Metals - Metallurgic Hardness Testing

We have keyed in on a new buzz word for the '90's in bike metallurgy. You know already for tires that "durometer hardness" is the current craze in advertising, even to the point of misleadingly, well for metals, specifically aluminum alloys, we are sure that hardness scales are going to be "hot" in ads. But the ads will use numbers and hardness scales that have no meaning to anyone outside the metal working industry, and the ads won't spend the time to educate you on their meaning. That's what this part of the article is about, so you won't simple mindedly make assumptions about what is harder. There are three types of tests used with accuracy by the metals industry, they are the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test. Hardness is the property of a metal which gives it 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. Since the definitions of metallurgic ultimate strength and hardness are rather similar, it can generally be assumed that a strong metal is also a hard metal. The way the three of these hardness tests measure a metal's hardness is to determine the metal's resistance to the penetration of a non-deformable ball or cone. The tests determine the depth which such a ball or cone will sink into the metal, under a given load, within a specific period of time.

Brinell Hardness Testing

The Brinell hardness test uses a desk top machine to press a 10mm diameter, hardened steel ball into the surface of the test specimen. The machine applies a load of 500 kilograms for soft metals such as copper, brass and thin stock. A 1500 kilogram load is used for aluminum castings, and a 3000 kilogram load is used for materials such as iron and steel. The load is usually applied for 10 to 15 seconds. After the impression is made, a measurement of the diameter of the resulting round impression is taken. It is measured to plus or minus .05mm using a low-magnification portable microscope. The hardness is calculated by dividing the load by the area of the curved surface of the indention, (the area of a hemispherical surface is arrived at by multiplying the square of the diameter by 3.14159 and then dividing by 2). To make it easier, a calibrated chart is provided, so with the diameter of the indentation the corresponding hardness number can be referenced. 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. Among the three hardness tests we discuss, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will better take into account multiple grain structures, and any irregularities in the uniformity of the alloy.

Rockwell Hardness Test

The Rockwell hardness tester measures resistance to penetration like the Brinell test, but in the Rockwell case, the depth of the impression is measured rather than the diametric area. With the Rockwell tester, the hardness is indicated directly on the scale attached to the machine. This dial like scale is really a depth gauge, graduated in special units. The Rockwell hardness test is the most used and versatile of the hardness tests. The desk top testing machine has a variety of attachments that make it capable of measuring the hardness of a wide variety of materials in many sizes and shapes. Among the Rockwell advantages are, tests can be made quickly, just a small mark left in the sample, and it's capable of testing very hard material. It uses two types of indenters for its "common scales". For soft materials such as copper alloys, soft steel,and aluminum alloys a 1/16" diameter steel ball is used with a 100 kilogram load and the hardness is read on the "B" scale. In testing harder materials, hard cast iron and many steel alloys, a 120 degrees diamond cone is used with up to a 150 kilogram load and the hardness is read on the "C" scale. The Rockwell test uses two loads, one applied directly after the other. The first load, known as the "minor", load of 10 kilograms is applied to the specimen to help seat the indenter and remove the effects, in the test, of any surface irregularities. In essence, the minor load creates a uniformly shaped surface for the major load to be applied to. The difference in the depth of the indentation between the minor and major loads provides the Rockwell hardness number. There are several Rockwell scales other than the "B" & "C" scales, (which are called the common scales). The other scales also use a letter for the scale symbol prefix, and many use a different sized steel ball indenter. A properly used Rockwell designation will have the hardness number followed by "HR" (Hardness Rockwell), which will be followed by another letter which indicates the specific Rockwell scale. An example is 60 HRB which indicates that the specimen has a hardness reading of 60 on the B scale. There is a second Rockwell tester referred to as the "Rockwell Superficial Hardness Tester". This machine works the same as the standard Rockwell tester, but is used to test thin strip, or lightly carburized surfaces, small parts or parts that might collapse under the conditions of the regular test. The Superficial tester uses a reduced minor load, just 3 kilograms, and has the major load reduced to either 15 or 45 kilograms depending on the indenter, which are the same ones used for the common scales. Using the 1/16" diameter, steel ball indenter, a "T" is added (meaning thin sheet testing) to the superficial hardness designation. An example of a superficial Rockwell hardness is 15T-22, which indicates the superficial hardness as 22, with a load of 15 kilograms using the steel ball. If the 120¡ diamond cone were used instead, the "T" would be replaced with "N".

Vickers Hardness Test

The Vickers testing method is similar to the Brinell test. Rather than using the Brinell's steel ball type indenter, and have to calculate the hemispherical area of impression, the Vickers machine uses a penetrator that is square in shape, but tipped on one corner so it has the appearance of a playing card "diamond". The Vickers indenter is a 136 degrees square-based diamond cone, the diamond material of the indenter has an advantage over other indenters because it does not deform over time and use. The impression left by the Vickers penetrator is a dark square on a light background. The Vickers impression is more easily "read" for area size than the circular impression of the Brinell method. Like the Brinell test, the Vickers number is determined by dividing the load by the surface area of the indentation (H = P/A). The load varies from 1 to 120 kilograms. To perform the Vickers test, the specimen is placed on an anvil that has a screw threaded base. The anvil is turned raising it by the screw threads until it is close to the point of the indenter. With start lever activated, the load is slowly applied to the indenter. The load is released and the anvil with the specimen is lowered. The operation of applying and removing the load is controlled automatically. Several loadings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness machines. A filar microscope is swung over the specimen to measure the square indentation to a tolerance of plus or minus 1/1000 of a millimeter. Measurements taken across the diagonals to determine the area, are averaged. The correct Vickers designation is the number followed "HV" (Hardness Vickers). 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. Although thoroughly adaptable and very precise for testing the softest and hardest of materials, under varying loads, the Vickers machine is a floor standing unit that is rather more expensive than the Brinell or Rockwell machines.