Hardness Testing

CONTENTS

1. CONSIDERATIONS ON HARDNESS TESTING

2. WHAT IS HARDNESS TESTING?

3. ROCKWELL HARDNESS TEST

4. ROCKWELL SUPERFICIAL HARDNESS TEST

5. THE BRINELL HARDNESS TEST

6. VICKERS HARDNESS TEST

7. MICROHARDNESS TEST

8. HANDHELD TESTERS

9. CONVERSIONS AND COMPARISONS OF HARDNESS VALUES

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1. CONSIDERATIONS ON HARDNESS TESTING

WHAT TYPE OF TESTER SHOULD I USE?

The more the operator can be removed from the test process, the better the result will be. A portable device should therefore only be used if the use of a bench tester is not going to be possible.

Bench testers offer greater stability, rigidity and consistency of operation. If your test piece is capable of being placed in a bench tester, or if you can take a sample for testing in a bench tester, then this will be the best option.

Better still are the digital and motorised machines that take out operator error by either instructing the user when to carry out each stage of the test, or remove any operator input apart from loading the sample and pressing “start”.

Portables are becoming very accurate, but they still require an operator to hold them in the correct orientation, and the relative low load is susceptible to any surface or material faults.

Portable testers that can fit in stands or rigid frames will produce better results than free hand operation – although sometimes a portable machine is the only way a test can be carried out.

If the component needs to be measured accurately to HV/10 for example, then the best option is to use a

machine that can test to HV/10. Comparative tests are possible, but as mentioned in section 9, they can leave room for errors. It is possible to make test samples to compare a bench test to a portable test to help reduce this problem.

ROCKWELL, VICKERS, BRINELL, LEEB, KNOOP, MICRO, MACRO, UCI, BENCH, PORTABLE...?

The amount of machines and methods available can be confusing. The important thing to do is choose the test method that bests suits your component.

For example, a Brinell 3000kg load behind a 10mm ball will be of no use for testing thin case hardness, and likewise a Micro-Vickers or Knoop test will be no good on coarse grained materials with a poor surface finish.

Using this guide will help you start to understand hardness testing, and go some way into helping you select the method that could be best suited for.

Bowers Metrology can help walk you through the various testers and methods to help you select the set up that will best suit your purpose.

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CV Rockwell Basic Digital Hardness Tester 600BD

Eseway® Premium Closed Loop Rockwell

Hardness Tester EW6000

2. WHAT IS HARDNESS TESTING?

WHAT IS HARDNESS?

Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting.

MEASUREMENT OF HARDNESS:

Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure.

Hardness of materials has probably long been assessed by resistance to scratching or cutting. An example would be material B scratches material C, but not material A. Alternatively, material A scratches material B slightly and scratches material C heavily. Relative hardness of minerals can be assessed by reference to the Mohs Scale that ranks the ability of materials to resist scratching by another material. Similar methods of relative hardness assessment are still commonly used today.

An example is the file test where a file tempered to a desired hardness is rubbed on the test material surface. If the file slides without biting or marking the surface, the test material would be considered harder than the file. If the file bites or marks the surface, the test material would be considered softer than the file.

The above relative hardness tests are limited in practical use and do not provide accurate numeric data or scales particularly for modern day metals and materials. The usual method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape, with a specific force applied for a specific time. There are three principal standard test methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Vickers, and Rockwell. For practical and calibration reasons, each of these methods is divided into a range of scales, defined by a combination of applied load and indenter geometry.

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CV Portable Digital Hardness TesterHardness Test Blocks

3. ROCKWELL HARDNESS TEST

The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load F0 (Fig. 1A) usually 10 kgf.

When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position.

While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration (Fig. 1B).

When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration (Fig. 1C).

The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number.

HR = E - e

F0 = preliminary minor load in kgfF1 = additional major load in kgfF = total load in kgfe = permanent increase in depth of penetration due to major load F1 measured in units of 0.002 mmE = a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenterHR = Rockwell hardness numberD = diameter of steel ball

Fig. 1.Rockwell Principle

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ROCKWELL HARDNESS SCALES

Scale IndenterMinor Load

F0kgf

Major LoadF1kgf

Total LoadF

kgf

Value ofE

A Diamond cone 10 50 60 100

B 1/16" steel ball 10 90 100 130

C Diamond cone 10 140 150 100

D Diamond cone 10 90 100 100

E 1/8" steel ball 10 90 100 130

F 1/16" steel ball 10 50 60 130

G 1/16" steel ball 10 140 150 130

H 1/8" steel ball 10 50 60 130

K 1/8" steel ball 10 140 150 130

L 1/4" steel ball 10 50 60 130

M 1/4" steel ball 10 90 100 130

P 1/4" steel ball 10 140 150 130

R 1/2" steel ball 10 50 60 130

S 1/2" steel ball 10 90 100 130

V 1/2" steel ball 10 140 150 130

TYPICAL APPLICATION OF ROCKWELL HARDNESS SCALES

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CV Rockwell Basic Digital Hardness Tester 600BD

Eseway Premium Rockwell Hardness Tester EW650

Advantages of the Rockwell hardness method include the direct Rockwell hardness number readout and rapid testing time.

Disadvantages include many arbitrary non-related scales and possible effects from the specimen support anvil (try putting a cigarette paper under a test block and take note of the effect on the hardness reading! Vickers and Brinell methods don't suffer from this effect).

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4. ROCKWELL SUPERFICIAL HARDNESS TEST

The Rockwell Superficial hardness test method consists of indenting the test material with a diamond cone (N scale) or hardened steel ball indenter.

The indenter is forced into the test material under a preliminary minor load F0 (Fig. 1A) usually 3 kgf.

When equilibrium has been reached, an indicating device that follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position.

While the preliminary minor load is still applied an additional major load, is applied with resulting increase in penetration (Fig. 1B).

When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained.

Removal of the additional major load allows a partial recovery, so reducing the depth of penetration (Fig. 1C).

The permanent increase in depth of penetration, e, resulting from the application and removal of the additional major load is used to calculate the Rockwell Superficial hardness number.

HR = E - e

F0 = preliminary minor load in kgfF1 = additional major load in kgfF = total load in kgfe = permanent increase in depth of penetration due to major load F1, measured in units of 0.001 mmE = a constant of 100 units for diamond and ball indentersHR = Rockwell hardness numberD = diameter of steel ball

Fig. 1.Rockwell Superficial Principle

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ROCKWELL SUPERFICIAL HARDNESS SCALES

Scale Indenter TypeMinor Load

F0kgf

Major LoadF1kgf

Total LoadF

kgf

Value ofE

HR 15 N N Diamond cone 3 12 15 100

HR 30 N N Diamond cone 3 27 30 100

HR 45 N N Diamond cone 3 42 45 100

HR 15 T 1/16" steel ball 3 12 15 100

HR 30 T 1/16" steel ball 3 27 30 100

HR 45 T 1/16" steel ball 3 42 45 100

HR 15 W 1/8" steel ball 3 12 15 100

HR 30 W 1/8" steel ball 3 27 30 100

HR 45 W 1/8" steel ball 3 42 45 100

HR 15 X 1/4" steel ball 3 12 15 100

HR 30 X 1/4" steel ball 3 27 30 100

HR 45 X 1/4" steel ball 3 42 45 100

HR 15 Y 1/2" steel ball 3 12 15 100

HR 30 Y 1/2" steel ball 3 27 30 100

HR 45 Y 1/2" steel ball 3 42 45 100

TYPICAL APPLICATION OF ROCKWELL SUPERFICIAL HARDNESS SCALES

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Eseway® Premium Closed Loop Rockwell Hardness Tester EW6000

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5. THE BRINELL HARDNESS TEST

The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg.

For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals.

The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation.

F= Force (Kgf)

B= Ball diameter (mm)

D= Diameter of Indentation

The diameter of the impression is the average of two readings at right angles and the use of a Brinell hardness number table can simplify the determination of the Brinell hardness. A well structured Brinell hardness number reveals the test conditions, and looks like this, "75 HB 10/500/30" which means that a Brinell Hardness of 75 was obtained using a 10mm diameter hardened steel with a 500 kilogram load applied for a period of 30 seconds.

On tests of extremely hard metals a tungsten carbide ball is substituted for the steel ball. Compared to the other hardness test methods, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structures and any irregularities in the uniformity of the material.

This method is the best for achieving the bulk or macro-hardness of a material, particularly those materials with heterogeneous structures.

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‘King’ Brinell Portable Hardness Tester

CV Brinell Digital Hardness Tester 3000JDB

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6. VICKERS HARDNESS TEST

The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf.

The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated.

The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.

F= Load in kgfd = Arithmetic mean of the two diagonals, d1 and d2 in mm

HV = Vickers hardness

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When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated from the formula, but is more convenient to use conversion tables. The Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of 800, was obtained using a 10 kgf force.

Several different loading settings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness testing methods.

The advantages of the Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments. The disadvantage of this type of test is that when using manual optical measuring devices, different operators can get different readings for the same indentation due to personal optical variations. Newer TV based optical systems have now made this less of a problem, and PC based systems have reduced the operator optical error to almost zero.

There is now a trend towards reporting Vickers hardness in SI units (MPa or GPa) particularly in academic papers. Unfortunately, this can cause confusion. Vickers hardness (e.g. HV/30) value should normally be expressed as a number only (without the units kgf/mm2).

Rigorous application of SI is a problem. Most Vickers hardness testing machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for calculating HV. SI would involve reporting force in newtons (compare 700 HV/30 to HV/294 N = 6.87 GPa) which is practically meaningless and messy to engineers and technicians.

To convert a Vickers hardness number the force applied needs converting from kgf to newtons and the area needs converting form mm2 to m2 to give results in pascals using the formula above.

To convert HV to MPa multiply by 9.807

To convert HV to GPa multiply by 0.009807

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7. MICROHARDNESS TEST

The term microhardness test usually refers to static indentations made with loads not exceeding 1 kgf. The indenter is either the Vickers diamond pyramid or the Knoop elongated diamond pyramid. The procedure for testing is very similar to that of the standard Vickers hardness test, except that it is done on a microscopic scale with higher precision instruments.

The surface being tested generally requires a metallographic finish; the smaller the load used, the higher the surface finish required. Precision microscopes are used to measure the indentations; these usually have a magnification of around X500 and measure to an accuracy of +0.5 micrometres. Also with the same observer differences of +0.2 micrometres can usually be resolved. It should, however, be added that considerable care and experience are necessary to obtain this accuracy.

KNOOP

Knoop Hardness Indenter Indentation

The Knoop hardness number KHN is the ratio of the load applied to the indenter, P (kgf) to the unrecovered projected area A (mm2)

KHN = F/A = P/CL2

Where:F = applied load in kgf

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A = the unrecovered projected area of the indentation in mm2

L = measured length of long diagonal of indentation in mmC = 0.07028 = Constant of indenter relating projected area of the indentation to the square of the length of the long diagonal.

The Knoop indenter is a diamond ground to pyramidal form that produces a diamond shaped indentation having approximate ratio between long and short diagonals of 7:1. The depth of indentation is about 1/30 of its length. When measuring the Knoop hardness, only the longest diagonal of the indentation is measured and this is used in the above formula with the load used to calculate KHN. Tables of these values are usually a more convenient way to look-up KHN values from the measurements.

MICRO-VICKERS

Vickers Pyramid Diamond Indenter Indentation

The Vickers Diamond Pyramid harness number is the applied load (kgf) divided by the surface area of the indentation (mm2)

Where:F= Load in kgfd = Arithmetic mean of the two diagonals, d1 and d2 in mm

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HV = Vickers hardness

The Vickers Diamond Pyramid indenter is ground in the form of a squared pyramid with an angle of 136o

between faces. The depth of indentation is about 1/7 of the diagonal length. When calculating the Vickers Diamond Pyramid hardness number, both diagonals of the indentation are measured and the mean of these values is used in the above formula with the load used to determine the value of HV. Tables of these values are usually a more convenient way to look-up HV values from the measurements.

KNOOP VS. VICKERS

Comparing the indentations made with Knoop and Vickers Diamond Pyramid indenters for a given load and test material:

Vickers indenter penetrates about twice as deep as Knoop indenter Vickers indentation diagonal about 1/3 of the length of Knoop major diagonal

Vickers test is less sensitive to surface conditions than Knoop test

Vickers test is more sensitive to measurement errors than knoop test

Vickers test best for small rounded areas

Knoop test best for small elongated areas

Knoop test good for very hard brittle materials and very thin sections

There is now a trend towards reporting Vickers and Knoop hardness in SI units (MPa or GPa) particularly in academic papers. Unfortunately, this can cause confusion.

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Vickers hardness (e.g. HV/30) value should normally be expressed as a number only (without the units kgf/mm2).

Rigorous application of SI is a problem. Most Vickers hardness testing machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for calculating HV. SI would involve reporting force in newtons (compare 700 HV/30 to HV/294 N = 6.87 GPa) which is practically meaningless and messy to engineers and technicians.

To convert a Vickers hardness number the force applied needs converting from kgf to newtons and the area needs converting form mm2 to m2 to give results in pascals using the formula above.

To convert HV to MPa multiply by 9.807 To convert HV to GPa multiply by 0.009807

8. HANDHELD TESTERS

THE SCLEROSCOPE AND LEEBS TEST (REBOUND HARDNESS TESTING)

The Scleroscope test consists of dropping a diamond tipped hammer, which falls inside a glass tube under the force of its own weight from a fixed height, onto the test specimen.

The height of the rebound travel of the hammer is measured on a graduated scale. The scale of the rebound is arbitrarily chosen and consists on Shore units, divided into 100 parts, which represent the average rebound from pure hardened high-carbon steel. The scale is continued higher than 100 to include metals having greater hardness.

In normal use the shore scleroscope test does not mark the material under test. The Shore Scleroscope measures hardness in terms of the elasticity of the material and the hardness number depends on the height to which the hammer rebounds, the harder the material, the higher the rebound. The Scleroscope is a difficult tester to use and has largely been superseded by the Leeb style tester.

The Leeb test is a modern version of the Scleroscope. It uses a spring loaded carbide ball hammer rather than the gravity system of the Scleroscope.

An electronic sensor measures the velocity of the hammer as it travels toward and away from the surface of the material being tested. The obtained figure is a Leeb hardness that can be related to other hardness scales as such a majority of Leeb testers have the inbuilt ability in their electronics to convert to more common hardness scales such as Brinell, Vickers and Rockwell.

LEEB VALUE = HAMMER REBOUND VELOCITY / IMPACT VELOCITY x 1000

The main limitations are that the items to be tested must have a

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CV Portable Hardness Tester - TH-160

certain mass and thickness to ensure correct readings. Leeb testers are portable and can cover a wide range of material test circumstances. They can be used at different angles as long as they are perpendicular to the test surface, as many testers have inbuilt angle correction.

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THE DUROMETER

The Durometer is a popular instrument for measuring the indentation hardness of rubber and rubber-like materials. The most popular testers are the Model A used for measuring softer materials and the Model D for harder materials.

The operation of the tester is quite simple. The material is subjected to a definite pressure applied by a calibrated spring to an indenter that is either a cone or sphere and an indicating device measures the depth of indentation.

THE UCI METHOD

The Ultrasonic Contact Impedance (UCI) hardness test method uses a spring to a set load to a Vickers indenter, which is in turn attached to the end of a resonating probe.

As the probe and Vickers indenter penetrate the test sample the frequency of vibration changes in the probe. This change is measured and can be related to the depth of penetration of the Vickers indenter into the sample.

The results are electronically converted to other hardness scales such as Brinell, Vickers and Rockwell. The advantage to these instruments is the accuracy, portability and range of materials that can be tested - although the test sample has a smooth surface and be at least 12mm thick.

DIRECT LOAD METHOD

This mechanical system uses a direct load of about 150N onto an indenter. The machine is operated by pressing fully down on the handles on either side of the tester. The reading of the indenter depth is then registered on either an analogue or

digital display.

Some of these direct load handheld testers have the ability to convert to several different scales. Due to the mechanical construction and operation, these testers can be used in any orientation, although they require a stable sample surface to work correctly. For more accurate readings and for measuring smaller components a light weight portable bench stand can be used.

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Vickers Portable Hardness Tester 'Ultramatic' CV-HV400

CV Digital Durometer and Stand

CV Instrumatic Portable Analogue

Hardness Tester

CV Portable Digital Hardness Tester

9. CONVERSIONS AND COMPARISONS OF HARDNESS VALUES

Hardness conversion between different methods and scales cannot be made mathematically exact for a wide range of materials.

Different loads, different shape of indenters, homogeneity of specimen, cold working properties and elastic properties all complicate the problem.

All tables and charts should be considered as giving approximate equivalents, particularly when converting to a method or scale which is not physically possible for the particular test material and thus cannot be verified.

For example, a thin item may not stand up to HRC testing, but a comparison can be made from HV/10 or HR15T to HRC.

Problems can occur though. In the diagram below the HRC and Vickers test on the sample with no surface hardness should give a good comparison, as the material is even throughout.

The first test shows a good comparison:

The HRC test may give a comparative reading of 55HRC - 602HV/10.

The HV test may give a comparative reading of 601HV/10 – 55.3HRC.

The surface hardened example though demonstrates why it is always best (where possible) to use the correct hardness test for the material in question. The HRC indenter penetrates the surface hardened layer and measures into the softer material below. The Vickers test fails to penetrate the hardened surface and therefore only measures the hard top surface.

The second test shows a poor comparison:

The HRC test may give a comparative reading of 59HRC - 680HV/10.

The HV test may give a comparative reading of 880HV/10 – 66.4HRC.

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HARDNESS CONVERSION TABLES AND CHARTS

Hardness Conversion Table

Hardness Scale Relationship Chart

Rockwell Hardness Comparison Chart

Brinell and Vickers Hardness Scale and Tensile Strength Equivalents

Brinell Hardness, Vickers Hardness and Tensile Strength Equivalents

Hardness Conversion Table - Rockwell C Hardness Scale (hard materials)

Hardness Conversion Chart - Rockwell C Hardness Scales (hard materials)

Estimated Hardness Equivalent Chart - Rockwell C and Vickers (hard materials)

Hardness Conversion Table - Rockwell B Hardness Scale (soft metals)

Hardness Conversion Chart - Rockwell B Hardness Scale (soft metals)

Hardness Charts with Calculators for Hardness and Depth (.xls file)

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