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Hardness Laboratory

Laboratory Performed on September 23, 2008 by the Late Tuesday Lab Group:

Ryan Carmichael, Rachel Cohen, Zach Eichenwald, Anne Krikorian, Jing Ng, Meghan Whalen

Report by Ryan Carmichael and Anne Krikorian

E59 Laboratory Report – Submitted October 7, 2008

Department of Engineering, Swarthmore College

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Abstract:

In this lab, Rockwell hardness tests were performed on two unknown steel specimens and

one unknown brass specimen, with the intent of identifying the metals. The results of our

experiment, shown in Table 1 below, correlated to several possible compositions for the

unknown steel specimens and one likely composition for the brass specimen. These results

suggest that the Rockwell hardness test is a poor test to determine composition of specimens,

although, in practice, hardness tests are useful for other purposes.

Theory:

Hardness is a material quality with many definitions and applications. In this lab, we

define hardness as resistance to indentation under an applied load, which corresponds with

deformation of the material under a load. Other definitions of hardness include resistance to

scratching, resistance to abrasion, resistance to cutting or drilling, or energy absorption under

impact loads. Definitions of hardness are essentially arbitrary, and different hardness tests reflect

this, measuring a variety of different qualities.

Possible Materials Tested

Brass Steel 1 Steel 2 ~HRB 18.6 ~HRB 89.2 ~HRB 87.2

Carbon steel 1042 cold drawn Carbon steel 1040 cold drawn Carbon steel 1042 normalized, cold drawn Carbon steel 1045 annealed, cold drawn Carbon steel 1043 cold drawn Carbon steel 1046 hot rolled

Cartridge brass, 70% UNS C26000 annealed Carbon steel 1045 cold drawn Carbon steel 1140 cold drawn Carbon steel 1046 annealed, cold drawn Carbon steel 1145 hot rolled Carbon steel 1049 hot rolled Carbon steel 1146 hot rolled Carbon steel 1050 hot rolled Austenitic stainless steel 201 annealed Carbon steel 1137 hot rolled

Table 1: Possible Specimen Compositions

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Hardness is related to ductility, tensile strength, and many other material properties, but

does not have a defined relationship with any of them. As such, the hardness test is by and large

used to compare the hardness values of different materials and not in calculations to determine

other properties. Still, there are multiple applications of hardness tests. Hardness test results can

sometimes differentiate between materials, and materials may be graded according to hardness.

Hardness can also be used to roughly predict material properties, but only roughly. In

production, hardness tests may be performed to check for consistency or the effects of a

hardening treatment. This is the most common use of hardness tests.

Many different hardness tests exist; for the purpose of this lab, we will focus on

indentation hardness tests, but not exclusively. The main tests used are the Brinell, Rockwell,

Knoop, Monotron, and Shore scleroscope tests. All of these tests use an indenter, whether a ball,

cone, or pyramid made out of a hard substance such as hard steel, diamond, or, more modernly,

tungsten carbide. All but the Shore scleroscope test are static tests, meaning that the load is

applied gradually- it is the depth of indentation achieved by applying a steady load to the

indenter that is of interest. The Shore scleroscope, meanwhile, applies a load by dropping it or

propelling it with a spring, and the height of rebound of the indenter is the concern (this test

measures energy absorption more than resistance to indentation). In most hardness tests,

variations in the size or shape of the specimen tested can cause large variations in the results of

the tests, even if they are made of the same material. As a result, care needs to be taken that the

thickness of each specimen is within a testable range, and that the surface area being tested is

large enough for the proximity of the edges to the indention to not affect results.

Each hardness test has its own limitations. The Brinell test uses a hard ball as an indenter

and applies relatively large loads, with 3000 kg common. The Rockwell test (tester shown in

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Figure 1) is similar, but uses various

types of indenters and various

loads, both smaller than those used

in the Brinell test. The Knoop

indenter is made of diamond and

shaped to produce a diamond-

shaped indention, and is often used

for the testing of very small

specimens and requires microscopic

examination of the sample. The

Monotron test measures the load

required to achieve a standard

indentation depth, and uses a

diamond-tipped cone indenter with

the load applied by a hand lever.

The Shore scleroscope, ass mentioned before, measures the rebound hardness of a material, and

depends on the resilience of the hammer for consistent results.

In the United States, the two most commonly used tests are the Brinell and Rockwell

tests: we will use the Rockwell test method. The Rockwell test is particularly quick and

convenient, leading to its popularity in production and manufacturing uses. The test makes use

fifteen different scales, according to the hardness range of the material being tested and

determined by the type of indenter and the load applied. It can, as a result, be used to find

Figure 1: Rockwell Tester (formerly Troxell figure 12.7)

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hardness values that fall outside of other test method’s scales, for instance those higher than the

Brinell scale reaches. In the lab, we use the two most common Rockwell scales: B and C.

In general, hardness results can be converted from one scale to another, so long as the test

is examining the same type of hardness. These conversions can be complicated, though, due to

the inherent differences in the tests themselves. Even within the Rockwell testing methods,

results fall outside of the expected range for a certain scale cannot be converted to a more

appropriate scale with high accuracy.

Procedure: (Adapted from Troxell)

Figure 2: Procedural Diagrams (originally Troxell 12.8)

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• As shown in Figure 2, we first examined the Rockwell hardness tester, insuring that the

operating lever was in the position closest to the operator and that the correct major load was

in position to be applied.

• We then inserted the proper indenter into the testing machine and placed the test specimen on

the anvil.

• Next, we turned the elevating screw, raising the specimen into contact with the indenter. We

continued to elevate the specimen until the initial load was fully applied, determined by the

pointer being within 5 scale divisions of its upper vertical position. We then turned the bezel

of the gage so that the B 30 mark, distinguishable by a red arrow, was directly above the

pointer.

• After checking that the specimen was held firmly in place by the initial load, we released the

operating lever with a smooth motion. Allowing the handle to move without interference

until the major load was applied, we waited until the handle and pointer came to a rest.

• We then returned the operating load to its original position with a smooth, gentle motion.

• Finally, we recorded the result of the test off of the appropriate dial.

• We repeated this test for a total of five tests on each specimen.

Results:

In order to identify the composition of each specimen, we calculated the mean of the five

tests and compared these results to standard hardnesses for steels and brasses, respectively,

published by ASM (Appendices A and B). In order to make a direct comparison, we need to

convert our result in Rockwell B hardness to Brinell hardness. For the steel specimens we

converted via the formula HB = 7300 / (130 – HRB) found in ASM Handbook Vol. 8

Mechanical Testing and Evaluation. For the brass specimen, we converted to Rockwell F

hardness by using a conversion graph (Appendix C). The results obtained are presented below in

Tables 1 and 2.

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Discussion: Our results yield a number of possible materials that the unknown samples could have

been composed of. For both of the steel samples, we found a range of steels that matched our

results: for steel 1, carbon steel 1042-1046 cold drawn or 1049, 1050, or 1137 hot-rolled; and for

steel 2, 1040, 1045, or 1140 cold drawn, 1046, 1145, or 1146 hot-rolled, or austenitic stainless

steel 201 annealed. The confidence we have in these possibilities varies, as the raw data for the

unknown steel 2 has the largest standard deviation while unknown steel 1 has the lowest. For the

Hardness Test Results

Unknown Brass Unknown Steel 1 Unknown Steel 2

Test Rockwell Result Test Rockwell

Result Test Rockwell Result

1 B 18 1 B 88 1 B 84 2 B 19 2 B 89 2 B 78 3 B 18 3 B 90 3 B 90 4 B 17 4 B 90 4 B 92 5 B 21 5 B 89 5 B 92

Mean B 18.6 Mean B 89.2 Mean B 87.2 Standard Dev. B 1.517 Standard Dev. B 0.837 Standard Dev. B 6.010

Rockwell F equiv. F 70 Brinell Equiv. HB 178.9 Brinell Equiv. HB 170.6

Table 2: Test Results

Possible Materials Tested

Brass Steel 1 Steel 2 ~HRB 18.6 ~HRB 89.2 ~HRB 87.2

Carbon steel 1042 cold drawn Carbon steel 1040 cold drawn Carbon steel 1042 normalized, cold drawn Carbon steel 1045 annealed, cold drawn Carbon steel 1043 cold drawn Carbon steel 1046 hot rolled

Cartridge brass, 70% UNS C26000 annealed Carbon steel 1045 cold drawn Carbon steel 1140 cold drawn Carbon steel 1046 annealed, cold drawn Carbon steel 1145 hot rolled Carbon steel 1049 hot rolled Carbon steel 1146 hot rolled Carbon steel 1050 hot rolled Austenitic stainless steel 201 annealed Carbon steel 1137 hot rolled

Table 1: Possible Specimen Compositions

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unknown brass sample, we found one particular material that we considered likely. While we

initially could not find a material with matching properties due to conversion problems, we later

found a graph detailing the conversion between the scales for aluminum alloys and brasses,

which gave more reasonable results than the formulas we found elsewhere. With the new

conversion, we found that Cartridge brass, 70% UNS C26000, annealed is the most likely

material, although the hardness numbers do not match precisely (HRF 70 compared to HRF 72).

In this lab, error came from many sources above and beyond human error. The Rockwell

hardness test is easily influenced by the cleanliness and consistency of the machine and the

specimen. Dirt or dust build-up on the anvil can result in incorrect results, as can grime or

oxidation on the specimen. Also, while a single specimen can be tested many times if the size is

sufficient, proximity to previous indentations and to the edges of the sample can affect results.

Impurities or inconsistent shape in the indenter, particularly for the steel-ball indenter, will yield

incorrect values, too. Beyond material factors, the value determined for the brass specimen is

inaccurate because of the scale used: we used scale B when scale F would have been more

appropriate, and so our results were too small to accurately convert to the proper scale.

Conclusion:

Overall, we found in this lab that using the hardness number or rating of a material was a

poor method of identifying the material tested. Discrepancies between different testing methods

and scale conversions, along with the indistinctive nature of the hardness values, prevent

hardness form being a distinguishing property. However, we did determine the likely identities

of the unknown samples we tested. In order to obtain more specific results, more testing would

have to be preformed.

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References:

“Aluminum Alloys and Brasses – Hardness Conversions.” Hardness Conversion Charts. Granta

Design. 6 Oct 2008. < http://www.grantadesign.com/images/hardness.al1.gif>

ASM Handbook, Volume 20, Materials Selection and Design. Materials Park, OH: ASM

International, c1990-.

ASM Handbook, Volume 8, Mechanical Testing and Evaluation. Materials Park, OH: ASM

International, c1990-.

ASTM E18-03: Standard Methods for Rockwell Hardness and Rockwell Superficial Hardness of

Metallic Materials. West Conshohocken, PA : ASTM International, 2004.

Davis, Harmer Elmer, G. Hauck, and G. Troxell. The Testing of Engineering Materials. Boston:

Mcgraw-Hill College, 1982.

Siddiqui, Faruq. “Mechanics of Solids: Hardness Test.” Swarthmore College, 2008.

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Appendix A: Steel Composition and Property Chart

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Appendix B: Brass Composition and Property Chart

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Appendix C: Rockwell Hardness Conversion Graph