Rockwell Hardness Test for Metals – Definition – Principles, Standards and Applications

What is Rockwell Hardness test?

Rockwell hardness tester gives hardness numbers directly. Minor load of 10 kgf is first applied and then a major load (60, 100, 150 kgf) according to the hardness of the materials, is applied for 5 – 10 sec and after removing both loads gradually, the two observed depths are then subtracted to have the Rockwell hardness number. Diamond cone or steel ball with four sizes are used as indenters in this tester with 30 different scales. A copper and two plain carbon sheets of steel (AISI 1040). Rockwell hardness tester gives hardness numbers directly.

Minor load of 10 kgf is first applied and then a major load (60, 100, 150 kgf) according to a hardness of the materials, is applied for 5 – 10 sec and after removing both loads gradually, the two observed depths are then subtracted to have the Rockwell hardness number. Diamond cone or steel ball with four sizes are used as indenters in this tester with 30 different scales. A copper and two plain carbon sheets of steel (AISI 1040 & 1045) samples are evaluated, obtained hardness numbers are compared to get results.  

Rockwell hardness test is a non-destructive test used for soft as well as hard samples. For identification of phases, Vicker Hardness Test is used. Other than the Rockwell hardness test, the Brinell hardness test is also used industrially. But, the Brinell hardness test is considered s destructive hardness test while the Rockwell test is considered a non-destructive test.

Rockwell Hardness Testing

One of the widely used methods for hardness measurement is Rockwell hardness test because it gives direct final reading. Its Basic characters which make the process efficient are:

• Fast speed of testing
• relatively inexpensive
• lesser deformation size

Rockwell Hardness Test Standard

The following are the standards for the Rockwell Hardness test, which describes the testing procedure and its specifications.

• ASTM E 18
• ISO 6508

On comparing with other testing methods, the Rockwell testing can also yield pieces of information about metallic materials` properties, like tensile strength, wear hardness, and draw- ability.

Rockwell hardness tester Principle

In the case of Rockwell hardness testing, hardness is measured as the function of a depth of plastically deformed region as the load is applied. Hardness is described as a unitless number enlisted on a scale. Scale gives a range of hardness. Unlike Brinell hardness testing, it is achieved by differentiating the depths of indentations when minor and major loads are applied for a specific time and at a specific rate.

Theory of Rockwell hardness test

Rockwell Hardness Test is carried out by making contact of material with the indenter and applying a 10 kg minor load, this results in an indenter to hold the specimen firmly at a place. After that depth of impression is observed. After then resetting the dial to zero position, a constant major load is applied for some time called the dwell time. This results in the formation of the indent of a specific shape.

The major load is then removed and keeping the sample under minor loading, depth of indentation is measured. After measuring depth for the second time, the minor load is also removed and the Rockwell hardness number is measured subtracting the two depths observed. By measuring the depth actually dial gives direct hardness reading.

Rockwell hardness scales

Due to diversity in the metals or generally materials (based on hardness), ASTM defines about 30 scales which are developed to accommodate these materials of different hardness. These scales consist of indenter type, specifications, load range and time for testing.

These 30 scales are divided into subgroups named as regular and superficial Rockwell scales. The difference in both scales is the application of load and samples to be tested. Relatively lighter loads and thin specimens are preferred for superficial scales and larger loads along with relatively thick samples go under regular Rockwell scales. For ease generally, 3 of these scales are used i.e. A- scale, B-scale, and C-scale. Further detail on Rockwell scale can be studies on wikipedia.

Limitations of rockwell hardness test

• Diamond indenters are not calibrated below values of 20. For less than 20 shift to the B scale.
• For 100 to 130 we can use B scale but it may damage the indenter. Due to which sensitivity of Rockwell machine decreases.
• C scale cannot be used on the tungsten carbide, material may damage. For this A scale can be used.

Rockwell Indenters

Two types of indenters are usually available for Rockwell hardness testing.

• Brale

It is a diamond shaped spheroconical indenter with 120^o included angle between opposite faces and a very fine tip of round 0.4mm diameter

• Hardened ball

Steel ball with different diameters can be used. For example (1/16 of an inch, 1/8 of an inch, 1/4 of an inch and 1/2 of an inch).

Is Rockwell Hardness Test Destructive?

In the Brinell hardness test, an indent on a sample is too large which results in loss of not only the aesthetics of component but also results in loss of tribological behavior. While in the Rockwell test, the indent is too small to become visible under normal circumstances. That’s why the Rockwell hardness test is considered one of the efficient hardness test for non-destructive hardness on an industrial scale.

Working of dial gauge

The depth of indentation is recorded on a dial gauge in terms of Rockwell hardness number. The dial contains 100 divisions. The dial gauge of the machine is provided with red and black scales with a long pointer. The red scale is used for hardness readings obtained with ball indenter and the black scale is used for diamond indenter. The reading on the dial is inversely proportional to the depth of indentation. In the case of soft materials, the depth of indentation is more, but the dial shows the smaller hardness number, Whereas, in case of hard materials , depth of indentation is less and dial gives higher Rockwell hardness number,

Depth measurement for B and C scales Total number of divisions on dial gauge = 100 1division = 0.1mm

Depth of 1 division = 0.1 x 0.02

= 0.002 mm

For B scale

Depth of indent = (130 – HRB) x 0.002 mm

Where HRB is Rockwell hardness number by B scale

For C scale

Depth of indent = (100 – HRC) x 0.002 mm

Where HRC is Rockwell hardness number by C scale

Dial Errors

Upon time if rockwell hardness testing machine lose its calibration, then the percentage error is calculated by using a calibration sample. The calibration sample of known hardness is tested onto the machine and %age factor is calculate using the observed and the known hardness. For further readings percentage factor is added to obtain original hardness.

Procedure for Rockwell hardness test

1. Suitable steel and copper specimen were selected.
2. These samples were ground in order to remove grease or any foreign material.
3. The sample was then placed on the object table under the indenter of testing machine.
4. The specimen was then brought in contact with the indenter by turning the wheel of the machine until the small needle on the dial stopped at the red dash.
5. A minor load of 10kgf was applied by doing so.
6. The major load was then applied by pushing back the crank handle.
7. Due to this, the needle moved in the anti-clockwise direction and when it was on rest position, the load was removed by pulling the crank handle again in forward direction.
8. The dial reading was our required Rockwell hardness number.

Observations and Calculations

For steel (AISI 1040)

Indenter: Diamond cone Scale used:

C – scale (Black) Minor load = 10 kgf

Major Load = 140 kgf

Rockwell hardness No. = 29 HRC

Depth of Indent:

h = (100-HRC) × 0.002

h = 0.142mm

For Steel (AISI 1045)

Indenter: Diamond cone Scale used:

C – scale (Black) Minor load = 10kgf

Major Load = 140 kgf

Rockwell hardness No. = 30.5 HRC

Depth of Indent:

h = (100-HRC) × 0.002

h = 0.139mm

For Copper

Indenter: 1/16 inches steel ball Scale used:

B – scale (Red) Minor load = 10 kgf

Major Load = 90 kgf

Rockwell hardness No. = 64.5 HRB

Depth of Indent:

h = (130 – HRB) × 0.002

h = 0.131mm

Precautions

• Avoid testing of too thin samples because due to this the steel anvil may be damaged due to indentation and cannot be used for further testing.
• In order to check the hardness of a surface feature such as case hardened surface, the scale should be chosen according to the thickness of this surface feature.

Advantages of Rockwell Hardness Test

• Hardness value can be achieved in lesser time as compared to other hardness testing methods.

• Direct reading of hardness number is obtained through the dial of machine.
• This machine is used very often for routine work.
• It gives the hardness values by considering the depth of the indent.
• It can also be used to check the hardness of cylindrical specimens.
• A wide range of sample stages can be adjusted into this machine.
• It is less destructive test as compared to Brinell hardness test due small size of indent.

Limitations of Rockwell Hardness Test

• Due to large number of scales, care must be taken in order to select a suitable scale for a suitable material.
• Sensitivity of test is low in case of ball indenters as the possibility of the ball flattening is increased.
• At high hardness ends of the scales, the life of diamond indenter may be reduced by fracturing.

Discussion

Comparison with Literature

For copper, the Brinell equivalent hardness number for 64.5 HRB is 119 HBN. This is deviated from the literature i.e. 16-100 HB. The possible reasons could be that the copper is alloyed or is impure.

For steel specimens, the Brinell equivalent hardness number for 29 HRC and 30.5 HRC is 277.81 HBN and 287 HBN respectively which coincide with literature that ranges from 48-300 HB.

Comparison with One-another

The copper specimen is softer than steel AISI 1040 and AISI 1045, so B scale is used. The HRC Equivalent of obtained hardness value 64.5 HRB is << 21, hence it is proved that copper is very much softer that the two steel specimens

For steel specimens, AISI 1045 contains 0.05% carbon greater that AISI 1040 steel so it would be harder. This is proved by this experiment as the HRC for AISI 1045 is 30.5 which is greater than HRC 29, the hardness number for AISI 1040. Destructive test basically results in either sample failure, loss of functionality or some tribological property.