Hardness

ME-205 and ME-215 Hardness Experiment ME Dept. at KFUPM

1

Hardness Experiment Reading Assignment Callister Section: 6.10. Introduction Testing for hardness is a quick and non distractive test that provides useful information about the strength of materials. The hardness of a material indicates its resistance to surface deformation such as scratching or load indentation. The higher the hardness of a material, the more it is difficult to make an indentation or a scratch. There is no absolute scale for hardness measurement. However, there are various techniques to measure the material hardness with each having its own scale. Hardness tests for metals involve the use of a standard small indenter which is forced into the surface of a material to be tested. The applied load and the depth or size of the resulting indentation are measured and assigned a hardness number according to the related technique. Note that the measured hardness is only relative, and care should be exercised when comparing values determined by different techniques. In generally, it can be concluded that the softer the material, the larger and deeper is the indentation, and the lower the hardness index number. The most common standard techniques include: the Brinell test, the Rockwell test, and the Vickers test. The indenter size and shape, the load range, and the hardness formula for each technique are shown in Figure 1. For most steels, the Brinell hardness (HB) and the tensile strengths are proportionally related as shown in Figure 2 with a slope of 3.45 when the tensile strength is in MPa. Objective The main objective of this experiment is to use different standard techniques to measure the hardness of metallic materials. Instrument Three different instruments will be used:

a) The Brinell hardness testing equipment, which consists of a hydraulic press and a 10 mm ball indentor.

b) The Rockwell hardness testing equipment with different indenters and loads. c) Vickers hardness testing equipment with diamond pyramid and different loads.

All instruments are calibrated by the lab technician; however, the students are encouraged to conduct at least one calibration for one instrument. Materials Three metal samples will be used to measure their hardness using the three techniques. The samples can be: low carbon steel; medium carbon steel; high carbon steel; cast iron; aluminum; brass; copper; or stainless steel. Experimental Procedure At least three different materials will be tested using different hardness techniques. The test parameters and conditions will be selected according to the availability of materials. For each type of materials a total of 10 measurements are required for statistical analysis. In order to obtain uniform and accurate results, it is important that all tests have to be conducted under standard conditions. The American Standard for


2

Testing and Materials (ASTM) has set up standard methods of hardness tests, which are included in the reference. Proceed with the following steps:

a) Identify the material of each specimen used. b) Clean the surface of the specimen using an emery cloth, if needed. c) Position the specimen under the indenter of the instrument. d) Apply a specific load. e) Measure the indentation (imprint on the specimen surface) using a microscope

or an eye-magnifier. If the indentation is very shallow (not deep) in the specimen then increase the load. If the indentation is very deep in the specimen then decrease the load. If the indentation is within the range then proceed with the next step, otherwise go back to step (b).

f) Record your observations and measurements. g) Take other hardness measurements at different locations.

In all cases, be sure to write your observations in your notebook for each test. You need to include these observations in your report. Analysis of Results

1. Calculate the Brinell hardness number for each test. 2. Report the Rockwell hardness number for each test. 3. Calculate the Vickers hardness number for each test. 4. Report the above values for all tested specimens in a table format. One column

should indicate the average values, and an adjacent column should show the standard deviations of the results.

5. Convert the measured Rockwell hardness numbers to Brinell hardness numbers, using the conversion chart (Figure 3). Compare these values with measured Brinell hardness numbers for all materials tested by both hardness methods.

6. Predict the tensile strength from the Brinell and Rockwell hardness numbers for steel specimens using Figure 2.

Points of Discussion

a) Comment on the usability of the hardness experiment (specify few examples if possible).

b) Compare the different hardness techniques. Specify their advantages and disadvantages according to your observations.

c) Compare the measured hardness of the three materials tested. Were you able to measure the hardness of all materials?

d) Discuss the behavior of the different materials, which you have tested.


3

g) Explain why the Brinell hardness-testing method is commonly preferred in foundries that produce castings, whereas the Rockwell hardness-testing method is commonly preferred in heat-treating laboratories.

h) Specify the possible sources of errors for each technique. How much error do these factors contribute to the results obtained?

i) Write your observations and comments whenever possible in your discussion.

Hardness Standards

ASTM E-10: “Standard Test Method for Brinell Hardness of Metallic Materials.”

ASTM E-18: “Standard Test Method for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials.”

ASTM E-1842: “Standard Test Method for Macro-Rockwell Hardness Testing of Metallic Materials.”

ASTM E-92: “Standard Test Method for Vickers Hardness of Metallic Materials.”

ASTM E-384: “Standard Test Method for Micro-indentation Hardness of Materials.”

ASTM E-140: “Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness.”

Figure 1: Hardness Testing Techniques.


4

Figure 2: Relationship between tensile strength and HB.

Figure 3: Comparison of several hardness scales.

Documents

Hardness