Fuente: Roberto León, Departamento de Ingeniería Civil y ambiental, Virginia Tech, Blacksburg, VA
Prueba de la dureza es uno de los ensayos mecánicos más universalmente valiosos disponibles para ingenieros, ya que es simple y relativamente barata para la riqueza de la información y los datos que produce. Ensayos de dureza, generalmente bajo la forma de un ensayo de penetración superficial, es más rápido y menos destructivos que los ensayos de tracción. Dureza proporciona una relación lineal con resistencia a la tracción sobre una amplia gama de fortalezas para muchos materiales, tales como el acero. Pruebas de dureza son empíricas, más que derivadas de la teoría, como los resultados confunden los efectos de muchas propiedades de diferentes materiales (módulo de Young, resistencia, etcetera).
Dureza es una característica de un material que se utiliza para describir cuánto plástica deformación (rendimiento) que un material se experimentan cuando se aplica una fuerza conocida). Uno puede caracterizar dureza de tres maneras: rayado, indentación y dureza del rebote. Un ejemplo común de principios de una prueba de dureza (cero) es la escala de Mohs (1820), derivada de minerales, y en que talco tiene un valor de 1 y el diamante de un valor de 10. En muesca prueba utilizando el método Rockwell, se utilizan pequeños penetradores con cargas diferentes. Los más comunes son el B Rockwell de dureza (HRB), que utiliza un 1/16 de pulgada habían endurecido a penetrador de bola de acero con un peso de 100 kg y C de la dureza Rockwell (HRC), que utiliza a un penetrador de cono de diamante con un peso de 150 kg. HRB las pruebas se realizan para materiales con dureza de gama baja, tales como aluminio, latón y aceros blandos, mientras que pruebas HRC se utilizan para materiales con dureza alta gama, como los aceros más duros. Pesos más pequeños (15 a 45 kg) se utilizan para la prueba de dureza superficial Rockwell como HR15W, que utiliza una bola de acero 1/8 de pulgada con un peso de 15 kg. Con su carga baja y poco profunda impresión, una prueba de dureza superficial de Rockwell es ideal para materiales muy delgados o quebradizos. Un ejemplo de una prueba de rebote es el martillo de Schmidt, utilizado para medir la resistencia del hormigón. En esta prueba, una masa de acero se dispara en la superficie con una fuerza conocida y se mide el rebote de la bola. En todos los tipos de prueba de la dureza, es imprescindible para llevar a cabo calibraciones extensas si se quieren obtener resultados fiables.
Para este experimento analizaremos la prueba de dureza Rockwell, que mide la dureza de indentación de aluminio sin tratar y tratada térmicamente.
A typical result of Rockwell testing on a Jominy specimen for a HR C test is shown in the video. The hardness decrease appreciably as one moves away from the end subjected to the water jet. The hardness test value can be converted to a tensile strength through charts provided by the testing machine manufacturer. The results show that the steel varies considerably in both hardness and strength as one moves away from the quenched end.
Hardness testing is one of the principal tests to garner important engineering information about a material. Rockwell hardness testing is the preferred method of hardness testing, as it eliminates the need for advanced optical equipment, but instead employs basic laboratory equipment to accurately, inexpensively, and quickly measure the hardness of a material. More importantly, this method is easily translated and reproducible between labs and testing personnel because of its relative simplicity. The procedure requires that after taking an initial reading in preloading conditions, one applies the major load and measure the change in position from the initial value. This value can then be calculated into a hardness value, which can also be transformed and approximated to other hardness scales by a series of equations. The Rockwell hardness test was used to show how the rate of cooling affect the martensitic structure of steels, and how the rate of cooling results in materials with significantly different strengths.
Hardness tests enable us to utilize proper materials that will ensure high quality of performance in their use. Every product that we use in our daily lives has been tested for hardness. Consider the following amusing scenario is just one of many daily examples as to how hardness testing allows us to enjoy materials that are both useful and safe. Imagine the plethora of times hardness testing was used in order for you to throw a backyard barbeque for your family reunion. The metal lawn chairs certainly had to be hardness tested to make sure that the metal was strong enough, so that Aunt Bessie did not go tumbling to the ground when she sat down. The metal grates on the grill also had to be hardness tested over a wide range of temperatures (given that your mother likes her steak well done on high heat) to ensure they would not fracture or give when inevitably you dropped the grilling tongs on the grill. The knives you use to cut the meat are all hardened steel also. Finally, the metal ladder in the pool had to hardness tested in aquatic conditions to make sure that when your younger cousin tried to climb out of the pool, he didn’t fall back in when the metal failed. There are a lot of saved hamburgers, brats, and hotdogs, not to mention human lives, thanks to hardness testing.
Hardness testing is a simple and relatively inexpensive test. Quicker and less destructive than tensile testing, it is considered one of the most universally valuable mechanical tests available to engineers.
Hardness testing values are empirical, and yet results provide a very good correlation with material strength over a wide range of materials. When a known force is applied in a hardness test, the amount of plastic deformation the material undergoes determines the hardness value.
In Rockwell-type testing, loaded indenters of various sizes and shapes measure hardness. For this experiment, we will measure and compare the hardness of untreated and heat-treated steel using the Rockwell hardness test.
Several common methods to measure the hardness of metals include Brinell, Vickers, Knoop, and Rockwell hardness BNC. Each of these methods utilizes a penetrator, either in the shape of a sphere, a cone, or a diamond pyramid.
An indentation is made into the surface of a metal and a hardness reading is displayed. Of this list, the Rockwell hardness test is the most popular one for structural steels. A standard manual Rockwell tester consists of a lever system to apply the load and an analog readout showing the hardness number.
In the typical Rockwell test, a zero point is established to account for surface variations by applying a preliminary load and measuring an initial penetration depth. Next, the major load is applied to the indenter. Finally, the load is removed and the final penetration depth is measured. The dial gauge on the top of the machine uses the difference between these two values to display a Rockwell hardness value. The harder the material is, the less the indenter will penetrate, resulting in a higher Rockwell hardness value. Thus, the values for Rockwell B and Rockwell C hardness are based on the depth of the penetration, and therefore the test machines are calibrated often using calibration test blocks for specific hardness ranges.
Rockwell hardness testing can be used to evaluate how the strength of a material is changed through processes such as heat treatment or cold rolling. Cold rolling tends to result in stronger, harder materials. Heat treating can result in softer materials through heating but harder structures through quenching.
For example, in the Jominy End Quench Test, a cylindrical specimen is heated uniformly. One end is then quenched with a stream of water. Changes along the length of the specimen from the quenched end to the unquenched end can be seen in hardness values, which are representative of changes in the microstructure.
In the next section, we will measure the hardness along the length of a steel Jominy End Quench Test specimen to observe the transition from untreated to heat-treated steel using the Rockwell hardness test.
Before you begin, familiarize yourself with the testing machine. The anvil, which can be raised or lowered by the capstan handwheel, supports a sample underneath the interchangeable indenter.
A test load is chosen using the selector on the side of the machine and is applied by turning the load lever from the unloading position to the loading position. Correct pre-loading and final measurements are determined using the dial gauge on the top of the machine.
Before inserting your test specimen, confirm that the Rockwell C diamond cone indenter is installed and the test load is set to 150 kilograms. Secure the specimen in the machine with the flat surface against the anvil. For this demonstration, we’ll use a Jominy End Quenched specimen that has undergone water cooling treatment.
Move the load lever to the unloading position, and then raise the anvil to bring the specimen close to the indenter. Adjust the specimen position so that the indenter is one-sixteenth of an inch from the end. When the position is correct, re-secure the specimen so that it will not move during testing. Now, raise the sample once again until the needle on the front dial begins to move slightly, indicating that contact with the indenter is established.
Apply the pre-load by continuing to slowly raise the specimen until the needle on the dial has completed three full turns. Stop when the needle has completed the third turn. Adjust the outside ring of the dial gauge so that the initial reading is zero. Then move the load lever to the loading position to apply the test load. The needle will settle to a new value as the load is applied. Wait until it stops moving and then move the load lever back to the unloading position.
Record the Rockwell C hardness from the dial gauge and then lower the anvil to move the sample away from the indenter. Repeat this test along the length of the specimen. The ASTM-A255 guidelines specify that readings should be taken at one-sixteenth inch intervals for the first half inch and at one-eighth inch intervals for the next half inch.
Plot the Rockwell hardness of the specimen as a function of position along the specimen. The hardness clearly decreases as distance from the quenched end increases.
The Rockwell hardness test was used to show that due to the rate of cooling, the change in internal structure of the material affected the hardness of the material, which in turn indicates the strength of the material.
Now that you appreciate the hardness test for its ease of use, let’s take a look at how it is applied to assure the quality of everyday products.
Hardness values can easily be converted to strength values using charts derived from empirical equations. As expected, softer materials have lower strength values and harder materials have higher strength values. Because of this, hardness values can be used in place of more costly tensile testing to predict the strength of things we use every day.
Just looking around at a backyard barbecue, you will see many products that are deemed safe due to hardness testing. Metal lawn chairs, grates on the grill over a wide range of temperatures, hardened steel knives, and the metal pool ladder in aquatic conditions. All were likely hardness tested to assure consumer safety.
You’ve just watched JoVE’s introduction to the Rockwell hardness test. You should now understand why hardness testing is commonly used, how to perform hardness testing, and how to analyze the results obtained.
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