Contents:
1 General notes on hardness testing 2 The Rockwell procedure    2.1 Standard Rockwell    2.2 Superficial Rockwell    2.3 Fields of application with different Rockwell scales    2.4 Tests on cylindric and spherical surfaces    2.5 Pros and cons of the Rockwell procedure    2.6 Variations of the Rockwell procedure 3 The Brinell method    3.1 The labelling of Brinell tests    3.2 Different applications of Brinell tests    3.3 Pros and cons of the Brinell procedure    3.4 Rockwell tests with Brinell loads and Brinell penetrators 4 The Vickers method    4.1 Applications of different Vickers test loads    4.2 Pros and cons of the Vickers procedure 5 Other hardness testing procedures    5.1 The Shore procedure (for metals)    5.2 The Knoop method 6 Revaluation tables and hardness comparison plates    6.1 The use of revaluation tables    6.2 The use of hardness comparison plates

1 General Notes on Hardness Testing     Among the different kinds of measurements that are carried out in a laboratory, hardness testing is one of the most complex ones. On the one hand, there are different measurement procedures; on the other hand, it is necessary to measure large, small, hard, soft, thin or thick metal parts. Considering the different procedures and the large number of scales, it is understandable that even very experienced persons can be challenged by hardness testing tasks. As in so many other areas of application, electronic development has led to a significant simplification of hardness testing. With computer-aided hardness testers, a higher precision during result readout, data storage and the possibility of data processing to statistics, graphic representations, documentation, etc. have become a matter of course. However, electronics are still only used for results readout (and, if necessary, automation of the measuring / drive), while the different mechanic testing results are still applicable. Although later we will be talking about definitions, advantages and disadvantages of the Rockwell, Brinell, and Vickers procedures, it is still beneficial to deal briefly with the most important features that should be considered before buying such a device, here in the introduction:  1) the total test load; 2) the hardness range; 3) accuracy, 4) the flexibility of the device with regard to forms and dimensions of specimens, and 5) economic aspects.  Total test load On the one hand, there is the general rule to use a test load as high as possible. This allows for higher accuracy (because the measurement is less sensitive to the surface texture with a higher test load). On the other hand, the indentation should not be deeper than 1/10 of the thickness of the specimen or the hardened surface. The degree of homogeneity of the material is also an important criterion: a typical example is cast iron, which is usually only tested with a high total load, except in the ranges where it has been induction hardened, e.g. machine tool bases. Hardness range Above a hardness of approximately 650 HB/30, a diamond penetrator should be used; below that value it is also possible to use a penetrator made of steel or hard metal. The Brinell method, which does not allow diamond penetrators, cannot be used for hardened steel. The Rockwell method is more universal, because it allows for the used of diamond cone and steel ball penetrators. The Vickers method, which only allows for a diamond pyramid penetrator, can be employed in the entire hardness range. However, it is most suitable for tests in laboratories compared to tests in workshops. Accuracy The precision of the measurement is heavily dependent on the accuracy employed by the operator. This also includes well-ground surfaces, sufficient measurement periods and frequent revisions of the testing device with reliable test plates. If possible, the use of static systems should be preferred to dynamic systems. Using very low testing loads is a particular restriction to the precision of the measurements. Flexibility of the device with regard to forms and dimensions of specimens The specimen can be put on the device, or the device can be put on the specimen. The first case describes stationary devices, which have enough capacity to hold the specimen. Stationary devices are, thus, primarily suitable for tests on small and medium-sized specimens. Portable devices can be clamped to the specimens (clamping jaw, chain, etc) or – when testing large or bulky specimens – just put on the specimens. Portable devices can only be dynamic when using high testing loads. When the testing loads are smaller, they can also be static. It is possible to find customer-specific solutions for special cases. Economic aspects This includes the following elements: - the purchase price of the device, - the universalism of the application, - the measurement period, and - the qualification needed to operate the device. The first two aspects are important when specimens of different forms and with different surface treatments are tested. This is usually the case in technical companies and in small-scale industries. In companies that do their tests serially the quickness of the measurements and the possibility to employ unskilled staff are very important. Here, such devices are preferred that do not need special clamping equipment.

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2 The Rockwell method For a better understanding, the sequence of the Rockwell method, which is described below, is also shown in the numbered images below (figure 1). The meter, which is connected to the penetrator and displays the penetrators shifts on a larger scale, is also included in the figure. The tested surface is exposed to the penetrator and the first test load Fo (preload) is applied. A small indentation appears. At this point, the meter is set to zero. Slowly and without shocks the load F1 is applied additionally. Together with the preload this is defined as total test load F. With this load the penetrator enters the material more or less deep, depending on the hardness of the material. This position needs to be kept to reach the final penetration (when testing hard materials the penetration is almost immediate; with soft materials it is necessary to wait for a number of seconds). The penetration procedure can also be observed on the indicator of the meter. When the indicator of the meter finally stops moving, the additional load F1 is removed until the preload is applied respectively. This way, the penetrator remains in the imprint and all elastic deformations, which were caused by the application of the total test load, are eliminated; thus, the meter only shows the remaining penetration depth (as difference between preload and total test load).  The penetrator, preloads, test loads, and the units are standardised in the Rockwell method and can be divided into two groups: standard Rockwell (method N) and superficial Rockwell (method T).

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2.1 Standard Rockwell

The standard Rockwell procedure is intended for the use of one single diamond cone penetrator of 120° with a rounded off peak of 0.2 mm radius (see figure 2), or different ball penetrators made from hard metal with diameters of 1/16"; 1/8"; 1/4"; 1/2" (inch).

 

 

Figure 2 - profile of the Rockwell penetrator with diamond cone

The preload is unchanging: 98.07 N.The total test loads are (preload + additional load): 588.4 N; 980.7 N; 1,471 N.The measuring unit in standard Rockwell corresponds to 0.002 mm penetration.The hardness value increases with the hardness of the material, but at the same time the penetration difference between the preload and the total testing load decreases the harder the material is. Thus, Rockwell hardness values are calculated by subtracting the penetration depth (per 0.002 mm) from 100 (when using the diamond penetrator) or from 130 (when using any ball penetrator). 

Example:
with a diamond penetrator and a penetration depth of 0.082mm this makes
                100 – 0.082/0.002 = 59 Rockwell;
the same penetration depth measured with a ball penetrator makes
                130 – 0.082/0.002 = 89 Rockwell 

When using analogue devices with dial gauges, which usually have 100 partitions (one rotation = 0.2 mm), the Rockwell values can be read directly from the dial. The dial then usually has 2 series of numbers: the black ones are for diamond penetrators and the red numbers are made for ball penetrators. For the zero position always use the black 0 (or the red 30). When using a digital device, the data are displayed after the complete measuring cycle was run through. Due to the different combinations of penetrators and test loads, there is a great number of scales, which are labelled with different letters (see Table 1).

Table 2 – superficial Rockwell scales, F=total test load (Newton)
*) W, X, Y are not standardised

The ERNST devices NR3SR, AT130ASR and AT130DSR all work with the superficial Rockwell procedure.

 

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2.2 Superficial Rockwell

Although the superficial Rockwell method uses the same penetrators as the standard Rockwell procedure, the method requires a more precisely shaped diamond cone penetrator. This regards not only the conicity of the 120° cone peak, but also its rounding off of 0.2 mm. With this method, smaller total loads are used to create smaller indentations, so that the smallest shape defects on the peak would falsify the measuring results.The preload is unchanging: 29.42 NThe total test loads are (preload + additional load): 147.1N; 294.2N; 441.3 NThe measuring unit in superficial Rockwell corresponds to 0.001 mm penetration depth. In contrast to standard Rockwell, the zero point is set to 100 (0 on the dial gauge) in the superficial Rockwell procedure (both with the diamond penetrator and with the ball penetrator). The dial only has one series of numbers and 100 partitions. One rotation of the index equals 0.1 mm.

 

 

Example:
With a diamond or ball penetrator and a penetration depth of 0.082mm this makes 100 – 0.082/0.001 = 18 superficial Rockwell. Due to the different combinations of penetrators and test loads, there is a great number of superficial Rockwell scales, which are labelled with different letters. The respective letter is also preceded by a number which indicates the total load used in the test (see Table 2).

 

	HR Scale
Penetrator	Diamond Cone	Ball 1/16" 1,5875mm	Ball 1/8" *	Ball 1/4" *	Ball 1/2" *
F=441,3N	45 N	45 T	45 W	45 X	45 Y
F=294,2N	30 N	30 T	30 W	30 X	30 Y
F=147,1N	15 N	15 T	15 W	15 X	15 Y

superficial Rockwell scales, F=total test load (Newton)
*) W, X, Y are not standardised

 

The ERNST devices NR3SR, AT130ASR and AT130DSR all work with the superficial Rockwell procedure.

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2.3 Fields of application with different Rockwell scales

As we have seen, there is a considerable number of Rockwell-scales. Which scale to choose is a question depending on the hardness of the material, and the thickness of the specimen or hardened surface (in cases where there have been surface treatments such as carburisation, nitriding etc.).The hardness of the material determines the choice of the penetrator: diamond cone or ball.

 

The diamond cone is solely used for tempered or hardened steel and hard metal. It is not recommendable for steel with a solidity below 785 N/mm2 (20 HRC, 230 HB/30).

The steel ball penetrator is used for softer materials. The softer the material, the larger should be the diameter of the ball and / or the smaller should be the total test load. For instance, the materials that can be tested with the HRB scale (ball 1/16" – total test load 980.7N) are harder than the materials tested with the HRL scale (ball 1/4 "- total test load 588.4N).

The large balls are solely used for the testing of plastics and materials alike. Flowing plastics can be measured as well, if certain steps are taken, with the help of the total test load.

Again, we would like to mention that the HR hardness testing also requires a minimum thickness of the sample. However, there is no hard rule for this minimum thickness. It is usually estimated by calculating 10 x the penetration depth (see Table 3). This principle is also valid for hardened surfaces (carburisation etc.), which are usually measured with the smallest total test load (scale HRA).

 

 

F	HRC
	20	30	40	50	60	70
147,1N	0.41	0.33	0.26	0.19	0.14	0.09
294,2N	0.69	0.58	0.47	0.36	0.26	0.17
441,3N	0.91	0.77	0.63	0.50	0.37	0.25
588,4N	1.0	0.9	0.8	0.7	0.6	0.5
1471N	1.8	1.6	1.4	1.2	1.0	0.8

Table 3 - measurable minimum thickness for Rockwell tests with diamond penetrators

 

Most frequently used are the following Rockwell scales:

1. HRC (diamond cone - 1471N)
HRC is the most characteristical Rockwell scale for testing hardened, tempered and carburised samples.
When talking about "Rockwell hardness" in general, this usually means the HRC scale. This might cause a certain confusion, because sometimes a hardness of the HRC scale is ordered, although the small dimensions of the sample make tests with a total test load of 1471 N impossible. In such cases, other Rockwell scales or other measuring procedures are used to determine the hardness, which is then revalued to HRC with the help of charts.
As we will see later, those revaluation tables can only give approximated values. That is why it is recommended to use only hardness values that can be measured in reality when entering them into drafts, orders, etc.

 

2. HRA (diamond cone – 588.4N)
Mainly used for carburised materials and hard metals, whose high carbide hardness might damage the diamond.

 

3. HRB (ball 1/6" – 980.7N)
In Europe, this scale is usually used for copper alloys (brass, bronze etc.); in the U.S., it is also used for steel up to approx. 686 N/mm².

 

4. Rockwell N and T (superficial Rockwell)
The scales HR 15N, HR 30N, HR 45N (diamond cone) are used for samples with thin carburisation; the scales HR 15T, HR 30T, HR 45T (ball 1/16") are used for thin metal sheets. The general notes concerning the choice of the total test load always have to be attended to.

 

 

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2.4 Tests on cylindric and spherical surfaces

It is clear that the conditions for hardness measurements on cylindric or spherical surfaces are
different from those on flat surfaces. The differences are not as crucial with larger diameters
because then the bending of the surface is small and approximates a flat.

When working with smaller diameters (with higher bending) it is necessary to keep in mind that
the penetrations get an oval shape (for cylinders) when viewed from above and that the vertical
profile of the penetrated area has different thicknesses. That is why the tester has to add correction
values to the results, which depend on the hardness and the diameter of the sample (see Table 4).

 

Cylindric sample surfaces

Hardness scales and read off hardness		Cylinder diameter d in mm
		3	6,5	9,5	11	12,5	16	19
Hardness HR A-C-D diamond cone	80	0,5	0,5	0,5	0	0	0	0
	70	1,0	0,5	0,5	0,5	0,5	0	0
	60	1,5	1,0	0,5	1,0	0,5	0	0
	50	2,5	1,5	1,0	1,5	0,5	0,5	0,5
	40	-	2,0	1,0	2,0	1,0	0,5	0,5
	30	-	2,5	1,5	2,5	1,0	1,0	0,5
	20	-	-	2,0	3,5	1,5	1,0	1,0

Hardness scales and read off hardness		Cylinder diameter d in mm
		3	5	6,5	8	9,5	11	12
Hardness HR B - F - G Ball penetrator	90	4,0	3,0	2,0	1,5	1,5	1,5	1,0
	80	5,0	3,5	2,5	2,0	1,5	1,5	1,5
	70	-	4,0	3,0	2,5	2,0	2,0	1,5
	60	-	5,0	3,5	3,0	2,5	2,0	2,0
	50	-	-	4,0	3,5	3,0	2,5	2,0
	40	-	-	4,5	4,0	3,0	2,5	2,5

 

Spherical sample surfaces

Hardness scales and read off hardness		Ball diameter d in mm
		4	6,5	8	9,5	11	15	25
Hardness HRC Diamond cone	65	5,2	3,2	2,6	2,2	1,9	1,4	0,8
	60	5,8	3,6	2,9	2,4	2,1	1,5	0,9
	55	6,4	3,9	3,2	2,7	2,3	1,7	1,0
	The correctional value delta-H, which needs to be added to the measured value, can be calculated with the values from this table and the following formula:

Table 4 – correctional values for Rockwell measurements on cylindric and spherical surfaces with diamond and ball penetrators. (The correctional values must be added to the values on the display or the dial gauge.)

 

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2.5 Pros and cons of the Rockwell procedure

• Among the known hardness testing procedures Rockwell is the only one that allows for the direct reading of the hardness value without optical measuring. Thus, this procedure is not only quicker, it is also the only hardness testing procedure that can be automated completely.
  
• Hardness testing devices that work according to the Rockwell procedure are the most widespread. The most important advantages are: deviations due to personal estimations are avoided; it is less sensitive to rough surfaces (even though the standard says that surfaces have to be flat and carefully abraded).
  
• The most important limits of the application range are the possible total test loads. The minimum is 147.1N (HR15N / HR15T) and the maximum is 1,471N. However, workshops or foundries often require test loads of 10N or less, or of up to 30,000N. There is no Rockwell scale for tests on cast iron, nor is there one for steel sheets thinner than 0.15mm. In order to close this gap, there are devices that work with the Rockwell procedure (with pre- and total test loads), but with much larger (or smaller) and thus non-standardised test loads.
  
• While there are many scales, a different solution has been established for some materials (e.g. untreated steel). They are usually tested with a Rockwell device, which is equipped with a Brinell penetrator and uses Brinell testing loads (in this regard refer to chapter 3.4).

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2.6 Variations of the Rockwell procedure

A profound disadvantage of the traditional Rockwell devices is that the accuracy of the measurement is mainly based on the perfect contact between the test specimen and the contact surface.

 

When using the Rockwell procedure (Fig. 1, point 3; removal of additional test loads and return to preload), the only deformation displayed on the dial gauge should be the indentation itself.

However, this is only the case if the specimen is supported perfectly. If the supporting surface is polluted with an oil film, grease or other contaminants, there is a plastic deformation when the test load is applied and the result of the indentation measurement is falsified. This is an obvious limitation for workshops and hardening shops, because, of course, they cannot guarantee to work under ideal conditions for those devices.

In order to solve this problem, new devices have been developed that work according to the Rockwell procedure. Their penetrators are surrounded by a ferrule which allows for another contact with the specimen’s surface as the reference for the measurement (see Fig. 3). Thus, a possible resilience of the specimen, the spindle or other moveable parts of the stand cannot influence the result. In this regard, the advantages are the same as if one would use the Brinell or Vickers procedures (as we will see later).

 

Figure 3 – variation of the Rockwell procedure (ferrule

• 0: exchange of specimens: in rest position, the point of the penetrator (a) pokes out of the ferrule (b).
  
• 1: Both the penetrator and the ferrule lower down on the specimen and the penetrator recedes because of the applied load. The zero position is set automatically.
  
• 2: The total test load is applied.
  
• 3: The total test load is released and the displacement (intrusion) of (a) compared to (b) is determined.

If the specimen yields, the relation between (a) and (b) stays unchanging and the possibility of the typical Rockwell error is eliminated. This procedure can also be applied with superficial Rockwell tests.

Some ERNST-hardness testers include a third element which should not be confused with the ferrule (b). This element is called clamping hood and it can be found in devices with a stand. It is used to clamp specimens so that no extra tools are necessary to fix the specimens. It is also very easy to remove that clamping hood if it is not needed for tests.

Portable devices also have an element similar to this, which is called measuring foot. It can be exchanged and helps to get an optimum contact.

&nb

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3 The Brinell method

The Brinell method involves ball penetrators of different diameters (always in mm, in contrast to the Rockwell dimensions in inch), which are pressed with a certain load onto a smooth and even surface for a certain amount of time (10 to 15 seconds).

The emerging indentation, which has the shape of a spherical cup, is measured with optical devices (microscope or projector).

 

Figure 4 - hardness testing according to Brinell

 

The Brinell hardness (HBW) is determined by the relation between the applied testing load and the surface of the spherical cup. This is the formula:

 

where F is the test load in N , D is the diameter of the ball penetrator in mm and d is the diameter of the indentation in mm.In practice, tables are used that give the Brinell hardness values subject to the test load, ball diameter and the diameter of the indentation.Usually, the Brinell method uses the following standardised ball penetrators:

 

 

 

Ball diameter:

10mm

5mm

2,5mm

1mm

The standard test loads are:

Abbreviation HBW 10	Ball diameter	Test load (N)	Abbreviation HBW 5	Ball diameter	Test load (N)
HBW 10/3000	10 mm	29420	HBW 5/750	5 mm	7335
HBW 10/1500	10 mm	14710	HBW 5/250	5 mm	2452
HBW 10/1000	10 mm	9807	HBW 5/125	5 mm	1226
HBW 10/500	10 mm	4903	HBW 5/62,5	5 mm	612,9
HBW 10/250	10 mm	2452	HBW 5/25	5 mm	245,2
HBW 10/100	10 mm	980,7

   

Abbreviation HBW2.5	Ball diameter	Test load (N)	Abbreviation HBW 1	Ball diameter	Test load (N)
HBW 2,5/ 187,5	2,5 mm	1839	HBW 1/30	1 mm	294,2
HBW 2,5/ 62,5	2,5 mm	612,9	HBW 1/10	1 mm	98,07
HBW 2,5/ 31,25	2,5 mm	306,5	HBW 1/5	1 mm	49,03
HBW 2,5/ 15,625	2,5 mm	153,2	HBW 1/2,5	1 mm	24,52
HBW 2,5/ 6,25	2,5 mm	61,29	HBW 1/1	1 mm	9,807

Table 5 – Brinell abbreviations, ball diameter and test loads (see ISO 6506-1)

 

The following points have to be considered for the Brinell method:

1. The standard (EN ISO 6506-1) requires the diameter of the indentation to be between 0.24 and 0.6 of the diameter of the ball penetrator.
   In order to meet this requirement there has to be a certain degree of loading. If a small ball penetrator is pressed on a soft material with a high load, the indentation will be to deep, of course. Then again, if a larger ball penetrator is pressed on a hard material with a small load, the indentation might be smaller than 0.24 of the ball diameter. It is thus, almost illegible and not admissible.
   
2. For the Brinellmethod there is a basic formula to determine the degree of loading: 1.02 F/D² between test load (N) and diameter (mm) of the ball penetrator squared. The harder the material, the higher must the degree of loading be.

Degree of loading: 1.02 F / D²
30	15*	10	5	2,5	1

Table 6 – degree of loading

*) The degree 15 is only standardised for HBW10/1500, all the other degrees of loading can be used for all tests

 

The degree of loading 1.02F/D² is important because there are different results depending on which degree of loading was used. For example: a Brinell hardness value determined with a 10 mm ball and 9,807N (degree of loading 10) for a material is different from the hardness value determined with a 10 mm ball and 4,903N (degree of loading 5). However, if the same material is measured with a 2.5 mm ball and a total test load of 612,9N (degree of loading 10) the resulting hardness value is the same as in the first measurement because the degree of loading is the same (provided that the material is homogeneous and has no layers of different hardnesses).

 

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3.1 The labelling of Brinell tests

The abbreviation HBW stands for Brinell hardness. The Brinell hardness stands before the abbreviation and is followed by the ball diameter in mm, the test load according to the table and the testing time in seconds, if it differs from the standard time (10-15 seconds).

 

Example: 305 HBW 2.5 / 187.5: Brinell hardness 305, determined with ball penetrator 2.5mm, 1,839N test load and 10-15 seconds load application time

Example: 305 HBW 2.5 / 187.5 / 20: Brinell hardness 305, determined with ball penetrator 2.5mm, 1,839N test load and 20 seconds load application time

 

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3.2 Different applications of Brinell tests

As mentioned before, it is the hardness of the material which determines the degree of loading that is applied. When the most suitable degree is chosen, the test load is chosen according to the following elements:

1. the thickness of the tested specimen; because the considerations mentioned in the Rockwell chapter, which say that the minimum thickness should be 10x the depth of the indentation, also apply for Brinell tests (see table 7).

Ball D in mm	Centered indentation diameter
	0,5	1,0	1,5	2,0	3,0	4,0	5,0	6,0
1	0,54
2	0,25	1,07
2,5		0,83	2,00
5			0,92	1,67	4,00
10					1,84	3,34	5,36	8,00

Table 7 - measurable minimum thickness for Brinell tests (see ISO 6506-1)

2. the homogeneity of the material; because for less homogeneous materials it is recommended to use high test loads.
   
3. convenience of the readout; because the determination (either with a microscope or projector) of the indentation’s diameter is easier with large indentations.

For the following materials there are standard Brinell tests:

 

Steel: almost always HBW x | 3000 (x=ball diameter).
For steel, the Brinell method is very important because there is a constant, quite accurate relation between the Brinell hardness and the tensile strength (with a ratio of 3.53 for carbon steel, chromium steel and chromium-manganese steel; for chromium-nickel steel it is 3.33).

Example: 225 HBW x | 3000 à 225 x 3.53 = 794.3 N/mm² (see DIN 50150)

This is the only possibility to determine the tensile strength of steel non-destructively.However, the Brinell method cannot be used for hardened steel. As there is no diamond penetrator intended for the Brinell procedure, tests on treated steel with more than 1765 N/mm² are not possible. Soft iron is usually tested with HB x | 3000, although the indentation diameter exceeds 0.6 of the ball diameter.

 

 

 

Cast iron: always use HBW x | 3000. Due to the smaller homogeneity, it is recommended to use the highest total test load of 29,420 N.

 

Light metal: usually HBW x | 10 or HBW x | 5; for very soft alloys it is also possible to use HBW x | 2.5. The fact that it is possible to use different degrees of loading for medium hardness values might easily cause confusion. Thus, it is important to indicate the kind of test exactly (opposed to ferrous alloys, for which HBW x | 30 is always used).

 

Copper alloys: For bronze use HBW x | 10 (if it is very hard, use HBW x | 30), and HBW x | 10 or HBW x | 5 for brass. Apart from that, also consider the principles mentioned for light metals.

 

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3.3 Pros and cons of the Brinell procedure

• The main advantage of the Brinell method is that it uses very high test loads generated by relatively simple and robust devices.
• Furthermore, the indentation can be measured with the help of a simple microscope or even with a measuring magnifier.
• Measurements can also be carried out when the conditions are not ideal, because (contrary to the Rockwell method) a possible drawback of the specimen does not influence the result.
• The Brinell value can be multiplied with a certain coefficient, which is specific for every material, to determine the material’s tensile strength.

• One of the most serious disadvantages of the Brinell method is the fact that the indentation is measured optically, which includes the danger for measuring errors. Modern, automatic image evaluation computer systems reduce this source of errors significantly.
• Although high test loads are used, the surface must be well prepared in order to achieve the high accuracy needed for the measurement of the indentation.
• Thus, Brinell testing is not a quick task and not suitable for routine tests. To avoid this disadvantage, the Rockwell method is often used with Brinell penetrators and Brinell test loads (see next chapter).
• Tests on cylindrical surfaces are not possible. If such specimens need to be testes, the surface must be prepared so that an even area is produced for the measurement (10).

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3.4 Rockwell tests with Brinell loads and Brinell penetrators In order to avoid the different disadvantages and to find as much applications for the Rockwell devices as possible, these devices are often used with Brinell penetrators and test loads. Most of these devices also offer the loads 612.9, 1226 and 1839 N, apart from the usual Rockwell loads. Thus, they are suitable for Brinell tests. But they measure hardness according to the Rockwell method, with the depth of the indentation and not by measuring the diameter. The result can be read off a Rockwell dial gauge or displayed immediately. With the help of tables this result can then be converted to the Brinell value. However, this relatively quick method cannot be considered a genuine Brinell test. As a matter of fact, the converted results are not the same for each material (for instance, the conversion for steel is not the same as that for cast iron). This method should be preferred for routine tests or when there is no possibility for optical measurements. It also offers the advantage that the surface must not be prepared as well as for optical analysis. And the tensile strength for steel can be measured reliably. To achieve better accuracy during routine tests, the ERNST-devices offer the possibility to calibrate the Brinell scale beforehand with an optical test measurement.

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4 The Vickers method

(NOTE! Some of the details in this chapter are out of date! It is being revised.)This method is similar to the Brinell method, but it uses a diamond penetrator in the shape of a pyramid with a square base and an angle of 136°. Thus, the indentation looks like a concave (negative) pyramid with a square base. The length of the two diagonals of the indentation is measured (mean value).

 

 

 

Figure 5 - principle of hardness testing according to Vickers (see ISO 6507-1, -2, -3)

 

Similar to the Brinell method, the Vickers hardness value HV is determined by the ratio between the applied test load and the surface of the indentation.The test loads most often used are: 9.81, 19.62, 49.05, 98.10, 294.30 N. It is also possible to use test loads below 9.81 N, which means entering the domain of micro hardness and applications in metallographical laboratories.The Vickers hardness is calculated with the following formula, whereas d is the mean value of the indentation’s diagonals (accuracy: +/- 0.002 mm):

 

 

The labelling for Vickers tests is HV (H= hardness, V = Vickers), then the test load and the test time. The test load is indicated in the usual kp numerical values. That is why the actual test load must be divided by 9.81 to get the Vickers label (e.g. HV50: 50 = 490.5N / 9.81). Thus, a Vickers hardness value might look as follows:

  210 HV50/30   Vickers hardness 210, test load 490.5N, test time 30 seconds

 

Usually, the test load is applied within 15 seconds and effective for another 30 seconds. Soft materials require longer test times, steel with a hardness of 140 HV or more requires only 10 seconds.

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4.1 Applications of different Vickers test loads

The values obtained with different test loads are comparable because the Vickers method allows for only one penetrator and the Vickers value is the specific test load per mm² of the surface.For instance, when a material is tested with a test load of 294.30N and then a second time with 9.81N the results are the same (of course, only if the material is homogeneous and without layers of different hardness values).The Vickers method is also suitable for materials with different layers. Increasing test loads are applied subsequently to determine the thickness of certain surface layers, e.g. after nitration hardening.Apart from that, all the rules mentioned above for the other methods (minimum thickness = 10 x indentation depth) also apply to Vickers. In other words, the diagonal must not be longer than 2/3 of the specimen thickness.The Vickers method is especially suitable for tests of small and thin parts or components with any kind of surface treatment, i.e. for tests with low test loads.However, the Vickers method should not be used for inhomogeneous materials, like cast iron.

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4.2 Pros and cons of the Vickers procedure 

• The biggest advantage of Vickers is its scale, which comprises the smallest and the highest hardness values in one scale. It is thus very suitable for laboratory tests.

• Most of the disadvantages of Vickers are based on the long duration of the whole procedure because the indentation must be measured optically (with the help of a miscroscope or projector). This, of course, also is a source for measuring errors. However, modern, automatic image evaluation computer systems reduce this source of errors significantly.
• The surface must be well prepared and the penetrator must be applied evenly. Otherwise, the smallest inclination would cause irregularities in the indentation. Thus, the Vickers procedure is not suitable for routine tests.
• The indentation is not well readable on some materials because of the irregular distribution of the load (more load on the edges than on the sides of the pyramid).

To put it in a nutshell, the Vickers procedure is more suitable for applications in laboraties than in workshops.ERNST devices offer possibilities to read Vickers hardness values quicker and more direct so that some of these limitations can be overcome.

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5 Other hardness testing procedures

5.1 The Shore procedure (for metals)

This methods is based on the principle that a ball (or a shaft with ball point) is dropped on the specimen and rebounds more or less, dependingon the hardness of the material and the drop height. However, this method is used seldomly because the precision of the results is very much depending on the mass of the specimen and on the perfectly vertical falling axis. The hardness values are then given in Shore points and are only standardised for big, dressed to size cylinders (calenders).

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5.2 The Knoop method

This procedure is similar to Vickers, with a pyramid-shaped diamond penetrator and a rhombical base area (diagonals at a ratio of 1 : 7.), and it is only used in laboratories with a few grammes as total test load.

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6 Umwertungstabellen und Härtevergleichsplatten

6.1 The use of revaluation tables

As there is no mathematical interrelationship between the different hardness scales, the revaluation tables had to be compiled with the help of empirical tests. There are different tables which might have considerable differences. Usually, the tables also offer the tensile strength in N/mm² for steel.The values taken from the revaluation tables must be considered an orientation; they are not absolute.

 

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6.2 The use of hardness comparison plates

Usually, the equipment of a hardness tester includes one or more hardness comparison plate. They must be made from very homogeneous and appropriately treated materials. To achieve the highest possible accuracy they should only be evaluated on one side. It is very important to test the hardness testing device regularly with the help of the comparison plates to ensure that it works properly.

 

The distance between two indentations on one plate is measured between the middle of each indentation or between the middle of an indentation and the edge of the specimen. It should be no less than the following:

 

• for Rockwell tests: min. 4 x the indentation’s diameter; not less than 2 mm
  (edge distance: min. 2.5 x the indentation’s diameter; not less than 1 mm)
  
• for Brinell: min. 3 x the indentation’s diameter
  (edge distance: min. 2,5 x the indentation’s diameter
  
• for Vickers: min. 3 x the indentation’s diameter
  (edge distance: min. 3 x the indentation’s diameter

When there are so many indentations that the surface is covered with them, do not grind them to re-use the plate. The structures of the layers below the indentations (approximately 8x the indentation depth) are usually altered because of the load application and thus, measuring results would not be accurate.

 

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