MPE Open Ended Laboratory Sheet for Materials Science Lab

1.0 TITLE Sample Preparation and Metallography of Carbon Steel

2.0 OBJECTIVE The objectives of this experiment are: (a) To learn the techniques of sample preparation. (b) To investigate the metallography of carbon steels that have undergone different kind of heat treatments.

3.0 THEORY Carbon is the most important element in steel alloy. Increasing carbon content would increase hardness and strength of the alloy. Typically, carbon steels are stiff and strong. But high carbon content also increases brittleness and reduces weldability because of its tendency to form martensite structure. This means that presence of carbon can be both a blessing and a curse when it comes to a commercial steel. Steels are classified into three groups:

1. Low carbon steels 2. Medium carbon steels 3. High carbon steels

Often called mild steels, low-carbon steels have less than 0.30% Low carbon and are the most commonly used grades. Good machining and weld nicely and are more ductile than higher-carbon steels. Medium-carbon steels have carbon content from 0.30 to 0.45%. Medium Increased carbon means increased hardness and tensile strength, decreased ductility, but more difficult in machining. High-carbon steels have 0.45 to 0.75% carbon, these steels can be challenging to weld. Preheating, postheating (to control cooling rate), High and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding. Iron-carbon phase diagram, also known as Fe-Fe3C phase diagram is the most important binary system since most primary structural materials in every technology are based on iron-carbon alloys. In materials development, a relationship between composition, microstructure and process, ie. heat treatment would affect the resultant properties of the materials. Figure 1 shows the factors of temperature and percentage of carbon that affect the outcome phase of the steel alloy and the phases are summarized in table 1.

Figure 1. Fe-Fe3C phase diagram

Table 1. Solid phase in Fe-Fe3C phase diagram As stated earlier, microstructure or phase alloy can be affected by heat treatment or cooling rate. When specimen of carbon steel is cooled at slower rate (slow cooling), the carbon steels undergo phase transformations as in the above-mentioned phase diagram. While in quenching process, a steel cool at a rapid rate thus resulting in smaller grain sizes, and caused the formation of martensite.

4.0 EQUIPMENTS

(a) Grinding and Polishing Machine:

Grinding and polishing are the final stage in the sample preparation process and consists of several steps. Each step uses finer abrasive than the previous one, the ultimate goal being to produce a deformation-free, scratch-free and highly reflective sample surface. The sample may need to be examined, either etched or unetched, to reveal the microstructure of the material. It frequently happens that contrasts in the material are only visible after a successful etching step.

Figure 2. Grinding/polishing step in preparing sample

(b) Optical Microscope:

The optical microscope, often referred to as the “light optical microscope,” is a type of microscope that uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the 17th century. Basic optical microscopes can be very simple, although there are many complex designs that aim to improve resolution and sample contrast. Historically, optical microscopes were easy to develop and are popular because they use visible light, so samples may be directly observed by the eye. The image from an optical microscope can be captured by normal light-sensitive cameras to generate a micrograph.

5.0 EXPERIMENT

Design the experiment in order to meet the given objectives. Carbon steel samples: (i) Specimen 1 - Course pearlite (ii) Specimen 2 - Fine pearlite. (iii) Specimen 3 - Martensite. (iv) Specimen 4 - Tempered martensite. v) Specimen 5 - Bainite.

The steps of sample preparation such as mounting, grinding, polishing and etching will be shown for a demonstration purpose only. Examine every specimen provided under the optical microscope and sketch the microstructures observed.

6.0 RESULTS AND DISCUSSION Show the results appropriately in the form of table, graph or others. Conduct the appropriate analysis and discuss the finding.

7.0 REPORT Submit the FORMAL REPORT within 7 days from this experiment. Report must be typed. Similarity test will be conducted using Turnitin where similarity index of 20% is considered passing mark. Formal report must contain the following standard content: 1. Title 2. Objective 3. Introduction and Theory 4. Apparatus 5. Procedures 6. Data and results 7. Analysis and discussion 8. Conclusion

9. References Refer to the provided front cover for the distribution of marks.

*Submit the FORMAL REPORT within 5 days from this experiment