Final Project: Effects of Super Cooling on Free Cut Brass, Hardness, Impact Energy, and Ductility.

ENGR 2322 – Material ScienceAdvisor: Dr. Magesh Thiyagarajan

Date Submitted : May 13, 2013

Shawn Robinson

AbstractThe objective of this lab is to observe the effect of super cool heat treatment on free cut Brass (Alloy 360) material properties. The material properties we will be paying attention to will be hardness, impact energy, and ductility. The brass will be heat treated and quenched in dry ice (-78.5C), and liquid nitrogen (-275C). Then a hardness test and impact test will be performed on the post treated samples.Materials

- Oven- Dog bone Brass sample (152mm*12.7mm*3.4mm)

- Face shield- Thermal shielding gloves- Safety goggles- Rockwell Hardness tester- Charpy Impact tester - End mill,- Ban saw- Sanding wheel- Liquid Nitrogen- Dry Ice- 2 Styrofoam Coolers

Introduction

Heat treating is a methodology to increase hardness, toughness, relieves stress or ductility of a material. In general, heat treatment is a process used to alter physical or sometimes chemical properties of a material. To combine, hardenability of any metal alloy is its capability to be hardened by heat treatment. Hardness is the resistance of the material to indentation or scratch. It is a qualitative indication of the materials strength. The higher the hardness of a material, the more it is difficult to make an indentation or a scratch, and vice versa.Annealing temperatures for this brass are 795-1100°F. In this lab the effects of extreme cooling rates will be investigated by quenching in liquid nitrogen and dry ice. Super cooling sends the brass into extreme temperature shock, thus forcing out impurities and making it more ductile. The first test that will be done is the Rockwell hardness test, to

Fig.1. Brass sample

Fig.2. Rockwell hardness tester

find the hardness of brass before heat treating. Hardness is a materials ability to resist surface wear by indentation or scratching. These hardness tests are used throughout the field of material science to help determine the surface properties of a material to facilitate the requests of the engineers desires for certain surface properties for a part or job. These tests are also used to help determine the effectiveness of a manufacturing process such as quenching and other heat treating methods. Rockwell hardness test has different scales for different materials based on their degree of resistance to wear. These scales differ mainly by the amount of load applied to the material, the indenter used to make the indention in the material and the formula used to calculate the resulting hardness reading. For this experiment we used the HRB scale with the Diamond Cone indenter and a major load of 100 kg to identify the effects of heat treating on hardness. The last test that will be conducted is the Charpy Test. The Charpy test is “a standardized high strain-rate test which determines the Yield Strength, amount of energy absorbed by a material during fracture”. The energy absorbed is measured by the toughness or the ability of the material to absorb this energy and resist lateral fracture. The Charpy apparatus that was used in this experiment is the Universal Pendulum Impact Test machine. The pendulum acts as a hammer that is used in order to break the material. This apparatus will be used to test the toughness and ductility of the dry Ice quenched brass, and compare it to the liquid nitrogen quenched brass. This will give a good indication of how the ductility of brass is comparatively affected by the different cooling rates.

Material CharacteristicsBrass is a binary alloy composed mainly copper and zinc, with copper being the majority of the mixture, and the zinc content being anywhere form a few percent to 40%. Most brass alloys have a tiny bit of lead in their composition to help improve machinability. Free-Cutting Brass is a common alloy. It is one of the most widely used of the copper metals, with hundreds of millions of pounds of it are consumed annually in the USA. It is commonly used as a substitute for leaded steel in screw machine parts because of its higher strength and cheaper cost. Brass has a density of 8.49 g/cc, a melting point of 1652-1724 °F (900-940 °C), and a crystal structure of face centered cubic. It is commonly used in, condenser plate, marine hardware, valve stems, balls, nuts, bolts, and rivets fittings.

Free-Cutting Brass, UNS C36000

COMPONENT WT. %C 60 - 63Fe Max 0.35

Other Max 0.5

Pb 2.5 - 3.7Zn 35.5

PHYSICAL PROPERTIES

METRIC ENGLISH COMMENTS

Density 8.49 g/cc 0.307 lb/in³ at 20°C (68°F)

MECHANICAL PROPERTIES

METRIC ENGLISH COMMENTS

Tensile Strength, Ultimate

338 - 469 MPa

49000 - 68000 psi

Tensile Strength, Yield

124 - 310 MPa

18000 - 45000 psi

Depending on temper

Elongation at Break 53% 53% in 457.2 mmModulus of Elasticity

97 GPa 14100 ksi

Bulk Modulus 140  GPa 20300 ksi Typical for SteelPoisson's Ratio 0.31 0.31 CalculatedMachinability 100% 100%  UNS C36000 (free-cutting

brass) = 100%Shear Modulus 37 GPa 5370 ksi

THERMAL PROPERTIES

METRIC ENGLISH COMMENTS

CTE, linear 250°C 20.5 µm/m-°C

11.4  µin/in-°F

from 20-300°C (68-570°F)

Thermal Conductivity

115  W/m-K 798  BTU-in/hr-ft²-°F

at 20°C (68°F)

Melting Point 885 - 900 °C 1630 - 1650 °F

Solidus 885 °C 1630 °FLiquidus 900 °C 1650 °F

Experimental Procedure

Step 1. Test the hardness using HRB scale before heat treatment.1. Mount the Anvil on the Rockwell Hardness tester 2. Turn the power ON switch located on the lower rear side.

Table.1 Material Properties of brass [3]

3. Clear the memory of previous data set by clicking the delete button. 4. Identify the sample to be tested and choose the corresponding indenter. Mount the diamond cone intender on the Rockwell Hardness tester. 5. Set the scale to HRC. 6. If the “major load” is flashing on the screen, it indicates incorrect load position. Set the correct major load by turning the knob on the right side. 7. Once the indenter, scale and load is set, you can proceed to the test by raising the specimen on the anvil by turning the wheel. 8. When the sample makes contact with the indenter the initial preload will be applied as your raise further and you can see the increase initial load and you must stop when it reaches a SET value followed by a clicking sound. 9. Then the machine will automatically apply the major load and calculate the harness number in the corresponding scale 10. Record the results and lower the anvil and move the specimen to test on a different spot. 11. Then repeat the procedures to determine 10 data points.

Step 2. Brass dog bone sample needs to be machined into two charpy impact samples with the following dimensions: 3"x.5"x.03" with 60 degree notch angle.

1. Use the ban saw to cut the sample in two, at half of the length.

2. Grind off the fares of the samples to give a uniform width & length. Note that a cup of water should be used to periodically dip the sample in because it gets hot.

Fig.4. Cutting sample on ban saw

Fig.5. Cut sample

3. Use the end mill to manually cut the 60 degree notch angle in the middle of the length side of the samples, making each pass at 3-5 ten thousandths of an inch.

Fig.6. Grinding the samples

Fig.7. Sample after grinding

Step 3. Put the two sample in the oven at 1000F for 30minsStep 4. While the samples are in the over, prepare the two Styrofoam coolers, one with 10lbs of dry ice, and the other with 1L of liquid nitrogen.

Step 5. Retrieve each sample from the oven, safely quenching them in their respective containers. Note: the sample that is lowered into the liquid nitrogen should be done slowly to ensure the sample does not burn through the container. Let samples sit in their respective containers for 15mins

Fig.8. Cutting 60 degree notch angle

Fig.9. Preparing the “baths”

Step 6. Remove samples form containers and let them sit for 15mins to reach room temp.

Fig.10 quenching the sample

Step 7. Repeat step 1 for the heat treated samples.Step 8. Perform the Charpy impact test for the heat treated Samples.

1. Raise the pendulum arm to the right until it is firmly supported by the latching mechanism. Caution: Make sure the safety latch is in the clear when raising the pendulum arm into this test position. 2. Place the Charpy specimen horizontally across supports with the notch away from the hammer. Make sure the specimen is centered within the anvil jaws (see Figure 1). 3. Slide the indicator pointer to the left until it indicates the maximum energy range on the upper Charpy Tension scale. 4. Ensure all participants are clear and behind the caution stripe. The test conductor shall then release the pendulum by pushing up on the release knob. The hammer will drop and attain a striking velocity of 16.8 ft/s, striking the specimen, with a swing through dependent on the amount of energy absorbed by the test specimen. The indicator will move and stop when peak swing through is registered, providing a direct reading of the energy absorbed by the specimen. Read the indicated value from the Charpy scale and record. 5. Apply manual brake until the pendulum has returned to its stable hanging vertical position. 6. Record the scale and readings.

Fig.11 samples left in containers

Fig.12. Samples post quenching Dry Ice (top) & liquid nitrogen (bottom)

7. Remove the specimen from the testing area and observe the failure surface. 8. Take pictures of your broken samples for analysis. Microscopes can be used to study the broken surfaces. Leave pendulum in the down hanging vertical position until another test is to be performed.

Experimental Data

Unheattreated Heat Treated Dry Ice

Heat Treated Liquid Nitrogen

Material Type: Brass

Indenter: 1/16 dial

steel sphere

Scale: HRB Scale

Major Load: 100 kg

Test Number

HRB HRB HRB

1 73.0 *-10.3 -14.42 72.7 *0.9 -17.43 73.4 4.6 -13.64 72.7 4.5 -12.75 72.7 6.5 -12.46 72.6 7 *-6.37 72.8 7 *-2.88 73.0 6.2 *2.99 72.9 6 *2.7

10 72.9 3.8 *3.4

Mean 72.9 5.7 -14.1Standard Deviation

0.219317122 1.152171862 1.793320942

Min 72.6 3.8 -17.4Max 73.4 7 -12.4

Table.2. Hardness test data

Test Number

1 2 3 4 5 6 7 8 9 10

-40

-20

0

20

40

60

80

Rockwell Hardness for different heat treatment processes

UnheattreatedHeat Treated Dry IceHeat Treated Liquid Nitrogen

Test number

hardness (HRB)

Sample Test Result – Images

Graph.1. Hardness graph for all samples

Fig.13. Post quenching Dry Ice sample

Leeching on left end

Data AnalysisThe acquired data shows that the hardness for the stock sample (pre-heat treated), had an average HRB value of 72.9 [table.2]. It is important to note that the HRB scale is dimensionless and is just used to express the relative hardness of a material. Most sources show that this is an accurate hardness with most being around 75-78 [5]. When the dry ice sample was quenched, the sample made a loud vibrating sound for a few seconds and stopped. This happened because of the rapid cooling of the material, caused the excess energy stored in the momentum of the particles, to be released in the form of sound energy [6]. The average hardness of the dry ice quenched sample was 5.7 on the HRB scale, with a standard deviation of 1.15 [table.2]. This data shows that when brass is quenched, It becomes softer. This is due to the grain structures becoming larger, and making the material softer [7]. It is important to note that the end of the sample that was quenched first experienced leeching on one end [fig.13]. Leeching is where the zinc is forced out of the alloy [4] leaving behind the spongy soft copper. This would explain the softer data points to one end and the higher standard deviation. The liquid nitrogen quenched sample had an average hardness of -14.2 on the HRB scale and had a standard deviation of 1.79. The data shows that the liquid nitrogen quenching process causes the brass to become even softer than the dry ice quenched sample. This is due to the grain structures as discussed earlier, as well as forces from temperature shock. Since the cooling rate for liquid nitrogen in much higher than dry ice, the compressive force from temperature shock caused more impurities to be squeezed out and the grain structure larger [7] making the brass even softer. It was also noted that the end which the sample was gripped with the iron tongs and lowered into the liquid nitrogen, had higher hardness values than the rest of the material [fig.14]. Since liquid nitrogen is close to 0K, meaning no energy, the slightest

Fig.14. Post quenched Liquid Nitrogen sample; suface texture gradient on right end

Fig.15. Post impact test (dry Ice sample) Fig.16. Post impact test (Liquid Nitrogen sample)

momentum transfer between the molecules of the tongs and brass sample cause a raise in energy, almost creating a jominy end quench result. This gives reason to the higher standard deviation. During the experiment the Charpy impact test was performed on the post heat-treatment samples, yielding impact energies 138ftxlbs and 120ftxlbs, for the dry ice and liquid nitrogen samples respectively. I believe my charpy test yielded invalid results due to several reasons. The first reason is due to the fact that both samples did not break, and simply bent around the hammer [fig. 15 & 16]. This happened because the effects of super cooling brass left it extremely soft and ductile. Also another reason is that the sample should have been cut into smaller pieces, to be able to run multiple trials, create control data, and to ensure that the samples would break due to them being so soft. I also feel that the notch should have been cut deeper to compensate for the softness. However, a quick comparative analysis can be done on these two values. Since the impact energy for the liquid nitrogen was lower than that of the dry ice, it supports our data of the liquid nitrogen being softer and more ductile than that of the dry ice, because it gave less resistance to impact and molded more easily around the hammer. In the end, the higher cooling rate did have a higher impact on the hard and ductility of brass, which is what we were looking for [graph.1]. Smaller test samples should be used next time to achieve more controlled and consistent results.

ConclusionThe effects of super cooling had a drastic effect on the material properties of brass. It became much softer and ductile as the cooling rate was increased [graph 1]. This is caused by the massive compressive force due to temperature shock coupled with the conductive material properties of brass. It is important to note from this lab that when working with extremely low temperatures, that any minute variations on the macro along the sample have drastic variations on microstructure of the material and what happens. To improve this lab, smaller sample sizes should be used to decrease variations in each sample and the effects of error on the macro level, as well as get more normalized data and reliable data from the charpy impact test.

References

1 James F. Shackelford, “Introduction to Material Science For Engineers”, 7th Ed, Prentice hall. pp. 153-155

2 http://www.copper.org/applications/rodbar/alloy360/alloy360.html

3 http://www.ezlok.com/TechnicalInfo/MPBrass.html

4 Heat Treater's Guide: Practices and Procedures for Nonferrous Alloys

5 http://www.speedymetals.com/information/Material13.html

6 CRACKING PERCEPTION BY EMITTED SOUND DURING QUENCHING Franc RAVNIK 1, Janez GRUM* 2 University of Ljubljana, Faculty of Mechanical Engineering,

7 Science of Conflict Metallurgy-Iron Quenching