Meyer hardness test
The Meyer hardness test is a used hardness test based upon projected area of an impression. This is a more fundamental measurement of hardness than other hardness tests which are based on the surface area of an indentation; the principle behind the test is that the mean pressure required to test the material is the measurement of the hardness of the material. The mean pressure is calculated by dividing the load by the projected area of the indentation; the result is called the Meyer hardness. An advantage of the Meyer test is that it is less sensitive to the applied load compared to the Brinell hardness test. For cold worked materials the Meyer hardness is constant and independent of load, whereas for the Brinell hardness test it decreases with higher loads. For annealed materials the Meyer hardness increases continuously with load due to strain hardening. Based on Meyer's law hardness values from this test can be converted into Brinell hardness values, vice versa; the Meyer hardness test was devised by Eugene Meyer of the Materials Testing Laboratory at the Imperial School of Technology, Germany, circa 1908.
Tabor, The Hardness of Metals, Oxford University Press, ISBN 0-19-850776-3
The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, denoted by a single letter, that use different indenters; the result is a dimensionless number noted as HRA, HRB, HRC, etc. where the last letter is the respective Rockwell scale. When testing metals, indentation hardness correlates linearly with tensile strength; this important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight portable equipment, such as hand-held Rockwell hardness testers. The differential depth hardness measurement was conceived in 1908 by a Viennese professor Paul Ludwik in his book Die Kegelprobe; the differential-depth method subtracted out the errors associated with the mechanical imperfections of the system, such as backlash and surface imperfections.
The Brinell hardness test, invented in Sweden, was developed earlier – in 1900 – but it was slow, not useful on hardened steel, left too large an impression to be considered nondestructive. Hugh M. Rockwell and Stanley P. Rockwell from Connecticut in the United States co-invented the "Rockwell hardness tester," a differential-depth machine, they applied for a patent on July 15, 1914. The requirement for this tester was to determine the effects of heat treatment on steel bearing races; the application was subsequently approved on February 11, 1919, holds U. S. Patent 1,294,171. At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT. New Departure was a major ball bearing manufacturer which in 1916 became part of United Motors and, shortly thereafter, General Motors Corp. After leaving the Connecticut company, Stanley Rockwell in Syracuse, NY, applied for an improvement to the original invention on September 11, 1919, approved on November 18, 1924.
The new tester holds U. S. Patent 1,516,207. Rockwell moved to West Hartford, CT, made an additional improvement in 1921. Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines. Stanley started a heat-treating firm circa 1923, the Stanley P. Rockwell Company, which still exists in Hartford, CT; the later-named Wilson Mechanical Instrument Company has changed ownership over the years, was acquired by Instron Corp. in 1993. Rockwell hardness tester: HRA, HRB, HRC Superficial Rockwell hardness tester: 15N, 30N, 45N, 15T, 30T, 45T, 15W, 30W, 45W, 15X, 30X, 45X, 15Y, 30Y, 45Y Plastic Rockwell hardness tester: HRE, HRL, HRM Twin Rockwell hardness tester: HRA, HRB, HRC,15N, 15T, 15W, 15X, 15Y, 30N, 30T, 30W, 30X, 30Y, 45N, 45T, 45W, 45X, 45Y The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load; the minor load establishes the zero position.
The major load is applied removed while still maintaining the minor load. The depth of penetration from the zero datum is measured from a dial, on which a harder material gives a higher number; that is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques; the equation for Rockwell Hardness is H R = N − d s, where d is the depth, N and s are scale factors that depend on the scale of the test being used. It is used in engineering and metallurgy, its commercial popularity arises from its speed, robustness and small area of indentation. Legacy Rockwell hardness testers operation steps: Load an initial force: Rockwell hardness test initial test force is 10 kgf. Load main load: reference below form / table'Scales and values'. Leave the main load for a "dwell time" sufficient for indentation to come to a halt. Release load.
In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. Readings should be taken from a flat perpendicular surface, because convex surfaces give lower readings. A correction factor can be used. There are several alternative scales, the most used being the "B" and "C" scales. Both express hardness as an arbitrary dimensionless number. Except for testing thin materials in accordance with A623, the steel indenter balls have been replaced by tungsten carbide balls of the varying diameters; when a ball indenter is used, the letter "W" is used to indicate a tungsten carbide ball was used, the letter "S" indicates the use of a steel ball. E.g.: 70 HRBW indicates the reading was 70 in the Rockwell B scale using a tungsten carbide indenter. The superficial Rockwell scales use lower loads and shallower impressions on brittle and thin materials; the 45N scale employs a 45-kgf load on a diamond cone-shaped Brale indenter, can be used on dense ceramics.
The 15T scale employs a 15-kgf load on a 1⁄16-inch-diameter hardened steel ball, can be used on sheet metal. The B and C scales overlap, such that readings below HRC 20 and those above HRB 100 consi
Steel is an alloy of iron and carbon, sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, tools, automobiles, machines and weapons. Iron is the base metal of steel. Iron is able to take on two crystalline forms, body centered cubic and face centered cubic, depending on its temperature. In the body-centered cubic arrangement, there is an iron atom in the center and eight atoms at the vertices of each cubic unit cell, it is the interaction of the allotropes of iron with the alloying elements carbon, that gives steel and cast iron their range of unique properties. In pure iron, the crystal structure has little resistance to the iron atoms slipping past one another, so pure iron is quite ductile, or soft and formed. In steel, small amounts of carbon, other elements, inclusions within the iron act as hardening agents that prevent the movement of dislocations that are common in the crystal lattices of iron atoms; the carbon in typical steel alloys may contribute up to 2.14% of its weight.
Varying the amount of carbon and many other alloying elements, as well as controlling their chemical and physical makeup in the final steel, slows the movement of those dislocations that make pure iron ductile, thus controls and enhances its qualities. These qualities include such things as the hardness, quenching behavior, need for annealing, tempering behavior, yield strength, tensile strength of the resulting steel; the increase in steel's strength compared to pure iron is possible only by reducing iron's ductility. Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the production of blister steel and crucible steel. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began; this was followed by the Siemens–Martin process and the Gilchrist–Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron.
Further refinements in the process, such as basic oxygen steelmaking replaced earlier methods by further lowering the cost of production and increasing the quality of the final product. Today, steel is one of the most common manmade materials in the world, with more than 1.6 billion tons produced annually. Modern steel is identified by various grades defined by assorted standards organizations; the noun steel originates from the Proto-Germanic adjective stahliją or stakhlijan, related to stahlaz or stahliją. The carbon content of steel is between 0.002% and 2.14% by weight for plain iron–carbon alloys. These values vary depending on alloying elements such as manganese, nickel, so on. Steel is an iron-carbon alloy that does not undergo eutectic reaction. In contrast, cast iron does undergo eutectic reaction. Too little carbon content leaves iron quite soft and weak. Carbon contents higher than those of steel make a brittle alloy called pig iron. While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel.
Common alloying elements include: manganese, chromium, boron, vanadium, tungsten and niobium. Additional elements, most considered undesirable, are important in steel: phosphorus, sulfur and traces of oxygen and copper. Plain carbon-iron alloys with a higher than 2.1% carbon content are known as cast iron. With modern steelmaking techniques such as powder metal forming, it is possible to make high-carbon steels, but such are not common. Cast iron is not malleable when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to make malleable iron or ductile iron objects. Steel is distinguishable from wrought iron, which may contain a small amount of carbon but large amounts of slag. Iron is found in the Earth's crust in the form of an ore an iron oxide, such as magnetite or hematite. Iron is extracted from iron ore by removing the oxygen through its combination with a preferred chemical partner such as carbon, lost to the atmosphere as carbon dioxide.
This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at about 250 °C, copper, which melts at about 1,100 °C, the combination, which has a melting point lower than 1,083 °C. In comparison, cast iron melts at about 1,375 °C. Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore in a charcoal fire and welding the clumps together with a hammer and in the process squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. All of these temperatures could be reached with ancient methods used since the Bronze Age. Since the oxidation rate of iron increases beyond 800 °C, it is important that smelting take place in a low-oxygen environment. Smelting, using carbon to reduce iro
Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials are harder than others. Macroscopic hardness is characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex. Hardness is dependent on ductility, elastic stiffness, strain, toughness and viscosity. Common examples of hard matter are ceramics, certain metals, superhard materials, which can be contrasted with soft matter. There are three main types of hardness measurements: scratch and rebound. Within each of these classes of measurement there are individual measurement scales. For practical reasons conversion tables are used to convert between another. Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object; the principle is that an object made of a harder material will scratch an object made of a softer material. When testing coatings, scratch hardness refers to the force necessary to cut through the film to the substrate.
The most common test is Mohs scale, used in mineralogy. One tool to make this measurement is the sclerometer. Another tool used to make these tests is the pocket hardness tester; this tool consists of a scale arm with graduated markings attached to a four-wheeled carriage. A scratch tool with a sharp rim is mounted at a predetermined angle to the testing surface. In order to use it a weight of known mass is added to the scale arm at one of the graduated markings, the tool is drawn across the test surface; the use of the weight and markings allows a known pressure to be applied without the need for complicated machinery. Indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object. Tests for indentation hardness are used in engineering and metallurgy fields; the tests work on the basic premise of measuring the critical dimensions of an indentation left by a dimensioned and loaded indenter. Common indentation hardness scales are Rockwell, Vickers and Brinell, amongst others.
Rebound hardness known as dynamic hardness, measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material. This type of hardness is related to elasticity; 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. There are five hardening processes: Hall-Petch strengthening, work hardening, solid solution strengthening, precipitation hardening, martensitic transformation. In solid mechanics, solids have three responses to force, depending on the amount of force and the type of material: They exhibit elasticity—the ability to temporarily change shape, but return to the original shape when the pressure is removed. "Hardness" in the elastic range—a small temporary change in shape for a given force—is known as stiffness in the case of a given object, or a high elastic modulus in the case of a material. They exhibit plasticity—the ability to permanently change shape in response to the force, but remain in one piece.
The yield strength is the point. Deformation in the plastic range is non-linear, is described by the stress-strain curve; this response produces the observed properties of scratch and indentation hardness, as described and measured in materials science. Some materials exhibit both viscosity when undergoing plastic deformation, they fracture—split into two or more pieces. Strength is a measure of the extent of a material's elastic range, or elastic and plastic ranges together; this is quantified as compressive strength, shear strength, tensile strength depending on the direction of the forces involved. Ultimate strength is an engineering measure of the maximum load a part of a specific material and geometry can withstand. Brittleness, in technical usage, is the tendency of a material to fracture with little or no detectable plastic deformation beforehand, thus in technical terms, a material can be both strong. In everyday usage "brittleness" refers to the tendency to fracture under a small amount of force, which exhibits both brittleness and a lack of strength.
For brittle materials, yield strength and ultimate strength are the same, because they do not experience detectable plastic deformation. The opposite of brittleness is ductility; the toughness of a material is the maximum amount of energy it can absorb before fracturing, different from the amount of force that can be applied. Toughness tends to be small for brittle materials, because elastic and plastic deformations allow materials to absorb large amounts of energy. Hardness increases with decreasing particle size; this is known as the Hall-Petch relationship. However, below a critical grain-size, hardness decreases with decreasing grain size; this is known as the inverse Hall-Petch effect. Hardness of a material to deformation is dependent on its microdurability or small-scale shear modulus in any direction, not to any rigidity or stiffness properties such as its bulk modulus or Young's modulus. Stiffness is confused for hardness; some materials are stiffer than diamond but are not harder, are prone to spalling and flaking in squamose or acicular habits.
The key to understanding the mechanism behind hardness is understanding the metal
Vickers hardness test
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials; the Vickers test is easier to use than other hardness tests since the required calculations are independent of the size of the indenter, the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe a material's ability to resist plastic deformation from a standard source; the Vickers test has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number or Diamond Pyramid Hardness; the hardness number can be converted into units of pascals, but should not be confused with pressure, which uses the same units. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, is therefore not pressure, it was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size.
A diamond in the form of a square-based pyramid satisfied these conditions. It had been established; as two tangents to the circle at the ends of a chord 3d/8 long intersect at 136°, it was decided to use this as the included angle between plane faces of the indenter tip. This gives an angle from each face normal to the horizontal plane normal of 22° on each side; the angle was varied experimentally and it was found that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load. Accordingly, loads of various magnitudes are applied to a flat surface, depending on the hardness of the material to be measured; the HV number is determined by the ratio F/A, where F is the force applied to the diamond in kilograms-force and A is the surface area of the resulting indentation in square millimeters. A can be determined by the formula. A = d 2 2 sin , which can be approximated by evaluating the sine term to give, A ≈ d 2 1.8544, where d is the average length of the diagonal left by the indenter in millimeters.
Hence, H V = F A ≈ 1.8544 F d 2,where F is in kgf and d is in millimeters. The corresponding units of HV are kilograms-force per square millimeter. To calculate Vickers hardness number using SI units one needs to convert the force applied from newtons to kilogram-force by dividing by 9.806 65. This leads to the following equation: H V ≈ 0.1891 F d 2, where F is in N and d is in millimeters. A common error is that the above formula to calculate the HV number does not result in a number with the unit Newton per square millimeter, but results directly in the Vickers hardness number, in fact kilograms-force per square millimeter. To convert the Vickers hardness number to SI units the hardness number in kilograms-force per square millimeter has to be multiplied with the standard gravity to get the hardness in MPa and furthermore divided by 1000 to get the hardness in GPa. Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or xxxHVyy/zz if duration of force differs from 10 s to 15 s, e.g. 440HV30/20, where: 440 is the hardness number, HV gives the hardness scale, 30 indicates the load used in kgf. 20 indicates the loading time if it differs from 10 s to 15 sVickers values are independent of the test force: they will come out the same for 500 gf and 50 kgf, as long as the force is at least 200 gf.
For thin samples indentation depth can be an issue due to substrate effects. As a rule of thumb the sample thickness should be kept greater than 2.5 times the indent diameter. Alternatively indent depth can be calculated according to: h = d 2 2 tan θ 2 ≈ d 7.0006, When doing the hardness tests the minimum distance between indentations and the distance from the indentation to the edge of the specimen must be taken into account to avoid interaction between the work-hardened regions and effects of the edge. These minimum distances are different for ISO ASTM E384 standards. If HV is expressed in kgf/mm2 the tensile strength of the material can be approximated as σu ≈ HV×c ≈ HV/0.3, where c is a constant determined by yield strength, Poisson's ratio, work-hardening exponent and geometrical factors - ranging between 2 and 4.. In other words, if HV is expressed in N/mm2 then