35.5: Example Test Data
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A plain carbon steel and an alloy steel were assessed using the Jominy end quench test.
The hardness of the samples was measured as a function of the distance from the quenched end to demonstrate the different hardenability of the two steels. The data is shown as Vickers and Rockwell hardness.
The alloy compositions are given in the table below.
|Plain carbon Steel||0.3||0.7||0.1||0.14||0.26||0.03||0.003||0.02|
The Vickers hardness test uses a square pyramidal diamond indentor. The recorded hardness depends on the indentation load and the width of the square indentation made by the diamond. The indentation load is typically between 10 and 30 kg. The hardness number is usually denoted by HV20 for Hardness Vickers 20 kg, for example. The Vickers test is most commonly used in the UK. The Rockwell hardness of a metal can also be determined using a similar technique.
The variation of hardness was measured with distance from the quenched end. The results are plotted in the graph below. Click on the circled data points to see how the microstructure varies with distance from the quenched end.
The alloy steel clearly has the highest hardenability, forming martensite to a greater depth than the plain carbon steel. Look at both the microstructures at high magnification, and try to observe the relationship between the volume fraction of martensite and the hardness of the steel.
The Vickers hardness scale is not the only scale used to measure hardness in metals.
The Rockwell hardness test measures a number which depends on the difference in the depth of an indentation made by two loads, a minor load followed by a major load. There are different scales for the Rockwell hardness test. For example, the commonly used Rockwell C test uses a minor load of 10 kg, followed by a major load of 150 kg. The number is denoted as HRC for Hardness Rockwell C scale. The indentor is either a conical diamond pyramid, or a hardened steel ball. The Rockwell test is commonly used in the USA.
Other tests include the Brinell and Knoop hardness tests.
There are conversion charts between the hardness scales. These can be found in standards, such as the British Standards, and reference works such as the ASM Metals handbook. It's important to use the correct conversion chart for different materials, since the hardness test causes plastic strain, and therefore varies with the strain hardening properties of the material.
The graph below gives the Jominy end quench data in terms of the Rockwell hardness number. Clicking on the circled data points will take you to images of the microstructure at that location in the sample.