Hardness Testing – A Guide to Best Practices

Hardness, as applied to most materials, and in particular metals, is a valuable, revealing, and widely used mechanical test that has been in use in different forms for more than 250 years. As a material property, its importance and value cannot be understated; the information from a hardness test can be used for providing critical material performance information and insight to the strength, flexibility, durability, and capabilities of a wide range of component types from raw materials to finished goods, and prepared specimens. Hardness testing is extensively used in a wide range of industries and plays a particular significance in aerospace, automotive, structural, failure analysis, quality control, and several other forms of manufacturing.

What is Indentation Hardness Testing?

The most fundamental and commonly used definition is the resistance of a material to permanent, plastic deformation. It is measured by loading an indenter of specified geometry and properties onto the material for a definite length of time, and measuring either the depth of dimensions or penetration of the resulting impression or indentation. Rockwell testing is the most frequently used method by virtue of the quick results delivered and is typically used on alloys and metals. It produces a value based on indentation depth or un-recovered indentation.

Importance of Good Practice

Paramount to reliable and accurate Rockwell hardness results is guaranteeing that the operators and the methods employed are following proper test technique and practice. The accurate nature and exactness of a Rockwell test requires following strict hardness protocol and adherence to standards. With the unit of measurement for one regular Rockwell point equal to only 0.002 mm (approx. 0.00008 inch), it becomes clear that such exact measurement needs an extremely precise measuring system and process.

Failure to properly prepare and then execute a Rockwell hardness test can lead to false readings or compromised test data, potentially contributing to the production and delivery of a substandard product. This could have catastrophic and detrimental effects on the performance and the integrity of the goods they are used in.

Rockwell Test Techniques - Test Scale

  • Type of material
  • Specimen thickness
  • Area/width
  • Test location
  • Material homogeneity
  • Scale limitations

Material Type

Following sound practice and complying with applicable standards is comparatively straightforward and will significantly contribute to true and accurate results. Foremost to any Rockwell test process is the identification of the proper hardness scale to be employed on the component to be tested. There are 30 different Rockwell scales with most of the applications covered by the Rockwell HRC and HRB scales for testing most brass, steels, and other metals. With the increasing use of materials other than common steel and brass, and also requirements to test thin materials and sheet steel, a fundamental knowledge of the factors that must be considered in selecting the correct scale to guarantee an accurate Rockwell test is necessary.

The choice is not only between the superficial hardness test and regular 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. An engineering specification is often established at the material design phase and the operator can depend on documented scale requirements. If no specification exists or there is doubt about the appropriateness of a predetermined scale, an analysis should be made of the following factors that control scale selection:

Material Thickness

In the absence of a specified hardness scale, the material type should be identified and then compared with different tables that list the typical type of scale that is relevant to a given material. Generally, this is based on empirical testing information and historical data. As a rule of thumb, using the heaviest load that the material can withstand is advisable as the larger indent will offer the greatest integrity and be minimally affected by the material surface condition.

Usually, diamond scale indenters are used on hardened steels and other extremely hard materials while the ball scales are more applicable to the copper alloy, brass, and aluminum type of materials. While material composition knowledge is a necessary tool in scale selection, there are other highly significant material parameters that play a vital role in determining the proper test method and technique to be followed.

Support

The material thickness is of main importance in scale selection. Since the 30 Rockwell scales are distinguished by total test force, including the indenter type, a force or load that is excessive for the material thickness will be eventually influenced by the support anvil. Interruption in material flow such as this can will lead to erroneous readings and significant misinterpretation of the actual material hardness. ASTM provides scale thickness requirements both in graphical as well as tabular form. It is recommended that these are used as a reference guide in deciding a 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 needed, the actual depth of any indentation can be calculated in order to confirm that this requirement is being met, but this is generally not necessary as the reference tables and graphs offer sufficient information to make an educated decision. As a final rule, no deformation of the material should be obvious on the supporting (underside) surface of the material.

Perpendicularity

Specimen support is also very critical in Rockwell testing because of the fact that the method involves depth measurement. Any specimen movement is transferred to the indenter and the measurement system, leading to an error being introduced into the test. With the precise nature of the test (bearing in mind that one Rockwell point on the regular scale equals 0.002 mm or 0.00008 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 discovered including anvil hardness recommendations. The supporting shoulder and the surface that the specimen sits on must be parallel to each other, and it is necessary for the anvil to present the test specimen perpendicular to the indenter. Both the shoulder and the supporting surface must be free of scratches, nicks, and dirt, and be of sufficient design in order to properly support the material under test. Anvils should be checked on a regular basis, usually before each use, and they should be replaced if found too compromised.

Damaged, dirty or nicked indenters can cause significant drift and repeatability problems in hardness readings. A wide range of standardized, and also custom made fixtures, exist in order to accommodate the different specimen geometries that are tested. Some of the more common anvils include flat or plane anvils for supporting flat surfaces, the cylindron anvil for larger diameter parts, and the "V" style anvil for supporting cylindrical work. Another commonly employed anvil is the pedestal spot anvil that has a small raised flat spot and is used when checking thin, small or irregularly shaped pieces and also test materials without a truly flat bottom.

As it is essential that contact is made between the piece being tested and the part of the anvil instantly beneath the indenter, the small raised spot reduces the effect that could be realized with non-flat test pieces by decreasing the surface area of contact. Test pieces that are not flat should be located on the spot anvil with the curved side down in order 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 suggested; this anvil comprises of a slightly raised, flat, polished diamond surface capable of supporting the test piece and preventing damage and influence that could occur with a standard anvil.

This anvil is just used with the 15 T or 30 T Rockwell scales. It is never recommended to use a diamond indenter with a diamond spot anvil, because breakage of both the anvil and indenter is possible. It is recommended to use the gooseneck anvil for testing outside diameter surfaces of thin-walled tubing. It usually threads onto the tester lead screw or support holder and comprises of a mandrel at the top to support the part to be tested and placed over this mandrel in order 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 weight and size of the "T" slot table, they can only be used with Rockwell® testers that actuate the indenter down to the stationary table that is affixed to the base of the tester as opposed to presenting the part to the diamond through lead screw actuation. Another useful fixture is the Vari-Rest fixture that extends horizontally in order to support elongated pieces.

Indent Spacing

It is a basic requirement that the surface to be indented is perpendicular to the direction of travel of the indenter and that the test piece does not slip or move during the test cycle. A study demonstrated that the effect on the HRC scale indicated a tilt angle of one degree between the specimen surface and the indenter’s axis could result in a 5% error in hardness. Tilt angle should never surpass 2 degrees to guarantee accurate testing. The perpendicularity of the indenter to the specimen is influenced by a number of factors, such as the supporting anvil, the opposing surfaces of the material, and the mechanical components in the tester. Furthermore, the indenter holder and indenter play a vital role in perpendicularity.

Cylindrical Testing and Correction Factors

During coupon block verification or specimen testing, the spacing between indents, as well as from the material edge, must be maintained properly in order to prevent any adjacent indents or worked edge from affecting the next test. The acknowledged criterion is that the distance from the center of any indentation shall be at least three times the diameter of the indentation. Referring 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 a half times the diameter of the indentation. The purpose for these distances is to guarantee that any indentation made is not influenced by work hardening and flow of material around the earlier indentation. Furthermore, the edge distance requirement guarantees that the indentation's area of contact allows proper support.

Surface Finish

When testing on cylindrical surfaces the results will generally show a lower hardness value than if the material was flat. This condition is because of the curvature of the test piece and is dependent upon the hardness of the material; the applied force; the diameter of the test piece; and the shape and size of the indentation. If testing is to be used only for control purposes and all other factors are kept equal, (specimen diameter, indenter and scale), there will be adequate information so that comparative data and subsequent testing is benchmarked.

However, in a number of cases, it is indeed better to compare the hardness of the rounded material with the hardness value of a flat piece, making correction factors compulsory. In a cylindrical piece, the reduction in lateral support will result in the indenter penetrating further into the material which translates to reduced hardness readings. If the diameter of the material is greater than 25 mm (1 inch), the surface will offer suitable surface structure for testing and corrections are not needed. Lower diameter materials will require the correction factor added to the test result.

Most digital Rockwell testers available offer 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 in order to determine the correct factor to adjust by. Alternatively, and in contrast to convex surfaces, concave surfaces will offer higher material support because of the curvature towards the indenter and result in apparently harder material due to production of a shallower indent. A correction factor must be subtracted in this case. It should be noted that all corrections generate approximate results and should not be expected to meet the exact specification. Also, it is vital to guarantee the exact alignment of the indenter to the radius when cylindrical testing.

Other Important Factors to Consider

As good practice, the material tested should be smooth, clean and even. The degree of specimen surface roughness that can affect the hardness results is based on the Rockwell scale being used. Generally, the regular scale is capable of tolerating a finished ground surface in order to attain accurate results. However, as the applied forces become lighter, the surface's requirements become increasingly influential and the need for a smoother surface becomes more vital. For the lowest hardness test force, the 15 Kgf scales, a lapped or polished surface is recommended. Care should be taken when finishing any material before testing in order to avoid the possibility of causing a work hardened condition to the material.

Many basic but significant factors should also be considered when conducting Rockwell tests.

  • Cleanliness of the material, the indenters, supporting anvils and any contacting surfaces, as well as the overall condition of the machine are important elements.
  • The instrument's environment should also be taken into account. Care to avoid areas where excessive vibration occurs will prevent any effects on the hardness readings and tester performance. Maintaining a steady temperature range where the tester is placed should be guaranteed, ASTM suggests testing at ambient temperatures from 50-95 °F (10-35 °C). Operating the tester in temperature extremes may have adverse results on test data.
  • Daily indirect performance verification of the testing instrument is also vital; the scales being used should be verified with the help of standardized test blocks or coupons. If possible, it is suggested that the system is verified with each scale change and at each shift start-up. Blocks should be selected that are in the estimated range of the material being tested and used only on the calibrated side. Two "seating" indents should be made to set the anvils, block, and indenter. These values should be discarded before actual recording of results. Five total readings should be made in the verification process; the measured values must fall within the tolerance stated on a block and the block certificate. If verification fails, the machine should be removed from operation until the appropriate repairs or adjustments are made. Periodic visual inspection of the diamond and ball indenters for damage that could take place during testing should be carried out and if compromised should they be replaced.
  • Finally, maintenance and authorized verification of the instrument is imperative to the nonstop smooth operation and assurance that the system fulfills the precision requirements of a Rockwell test. ASTM recommends annual maintenance and verification of a Rockwell tester and more frequent verification under extreme conditions or heavy use. Verification should be carried out by an accredited verifying agency and the report should follow and reference ASTM E18 Rockwell test method.
  • Hardness testing is a vital and useful tool in quality control and acceptance, materials testing, and performance of materials. The data produced is essential to verify structural integrity, heat treatment, and quality of components in order to determine if a material has the properties necessary for guaranteeing that the materials used in the things used daily contribute to a well-engineered, safe, and efficient world. Proper technique, strict adherence to standards, procedure, and following good practice will immensely contribute to the usefulness and accuracy of Rockwell testing.

Buehler

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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