Brinell Hardness Test(new)

Experiment # 1 Objective Measurement of Hardness by using Brinell Hardness Test

Abstract Brinell tests have long been the preferred method of assaying the hardness of metals during forming operations. Methods and apparatus for performing Brinell hardness test measurements produce hardness numbers and average Brinell indentation diameter readings automatically and minimize operator intervention and hence, operator error.

Theory (1) To understand what hardness is, and how it can be used to indicate some properties of materials (2) To conduct typical engineering hardness tests and be able to recognize commonly used hardness scales and numbers (3) To be able to understand the correlation between hardness numbers and the properties of materials (4) To learn the advantages and limitations of the common hardness test methods Experime ntal Condition: Temperature: 25o C Load (P) =3000kg Indenter Diameter (D) = 10mm Introduction It is a common practice to test most materials before they are accepted for processing, and before they are put into service to determine whether or not they meet the specifications required. One of these tests is hardness. The Rockwell, Brinell and durometer machines are those most commonly used for this purpose.

What is Hardness? The Metals Handbook defines hardness as "Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation. In materials science, there are three principal operational definitions of hardness: Scratch hardness: Resistance to fracture or plastic (permanent) deformation due to friction from a sharp object Indentation hardness: Resistance to plastic (permanent) deformation due to a constant load from a sharp object Rebound hardness: Height of the bounce of an object dropped on the material, related to elasticity. The equation based definition of hardness is the pressure applied over the projected contact area between the indenter and the material being tested. As a result hardness values are typically reported in units of pressure Material Science Hardness is a characteristic of a solid material expressing its resistance to permanent deformation. Hardness can be measured on the Mohr’s scale or various other scales. Some of the other scales used for indentation hardness in engineering—Rockwell, Vickers, and Brinell—can be compared using practical conversion tables. Scratch Hardness In mineralogy, hardness commonly refers to a material's ability to penetrate softer materials. An object made of a hard material will scratch an object made of a softer material. Scratch hardness is usually measured on the Mohr’s scale of mineral hardness. One tool to make this measurement is the sclerometer. Pure diamond is the hardest readily-available natural mineral substance and will scratch any other natural material. Diamond is therefore used to cut other diamonds; in particular, higher-grade diamonds are used to cut lower-grade diamonds.

Indentation Hardness Indentation hardness tests are primarily used in engineering and metallurgy fields. The tests work on the basic premise of measuring the critical dimensions of an indentation left by a specifically dimensioned and loaded indenter. Rebound Hardness Also known as dynamic hardness, rebound hardness measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material. The device used to take this measurement is known as a scleroscope. Two scales that measures rebound hardness are the Leeb rebound hardness test and Bennett hardness scale.

How to Measure Hardness? Moh’s scale of hardness The Moh’s scale of hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. Moh’s Hardness

Mineral

Absolute Hardness

1

Talc

1

2

Gypsum

3

3

Calcite

9

7

Quartz

100

10

Diamond

1600

Hardness Measurement Method There are three types of tests used with accuracy by the metals industry; they are the Rockwell hardness test, the Brinell hardness test, and the Vickers hardness test. The way the three of these hardness tests measure a metal's hardness is to determine t he metal's resistance to the penetration of a non-deformable ball or cone. The tests determine the depth or area which such a ball or cone will sink into the metal, under a load, within a specific period of time. The followings are the most common hardness test methods used in today`s technology: 1. Rockwell hardness test

3. Vickers

2. Brinell hardness

4. Knoop hardness

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. Brinell testing is typically done on iron and steel castings using a 3000Kg test force and a 10mm diameter carbide ball. Aluminum and other softer alloys are frequently tested using a 500Kg test force and a 10 or 5mm carbide ball. Therefore the typical range of Brinell testing is 500 to 3000kg with 5 or 10mm carbide balls.

Standards Brinell Test methods are defined in the following standards: ASTM E10 ISO 6506

Brinell Hardness numbers Materials

Hardness

Aluminium

15 HBS 10/100

Copper

35 HBS 10/100

Mild Steel

120 HB

Stainless Steel

200 HB

Glass

1550 HB

Hardened Tool Steel

1500-1900HB

Brinell Test Method All Brinell tests use a carbide ball indenter. The test procedure is as follows: The indenter of Diameter (D) 10mm is pressed into the sample by an accurately controlled test force. The force of 3000kg is maintained for a specific dwell time, normally 15 - 30 seconds. After the dwell time is complete, the indenter is removed leaving a round indent in the sample. The size of the indent is determined optically by measuring two diagonals of the round indent using either a portable microscope or one that is integrated with the load application device. The Brinell hardness number is a function of the test force divided by the curved surface area of the indent. The indentation is considered to be spherical with a radius equal to half the diameter of the ball. The average of the two diagonals is used in the following formula to calculate the Brinell hardness:

Where: P = applied force (kgf) D = diameter of indenter (mm) d = diameter of indentation (mm) 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 HBW 10/3000/30" which means that a Brinell Hardness of 75 was obtained using a 10mm diameter hardened steel with a 3000 kilogram load applied for a period of 30 seconds and the W indicates that a carbide ball was used. On tests of extremely hard metals a tungsten carbide ball is substituted for the steel ball. The Brinell number, which normally ranges from HB 50 to HB 750 for metals, will increase as the sample gets harder. Tables are

available to make the calculation simple. A typical Brinell hardness is specified as follows: 356HBW Where 356 is the calculated hardness and the W indicates that a carbide ball was used. Previous standards allowed a steel ball and had an S designation. Steel balls are no longer allowed.

Result for Mild Steel Readings

Center line for the notch part d (mm)

Brinnel Hardness Test number (BHN)

1

3.25

117

2

3.20

121

3

3.20

121

Average

119.7

Application Because of the wide test force and Indenter diameter range the Brinell test can be used on almost any metallic material

Strength One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same. A wide range of test forces and ball sizes to suit every application. Nondestructive, sample can normally be reused. 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.

Weaknesses The main drawback of the Brinell test is the need to optically measure the indent size. This requires that the test point be finished well enough to make an accurate measurement. Slow testing can take 30 seconds, not counting the sample preparation time. Because of the relatively large indentations, the work piece may not be usable after testing

Limitations However, because of the large ball diameter the test cannot be used to determine the hardness variations in a welded joint for which the Vickers test is preferred. Very hard metals, over 450BHN may also cause the ball to deform resulting in an inaccurate reading. To overcome this limitation a tungsten carbide ball is used instead of the hardened steel ball but there is also a hardness limit of 600BHN with this indenter. The oxide layer should be remove from the sample by using sand paper or grinders otherwise it would cause variation in hardness number. We cannot perform this operation on a very thin sheet of metal usually having thickness less than 10mm.For this we need to pile few sheets accurately without any impurity or oxide layer.

Errors in Hardness Testing There are many factors that can affect the accuracy of the hardness test. Some of these such as flatness and surface finish have already been mentioned above but it is worth reemphasizing the point that flatness is most important - a maximum angle of approximately ± 1° would be regarded as acceptable. To achieve the required flatness tolerance and surface finish surface grinding or machining may be necessary. The correct load must be applied and to achieve this there must be no friction in the loading system otherwise the impression will be sma ller than expected - regular maintenance and calibration of the machine is therefore essential. The Brinell ball will deform over a period of time and inaccurate readings will result. This deterioration will be accelerated if a large proportion of the work is on hard materials. The length of time that the load is applied is important and must be controlled. The specimen dimensions are important - if the test piece is too thin the hardness of the specimen table will affect the result. As a rule of thumb the specimen thickness should be ten times the depth of the impression for the Brinell test and twice that of the Vickers diagonal. Similarly, if the impression is too close to the specimen edge then low hardness values will be recorded - again as a rule the impression should be some 4 to 5 times the impression diameter from any free edge. The specimen table should be rigidly supported and must be in good condition - burrs or raised edges beneath the sample will give low readings. Impact loading must be avoided. It is very easy to force the indenter into the specimen surface when raising the table into position. This can strain the equipment and damage the indenter. Operator training is crucial and regular validation or calibration is essential if hardness rest results are to be accurate and reproducible

Comments: Indentations should not be made on a curved surface having a radius of less than 1 inch. The load should be applied in such a way that the direction of loading and the test surface are perpendicular to each other within 2o. The thickness of the work piece being tested should be such that no bulge or mark showing the effect of the load appears on the side of the work piece opposite the indentation. In any event, the thickness of the specimen shall be at least 10 times the depth of the indentation

References 1. ^ ASTM E10 - 08 Standard Test Method for Brinell Hardness of Metallic Materials 2. ^ ISO 6506-1:2005 Metallic materials - Brinell hardness test - Part 1: Test method 3. http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness 4. http://en.wikipedia.org/wiki/Brinell_hardness_test 5. http://www.pdfcoke.com/doc/18004150/Hardness-Report?autodown=pdf 6. ―Technical Metallurgy‖ by Cliffe (page 150 – 154) 7. ―Material, Their Nature, Fabrication and Properties‖ by Sergal (page 143 – 145, 71 –72) 8. ―Metallurgy for Engineers‖ by Rollesan (page 15)