5 Must-Have Features in a Digital Rockwell Hardness Testing Machine

Following sound practice and complying with applicable standards is relatively straightforward and will greatly contribute to true and accurate results. Foremost to any Rockwell test process is identification of the proper hardness scale to be used on the component to be tested. There are 30 different Rockwell scales with the majority of applications covered by the Rockwell HRC and HRB scales for testing most steels, brass, and other metals. With the increasing use of materials other than common steel and brass, as well as requirements to test thin materials and sheet steel, a basic knowledge of the factors that must be considered in choosing the correct scale to ensure an accurate Rockwell test in necessary. The choice is not only between the regular hardness test and superficial hardness test, with three different major loads for each, but also between the diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter steel ball indenters. Often an engineering specification is established at the material design phase and the operator can rely on documented scale requirements. If no specification exists or there is doubt about the suitability of a predetermined scale, an analysis should be made of the following factors that control scale selection:

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Material Type

In the absence of a specified hardness scale the material type should be identified and compared with various tables that list the typical type of scale that is applicable to a given material. Usually this is based on historical data and empirical testing information. As a rule of thumb, using the heaviest load that the material can withstand is advisable as the larger indent will provide the greatest integrity and be minimally affected by material surface condition. Typically, diamond scale indenters are used on hardened steels and other very hard materials while the ball scales are more applicable to the brass, copper alloy, aluminum type of materials. While material composition knowledge is a necessary tool in scale selection, there are several other extremely significant material parameters that come into play in determining the proper test meth and technique to be followed.

Material Thickness

Of primary importance in scale selection is the material thickness. Since the 30 Rockwell scales are distinguished by total test force, as well as the indenter type, a load or force that is excessive for the material thickness will be ultimately influence by the support anvil. Interruption in material flow such as this can will result in erroneous readings and significant misinterpretation of the actual material hardness. ASTM provides scale thickness requirements both in tabular as well as graphical form. It is recommended that these are used as a reference guide in deciding suitable scale based on material thickness. A general, albeit approximate only, rule is that the material should be a minimum of 10 times the depth of the indentation when using a diamond type indenter and at least 15 times the depth when using a ball type indenter. If necessary the actual depth of any indentation can be calculated to confirm this requirement is being met, but this is generally not necessary as the reference tables and graphs provide adequate information to make an educated decision. As a final rule, no deformation of the material should be evident on the supporting (underside) surface of the material.

Support

Specimen support is also extremely critical in Rockwell testing due to the fact that the method involves depth measurement. Any specimen movement is transferred to the indenter and the measurement system, resulting in an error being introduced into the test. With the precise nature of the test (keeping in mind that one Rockwell point on the regular scale equals 0.002 mm or 0. of an inch) a movement of only 0.001 of an inch could cause an error of over 10 Rockwell points. The supporting anvil should be selected to match the specimen geometry and to provide full and uncompromised support and it is essential that the anvil is rigid enough to prevent any deformation during use. There are certain criteria that must be met on all anvils; a good reference is ASTM E18 where basic guidelines can be found including anvil hardness recommendations. The supporting shoulder and the surface that the specimen sits on must be parallel to each other, and the anvil must present the test specimen perpendicular to the indenter. Both the supporting surface and the shoulder must be free of nicks, scratches and dirt, and be of sufficient design to properly support the material under test. Anvils should be checked on a regular basis, typically prior to each use, and if found too compromised they should be replaced. Damaged, nicked or dirty indenters can cause considerable drift and repeatability problems in hardness readings. A variety of standardized, as well as custom made fixtures, exist to accommodate the various specimen geometries that are tested. Some of the more common anvils include plane or flat anvils for supporting flat surfaces, the &#;V&#; style anvil for supporting cylindrical work, and the cylindron anvil for larger diameter parts. Another commonly used anvil is the pedestal spot anvil that has a small raised flat spot and is used when checking small, thin or irregularly shaped pieces as well as test materials not having a truly flat bottom. As it is essential that contact is made between the piece being tested and the part of the anvil immediately beneath the indenter, the small raised spot minimizes the effect that could be realized with non-flat test pieces by reducing the surface area of contact. Test pieces that are not flat should be placed on the spot anvil with the curved side down to ensure that solid contact is made with the anvil at the point of test. For support of thin sheet type product the diamond spot anvil is recommended, this anvil consists of a slightly raised, flat, polished diamond surface that supports the test piece and prevents damage and influence that might occur with a standard anvil. This anvil is only used with the 15 T or 30 T Rockwell scales. Using a diamond indenter with a diamond spot anvil is never recommended, as breakage of both the indenter and anvil is possible. The gooseneck anvil is recommended for testing outside diameter surfaces of thin walled tubing. It typically threads onto the tester lead screw or support holder and includes a mandrel at the top to support the part to be tested is placed over this mandrel to prevent material compliance during testing. Larger parts can be supported using large diameter test tables or a &#;T&#; slot style table that can be used to clamp the test piece to the table. Due to the size and weight of the &#;T&#; slot table they can only be used with Rockwell® testers that actuate the indenter down to the stationary table that&#;s affixed to the base of the tester as opposed to introducing the part to the diamond via lead screw actuation. Another useful fixture is the Vari-Rest fixture that extends horizontally to support elongated pieces.

Perpendicularity

It is a fundamental requirement that the surface to be indented is perpendicular to the direction of travel of the indenter and that the test piece does not move or slip during the test cycle. A study showed that the effect on the HRC scale indicated a tilt angle of one degree between the specimen surface and the axis of the indenter could result in a 5% error in hardness. Tilt angle should never exceed 2 degrees to ensure accurate testing. The perpendicularity of the indenter to the specimen is influenced by many factors, including the opposing surfaces of the material, the supporting anvil, and the mechanical components in the tester. In addition, the indenter and indenter holder plays a crucial role in perpendicularity.

Indent Spacing

During specimen testing or coupon block verification, the spacing between indents, as well as from the material edge, must be properly maintained to prevent any adjacent indents or worked edge from influencing the next test. The accepted criteria is that the distance from the center of any indentation shall be at least three times the diameter of the indentation In regards to distance from material edge, the distance from the center of any indentation to the edge of the test piece shall be at least two and one-half times the diameter of the indentation. The purpose for these distances is to ensure that any indentation made is not influenced by work hardening and flow of material around the previous indentation. Also, the edge distance requirement ensures that the indentation&#;s area of contact permits proper support.

Cylindrical Testing and Correction Factors

When testing on cylindrical surfaces the results will usually show a lower hardness value than if the material was flat. This condition is due to the curvature of the test piece and is dependent upon the applied force; the hardness of the material; the size and shape of the indentation; and the diameter of the test piece. If testing is to be used for control purposes only and all other factors are kept equal, (specimen diameter, scale and indenter), there will be sufficient information so that comparative data and subsequent testing is benchmarked. However, in most cases, it is better to compare the hardness of the rounded material with the hardness value of a flat piece, making correction factors necessary. In a cylindrical piece, the reduction in lateral support will result in the indenter penetrating further into the material which translates to lower hardness readings. If the diameter of the material is greater than 25 mm (1 inch) the surface will provide suitable surface structure for testing and corrections are not required. Lower diameter materials will need the correction factor added to the test result. Most digital Rockwell testers available provide the means to meet the cylindrical diameter and the correction factor will automatically be added to the result. In manual dial gage testers ASTM correction tables must be referenced to determine the correct factor to adjust by. Alternatively, and in contrast to convex surfaces, concave surfaces will provide higher material support due to the curvature towards the indenter and result in apparently harder material due to production of a shallower indent. In this case a correction factor must be subtracted. It should be noted that all corrections produce approximate results and should not be expected to meet exact specification. Also, it is critical to ensure the exact alignment of the indenter to the radius when cylindrical testing.

Surface Finish

As good practice, the material tested should be clean, smooth, and even. The degree of specimen surface roughness that can affect the hardness results is dependent on the Rockwell scale being used. Usually, the regular scales can tolerate a finished ground surface to obtain accurate results. However, as the applied forces get lighter, the surface&#;s requirements become more influential and the need for a smoother surface becomes more important. For the lowest hardness test force, the 15 Kgf scales, a polished or lapped surface is recommended. Care should be taken when finishing any material prior to testing to avoid the possibility of causing a work hardened condition to the material.

It is an established notion in the field of manufacturing that a certain object is just as good as the materials it is made of. As such, production specialists need to make sure that every component, material, and material treatment used to build a certain device or machine is going to be strong and durable.

This is where the Rockwell Hardness test comes into play since it was specifically designed to measure the durability of a material. The system is designed for testing in accordance to ASTM E18 standard. But before you decide to invest on a Rockwell Hardness Tester, there are a few things about the test that you need to understand first.

What is the Rockwell Hardness Test?

The Rockwell Hardness Test is actually one of several tests aimed to gauge a material&#;s compatibility as a component to an object based on their strength and durability. The other tests in the series include the the Knoop, Brinell, and Vickers methods.

The concept of the Rockwell Hardness test is essentially similar to its counterparts. A sample material is fed on to either a digital or manual Rockwell hardness tester which would then drop a set load ending with an indenter on the surface. Then, the strength of the material will be determined by the nature of the indentation left on the sample.

However, the Rockwell method differs greatly from the Knoop, Vickers, and Brinell methods in one aspect. Where the others measure strength by the size or width of the indentation left, the Rockwell method measures strength through depth. The rationale behind the Rockwell method is that the durability of a material is better gauged on its ability to maintain structural integrity from damages that are not merely superficial. This is why the method is commonly used in the fields of metallurgy and engineering, where the integrity of a material cannot be simply observed at a surface level.

How is the Test Conducted?

As was stated, the Rockwell hardness test is essentially similar to other methods in the sense that strength is gauged through applying load on to a material. How the Rockwell test goes about doing this, however, is different.

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First, a minor load is applied to the surface which determines the zero position. Once the zero position is established, a heavier major load is then applied to the material which causes the indentation.

At the same time, the minor load is maintained all throughout the test. Some variations of the test would require that the major load remain on the material until it the indent it has made is deep enough.

Once the major load is removed, the depth of the penetration starting from the zero position is then measured. As with all hardness tests, the strength of the material is inversely proportional to the damage it sustained. As such, under the Rockwell method, materials tend to rate higher at the hardness scale if the indentation left on them is closer to the surface.

Why is the Rockwell Method Reliable?

In some cases, gauging the strength of materials through Rockwell Hardness Tester is preferable. This is due to a number of advantages as listed below.

1. No Material Preparation is Required

In some tests, the sample material has to be buffed, ground, cut up, and polished in order to get the most accurate of readings. Essentially, tests like the Knoop and Vickers method need for the sample surface to be as flat as possible with little to no variation.

In the Rockwell method, since the depth of the indentation is being measured, there is no need for the sample to undergo different processes to make the test accurate. This is also why the Rockwell method is used when testing unrefined materials like ore.

2. No Optical Evaluation

With the Rockwell method, the value of the hardness is directly readable since most hardness testers using this method don&#;t use diagonal scales to measure hardness. This is quite true if you opt for a digital Rockwell Hardness tester as the values are immediately displayed on screen after a few seconds of processing.

With the absence of optical evaluation, the chances of the operator making an error on the readings are greatly reduced.

3. Cost-Effective

Since Rockwell hardness testers do not need to be equipped with elaborate optical evaluation systems, they have rather short test cycles. This would allow for a more efficient testing process where operators can quickly move through one specimen to another.

Add to this the fact that Rockwell testers are considerably inexpensive compared to other instruments, the Rockwell method makes for a rather cost-effective process. And this is not the only way the Rockwell method helps its operators save up on costs.

4. Non-Destructive Nature

Despite the fact that the test leaves a sizeable indent on the material, the Rockwell method is a non-destructive and non-invasive process. After every test, the specimen itself can still be used for other purposes.

This means that the Rockwell method can effectively help its operators reduce their waste. If your workshop also processes raw materials, this could help in reducing operational costs.

How Accurate is the Rockwell Hardness Tester System?

The RockGen Rockwell Hardness Testing System was designed to test the hardness level of metal based on the indentation they leave on a tested material according to the Rockwell scale. The machine is capable of measuring the resolution of a material to a 0.001&#;. NextGen&#;s Rockwell hardness testers are guaranteed to provide one of the absolute highest accuracies on the market. The Rockwell Hardness Testing measurement is calculated by first applying the preliminary force test called a preload for a specified period of time called a dwell time.

After the specified period of time, an additional load, also known as the major load, will be applied. Now the total force has been applied and the measurements of the depth of the indentation are taken from the reference point to the final depth of the penetrator after it has undergone its major load. The distances between the two imprints are known as the Rockwell Hardness Values.

How Many Tests Are There for the Rockwell Regular Test Scales?

The Rockwell Hardness Tester has many different scales depending on the application of the tester on different materials. There is a total of 30 Rockwell hardness scales available to use for hardness testing and hardness value. The Rockwell Regular Scales Testing use a preload of 10kgf and a major load of 50kgf, 100kgf, or 150kgf. The Superficial Rockwell scales are for materials that are thin as 0.15mm or 0. inches and that use a preload of 3kgf and a major load of either 15kgf, 30kgf, or 45kgf.

What Are the Limitations of the Rockwell Testing Systems?

The Rockwell Testing systems depend on the material that is being tested including the hardness, thickness, indentation positioning, and scale limitations on the materials that are being tested. The material being tested must be at least 10x thicker than the indentation that the Rockwell indentation is going to be when it is made of the material.

NextGen&#;s Rockwell Hardness Testers are amongst the leading Rockwell Manufacturing machines and can provide you with all of the options your company needs including the Rockwell Twin Digital Testing machines. The Twin model is among the most popular products we have allowing the operator to test both Rockwell regular and Rockwell superficial scales on a single instrument. Enjoy the benefits of Rockwell hardness testing and the unprecedented accuracy it provides.

In Conclusion

Despite its advantages, the Rockwell method is not exactly applicable at all instances. However, it does not mean that a Rockwell Hardness Tester will not yield considerably accurate measurements which should help any workshop.

This is why it is important that you should find a rather reputable manufacturer who can then provide you with a well-build Rockwell tester. If calibrated right, these instruments can help you improve the quality of your facility&#;s output while reducing waste.

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