Lab Report

MT 724

Qualitative and Quantitative Metallography

Lab Report

For work done from 9th Aug to 30th Aug 2011

Objective:

To observe the microstructure of the given sample and determine the grain size and area fraction of the phases present in it.

Introduction:

Metallography: Metallography is the examination and analysis of microstructural features of the desired sample. The microstructural observation needs sample preparation as a pre-requisite. The sample preparation for microstructure is a crucial step. Selection of the sample should be such that the sample represents the parent material in all respects. The sample preparation includes preparing a surface with mirror finish without any scratches to reveal the microstructure by etching as a succeeding step. This may need an additional step called mounting.

Mounting of the sample is done to enable easier handling of the sample for manual or instrumental operations. Two kinds of mounting are done, viz. Hot Mounting and Cold Mounting. Hot mounting is done for better strength and rigidity of the sample which is useful in tests like hardness testing of the sample. Cold mounting too gives the necessary strength but mostly used for samples which are sensitive to heat.

The mounted sample is ground in order from rough to fine emery papers so as to get a good surface finish before polishing. Polishing is done on a disc polishing system on which a mirror finish of the sample surface can be obtained. The polishing once done satisfactorily, the sample can be etched to reveal the microstructure with a suitable etching agent. Etching is a preferential attack on the sample surface with varying energies of different locations.

Once sample is prepared, the micrograph obtained can be studied to get all the required information from the sample. To serve this purpose an optical microscope of good configuration can be used. Magnifications up to 1000X with good resolution are possible with optical microscopes.

Once the micrographs are obtained from the sample, quantitative measurements of the grain size and area fractions of phases present in it can be known.

Grain size measurement: Grain size of the given sample can be measured in Heyn’s lineal intercept method. In this method the number of grain boundaries intersecting the line is measured along with the length of corresponding line [1]. The grain size is calculated by taking length of the line per grain boundary.

Phase Analysis: Phase analysis of a two phase micrograph can be done by calculating the area of any one phase, either bright or dark, and dividing it with total area of the micrograph. The area fraction or the percentage of the phase can be indicated.

Experimental Details:

Sample Cutting: A sample cutter is used to slice the sample by maintaining appropriate rate of cutting. Sample is fixed to the sample holder by placing and fixing the screws in the sample holder. The sample is initially set to zero on the scale provided and 2.5 mm is measured 0.5 mm as compensation for cutting blade. Sufficient weight is put on the sample holder. Oil level is maintained in the machine to avoid any friction between the sample and the cutting blade.

After cutting, the sliced piece is cleaned and dimensional measurements are carried out. Up to 20 readings for each dimension are taken and standard deviation of all those readings is indicated. The values are tabulated in the following table.

Table 1: Dimensions of the cut sample

Sl. No. Thickness, t (mm) Diameter, d (mm) 1 1.55 7.93 2 1.59 7.94 3 1.58 7.95 4 1.66 7.95 5 1.69 7.96 6 1.76 7.96 7 1.81 7.95 8 1.92 7.94 9 1.99 7.93 10 1.99 7.92 11 1.94 7.93 12 1.97 7.92 13 1.75 7.93 14 1.68 7.93 15 1.60 7.93 16 1.58 7.93 17 1.58 7.94 18 1.58 7.94 19 1.65 7.94 20 1.77 7.96 Mean 1.73 7.94 Standard Deviation

0.16

0.01

Mounting: The sample after measurements is taken for hot mounting. Hot mounting involves the following steps

1. Firstly the sample is cleaned to remove any substrates on the surface. 2. Pressure if any, is released in the hot mounting system by rotating the side knob. 3. The top screw on the sample holder in hot mounting system is unscrewed and the sample

holder is brought upwards by operation of the side lever of hot mounting system.

4. Once the sample holder is elevated to the required height in the column, the sample is put in the middle of the holder required surface facing downwards. The depth of the holder in the column decides the thickness of the mount.

5. Sufficient Bakelite powder is added to cover the sample in the column and the holder is lowered again by operating the side knob.

6. The top screw is placed back to close the column and pressure is applied to the sample holder until it reaches an optimum pressure shown in the pressure gauge.

7. The hot mounting unit is configured from the menu for sufficient temperature, 170oC and sufficient heating and cooling rates. Once set, the cycle is started

8. Once the cycles are complete, the pressure in the column is released again by lowering the holder. The top screw is then opened again.

9. The holder is brought upwards again to remove the sample by slight impact force with a rubber hammer.

Grinding: The mounted sample thus obtained is ground using a series of emery papers of grit 240, 600, 800, 1000, and 1200 successively in order. The grinding of the sample is done by rubbing the sample on the emery paper with optimum force in one direction. The orientation of sample is changed in every successive emery paper grinding so as to clear the scratches produces in the previous paper. The grinding is done until fine parallel scratches are formed on the surface. If the scratches are deeper, rough grinding of lower grit is repeated.

Polishing: The surface after satisfactory grinding can be taken for polishing in disc polishing system after cleaning the sample with water. The sample is placed on a rotating disc with a velvet cloth, on which the sample is placed and optimum pressure is applied. The speed of the disc is maintained at around 350 rpm. The disc polishing system uses an abrasive suspended solution, alumina in water or diamond paste for polishing the sample. The abrasive solution is poured on the disc timely. The polishing is carried out in order of rough to fine of the particle sizes. The polishing of the sample is continued till a fine mirror finish of the sample surface is obtained.

When the desired surface is obtained the sample is examined in an optical microscope for fine scratches. When satisfactory surface is obtained, the sample is etched with a suitable etching agent, to reveal the microstructural features.

Etching: The etching agent preferred for brass samples is acidic ferric chloride. The required solution is prepared by adding 30 ml HCl, 10 ml Ferric Chloride to 100 ml of DM water. Once the solution is prepared, the sample etched with the solution by careful dipping of the sample in the solution for about 30 sec. The sample is then taken out and washed with water and later with acetone. Ultrasonication is done if necessary.

Microscopy: Etched sample is observed in an optical microscope. When the necessary details are visible in the microstructure, micrographs are taken and information is gathered from the micrographs. The micrograph of brass sample thus obtained is shown in Fig 1.

Figure 1: Optical microstructure of Brass sample with 500X magnification

Figure 2: Brass sample with grains and grain boundaries at 500X

The image of Brass sample taken at 500X magnification shows two phase microstructure with clearly distinguishable grains and grain boundaries in the bright phase. Some of the micro-structural features like twins can be observed in the microstructure. It can also be observed that the high contrast is a result of higher etching time. The micrograph shows about 75% of bright phase and 25% of dark phase. The grain size can be approximated to around 15 μm.

Grain Size Measurement: The grain size of the given sample is determined by Heyn’s intercept method. Up to 10 lines are drawn on the micrograph and number of grain boundaries intersecting the line are counted. The count being 432. The length of the lines combined 6245 μm is divided with this value to obtain the grain size as 14.46 μm.

Figure 3: Grain Size measurement of given sample, obtained as 14.46 μm

Phase Analysis: The phase analysis of the given image is done by calculating the area fractions of each phase. By doing the required area measurements, the area fraction of bright phase in the given image is found to be 69.80% or 0.69.

Figure 4: Phase analysis of the given sample, 69.8% of bright phase

Results and Discussions:

1. The given sample has been prepared satisfactorily and observed in the optical microscopes. Micrographs of the sample are taken.

2. Grain size measurement of the given micrograph is done and the grain size is found to be 14.46 μm.

3. Phase analysis of the given micrograph is done and the area fraction of brighter phase and darker phase are found to be 0.7 and 0.3 respectively.

Summary and Conclusions:

1. The role of microstructure in analysis of the properties of materials is understood. 2. The sample preparation techniques for microstructural observation have been understood. 3. Different features of a microstructure are observed. 4. Micrographs of a brass sample are taken and studied briefly. 5. Grain size measurements techniques have been used. 6. Phase analysis of a micrograph is done.

References:

1. George F Vander Voort, Metallography:Principles and Prractice, 4th Print, ASM International, June 2007, p-448,449.