals that may adhere to the indenter. In such situations, a suitable lubricant such as kerosene may be used. The use of a lubricant shall be defined on the test report.
Preparation shall be carried out in such a way that any alteration of the surface hardness of the test surface (for example, due to heat or cold-working) is minimized.

The repeatability R

In the performance of a Rockwell hardness machine at each hardness level, under the particular verification conditions, is estimated by the range of n hardness measurements made on a standardized test block as part of a performance verification.

Verification

Checking or testing to assure conformance with the specification. Rockwell testing machines shall be verified periodically using test blocks.

AFFRI ROCKWELL HARDNESS TESTERS:

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