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Metallurgical Testing

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Metallurgical Testing

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Metallurgical Testing

  1. 1. Metallurgical Testing 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 1
  2. 2. Non-Destructive Testing - This is carried out on components rather than on test pieces, they are designed to indicate flaws occurring due or after manufacture. They give no indication of the mechanical properties of the material. - Surface flaws may be detected by visual means aided by dye penetrant or magnetic crack detection. - Internal flaws may be detected by X-ray or ultrasonic testing. - In addition to this there are special equipment able to exam machine finish. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 2
  3. 3. Liquid Penetrant Methods - The surface is first cleaned using an volatile cleaner and degreaser. - A fluorescent dye is then applied and a certain time allowed for it to enter any flaws under capillary action. Using the cleaning spray, the surface is then wiped clean. - - An ultra violet light is shone on the surface, any flaws showing up as the dye fluoresce. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 3
  4. 4. Dye penetrant method - The surface is cleaned and the low viscosity penetrant sprayed on. - After a set time the surface is again cleaned. - A developer is then used which coats the surface in a fine white chalky dust, then the dye seeps out and stains the developer typically a red colour. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 4
  5. 5. Magnetic crack detection - A component is place between two poles of a magnet. - The lines of magnetism concentrate around flaws. - Magnetic particles are then applied, in a light oil or dry sprayed, onto the surface where they indicate the lines of magnetism and any anomalies/ abnormalities like the below figures. Limitation of Magnetic test: This method of testing has a few limitations. - Firstly it cannot be used on materials which cannot be magnetised such as austenitic steel and non-ferrous metals. - Secondly it would not detect a crack which ran parallel to the lines of magnetism. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 5
  6. 6. 5/27/2015 6 Tensile Test
  7. 7. - When a material is tested under a tensile load, it changes shape by elongating. - Initially the extension is in proportion to the increasing tensile load. - If a graph is plotted showing extension for various loads, then a straight line is obtained at first. - If the loading is continued the graph, deviates as shown. - Within the limit of the straight line, if the load is removed the material will return to its original length which is elastic limit of the specimen. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 7 Tensile Testing Method
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  9. 9. - When the test piece reaches the Yield point (Yu), there is a failure of the crystalline structure of the metal, not along the grain boundaries as it has been the case, but through the grains themselves. This is known as “slip”. - A partial recovery is made at the lower yield point (YL), then the extension starts to increase. - If the load is removed at any stage along the “Load-Extension” curve after Yield Point (YL), the material will have a corresponding permanent deformation. This termed “permanent set”. - Maximum loading occurs at the “ultimate Load” (S). and after Yield Point (YL) to Ultimate Load (S) is the plastic limit. - Ultimate Load (S) to Breaking point (B) this stage local wasting or extension will start which termed “necking”. Normally this starts at about the centre of the specimen and will rapidly be followed by failure up to breaking point (B). 9 Tensile Testing Method
  10. 10. Proof Test 5/27/2015 10
  11. 11. Proof Stress: For a material which does not have a marked yield point such as Aluminium, there is a substitute stress specified. This is termed “the proof stress”. - Proof stress is determined from a load/extension or stress/strain graph. 11
  12. 12. Proof Testing Method • Hard steels and non-ferrous metals (Aluminium) do not have defined yield limit, therefore a stress, corresponding to a definite deformation, (0.1% or 0.2%) is commonly used instead of yield limit. This stress is called proof stress or offset yield limit (offset yield strength): • σ0.2%= F0.2% / S0 • The method of obtaining the proof stress is shown in the picture. • As the load increase, the specimen continues to undergo plastic deformation and at a certain stress value its cross-section decreases due to “necking”. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 12
  13. 13. • At point S in the Stress-Strain Diagram the stress reaches the maximum value, which is called ultimate tensile strength (tensile strength): σt= FS / S0 • Continuation of the deformation results in breaking the specimen - the point B in the diagram (from Ultimate load S to breaking point B) • The actual Stress-Strain curve is obtained by taking into account the true specimen cross-section instead of the original value. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 13
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  16. 16. Creep Testing - Creep tests are carried out under controlled temperature over an extended period of time in the order of 10,000hrs. - The test piece is similar to the type used for tensile tests and creep is usually thought of as being responsible for extensions of metal only. In fact creep can cause compression or other forms of deformation 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 16
  17. 17. - Temperature of the test is at recrystallization point around 400oC. For other metals the recrystallization temperature is different (200oC for copper and room temperature for tin and lead). - At the start of the test the initial load must be applied without shock. - This load, normally well below the strength limit of the material, will extend the test piece slowly. - The load is kept steady through the test and the temperature is maintained accurately. - Extension is plotted and is seen to proceed in three distinct stages. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 17
  18. 18. HARDNESS TESTING The basis of the Brinell hardness testing is the resistance to deformation of a surface by a loaded steel ball. Oil is pumped into the chamber between the pistons until there is sufficient pressure to raise the Weight so that it is floating. The ball is now forced into the specimen material at the same force. The loading for steel and metals of similar hardness is 3,000Kg. The load is allowed to act for 15 sec to ensure that plastic flow occurs. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 18
  19. 19. - The surface diameter of the indentation is measured with the aid of a microscope which is traversed over the test piece on a graduated slide with a vernier. - Cross wires in the microscope, enable the operator to accurately align the instrument. - Both the loading and ball diameter (10mm) are known, by measuring the indentation diameter the hardness can be calculated. For softer materials the loading is reduced, Copper being 1000Kg and Aluminium 500Kg. The diameter of the indentation must be less than half the ball diameter. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 19
  20. 20. - The thickness of the specimen must be not less than 10x the depth of the impression. The edge of the impression will tend to sink with the ball if the surface has been work hardened; otherwise the local deformation will tend to cause piling up of the metal around the indent If the hardness test is used on very hard materials, the steel ball will flatten. This method is not reliable for reading over 600. It is used in preference to other methods where the material has large crystals, e.g. Cast iron. Mild Steel 130, Cast Iron 200, white cast iron 400, nitrided surface 750. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 20
  21. 21. Under low temperature conditions , impact or shock loading on a material can cause cracking in a material which is normally ductile at room temperature. To find out the Critical stressing in a material, Griffith equation, sc = Kic / ж Pc where, sc = the critical stress in a material Kic = the fracture toughness of a material Pc = the micro-crack length within the materials 21 BRITTLE FRACTURE TESTING
  22. 22. BRITTLE FRACTURE TESTING - The presence of these micro cracks (porous materials or defects) can act to cause transcrystalline type failures with a bright crystalline appearance. - Testing is carried out via the Charpy notched piece test at various temperatures between -200o to +200oC - To reduce the effects of brittle fracture the carbon content in carbon steels is kept as low as practical. - Grains within the materials are kept as small as possible by heat treatment and normalizing. - Alloying elements may also be added. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 22
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  24. 24. 24 Transition Temperatures • As temperature decreases a ductile material can become brittle - ductile-to-brittle transition – The transition temperature is the temp at which a material changes from ductile-to-brittle behavior • Alloying usually increases the ductile-to-brittle transition temperature. FCC metals remain ductile down to very low temperatures. For ceramics, this type of transition occurs at much higher temperatures than for metals.
  25. 25. Factors which affect the transition temperature are 1. Elements: - Carbon, silicon, phosphorus and sulphur raise the temperature. - Nickel and manganese lower the temperature. 2. Grain size: - the smaller the grain size the lower the transition temperature, hence grain refinement is beneficial. 3. Work hardening: - this appears to increase transition temperature. 4. Notches: - possibly occurring during assembly e.g. weld defects or machine marks. - Notches can increase tendency to brittle fracture. 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 25
  26. 26. 18/80 stainless steel 5/27/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 26 It is this property of stainless steel that makes it so suitable for use in LPG carriers. Hardness Testing

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