Optical Microscopy and Microstructures of Ferrous Alloys

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Optical Microscopyand

Microstructures ofFerrous AlloysLab 3

Thien Tran


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Abstract

This lab used various polishers and a metallurgical microscope to polish and photographthe microstructures of 1018, 1020, 1035, 1045, 1065, and 1090 carbon steels and cast iron

specimens !sing "#T$#% and &'cel to calculate the percentage of pearlite present in each

specimen and plot the resulting data, the e(uation y=0.8477

x+0.1676

)as produced,

demonstrating the percentage of pearlite * y + as a function of the percentage of carbon * x +

The percentage of pearlite therefore increased at roughl a 1-1 ratio to the percentage of carbon

.n addition, as the carbon content increased, the specimens/ strength increased and its ductilit

decreased

Introduction

"acroscopic properties of ferrous allos are directl affected b microstructural detailsThis is illustrated b the comparison of gra and ductile castiron #lthough the each contain

relativel the same amount of carbon, the graphite in the gra castiron appears as flaes, )hichtend to crac more easil The ductile castiron contains graphite in the form of nodular shapes,and therefore is more ductile than the brittle gra castiron 1

The microstructures of 1020, 1035, and 1065 carbon steels )ere e'amined tages of

polishing )ere used in order to bring the specimens into a smoothenough state such that thereflected light reveals microstructural information )hen vie)ed under a microscope o)ever,

since a smooth surface )ill not reveal an significant details )ithout undulations, an etchant )as

used to create these surface )rinles, thereb revealing the microstructures The structures that)e )ere most interested in )ere the amount of pearlite in comparison to the ferrite crstal

structure concentration

Method

1 The samples )ere mounted using the ari7ur o)der "ounting stem in order tofacilitate easier handling of the specimens

2 The return )aterpipe )as connected to the sin, and an abrasive sheet )as laid do)n

3 The specimen )as held flat )hile the )heel spun4 The polishing )heel )as then run )ith the )hite cloth, and polishing li(uid containing

abrasives of sie five micron )as poured on the cloth during the polishing of the

specimen

5 The cloth )as cleaned )ith )ater so that no residue )as visible

6 The specimen )as then polished on the )heel )ith the green cloth, and polishing li(uidcontaining abrasives of sie one micron )as poured onto the cloth in the process

: This )as then cleaned )ith )ater so that no visible residue remained8 The specimen )as then etched using an etchant containing 5; in


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>or the =bservation of "icrostructure

1 The stage )as positioned at a lo)er level to prevent damage to lenses )hen settingspecimen

2 The magnification )as set to 5' )ith the ob?ective magnification set at 10' for a total

magnification of 50'3 The mode of observation )as set to %> *bright field+ and the po)er level )as set to 3

4 The specimen )as set on the stage and the coarse ad?ustment nob )as used to slo)l

focus, and this )as stopped )hen an sort of image appeared5 The fine ad?ustment nob )as used to further focus on the microstructure

6 The illumination mode )as changed to dar field to observe the microstructure

: The magnification )as then increased progressivel to 10', 20', and 50'

8 pecial characteristics )ere noted, and the image of the magnifications )ith the bestmicrostructure depictions )ere saved

9 These steps )ere repeated for each allo sample

Results

The results from the e'periment )ere analed using "#T$#%, and organied into

graphs using &'cel The percentage of carbon )as determined from the name in the steel theallo >or e'ample, 1020 corresponded to 020; carbon The percentage of pearlite )as

determined b the contrasting shades in the images taen from the microscope These )ere

analed using image capabilities in "#T$#% The plot of pearlite vs carbon percentage in

each allo can be seen in figure 1 T)o trials )ere taen, resulting in 10 separate pictures to beanaled The ra) images can be seen in figures 1a through 12a in the #ppendi'

0 0.2 0.4 0.6 0.8 1

0

0.2

0.4

0.6

0.8

1

1.2

% Pearlite vs. % Carbon

Trial 1

Trial 2

Percentage of Carbon (%)

Percentage of Pearlite (%)

Figure 1. Percentage of Pearlite vs. Carbon in each sa!le. The increasing carbon sho"s an increasingtren# in !earlite

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The increase in carbon percentage had an associated general increase in pearlite

percentage, )ith the e'ception of the 1020 and 1065 steel The average of the t)o trials is a

better illustration of the e'perimental result, and can be seen in figure 2

0 0.$ 1

0

0.2

0.4

0.6

0.8

1

f%&' ( 0.8$& ) 0.1*

+, ( 0.*$

% Pearlite vs. % Carbon

-verage

Linear %-verage'

Linear %-verage'



Figure 2. -verage of the t"o trials to better re!resent the !earlite concentration as a function of carbonconcentration

@ith the ield strength of each material provided, as in Table 1 in the #ppendi', the ield

strength can be plotted as a function of the pearlite concentration The result of this function canbe seen in figure 3

0 0.20.40.60.8 1

0

100

200

300

400

$00

600

f%&' ( 11$.$* ln%&' ) $26.12

+, ( 0.8*

ield !trength vs. % Pearlite

Logarithic %'

Logarithic %'


ield !trength (MPa)

Figure 3. iel# strength as a function of !earlite concentration. The general tren# is an increase/ "ith#ecreasing slo!e

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# logarithmic function )as used to fit the data, and the natural log representation appears to be

accurate #s the percentage of pearlite increases, the ield strength also increases, but the trend

appears to flatten out as concentration continues to gro) The tensile strength )as also obtained,and the effect of this material propert can be observed in figure 4 2

0 0.2 0.4 0.6 0.8 1

0

200

400

600

800

f%&' ( 3$.06& ) 320.68

+, ( 0.84

Tensile !trength vs. Pearlite Concentration

Linear %'

Pearlite Percentage (%)

Tensile !trength (MPa)

Figure 4. Tensile trength vs. Percentage of Pearlite. Tren# follo"s the sae as iel# strength for ost!art

The trend is similar to that of ield strength, although the correlation seems to be more linear

than logarithmic

Discussion

The e(uation used to represent the relationship bet)een carbon and pearlite percentage isas follo)s-

y=0.8477 x+0.1676 *1+

This suggests that )ith ever 1; increase in carbon, there is a 084::; increase in pearlite This

correlation maes sense, since steels )ith less than appro'imatel 08; A *eutectoid point+ have

a ferrite and pearlite structureB as the carbon content increases, the amount of pearlite increasesThe slope of this e(uation is less than one, since the (ualit of the data points achieved from the

1020 and 1035 steels ma have been lo)

The effect of pearlite concentration on ield strength and tensile strength is representedb e(uations *2+ and *3+, respectivel-

y=115.57 ln(x)+526.12 *2+

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y=395.06 x+320.68 *3+

The natural log function has an increasing slope that graduall flattens out )ith increasingpearlite concentration This graph sho)s the effect of microstructure changes on the macroscopic

properties of the allo such as ield and tensile strength $o) carbon steels have a lo)er relative

strength, and this can be seen in figure 5

0 0.2 0.4 0.6 0.8 1

0

100

200

300

400

$00

600

*00

!trength vs. % Carbon

iel# trength

Tensile trength


!trength (MPa)

Figure $. ra!h sho"ing the relationshi! bet"een carbon concentration an# strength

#lthough the strength is lo)er for the lo)carbon steels, their ductilit remains high Their

strengths can be increased through heat treatment processes and (uenching, or through cold)or These processes )ould alter the microstructures in order to change these macroscopicproperties

The Cs(uared values from the graphs are close to unit, )hich suggest that the bestfit

correlations are accurate representations of the e'perimental trends

Conclusion

% polishing samples of 1018, 1020, 1035, 1045, 1065, and 1090 .ronAarbon steelallos and analing their microstructures under a metallurgical microscope, the percentage of

pearlite in each specimen )as established as )ell as some general trends in the data The plot of

the percentage of pearlite as a function of the percentage of carbon produces the e(uationy=0.8477x+0.1676 suggesting that the percentage of pearlite increases b 084::; for

ever 1; increase in Aarbon, or roughl a 1-1 ratio, proving that the percentage of pearlite is

directl dependent on the percentage of carbon present #s the percentage of carbon increased in

the specimens, the specimens/ tensile strength increased and its ductilit decreased .n addition,the change in the allo/s microstructure could be visualied )ith ever photograph taen The

pictures of each specimen sho) the progression in the transformation of the steel from a ferrite D

pearlite structure )ith nodule pearlite, to a cementite D pearlite structure )ith grain pearlite

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References

1 7o)ling,


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>igure 3a 1020 steel >igure 4a 1020 steel, trial 2


>igure :a 1045 steel >igure 8a 1045 steel, trial 2

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Optical Microscopy and Microstructures of Ferrous Alloys

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