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BEMIDJI STATE UNIVERSITY

Department of Technology, Art & Design

TADT 3217: Materials Science & Metallurgy

 

 

Hardness Testing

[A continued look at the Heat Treatment of Steel]

 

 

 

 

Background:

 

 

In Lab 2 we polished and imaged four 1045 steel samples:

1. Air Cooled
2. Slow Cooled
3. Quenched
4. Tempered

 

 

For more detailed information on these samples, please see Lab 2.

 

 

In this lab we used a Brinell hardness tester to obtain HB hardness values for the 4 steel samples. The air and slow cooled samples were indented with 1500 kgf and the quenched and tempered samples were indented with 3000 kgf. The theoretical maximum HV (Vickers Hardness) value for a given steel specimen is approximately given by Equation 1.

 

Theoretical Maximum HV = 200 + 1500 (wt% C)                                                                                                           (1)

 

You can convert your HB hardness values into an HV hardness value using the Hardness Comparison Table on Wikipedia.org.

 

 

Figure 1: HV and HRC hardness values vs. carbon wt% in low-mid carbon steels.Lines show hardnesstrends for variousmicrostructures that result from heattreatment.

 

You can then use Figure 1 to determine if your HB hardness values are in the right ballpark.

 

 

Procedure

 

1. The 4-sides of each sample are ground starting with 120 grit SiC, then with 240 grit SiC.
2. The Brinell Hardness Tester (Phase II model 900-352) is used for this hardness testing in this Lab.
3. Each sample is indented 4 times (1 indent per side). The diameter of each indent is measured with a hand held microscope with dial indicator. The left-right diameter AND the up-down diameter are measured then averaged.
4. Use the HB equation in the textbook (equation 6-17) to determine the Brinell hardness of each of the 4 indents on each of the 4 samples. Your hardness results should be within 5% for each sample.

 

Please help/work on the Data Analysis section:
 

Data Analysis

 

1. Discuss the relationship betweenthe observed microstructure and the measuredhardness for each of the 4 steel samples. You should consider both grain size and phases present. Keep the following in mind:
   1. Hardness will increase with decreasing grain size. This is related to the Hall-Petch equation. Therefore, coarser grained materials should be softer.
   2. Martensite is body-centered tetragonal. Think about the available slip systems and correlate this to hardness.
   3. Steel is a solid solution of Fe and C. When steel is quenched from the austenite phase, virtually all the C remains in solid solution. Think about how this correlates to hardness.
   4. The phases of steel that we have looked at are:
      1. Martensite
      2. Cementite
      3. Pearlite
      4. Ferrite

These are listed in order of decreasing hardness.

1. Make a plot of the Mean Hardness for each of the samples versus the heat treatment (Slow Cooled, Air Cooled, Quenched, and Tempered). The standard deviation should be given as vertical error bars on the plot.
2. Calculate the Ultimate TensileStrength (UTS) for each of the samples.Use equation 6-18 in the textbook to calculate the UTS (note that this equation should only be used for steels).
3. Based on your results and the fact that the HB hardness value for the as-purchased 1045 steel is 225 ± 10 HB, what heat treatment do you believe the as-purchased steel has undergone?
4. Compare your Hardness and UTS values for the various heat treated 1045 steel samples with literature values. Discuss.
5. Discuss potential sources of error from the Brinell Hardness Tester.
6. Discuss factors that may have contributed to any observed scatter in the hardness data.

 

Below are equations: 

 

Image transcription text

Hardness numbers are used primarily as a qualitative basis for comparison of materials, speci?cations
for manufacturing and heat treatment, quality control, and cor» relation with other properties of
materials. For example, Brinell hardness is related to the tensile strength of steel by the ap..

 

 

 

 

Image transcription text

2F HB (6-17) TTD D - VD2 - D.