To effectively use metals in various designs, it is important to know how durable they are. Hardness is the most commonly calculated quality characteristic of metals and alloys. There are several methods for its determination: Brinell, Rockell, Super Rockwell, Vickers, Ludwik, Shore (Monotron), Martens. This article discusses the Rockwell Brothers method.
What is a method?
The Rockwell method is called the method of testing materials for hardness. For the element under study, the penetration depth of the solid indicator tip is calculated. At the same time, the load remains the same for each hardness scale. Usually it is 60, 100 or 150 kgf.
The indicators in the study are balls made of durable material or diamond cones. They should be with a rounded sharp end and have an angle at the apex of 120 degrees.
This method is recognized as simple and quickly reproducible. Which gives him an advantage over other methods.
History
Vienna research professor Ludwig first proposed using an indenter to study hardness by penetrating the material and calculating the relative depth. His method is described in a 1908 paper, Testing by a Cone (Die Kegelprobe).
This method had disadvantages. The Hugh brothers and Stanley Rockwells proposed a new technology that eliminated errors of the mechanical imperfection of the measurement system (backlash and surface defects, contamination of materials and parts). Professors invented a hardness tester - a device that determines the relative depth of penetration. It was used to test steel ball bearings.
The determination of the hardness of metals by the Brinell and Rockwell methods deserved attention in the scientific community. But the Brinell method was inferior - it was slow and was not applied to hardened steels. Thus, it could not be considered the method of non-destructive testing.
In February 1919, the hardness tester was patented under the number 1294171. At this time, the Rockwells worked for a company manufacturing ball bearings.
In September 1919, Stanley Rockwell left the company and moved to New York State. There he applied for an improvement in the instrument, which was accepted. The new device is patented and improved by 1921.
At the end of 1922, Rockwell founded the Heat Treatment Enterprise, which still operates in Connecticut. Since 1993, it has been part of Instron Corporation.
Advantages and disadvantages of the method
Each method for calculating hardness is unique and applicable in any field. Brinell and Rockwell hardness testing methods are basic.
There are a number of advantages of the method:
- the possibility of conducting experiments with high hardness;
- minor surface damage during the test;
- A simple method that does not require measuring the diameter of the print;
- the test process is fast enough.
Disadvantages:
- in comparison with the Brinell and Vickers hardness testers, the Rockwell method is not accurate enough;
- carefully prepare the surface of the sample.
Rockwell Scale Structure
To test the hardness of metals using the Rockwell method, only 11 scales were derived. Their difference is in the ratio of the tip and load. The tip can be not only a diamond cone, but also a ball made of an alloy of carbide and tungsten or hardened steel in the shape of a sphere. The tip fixed in the installation is called the identifier.
Scales are usually denoted by the letters of the Latin alphabet: A, B, C, D, E, F, G, H, K, N, T.
Strength tests are carried out on the main scales - A, B, C:
- Scale A: diamond cone tests with a load of 60 kgf. Designation - HRA. Such tests are carried out for thin solid materials (0.3-0.5 mm);
- Scale B: steel ball test with a load of 100 kgf. Designation - HRB. Tests are conducted on annealed mild steel and non-ferrous alloys;
- Scale C: cone tests with a load of 150 kgf. Designation - HRC. Tests are carried out for metals of medium hardness, hardened and tempered steel or layers with a thickness of not more than 0.5 mm.
Rockwell hardness is usually denoted by HR with the third letter of the scale (for example, HRA, HRC).
Formula for calculation
The hardness of the material affects the penetration depth of the tip. The harder the test object, the less penetration.
To numerically determine the hardness of a material, a formula is needed. Its coefficients depend on the scale. To reduce the measurement error, the relative difference of the indenter penetration depth at the time of application of the main and preliminary (10 kgf) load should be taken.
The Rockwell hardness measurement method involves the use of the formula: HR = N- (Hh) / s, where the difference Hh denotes the relative depth of penetration of the indenter under loads (preliminary and main), the value is calculated in mm. N, s are constants, they depend on a specific scale.
Rockwell Hardness Tester
A hardness tester is a device for determining the hardness of metals and alloys by the Rockwell method. It is a device with a diamond cone (or ball) and the material into which the cone must enter. A load is also riveted to adjust the force of impact.
The time is displayed by an indicator. The process takes place in two stages: first, pressing is done with a force of 10 kgf, then stronger. For more pressing, a cone is used, for less - a ball.
The studied material is located horizontally. The diamond is lowered onto it with a lever. For a smooth descent, the device uses a handle with an oil shock absorber.
The main load time is usually from 3 to 6 seconds, depending on the material. The preload must be maintained until the test results are obtained.
The large hand of the indicator moves clockwise and reflects the result of the experiment.
The most popular in practice are such Rockwell hardness tester models:
- Stationary Metrotest devices of the ITR model, for example, ITR-60/150-M.
- Hardness testers Qness GmbH model Q150R.
- Stationary automated device TIME Group Inc model TH300.
Test procedure
The study requires careful preparation. When determining the hardness of metals by the Rockwell method, the surface of the sample should be clean, without cracks and scale. It is important to constantly monitor whether the load on the surface of the material is applied perpendicularly and whether it is stably placed on the table.
The imprint on the indentation of the cone must be at least 1.5 mm, and upon indentation of the ball, more than 4 mm. For effective calculations, the sample should be 10 times thicker than the indenter penetration depth after removal of the main load. At least 3 tests of one sample should also be carried out, after which the results should be averaged.
Test Steps
In order for the experiment to have a positive result and a small error, one should adhere to the procedure for its implementation.
Stages of conducting the Rockwell hardness test:
- Decide on the choice of scale.
- Set the required indenter and load.
- Conduct two test (not taken into account in the results) prints to correct the correct installation of the device and the sample.
- Place the reference block on the instrument table.
- Test the preload (10 kgf) and reset the scale.
- Apply the main load, wait for maximum results.
- Remove the load and read the resulting value on the dial.
The rules allow one sample to be tested when testing mass production.
What will affect accuracy
When conducting any test, it is important to consider many factors. Rockwell hardness testing also has its own characteristics.
Factors to pay attention to:
- The thickness of the test sample. The test rules prohibit the use of a sample that is less than ten times the depth of penetration of the tip. That is, if the penetration depth is 0.2 mm, then the material should be at least 2 cm in thickness.
- The distance between the prints on the sample must be maintained. It is three diameters between the centers of the near prints.
- You should consider the possible change in the results of the experiment on the dial, depending on the position of the researcher. That is, the result should be read from one viewpoint.
Mechanical properties in strength tests
It was possible to connect and study the strength characteristics of materials and the results of hardness testing using the Rockwell hardness method from material scientists such as N. N. Davidenkov, M. P. Markovets and others.
According to the results of indentation hardness testing, methods for calculating the yield strength are applied. This relationship is calculated for high-chromium stainless steels that have undergone multiple heat treatments. The average deviation, when using a diamond indenter, was only +0.9%.
Studies are also being conducted to determine other mechanical properties of materials related to hardness. For example, tensile strength (or temporary resistance), true fracture resistance and relative narrowing.
Alternative Hardness Testing Methods
Hardness can be measured not only by the Rockwell method. Consider the main points of each method and their differences. Statistical load tests:
- The studied samples. The Rockell and Vickers methods make it possible to test relatively soft and high-strength materials. The Brinell method is designed to study more soft metals with hardness up to 650 HBW. The Super Rockwell method allows testing for hardness at light loads.
- GOSTs. The Rockwell method corresponds to GOST 9013-59, the Brinell method - 9012-59, the Vickers method - 2999-75, the Shore method - GOST 263-75, 24622-91, 24621-91, ASTM D2240, ISO 868-85.
- Hardness testers. The devices of the Rockwell and Shore researchers are easy to use and small in size. Vickers equipment allows testing on very thin and small samples.
The experiments under dynamic pressure were carried out according to the method of Martel, Poldi, using a vertical coper Nikolaev, a spring device Schopper and Bauman and others.
Hardness can also be measured by scratching. Such tests were carried out using a Barb file, a Monters, Hankins instrument, a Birbaum microcharacterizer, and others.
Despite its shortcomings, the Rockwell method is widely used for hardness testing in industry. It is notable for its simplicity, mainly due to the fact that it is not necessary to measure the print under a microscope and polish the surface. But the method is not as accurate as the proposed research by Brinell and Vickers. Hardness, measured in various ways, is dependent. That is, Rockwell productive units can be converted to Brinell units. There are regulatory documents at the legislative level, such as ASTM E-140, which compare hardness values.