Preparation of Specimens for Metallographic Examination Exp.#1.

Objective To prepare the specimens surfaces to be examined by the microscope. To learn and to gain experience in the preparation of metallographic specimens. Exp.4: Preparation of Specimens 11/13/2018

Introduction Metallography is basically the study of the structures and constitution of metals and alloys, using metallurgical microscopes and magnifications, so that the physical and mechanical properties of an alloy can be related to its observed microstructure. Such microscopic studies can provide an abundance of constitutional information about the specimen under investigation, including the size and shape of the grains (crystallites), the presence of micro defects (such as segregation, hair cracks, and nonmetallic inclusions), and the nature and distribution of secondary phases. The metallographic examination can be used in quality control and to predict and/or explain the mechanical properties. Exp.4: Preparation of Specimens 11/13/2018

Introduction cont. Proper preparation of metallographic specimens to determine microstructure and content requires that a rigid step-by-step process be followed. In sequence, the steps include sectioning (cutting), ultrasonic cleaning, mounting, course grinding, fine grinding, and polishing, etching and microscopic examination. Specimens must be kept clean and preparation procedure carefully followed in order to reveal accurate microstructures Exp.4: Preparation of Specimens 11/13/2018

Equipment Minitom: Small, automatic precision cut-off machine for sectioning all metallographic and ceramic specimens. Metason: Ultrasonic Cleaning: For effective ultrasonic cleaning of metallographic, ceramic, and mineralogical specimens LaboPress-3 :Semi-automatic mounting press for hot mounting of all materials. Exp.4: Preparation of Specimens 11/13/2018

Equipment cont. LaboPol (grinding and polishing machine): Machines for grinding, lapping and polishing for 200 or 230 mm discs. LaboForce : Specimen movers for semi-automatic preparation of materialographic specimens on LaboPol grinding and polishing machines. LectroPol-5: Automatic, microprocessor controlled electrolytic polishing and etching of metallographic specimens Exp.4: Preparation of Specimens 11/13/2018

Hot Mounting In hot-mounting the sample is surrounded by an organic polymeric powder which melts under the influence of heat (about 200 oC). Pressure is also applied by a piston, ensuring a high quality mould free of porosity and with intimate contact between the sample and the polymer. Cylinder dia. Resin heating Cooling 30 mm Type Quantity Time Temp. Force Rate [spoons] [ml] [min] [oC] [kN] PolyFast 1.5 23 4-6 180 20 2-3 HIGH MultiFast (block) 2 30 Exp.4: Preparation of Specimens 11/13/2018

Optimizing the amount of resin It is important to supply enough resin during mounting. If the amount of resin is too small to cover the specimen after curing, the specimen, the ram and the cylinder may all be damaged. Min no. of Spoons = The factor 1.2 is multiplied to eliminate variation in the resin densities and the measuring during mounting. Height of 1 Spoon (15 ml) of Resin after Mounting in an Empty Cylinder Cylinder dia: 30 mm Resin: mm inch PolyFast 11.3 0.44 MultiFast (black) 8.9 0.35 Exp.4: Preparation of Specimens 11/13/2018

Grinding and Polishing The specimen must: 1 Grinding and Polishing The specimen must: 1. Be free from scratches, stains and others imperfections which tend to mark the surface. 2. Retain non-metallic inclusions. 3. Reveal no evidence of chipping due to brittle intermetallic compounds and phases. 4. Be free from all traces of disturbed metal. Indication Force (N) 1 2.2 2 5 3 7.5 4 10 12.5 6 15 7 17.5 8 20 Exp.4: Preparation of Specimens 11/13/2018

Grinding Surface MD-Primo 220 MD-Largo Abrasive SiC DP-Suspension Grit / Grain size - 9µm Lubricant Water Green/Blue rpm 300 150 Force(N) 120 180 Time (min) Until Plane 5 Exp.4: Preparation of Specimens 11/13/2018

Polishing Surface Abrasive Grit / Grain size Lubricant rpm Force(N) MD-Dac MD-Nap or MD-Chem Abrasive DP-Suspension OP-S or OP-U Grit / Grain size 3µm - Lubricant Green/Blue rpm 150 Force(N) 190 60 Time (min) 5 Exp.4: Preparation of Specimens 11/13/2018

Electrolytic Polishing Exp.4: Preparation of Specimens 11/13/2018

Electrolytes: Sample material Etchant Composition Remarks Carbon steel (usually 2%) (nitric acid) HNO3 1-5 ml Ethyl alcohol 100ml Immersion. Time ranges from a few seconds to a minute (picric acid) Picric acid 4g Immersion. Time ranges from a few seconds to Over a minute. Aluminum Hydrofluoric acid HF (concen) 0.5ml H2O 99.5ml Swab for 15 sec. Copper-based alloys 50% nitric acid 50% distilled water Exp.4: Preparation of Specimens 11/13/2018

Questions Describe the steps involved in the preparation of a metallographic sample: Sectioning (cutting) Plastic coating of the samples Mounting Grinding Polishing Etching Why should the specimen be roughly washed after each stage during either grinding or polishing? Why is fine grinding performed wet? What is the purpose of etching metallographic samples? What are the advantages of electrolytic polishing?   Exp.4: Preparation of Specimens 11/13/2018