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 . 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.

2 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. 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.

3 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. 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".

4 As a rule of thumb, a hard bondedwheel should be selected when sectioning soft stock, and a soft wheel for sectioning harder material. 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. 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.

5 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. 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.

6 When the epoxy cures the Specimen can be prepared. Caution must beexercised when cold mounting due to relatively poor adhesion between the Specimen edges and the epoxyplug; gaps often form which can degrade the quality of the Procedure Mounting Procedure(Hot-Mounting Equipment Currently Unavailable @ LSU!)Supplies for Cold-Mounting:- KoldMound Kit (Vernon-Benshoff Co.): Mix 2 parts powder with 1 part liquid- Plastic Mounting Ring with Removable Bottom- 8 Ounce Paper Cup with Stirrer- Weight Scale- Liquid Soap1. Apply liquid soap to the inner walls and base of the mounting Place the Specimen in the center/bottom of the plastic mounting ring base with the examination surface Attach the mounting ring cylinder to the base by simple pressing firmly in 3701, Materials of Engineering, LSU34. Mix the appropriate amount of KoldMount (2/1 Powder/Liquid - 15 grams of grams of Liquid perSample) in a paper Simply pour the epoxy mixture into the mounting ring; sample should be approximately " After one hour, remove the sample from the mounting ring and proceed with course GrindingIn view of the perfection required in an ideally prepared metallographic sample, it is essential that eachpreparation stage be carefully performed.

7 The Specimen must:1. Be free from scratches, stains and others imperfections which tend to mark the Retain non-metallic Reveal no evidence of chipping due to brittle intermetallic compounds and Be free from all traces of disturbed purpose of the coarse grinding stage is to generate the initial flat surface necessary for the subsequentgrinding and polishing steps. As a result of sectioning and grinding, the material may get cold worked to aconsiderable depth with a resultant transition zone of deformed material between the surface and theundistorted metal. Course grinding can be accomplished either wet or dry using 80 to 180 grit electricallypowered disks or belts, but care must be taken to avoid significant heating of the sample. The final objectiveis to obtain a flat surface free from all previous tool marks and cold working due to Specimen important factor throughout the Coarse Grinding and Fine Grinding Stages is that the scratches be uniformin size and parallel to each other in any one grinding stage.

8 Proper grinding involves rotation of the sample by90o between stages while the grinding angle must be held constant during the grinding at any one sample MUST be washed thoroughly before proceeding from one grinding stage to thenext ! ! !Failure to follow this basic rule will result in transferring abrasive particles between stages and will cause time-consuming, frustrating problems in removing unwanted Procedure-Coarse Grinding:*Note: Safety Glasses must be worn when operating the Course Grinding Equipment ! ! !1. Label the Specimen with your name so that it can be easily Turn the Motor On, then the water, adjust the flow to obtain a good film of water. Too much water willcause a spray when it contacts the Beginning with the 120 grit belt and using both hands to hold the Specimen , carefully place thesample face onto the exposed area of the belt being careful not to contact the rotating surface with asharp edge of the Specimen or your Applying moderate pressure evenly, move the sample left-and-right across the belt surface to obtainuniform grinding.

9 Use both hands to hold the Specimen ; unsecured specimens can "Catch an Edgeand FLY"!5. Lift the sample from the wheel periodically to determine the progress of grinding but do not rotate thesample. The 120 grit stage is complete when all the lines scratched in by the grinder are parallel onthe Specimen surface. If any line or scratch is not in the same direction as the other lines, continuegrinding until all of the lines are When all of the sample's scratches are parallel, carefully wash all of the debris from sample using tapwater and dry the Specimen immediately using a paper towel or pressurized air to avoid 3701, Materials of Engineering, LSU47. Proceed to the 180 grit stage with the scratches oriented approximately perpendicular to the intendedgrinding direction and repeat steps 3 thorugh When the 180 grit stage is complete, you're ready to move on to the medium/fine grinding and Fine GrindingMedium and Fine Grinding of metallurgical samples are closely allied with the Coarse Grinding whichprecedes them.

10 Each stage of metallographic sample Preparation must be carefully performed; the entireprocess is designed to produce a scratch free surface by employing a series of successively finer to be careful in any stage will result in an unsatisfactory idea is to carefully move from one stage to the next where the abrasives become finer at eachsuccessive stage. Movement from one stage to the next should only proceed when all of the scratches fromthe preceding stage are completely removed. In general, successive steps are 240, 320, 400 and 600 grit SiCand the grinding rate should steadily decrease from one stage to the next. Proper grinding involves therotation of the sample between stages while the grinding angle must be held constant during the grinding atany on stage. Recall that"The sample MUST be washed thoroughly before proceedingfrom one grinding operation to the next"!Wet grinding is generally applied in medium and fine grinding to avoid possible side affects due to heatingsuch as tempering, transformation, aging, incipient melting, etc.