Hardness1 ME 212 LABORATORY EXPERIMENT #3HARDNESS TESTING AND AGE HARDENING1.
OBJECTIVE:Our primary aim is to measure the Rockwell Hardness values for different materials and estimate ultimate tensile strengths by the aid of conversion tables.We will also focus on age hardening, the info on which can be found in the last part of this experiment sheet.2.
HARDNESS TESTING THEORY:Hardness is usually defined as the resistance of a material to plastic penetration of its surface.
There are three main types of tests used to determine hardness: Scratch tests are the simplest form of hardness tests.In this test, various materials are rated on their ability to scratch one another.Mohs hardness test is of this type.
This test is used mainly in mineralogy.In Dynamic Hardness tests, an object of standard mass and dimensions is bounced back from a surface after falling by its own weight.The height of the rebound is indicated.Shore hardness is measured by this method.
Static Indentation tests are based on the relation of indentation of the specimen by a penetrator under a given load.The relationship of total test force to the area or depth of indentation provides a measure of hardness.
The Rockwell, Brinell, Knoop, Vickers, and ultrasonic hardness tests are of this type.For engineering purposes, only the static indentation tests are used.BRINELL HARDNESS TEST: This test consists of applying a constant load, usually between 500 and 3000 kgf for a specified time (10 to 30 s) using a 5- or 10-mm diameter hardened steel or tungsten carbide ball on the flat surface of a workpiece.
2 Figure 1.
Brinell Hardness Test Schematic Hardness is determined by taking the mean diameter of the indentation and calculating the Brinell hardness number (BHM or HB) by dividing the applied load by the surface area of the indentation according to following formula :
()22HBDDdp=--where P is load in kg; D ball diameter in mm; and d is the diameter of the indentation in mm.Calculations have already been made and are available in tabular form for various combinations of diameters of impressions and load.The Brinell hardness number followed by the symbol HB without any suffix numbers denotes standard test conditions using a ball of 10 mm diameter and a load of 3,000 kg applied for 10 to 15 s.For other conditions, the hardness number and symbol HB are supplemented by numbers indicating the test conditions in the following order: diameter of ball, load, and duration of loading.
For example, 75 HB 10/500/30 indicates a Brinell hardness of 75 measured with a ball of 10 mm diameter and a load of 500 kg applied for 30s.
3 However, the BHN is not a satisfactory physical concept since the above equation does not give the mean pressure over the surface of the indentation.
Meyer suggested that a more rational definition of hardness than that proposed by Brinell, would be one based on the projected area of the impression rather than the surface area.The mean pressure between surface of the indenter and the indentation is equal to the load divided by the projected area of the indentation.Meyer proposed that this mean pressure should be taken as the measure of hardness.
It is referred to as the Meyer hardness
.24Meyer HardnessPdpVICKERS HARDNESS TEST: The Vickers hardness test uses a square base diamond pyramid as the indenter.The included angle between the opposite faces of the pyramid is l36.
The Vickers hardness tester operates on the same basic principle as the Brinell tester, the numbers being expressed in the terms of load and area of the impression.
As a result of the indenters shape, the impression on the surface of the specimen will be a square.The length of the diagonal of the square is measured through a microscope fitted with an ocular micrometer that contains movable knife-edges.
The Vickers hardness values are calculated by the formula: ()()22sin1.8544HVdd==where P is the applied load in kg, and d is the diagonal length in mm.
Microhardness Test: This term, unfortunately, is misleading, as it could refer to the testing of small hardness values when it actually means the use of small indentations.Test loads are between 1 and 1,000 g.Two types of indenters are used for Microhardness testing: the 136 square-base Vickers diamond pyramid described previously, and the elongated Knoop diamond indenter.
4 Figure 2.
Vickers Hardness Testing Schematic ROCKWELL HARDNESS TEST: This hardness test uses a direct reading instrument based on the principle of differential depth measurement.
Rockwell testing differs from Brinell testing in that the Rockwell hardness number is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy (major) load beyond the depth resulting from a previously applied (minor) load.Initially a minor load is applied, and a zero datum position is established.The major load is then applied for a specified period and removed, leaving the minor load applied.The resulting Rockwell number represents the difference in depth from zero datum position as a result of the application of major load.The entire procedure requires only 5 to 10 s.
Use of a minor load greatly increases the accuracy of this type of test, because it eliminates the effects of backlash in the measuring system and causes the indenter to break through slight surface roughness.The 120 sphero-conical diamond indenter is used mainly for testing hard materials such as hardened steels and cemented carbides.Hardened steel ball indenters with diameters 1/16, 1/8, 1/4, 1/2 in.are used for testing softer materials such as fully annealed steels, softer grades of cast irons, and a wide variety of nonferrous metals.In Rockwell testing, the minor load is 10 kgf, and the major load is 60, 100 or 150 kgf.
In superficial Rockwell testing, the minor load
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Internet: www.dura-bar.comE-Mail: email@example.comDuctile Iron Grades: All are equally heat-treatable, however in some cases 80-55-06 will respond to ame or induction hardening carburized in order to harden through heat treating.None of the Dura-Bar grades require carburizing.Minimal Growth:grades grow when heat treated, just like steel.
However the growth is very minimal and very predictable.
Heat treating does ductile iron.Temperatures:temperatures up to 1100F with no effect on microstructure or properties.The prole will be the same Cover: Furnace and basket photos courtesy of Solar Atmospheres..