METALLOGRAPHY

ME2151-2 Metallography Lab Report By Kabeer Ahmed Bin Mohamed Ismail A0096590U Lab Group 2F2 Date of experiment: 21st August 2013

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Objectives The objectives of this experiment include: 1. To obtain experience in the metallographic preparation of metallic specimens and 2. To observe the various microstructures in a welded mild steel joint

Experiment Results Fig 1 shows the general sketch of the 5 microstructures present in a welded steel. Fig 2.1 to Fig 2.5 show the detailed sketches of each of the 5 microstructural arrangements in various distinguishable zones under 200X magnification. All sketches are attached at the end of this report (Pages 5 and 6). Note that in all sketches, the shaded regions represent pearlite while the non-shaded regions represent ferrite.

Discussion A fusion weld is normally produced by the electric arc welding process. In this experiment, we will be looking at 5 zones that can be distinguished in the welded area and the parent metal adjacent to it (See Fig. 1, page 5). These zones are created since the various regions of the parent metal and the weld itself are subjected to different degrees of heat treatment during the welding process. The 5 zones in discussion today are: 1) Fusion Zone (Fig 2.1) 2) Extreme Grain Growth Zone (Fig 2.2) 3) Grain Refinement Zone (Fig 2.3) 2|Page

4) Transition Zone (Fig 2.4) 5) Unaffected Zone (Fig 2.5) Fusion Zone In the Fusion Zone, the parent metal was heated to the melting point and subsequently cooled. It is characterized by the presence of two microstructural features – Columnar grains and Widmanstatten structures as can be seen in Fig 2.1. As steel is a good conductor of heat, the weld is subjected to very rapid cooling from its molten state and the result is a chilled casting having the associated columnar grains. The Widmanstatten structure, however, appears as a result of large austenite grains being put through a moderately fast cooling rate. Extreme Growth Grain Zone The Extreme Grain Growth Zone exhibits the effect of high temperature treatment at temperatures slightly less than its melting point. The high temperature exposure leads to a significant growth of austenite grains. On cooling to room temperature, this effect is retained, giving rise to a region of coarse ferrite grains and Widmanstatten ferrite and pearlite, as can be seen in Fig 2.2. The demarcation between Extreme Grain Growth Zone and Fusion Zone is generally more distinct than between the other zones. Grain Refinement Zone In the Grain Refinement Zone, the parent metal is heated into the temperature range corresponding to the austenite phase in the

phase diagram.

Because of the relatively lower

temperatures, the austenite grains began to nucleate at many points to form smaller austenite grains which, on cooling, will result in the formation of fine ferrite and pearlite grains, as shown in Fig 2.3.

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Transition Zone In the Transition Zone, the parent metal during welding is heated to the region where ferrite and austenite coexist. In other words, mostly only pearlite grains have transformed to small austenite grains. On cooling, very fine pearlite grains with ragged looking boundaries are formed among the mostly untransformed original ferrite grains, as can be seen in Fig 2.4.

Unaffected Zone The Unaffected Zone represents the region of the parent metal that was not heated beyond the eutectoid temperature (727 deg. C) and there is, therefore, no observable structural change, as shown in Fig 2.5. Conclusion From this experiment I learnt how to prepare a specimen for metallographic inspection by a series of processes such as grinding, polishing and etching as well as how to operate the grinding and polishing machines with correct settings. I also learnt how to observe, differentiate and sketch the various microstructural zones present in a welded steel joint under magnification, and correlate the different zones to the Fe - Fe3C Phase Diagram. In conclusion, this experiment gave me hands-on experience in the metallographic preparation of metallic specimens, as well as a better understanding on how the different degrees of heat treatment results in the formation of zones of various microstructures.

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