Micro Structure of Materials

Title

Microstructure Examination of Steel

Objective

To observe the constituents and structure of metals and their alloys by means of an optical microscope

Introduction

            Metallography is the study of metals by optical and electron microscopes. Structures which are coarse enough to be discernible by the naked eye or under low magnifications are termed macrostructures. Those which require high magnification to be visible are termed microstructures. Microscopes are required for the examination of the microstructure of the metals.

The most commonly used microscope is the conventional light microscope. Many materials, however, do not transmit light and so we are restricted to looking at the surface of the specimens with an optical microscope.

Microscopy can give information concerning a material’s composition, previous treatment and properties. Particular features of interest are:

1. Grain size

2. Phases present

3. Chemical homogeneity

4. Distribution of phases

5. Elongated structures formed by plastic deformation

   

Grain size determination:   Grain size may be determined using an intercept method described below;

 Straight lines, all of the same length, are drawn through several photomicrographs that show the grain structure. The grains intersected by each line segment are counted; the line length is then divided by an average of the number of grains intersected, taken over all the line segments.

Materials/Apparatus

·         Metal sample

·         Grinding/polishing machine

·         Grinding paper

·         Alumina paper

·         2% natal acid

·         Alcohol

·         Tongs

·         Beaker

Procedure

There are 7 main steps before the microstructure observation.

First, we have to saw the section that we have to examine from the large part. If the sample is too small, we had to mount in resins. After that we did the coarse grinding and grinded progressively the surface   were going to examine by using the grinding machine that on a finer emery paper. That was a compulsory step we need to along this session. And then we polished the same surface which is grinded before using the same machine, alumina powder and diamond paste to make the surface soft. After that we etched the sample with diluted acid (2% nital for steel) and we washed it in alcohol and dried.

Finally, we examined it using a light microscope.

Calculations

Average grain size = (number of lines) (length of each line)

                                                  (total number of intersections)

Line 1 = 7.5                            

Line 2 = 5.75

Line 3 = 2.75

Line 4 = 6.25

Line 5 = 6 .00

Total   =28.25   

Number of lines = 5

Length of each line = 7cm

Total number of intersections = 28.25

Scale = 400:99

Average grain size =    5 x 7x 400 µm

                                                28.25 x 99

                                        =    50.058µm

Observations

Discussion

Errors we can occur

            We can find several errors that can be happen at the main steps. At the grinding step we should do it for a long time. If we hold it perpendicular to the rotation it can cause to have strips on the surface. When we are polishing also we have to do it for a long time because we have to observe the grains clearly. Before we observe the steel piece it needs to be dried, if not water drops can make the readings wrong. After that when we are determining the grain size, measurement errors and errors that can occur when taking the measurements.

Improved / measures to avoid errors

            To avoid the errors that can happen at the grinding step, we have to hold the piece with a little angle to the rotational direction. Before the observation steel piece must have been dried under any type of heat. To avoid the measurement errors, we have to take the measurements carefully and clearly.  To avoid the instrumental errors we have to use proper instruments without production errors.

Observed Phases of Microstructure

            We use phase diagrams to observe the microstructures and there are two types of phase diagrams.

1.      Single phase diagrams

Ex-: Metal

2.      System with two or more phase diagrams

Ex-: Ceramics

        Polymers

Usually, when we consider metal alloys, microstructure of them is characterized by number of phases present and the arrangement of them.

The microstructure of alloys depends on:

• Alloying element present

• Their concentration

• Heat treatment of alloy

The symbols used to mention each phase is given below.

Phase	Symbol
Austenite	γ
Allotriomorphic ferrite	α
Idiomorphic ferrite	αI
Pearlite	P
Widmanstätten ferrite	αw
Upper bainite	αb
Lower bainite	αlb
Acicular ferrite	αa
Martensite	α'
Cementite	θ

Austenite - This has a carbon close packed structure (Face -centered Cubic Structure -FCC)

Ferrite      - This has a body – centered crystal structure(BCC)

Cementite - Has an orthographic unit cell

       This unit cell has the formula -Fe3C

Hypereutectoid - Steel which contains less carbon concentration that is marked as s in phasor diagram.

Allotriomorphic - The shape of the ferrite doesn’t change its inside crystalline symmetry.

Idiomorphic - Has a crystallographically changed shape.

Figure 19: Allotriomorphic and Idiomorphic (Phase-trans.msm.cam.ac.uk, 2017)

Pearlite - A mixture of ferrite and cementite

   This is created by a parent austenite atom and a cooperative growth of the above   mentioned two phases.

Martensite - Generated when austenite is cooled to room temperature below Ms.

Barnite - Forms without any diffusion; but after words, diffusion is made with austenite

Ademastatine ferrite - Displosive mechanism is happened to create this. (αw/ϒ)

Figure 17: Austinite, ferrite and cementite and Phase diagram(Phase-trans.msm.cam.ac.uk, 2017)

Figure 18: Reconstructive and displace mechanisms (Phase-trans.msm.cam.ac.uk, 2017)

Importance of metallography

Metallography is very important in every main field.

1.      Engineering and industrial field

§  Welding - Done by applying an extra layer on top of the grain structures to stick some materials     together.

§  Annealing

2.      Medical Field

§  EPS (Energy Dispersive Spectroscope)

§  Titanium hip implant

3.      Archaeological Field

§  Adjustable Iris Diaphragms

§  Dust filled illuminator

4.      Material sciences

§  Extend of carburization

Conclusion

            Microstructure examination is conducted to be done carefully and neatly. We had good idea about how to work in a laboratory while doing the examination. Overall,  we had a good idea about using a light microscope, working with chemicals and also about using grinding/polishing machine. Finally, we observed the microstructure and took very important information about grain size and about phases of microstructure.

             

Refferences

Phase-trans.msm.cam.ac.uk. (2018). Metallography of Steels. [online] Available at: https://www.phase-trans.msm.cam.ac.uk/2008/Steel_Microstructure/SM.html [Accessed 21 Jul. 2018].

Diez, D. (2018). Metallography – an Introduction. [online] Leica-microsystems.com. Available at: https://www.leica-microsystems.com/science-lab/metallography-an-introduction/ [Accessed 23 Jul. 2018].

Www3.telus.net. (2017). Metallography. [online] Available at: http://www3.telus.net/BrentBeach/Sharpen/metallography.html [Accessed 23 Jul. 2018].