Hardness Testing of Materials

Title

Hardness Testing of Metallic Material

Objectives

To determine the hardness of hard steel using diamond tip

Introduction

·         It is very important to study hardness of metals for all fields specially engineering. Because without studying this human may face severe disasters. We experienced this all over the engineering history. So to measure the hardness, people used Rockwell hardness test since early 1900.

·        In this practical we take a number called Rockwell hardness number. We use diamond tip to measure Rockwell hardness number. 

Theory

For diamond tip-

HR=100-h/0.002

Pre load(N)-98, Indenter- Diamond(120 degree,0.2 tip radius), Total test force(N)-1471, Specimen material- Hard Steel.

Apparatus

·         Hard Steel

·         Diamond tip   

·         Rockwell Scale

     

Procedure

·         First, the crank was set to unload position.

·         And we have to choose a weight by rotating the nod.

·         Then, diamond tip was inserted to the Rockwell hardness tester.

·         The black(large) scale was pointed to ‘C’ on the scale.

·         After that, the hard steel piece was placed and the rotated the hand wheel till it touches the diamond tip without changing the scale.

·         Then, the hand wheel was rotated 3 rounds in the black scale.

(We can make sure that it has turned 3 circles by looking the small scale and it’s pointer needs to be in the red dot.)

·         After that, crank was set to a load position.

·         Finally, the readings were taken.

·         Average value of the hardness must be measured by taking multiple readings and calculating the average.

Observations

Sample	1st Value (HRC)	2nd Value (HRC)	3rd Value (HRC)	4th Value (HRC)	5th Value (HRC)	Average Value(HRC)
Hard Steel	82	77	79.5	73	74	77.10

Discussion

Results

            In this test we obtained 8 readings by doing the same procedure I mentioned earlier.

Hardness of a steel must be in 60-90HRC range. So, we had to ignore 3 readings because they were not possible. They were 93.5, 92.5 and 95.0. So, we took other 5 readings and calculated the average value for hardness. As the reason for those ignored values, depends on the people who did it. Those values were taken by 8 different students. Readings should be more accurate if it’s done by single person. At the end we took a general idea about Hardness of a material and the test was good enough to take a average value for the hardness. According to the reports and journals by readings are too high and I don’t think our test was successful.

Importance of Hardness Testing

             

            Hardness of a material is the property that enables it to resist deformation, bending, scratching, abrasion and cutting. Among those tests used in production industry and material selection to determine the characteristics of a material and its suitability for a given application, hardness testing is one of the most critical.

            There are lots of incidents happened in the history because of the wrong decisions about hardness. Mississippi River Bridge (2007) and Titanic Disaster are the most known examples.

·         Mississippi River Bridge (2007)

The main reason for this was use of wrong material for the structure. It was due to overweight of load for the structure. Then it was collapsed.

·         Titanic Disaster (1912)

Most people thinking that this happened due to collide with iceberg. This statement is true, but when consider in different angle this happened due to hardness of metal. Titanic was built ‘virtually unsinkable’ but it was sinking by colliding to iceberg. The reason was metals used for constructing titanic fractured in cold temperature. Titanic ship was built using one-inch-thick mild steel plates and two million steels and wrought iron rivets with latest technology at that time. But engineers didn’t think about the temperature effect to the metals. 

Ø  Wrong reading in a hardness test can cause thousands of deaths and resource damages. Hardness test is one of the most important test in material selection.

Disadvantages of Rockwell hardness testing method

            There are few advantages of Rockwell hardness test that affects the result.

·         It is not the most accurate hardness testing method, as given a slight error in measuring the depth difference can result in a significant error in the calculated hardness value.

·         The test location must be completely free of all contamination (scale/foreign bodies or oil) to achieve a meaningful test result.

·         The intender has unknown effects on the test results. As an example, if the intender is worn and point of the tip is no longer sufficiently acute.

·         With increasing hardness, it becomes increasingly difficult to distinguish between materials.

·         Repeatability

·         There are unstoppable errors because in Rockwell hardness test, all steps are done by students and those errors can affect to the readings slightly, but it can cause big problems.

How to improve the Accuracy

·         Most of errors can be avoid by trained operator. For example, when taking the readings by a trained operator the way person doing the work is more convenient than untrained employee.

·         Another method is check the results from another instrument. But this method is less practical in working situations. As an example, we can use a lens in the scale so we can take the reading more accurately.

·         We can use a digital display instead of using a numeric scale. So it can make the readings 100% unique. 

·         Maintain the device appropriately is the main thing we have to do for good test.

·         If the metal surface is curve/large or difficult to set under the diamond tip, we have to use those equipment’s to make it still.

Conclusion

Overall, we had a good idea about ‘What is hardness of a material’ and how to measure hardness of a material using Rockwell hardness method. Some of our readings were not good, but approximately our average value was fine. We had a good knowledge about Hardness, thanks to our instructors.

References

Berg Engineering Blog: NDT News From Around The Industry. (2018). The Importance of a Hardness Test in Materials Production - Berg Engineering Blog: NDT News From Around The Industry. [online] Available at: https://www.bergeng.com/blog/the-importance-of-a-hardness-test-in-materials-production/ [Accessed 27 Jul. 2018].

G&R Technology Inc. (2018). The Importance of Hardness Testing in Production - G&R Technology Inc.. [online] Available at: http://grhardnesstester.com/blog/importance-hardness-testing-production/ [Accessed 27 Jul. 2018].

Anon, (2018). [online] Available at: https://www.qualitymag.com/articles/91828-hardness-testing-assures-quality-in-advanced-materials [Accessed 27 Jul. 2018].

Emcotest.com. (2018). Theory of hardness testing. [online] Available at: https://www.emcotest.com/en/the-world-of-hardness-testing/hardness-know-how/theory-of-hardness-testing/rockwell-53/rockwell-test-procedure-182/ [Accessed 27 Jul. 2018].

Anon, (2018). [online] Available at: http://iopscience.iop.org/article/10.1088/0957-0233/20/6/065701/meta [Accessed 27 Jul. 2018].