Dr Preus Lab Assignment

8/3/2019 Dr Preus Lab Assignment

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Gary BywaterSTUDENT ID: 76041250

AEM & DTC ASSIGNMENT

04/12/2009

Identifying Steel Samples

Dr Preuss


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Outline of lab procedure

Firstly grinding of the samples was performed using 600, 800, and 1200 grinding papers in such

respective order. Grinding of the samples on each paper was performed for approximately a minuteat a time and checked under a microscope to ensure scratches on the samples were in a singledirection before moving onto the next grind paper. Upon using a different grind paper the grindsamples were rotated approximately 90 to make the new grinding direction distinct from theprevious when checked under a microscope. Between grinding, the samples required cleaning withdetergent and blow drying. A fter grinding the samples were polished on 6m, 1m, and 0.25m finepolishing plates and finally checked that no scratches remained before etching. After etchingmicrograph images were taken and a Vickers hardness test was performed using a 10kg load

Identifying the samples

The different types of samples are: normalised 0.1% carbon steel, drawn 0.1% carbon steel,normalised 0.4% carbon steel, water quenched 0.4% carbon steel and normalised 0.8% carbon steel.Figures 1-5 are the micrographs taken from the sample pieces given.

Table 1 Obtained Vickers hardness data (samples 1-5 refer to figures 1-5 respectively)

Sample d 1 m d2 m Averagedent m

Averagedent mm

Area mm 2 HVkgf/mm 2

1 2 3 1 2 31 373 380 379 368 378 370 374.6667 0.3747 0.0757 132.10322 267 267 270 267 272 276 269.8333 0.2698 0.0393 254.69023 309 285 285 305 296 296 296.0000 0.2960 0.0472 211.65084 260 261 258 258 261 248 257.6667 0.2577 0.0358 279.31035 255 187 187 230 187 186 205.3333 0.2053 0.0227 439.8297

Firstly, martensite is formed through the quenching of austenite which traps carbon and creates thebody centred tetragonal form of iron. Due to quenching the water quenched 0.4% carbon steelsample must be martensite. Martensite is a hard and brittle phase in comparison to pearlite andferrite, considering the Vickers hardness data the hardest sample was sample 5 (figure 5). Thereforesample 5 must be martensite, the water quenched 0.4% carbon steel.

Comparing figures 1-3 to figure 4: figures 1-3 have a similar microstructure with distinct phases of pearlite (dark) and ferrite (light) whereas figure 4 has a much different microstructure. The darketching of the pearlite in figures 1-3 is due to pearlites two phase lamellar structure containinglayers of alpha ferrite and cementite leading to intense etching at their grain boundaries. With morecarbon content contained in the steel more of the dark pearlite phase would be present due to morelayers of alpha ferrite and cementite.

With such consideration in mind, figures 1 and 2 must be the lower carbon content samples of 0.1%carbon due to the smaller amounts of pearlite when visually compared to figures 3 and 4. In orderto determine which of figures 1 and 2 are the normalised or drawn sample it can easily bedetermined by their Vickers hardness value as normalising steel increases the toughness of a


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material, making it the sample with the lower hardness of the two. Of sample 1 and 2, sample 2 hasthe higher Vickers hardness and therefore must be the drawn 0.1% carbon steel, and sample 1 mustbe the normalised 0.1% carbon steel.

Left to be matched up are the normalised 0.4% and 0.8% carbon steels. From the iron-cementite

phase diagram with 0.8% carbon the steel composition shifts from pearlite and ferrite to almost100% pearlite with minimal ferrite via the eutectoid reaction. This leads to the conclusion thatfigure 3 must be the normalised 0.4% carbon steel as at 0.4% carbon there is approximately evenamounts of pearlite and ferrite which is visible when compared to figure 4. Thus by process of elimination and that close to 100% pearlite has minimal white phase ferrite, figure 4 must be thenormalised 0.8% carbon steel sample, which is almost pure pearlite.

To reiterate the findings, the Vickers hardness calculations are helpful. Generally with greatercarbon content the hardness is increased, despite this the martensite sample having a modest 0.4%carbon is still very hard as martensite is a hard marterial and would be the hardest sample, which

was found to be true. The 2 nd hardest sample would be sample 4/figure 4, the pearlite whichcontains a high carbon content of 0.8%. 3 rd hardest would be figure 2/sample 2 as due to drawingthe microstructure is hardened, 4 th hardest would be figure 3/sample 3 as normalising reduceshardness and increases toughness, finally 5 th hardest would be figure 1/sample 1 as the lowercarbon content of 0.1% and normalising leads to a tougher sample, which in turn makes it thesample of lowest hardness.

Figure 1: Sample 1 Normalised 0.1% carbon steel


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Figure 2: Sample 2 Drawn 0.1% carbon steel

Figure 3: Sample 3 Normalised 0.4% carbon steel

Figure 4: Sample 4 normalised 0.8% carbon steel (practically 100% pearlite)


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Figure 5: Sample 5 water quenched 0.4% carbon steel (martensite)

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Dr Preus Lab Assignment