Experiment: Metallography Specimen Preparation and ...

1 ME 3701, Materials of Engineering, LSU1 Experiment: Metallography Specimen Preparation and ExaminationObjectives1. To learn and to gain experience in the Preparation of metallographic To examine and analyze the microstructures of metals and metallic Preparation of metallographic specimens to determine microstructure and content requires that a rigidstep-by-step process be followed. In sequence, the steps include sectioning, mounting, course grinding, finegrinding, polishing, etching and microscopic examination .

2 Specimens must be kept clean and preparationprocedure carefully followed in order to reveal accurate microstructures. Each student will prepare andexamine a brass, steel or aluminium sample for metallographic examination . Additionally, a brass, steel oraluminiumsample with different composition, a eutectoid steel sample and a eutectoid Pb-Sn sample will beexamined under the metallographic microscope. Photographs will be taken of the samples and the ASTM grain size number determined for each of the specimens if consists of the study of the constitution and structure of metals and alloys.

3 Much can belearned through Specimen examination with the naked eye, but more refined techniques require magnificationand Preparation of the material's surface. Optical microscopy is sufficient for general purpose examination ;advanced examination and research laboratories often contain electron microscopes (SEM and TEM), x-rayand electron diffractometers and possibly other scanning techniques in preparing a sample may result in altering the true microstructure and will most likelylead to erroneous conclusions.

4 It necessarily follows that the microstructure should not be altered. Hot or coldworking can occur during the Specimen Preparation process if the metallurgist is not careful. Expertise at themethods employed to produce high-quality metallographic samples requires training and practice. The basictechniques can be learned through patient persistence in a matter of hours. This module takes the studentthrough the metallographic sample Preparation process step-by-step with demonstrations and explanations ofsectioning, mounting, course & fine grinding, polishing, etching and microscopic such as shearing produce severe cold work, which can alter the microstructure of a cutting (sectioning) offers the best solution to eliminate these undesirable features; the resultantsurface is smooth, and the sectioning task is quickly accomplished.

5 Low-speed cut-off wheels are utilized incases where the heat created by standard abrasive cutters must be avoided. Ample coolant and proper speedcontrol are essential in all sectioning Cut-Off Wheels consist of abrasive grains (such as Aluminum Oxide or Silicon Carbide), bondedtogether with rubber or other materials in the form of a thin wheel. Rubber bonded wheels are mostextensively used for wet operation and resin bonds for dry cutting. When sectioning a piece of metal, theabrasive grains on the cutoff wheel become dull, therefore, the bond must be destroyed at the correct rate sothat new grains are always at the edge and are still held firmly by the bonding material.

6 The rate at which thebond breaks down is affected by several different factors, including:1) The hardness of the ) The hardness and workability of the metal ) The size and speed of the ) The power of the driving ) The type and amount of coolant and its method of ) The amount of pressure by which the wheel is applied to the ) The amount of vibration in the wheels are indexed from hard to soft and are referred to by "grade". As a rule of thumb, a hard bondedwheel should be selected when sectioning soft stock, and a soft wheel for sectioning harder material.

7 Areasonable amount of wheel pressure must be applied so the abrasive grains will "bite" and remove the metalME 3701, Materials of Engineering, LSU2in fine chips. An experienced operator can tell whether the wheel is cutting or merely "dragging" by the feel ofthe control arm and monitoring of the power consumption of the cutting cutting procedures must be followed; abrasive cut-off wheels will crack andbecome flying projectiles when used Procedure - Sectioning1. Place the sample on the slotted Table so that it is at the center of the swing of the Place the Locking Lever in the forward position to allow the Control Rod to be moved to the furthest the Rear Vise section to contact the rear face of the sample.

8 Move the Front Section to a position nearthe front face of the Tighten the Nuts that secure the Vise sections to the Push the Control Rod in to make firm contact with the sample and pull back the Locking Level to secure Turn on the coolant; it should flow readily to both cool and clean the cutting Apply steady moderate pressure until the Specimen is sectioned; avoid jerking motions that may cause theblade to samples are generally mounted in plastic for convenience in handling and to protect the edges of thespecimen being prepared.

9 Compression-type molding is commonly applied to encase specimens in 1 to diameter plugs of a hard polymer. Compression molding materials are classified as either thermosettingor thermoplastic:1) Bakelite is a low cost, relatively hard thermosetting polymer that is commonly ) Expensive Transoptic Thermoplastics are utilized when transparency is materials remain molten at maximum temperature and become transparent with an increase inpressure and a decrease in temperature. By definition, thermosetting materials require heat and pressureduring the molding cycle.

10 During mounting, the pressure and molding temperature are generally held constantthus time is the only variable. The metal sample is placed in the mounting cylinder which is then encased in apre-measured amount of powdered polymers. The mounting cylinder is then sealed, pressurized and heatedto complete the polymerization process necessary to solidly encase the metal inferior in quality to compression-type molding, cold molding (room temperature) is often used withepoxy to mount samples by simply mixing the epoxy and pouring it over a sample that is positioned face-down in a cold-mounting ring.