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Ut P5 (Product Tech.)

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Ut P5 (Product Tech.)

  1. 1. UT Product Technology TWI
  2. 2. Product Technology Steel Production Wrought Production Extrusion Forging Rolling Casting Welding Defects Inherent Processing Service Heat Treatment
  3. 3. 2 Stage Process <ul><li>Iron ore is reduced into pig iron assisted by other materials. </li></ul><ul><li>Carbon content of Pig Iron is lowered by reacting with oxygen </li></ul><ul><li>The molten metal is then cast into Ingots or continuously cast </li></ul><ul><li>Ingots are rolled into Blooms, Billets or Slabs </li></ul>Steel Production
  4. 4. 1st Stage <ul><li>Iron ore is reduced into pig iron assisted by other materials. </li></ul><ul><li>Raw materials Hematite (Fe 2 O 3 ) or Magnetite( Fe 3 O 4 ) + Coke Limestone Air </li></ul>Steel Production
  5. 5. 1st Stage <ul><li>Blast furnace reactions </li></ul>Steel Production <ul><li>Fe 2 O 3 + 3CO = 2Fe + 3CO 2 </li></ul><ul><li>Fe 2 O 3 + 3C = 2Fe + 3CO </li></ul><ul><li>SiO 2 + 2Cao = 2CaOSiO 2 </li></ul>Lime from limestone combines with impurities (mainly silica) in the ore to form fluid slag
  6. 6. Blast Furnace <ul><li>Charge </li></ul><ul><li>Ore 4000 </li></ul><ul><li>Limestone 800 </li></ul><ul><li>Coke 1800 </li></ul><ul><li>Air 8000 </li></ul><ul><li>14600 </li></ul><ul><li>Products </li></ul><ul><li>Pig Iron 2000 </li></ul><ul><li>Slag 1600 </li></ul><ul><li>Dust 200 </li></ul><ul><li>Furnace gas 10800 </li></ul><ul><li>14600 </li></ul>Steel Production
  7. 7. Blast Furnace Steel Production Product of Blast Furnace - Pig iron (>3% carbon)
  8. 8. Steel Production <ul><li>Pig iron converted to steel by blowing molten metal with oxygen or oxygen rich gases </li></ul><ul><li>Oxygen reacts with excess carbon </li></ul><ul><li>C + 2 O  CO 2 </li></ul><ul><li>C + O  CO </li></ul><ul><li>CO + O  CO 2 </li></ul>
  9. 9. Steel Production <ul><li>Bessemer </li></ul><ul><li>Open hearth process </li></ul><ul><li>Basic oxygen process </li></ul>
  10. 10. Steel Production <ul><li>Basic oxygen process </li></ul>Solid scrap
  11. 11. Steel Production <ul><li>Basic oxygen process </li></ul>Molten Pig Iron
  12. 12. Steel Production <ul><li>Basic oxygen process </li></ul>Oxygen lance
  13. 13. Steel Production <ul><li>Basic oxygen process </li></ul>Steel
  14. 14. Steel Production Molten steel poured into large molds (ingots) Ingots are used for further processing Hot top
  15. 15. Steel Production Molten steel poured into large molds (ingots) Ingots are used for further processing 2 types of mould - Narrow end up, Wide end up
  16. 16. Steel Production <ul><li>Metal solidifies from outside inwards </li></ul><ul><li>3 types of crystal formed </li></ul><ul><li>Chill or fine exui-axed </li></ul><ul><li>Columnar </li></ul><ul><li>Large equi-axed </li></ul>
  17. 17. Smelting Defects <ul><li>Pipes </li></ul><ul><li>Shrinkage </li></ul>Primary pipe/sink Secondary pipe
  18. 18. Smelting Defects <ul><li>Non-metallic inclusions </li></ul>
  19. 19. Smelting Defects <ul><li>Segregation of metals </li></ul>
  20. 20. Steel Production <ul><li>Alternative to ingots is Continuous casting </li></ul>Tundish Mold forming slab Water spray chamber Rollers
  21. 21. Steel Production
  22. 22. Advantages of Continuous casting <ul><li>Faster : 300 tons of steel in 45 mins compared to 12 hours </li></ul><ul><li>No piping problems </li></ul><ul><li>Cheaper : No ingot molds, handling </li></ul>Steel Production
  23. 23. Product Technology Casting
  24. 24. Casting Process <ul><li>Liquid metal is caused to fill a cavity and solidify into a useful shape </li></ul><ul><li>All materials used in metal manufacture cast at some time </li></ul>
  25. 25. Casting Process <ul><li>Stage 1 : A pattern of the finished item slightly over sized </li></ul><ul><li>Stage 2 : Mould constructed from the pattern </li></ul><ul><li>Stage 3 : Liquid metal poured through the channels to fill the mould </li></ul>
  26. 26. Casting Riser Pouring basin Runner Sprue Core
  27. 27. Casting Chaplets Chills
  28. 28. Casting <ul><li>Casting involves the solidification from liquid to solid </li></ul><ul><li>Solidification proceeds from outside to centre </li></ul><ul><li>Solidification involves shrinkage </li></ul>
  29. 29. Grain Growth
  30. 30. Casting Methods <ul><li>Sand casting </li></ul>
  31. 31. Sand Casting
  32. 32. Sand Casting
  33. 33. Sand Casting
  34. 34. Sand Casting
  35. 35. Sand Casting
  36. 36. Sand Casting
  37. 37. Casting Methods <ul><li>Sand casting </li></ul><ul><li>Die casting / Injection moulding </li></ul>
  38. 38. Die Casting Injection piston Casting cavity Die Fixed platen Moving platen
  39. 39. Casting Methods <ul><li>Sand casting </li></ul><ul><li>Die casting / Injection moulding </li></ul><ul><li>Investment casting / Lost wax process </li></ul>
  40. 40. Investment Casting Wax Pattern
  41. 41. Investment Casting Coat with refractory slurry
  42. 42. Investment Casting Reinforce with plaster backing (Investment)
  43. 43. Investment Casting Oven dry to liquify or vaporise pattern and dry mould
  44. 44. Investment Casting Pour metal
  45. 45. Investment Casting Remove investment material
  46. 46. Choice of Casting Method <ul><li>Dimensional Accuracy </li></ul><ul><li>Investment casting </li></ul><ul><li>Die casting </li></ul><ul><li>Sand casting </li></ul><ul><li>Cost </li></ul><ul><li>Sand casting </li></ul><ul><li>Die casting </li></ul><ul><li>Investment casting </li></ul>
  47. 47. Casting Defects <ul><li>Shrinkage cavities </li></ul><ul><li>Sinks </li></ul>Primary pipe/sink Secondary pipe
  48. 48. Casting Defects <ul><li>Blowholes and porosity </li></ul>Cross-sectional changes /corners
  49. 49. Casting Defects <ul><li>Inclusions </li></ul><ul><li>Scabs </li></ul><ul><li>Fins </li></ul>
  50. 50. Casting Defects <ul><li>Shrinkage </li></ul>
  51. 51. Casting Defects <ul><li>Scabs </li></ul>
  52. 52. Casting Defects <ul><li>Scabs- part of mould stuck to the casting </li></ul>
  53. 53. Casting Defects <ul><li>Fins </li></ul>Gaps
  54. 54. Casting Defects <ul><li>Fins- excess metal of casting </li></ul>Fin
  55. 55. Casting Defects <ul><li>Hot tears </li></ul>The larger section cools slower than the smaller section The grain are different between the sections Hot Tears
  56. 56. Hot Tears
  57. 57. <ul><li>Chills are used for: </li></ul><ul><li>Directional grain growth </li></ul><ul><li>Uniform cooling rate </li></ul>
  58. 58. Casting Defects <ul><li>Segregation </li></ul>
  59. 59. Product Technology Wrought Production Methods
  60. 60. Wrought Production <ul><li>Forging </li></ul><ul><li>Extrusion </li></ul><ul><li>Rolling </li></ul>
  61. 61. Wrought Production <ul><li>Forging </li></ul><ul><li>Metal confined under pressure to cause plastic flow </li></ul><ul><li>Extrusion </li></ul><ul><li>Metal forced through a die under a large load </li></ul><ul><li>Rolling </li></ul><ul><li>Thickness reduction through compression </li></ul>
  62. 62. Rolling Two-High Reversing Mill Ingots, slabs and billets rolled to produce long length products with uniform cross section PRIMARY ROLLING PROCESS / COGGING
  63. 63. Rolling Two-High Reversing Mill PRIMARY ROLLING PROCESS Secondary piping
  64. 64. Rolling Three-High Reversing Mill SECONDARY ROLLING PROCESS Lamination
  65. 65. Rolling Two-High Reversing Mill PRIMARY ROLLING PROCESS Non-metallic inclusion
  66. 66. Rolling Three-High Reversing Mill SECONDARY ROLLING PROCESS Stringers
  67. 67. Rolling Two-High Reversing Mill PRIMARY ROLLING PROCESS Segregation of metals
  68. 68. Rolling Three-High Reversing Mill SECONDARY ROLLING PROCESS Banding
  69. 69. Cold Rolling <ul><li>Initial rolling hot </li></ul><ul><li>Finishing by cold working </li></ul>Cluster mill 4 High mill
  70. 70. Rolling <ul><li>Bloom - Square c/s 150x150mm minimum </li></ul><ul><li>Slab - Rectangular c/s area greater than 14400 mm 2 </li></ul><ul><li>Billet - Square 50x50 up to 120 x 120mm </li></ul><ul><li>Primary rolling- ingot to blooms and slabs </li></ul><ul><li>Secondary rolling - blooms and slabs to plates , sheets etc </li></ul>
  71. 71. Forging Hammer Anvil Blacksmith
  72. 72. Forging <ul><li>6 basic actions </li></ul><ul><li>Upsetting </li></ul><ul><li>Swaging </li></ul><ul><li>Bending </li></ul><ul><li>Welding </li></ul><ul><li>Punching </li></ul><ul><li>Cutting out </li></ul>
  73. 73. Forging Hammer (Tup) Anvil Blacksmith / Open die forging
  74. 74. Forging <ul><li>Pressure forging </li></ul>
  75. 75. Forging <ul><li>Closed die </li></ul>
  76. 76. Extrusion <ul><li>Direct </li></ul><ul><li>Indirect </li></ul><ul><li>Impact </li></ul><ul><li>High loads used to shape ferrous and non-ferrous alloys </li></ul><ul><li>Items produced are of uniform cross section </li></ul>
  77. 77. Direct Extrusion Billet Ram Die
  78. 78. Indirect Extrusion Die Extruded item Billet
  79. 79. Impact Extrusion Die Blank Punch
  80. 80. Extrusion Defects <ul><li>Oxide films (‘Extrusion’ defect) </li></ul><ul><li>Surface cracks </li></ul><ul><li>Grain structure variation </li></ul>
  81. 81. Impact Extrusion
  82. 82. Wrought Production Defects <ul><li>Cracks </li></ul><ul><li>Laps </li></ul><ul><li>Seams </li></ul><ul><li>Stringers </li></ul><ul><li>Slugs </li></ul><ul><li>Bursts </li></ul><ul><li>Laminations </li></ul>
  83. 83. Wrought Production Defects <ul><li>Banding </li></ul><ul><li>Excessive flash </li></ul><ul><li>Lack of fill </li></ul><ul><li>Mismatch </li></ul><ul><li>Internal cracking </li></ul><ul><li>Mechanical marks </li></ul>
  84. 84. Other Wrought Processes <ul><li>Drawing </li></ul><ul><li>Material is reduced or changed in profile by pulling through a die </li></ul>Die Wire or rod Force
  85. 85. Other Wrought Processes <ul><li>Drawing </li></ul><ul><li>Material is reduced or changed in profile by pulling through a die </li></ul>Die Tube Force Mandrel
  86. 86. Product Technology Welding
  87. 87. A Weld : Definitions <ul><li>A union between pieces of metal at faces rendered plastic or liquid by heat,pressure or both. </li></ul><ul><li>BS 499 </li></ul><ul><li>A continuous defect surrounded by parent material </li></ul><ul><li>NASA </li></ul>
  88. 88. Welds <ul><li>An ideal weld must give a strong bond between materials with the interfaces disappearing </li></ul><ul><li>To achieve this </li></ul><ul><li>Smooth,flat or matching surfaces </li></ul><ul><li>Surfaces shall be free from contaminants </li></ul><ul><li>Metals shall be free from impurities </li></ul><ul><li>Metals shall have identical crystalline structures </li></ul>
  89. 89. Welding <ul><li>A union between pieces of metal at faces rendered plastic or liquid by heat,pressure or both. </li></ul><ul><li>BS 499 </li></ul><ul><li>Ultrasonics </li></ul><ul><li>Electron beam </li></ul><ul><li>Friction </li></ul><ul><li>Electric resistance </li></ul><ul><li>Electric arc </li></ul>Possible energy sources
  90. 90. Electric Arc Welding Power supply Work piece Electrode Clamp(Earth)
  91. 91. Electric Arc Welding <ul><li>Electric discharge produced between cathode and anode by a potential difference (40 to 60 volts) </li></ul><ul><li>Discharge ionises air and produces -ve electrons and +ve ions </li></ul><ul><li>Electrons impact upon anode, ions upon cathode </li></ul><ul><li>Impact of particles converts kinetic energy to heat (7000 o C) and light </li></ul><ul><li>Amperage controls number of ions and electrons, Voltage controls their velocity </li></ul>
  92. 92. Electric Arc Welding <ul><li>Arc Welding Processes </li></ul><ul><li>Manual metal arc </li></ul><ul><li>Tungsten Inert Gas </li></ul><ul><li>Metal Inert Gas </li></ul><ul><li>Submerged Arc </li></ul>Differences between them <ul><li>Methods of shielding the arc </li></ul><ul><li>Consumable or Non-consumable electrode </li></ul><ul><li>Degree of automation </li></ul>
  93. 93. Zones in Fusion Welds <ul><li>Fusion Zone </li></ul>
  94. 94. Zones in Fusion Welds <ul><li>Fusion Zone </li></ul><ul><li>Heat Affected Zone </li></ul>
  95. 95. Zones in Fusion Welds <ul><li>Fusion Zone </li></ul><ul><li>Heat Affected Zone </li></ul><ul><li>Parent Material or Base Metal </li></ul>
  96. 96. Joint Design Butt Weld
  97. 97. Joint Design Butt Weld Lap Joint
  98. 98. Joint Design Butt Weld Corner Joint Lap Joint
  99. 99. Joint Design Butt Weld Corner Joint Lap Joint Edge Weld
  100. 100. Joint Design Butt Weld Corner Joint Lap Joint T Joint Edge Weld
  101. 102. Manual Metal Arc (MMA) Consumable electrode Flux coating Core wire Arc Evolved gas shield Parent metal Slag Weld metal
  102. 103. Manual Metal Arc Welding <ul><li>Shielding provided by decomposition of flux covering </li></ul><ul><li>Electrode consumable </li></ul><ul><li>Manual process </li></ul><ul><li>Welder controls </li></ul><ul><li>Arc length </li></ul><ul><li>Angle of electrode </li></ul><ul><li>Speed of travel </li></ul><ul><li>Amperage settings </li></ul>
  103. 104. Tungsten Inert Gas (TIG) Non-consumable tungsten electrode Arc Parent metal Weld metal Gas shield Filler wire Gas nozzle
  104. 105. Metal Inert Gas (MIG) Consumable electrode(filler wire) Arc Parent metal Weld metal Gas shield Gas nozzle Reel feed
  105. 106. Submerged Arc Consumable electrode Reel feed Flux feed Flux retrieval Parent metal Weld metal Slag
  106. 107. Electroslag Filler wire Molten flux Weld metal Water cooled copper shoes
  107. 108. Welding Defects <ul><li>4 Crack Types </li></ul><ul><li>Solidification cracks </li></ul><ul><li>Hydrogen induced cracks </li></ul><ul><li>Lamellar tearing </li></ul><ul><li>Reheat cracks </li></ul>Cracks
  108. 109. Welding Defects <ul><li>Classified by Shape </li></ul><ul><li>Longitudinal </li></ul><ul><li>Transverse </li></ul><ul><li>Branched </li></ul><ul><li>Chevron </li></ul>Cracks <ul><li>Classified by Position </li></ul><ul><li>HAZ </li></ul><ul><li>Centreline </li></ul><ul><li>Crater </li></ul><ul><li>Fusion zone </li></ul><ul><li>Parent metal </li></ul>
  109. 110. Welding Defects <ul><li>Solidification </li></ul><ul><li>Occurs during weld solidification process </li></ul><ul><li>Steels with high sulphur content (low ductility at elevated temperature) </li></ul><ul><li>Requires high tensile stress </li></ul><ul><li>Occur longitudinally down centre of weld </li></ul><ul><li>eg Crater cracking </li></ul>Cracks
  110. 111. Welding Defects <ul><li>Hydrogen Induced </li></ul><ul><li>Requires susceptible grain structure, stress and hydrogen </li></ul><ul><li>Hydrogen enters via welding arc </li></ul><ul><li>Hydrogen source - atmosphere or contamination of preparation or electrode </li></ul><ul><li>Moisture diffuses out into parent metal on cooling </li></ul><ul><li>Most likely in HAZ </li></ul>Cracks
  111. 112. Welding Defects <ul><li>Lamellar Tearing </li></ul><ul><li>Step like appearance </li></ul><ul><li>Occurs in parent material or HAZ </li></ul><ul><li>Only in rolled direction of the parent material </li></ul><ul><li>Associated with restrained joints subjected to through thickness stresses on corners, tees and fillets </li></ul><ul><li>Requires high sulphur or non-metallic inclusions </li></ul>Cracks
  112. 113. Welding Defects <ul><li>Re-Heat Cracking </li></ul><ul><li>Occurs mainly in HAZ of low alloy steels during post weld heat treatment or service at elevated temperatures </li></ul><ul><li>Occurs in areas of high stress and existing defects </li></ul><ul><li>Prevented by toe grinding, elimination of poor profile material selection and controlled post weld heat treatment </li></ul>Cracks
  113. 114. Welding Defects <ul><li>Incomplete root penetration </li></ul><ul><li>Causes </li></ul><ul><li>Too large or small a root gap </li></ul><ul><li>Arc too long </li></ul><ul><li>Wrong polarity </li></ul><ul><li>Electrode too large for joint preparation </li></ul><ul><li>Incorrect electrode angle </li></ul><ul><li>Too fast a speed of travel for current </li></ul>
  114. 115. Welding Defects <ul><li>Root concavity </li></ul><ul><li>Causes </li></ul><ul><li>Root gap too large </li></ul><ul><li>Insufficient arc energy </li></ul><ul><li>Excessive back purge (TIG) </li></ul>
  115. 116. Welding Defects <ul><li>Lack of fusion </li></ul><ul><li>Causes </li></ul><ul><li>Contaminated weld preparation </li></ul><ul><li>Amperage too low </li></ul><ul><li>Amperage too high (welder increases speed of travel) </li></ul>
  116. 117. Welding Defects <ul><li>Undercut </li></ul><ul><li>Causes </li></ul><ul><li>Excessive welding current </li></ul><ul><li>Welding speed too high </li></ul><ul><li>Incorrect electrode angle </li></ul><ul><li>Excessive weave </li></ul><ul><li>Electrode too large </li></ul>
  117. 118. Welding Defects <ul><li>Incompletely Filled Groove </li></ul><ul><li>Causes </li></ul><ul><li>Insufficient weld metal deposited </li></ul><ul><li>Improper welding technique </li></ul>
  118. 119. Welding Defects <ul><li>Gas pores / Porosity </li></ul><ul><li>Causes </li></ul><ul><li>Excessive moisture in flux or preparation </li></ul><ul><li>Contaminated preparation </li></ul><ul><li>Low welding current </li></ul><ul><li>Arc length too long </li></ul><ul><li>Damaged electrode flux </li></ul><ul><li>Removal of gas shield </li></ul>
  119. 120. Welding Defects <ul><li>Inclusions - Slag </li></ul><ul><li>Causes </li></ul><ul><li>Insufficient cleaning between passes </li></ul><ul><li>Contaminated weld preparation </li></ul><ul><li>Welding over irregular profile </li></ul><ul><li>Incorrect welding speed </li></ul><ul><li>Arc length too long </li></ul>
  120. 121. Welding Defects <ul><li>Inclusions - Tungsten </li></ul><ul><li>Causes </li></ul><ul><li>Contamination of weld during TIG welding process </li></ul>
  121. 122. Welding Defects <ul><li>Burn Through </li></ul><ul><li>Causes </li></ul><ul><li>Excessive amperage during welding of root </li></ul><ul><li>Excessive root grinding </li></ul><ul><li>Improper welding technique </li></ul>
  122. 123. Welding Defects <ul><li>Arc Strikes </li></ul><ul><li>Causes </li></ul><ul><li>Electrode straying onto parent metal </li></ul><ul><li>Electrode holder with poor insulation </li></ul><ul><li>Poor contact of earth clamp </li></ul><ul><li>Spatter </li></ul><ul><li>Causes </li></ul><ul><li>Excessive arc energy </li></ul><ul><li>Excessive arc length </li></ul><ul><li>Damp electrodes </li></ul><ul><li>Arc blow </li></ul>
  123. 124. Steel Metallurgy <ul><li>Steel- Iron and carbon alloyed with other elements </li></ul><ul><li>Carbon- Strength, hardness, toughness, ductility </li></ul><ul><li>Manganese- Strength, hardenability </li></ul><ul><li>Silicon - Toughness </li></ul><ul><li>Molybdenum- Creep resistance, temper embrittlement </li></ul><ul><li>Chromium- Hardness, wear resistance, corrosion </li></ul><ul><li>Nickel - Ductility, strength, toughness </li></ul>
  124. 125. Steel Metallurgy <ul><li>Steel- Iron and carbon alloyed with other elements </li></ul>BCC FCC
  125. 126. Steel Metallurgy Low Stress Increased Stress Elastic Deformation Plastic Deformation
  126. 127. Heat Treatment <ul><li>Softening </li></ul><ul><li>Hardening </li></ul><ul><li>Tempering </li></ul><ul><li>Stress Relief </li></ul><ul><li>Post heat treatment performed to improve specific metallurgical or mechanical properties or stress relief </li></ul><ul><li>Controlled by </li></ul><ul><li>Heating rate </li></ul><ul><li>Temperature attained </li></ul><ul><li>Time at the elevated temperature </li></ul><ul><li>Cooling rate </li></ul>
  127. 128. Heat Treatment 900 850 800 750 700 Ar3 Ac1 Ac2 Ac3 Ar1 Ar2 1 2 Minutes to raise temperature by 10 C
  128. 129. Iron Carbide Diagram Ac3 Ac1 .2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2 Carbon % 1000 900 800 700 600
  129. 130. Iron Carbide Diagram Ac3 Ac1 .2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2 Carbon % 1000 900 800 700 600 Austenite Ferrite and Pearlite Pearlite and Cementite Austenite and Fe 3 C Austenite and Ferrite
  130. 131. Heat Treatment <ul><li>Hardening </li></ul><ul><li>Produce hard but brittle material </li></ul><ul><li>Heat to above transformation range </li></ul><ul><li>Cool very quickly ( quench ) in oil, water or brine </li></ul>
  131. 132. Heat Treatment <ul><li>Stress Relief </li></ul><ul><li>Relax stresses without significant changes in the metallurgical structure </li></ul><ul><li>Heat to 550-650 degrees C </li></ul><ul><li>Hold for 1 hour per 25mm thickness </li></ul><ul><li>Cool in air </li></ul>
  132. 133. Heat Treatment <ul><li>Full Annealing </li></ul><ul><li>Produces very soft low hardness material for machining or cold work </li></ul><ul><li>Heat to above 910 degrees C </li></ul><ul><li>Hold </li></ul><ul><li>Cool very slowly in furnace </li></ul><ul><li>Once reached 680 C , cool in air </li></ul>
  133. 134. Heat Treatment <ul><li>Sub Critical Annealing </li></ul><ul><li>Spheroidizing produces soft low hardness material cheaper than full anneal </li></ul><ul><li>Heat must not rise above 700 degrees C </li></ul><ul><li>Hold for recrystallisation to occur </li></ul><ul><li>Cool in air </li></ul>
  134. 135. Heat Treatment <ul><li>Normalising </li></ul><ul><li>Maintains and improves mechanical properties and modifies grain structure </li></ul><ul><li>Heat to above 910 degrees C </li></ul><ul><li>Hold </li></ul><ul><li>Cool in air </li></ul>
  135. 136. Nature and Origin of Defects <ul><li>Inherent </li></ul><ul><li>Processing </li></ul><ul><li>In Service </li></ul>
  136. 137. Heat Induced Defects <ul><li>Heat treatment cracks </li></ul><ul><li>Grinding cracks </li></ul><ul><li>Friction induced cracks </li></ul>
  137. 138. In Service Cracks <ul><li>Fatigue cracks </li></ul><ul><li>Stress corrosion cracks </li></ul><ul><li>Hydrogen induced cracks </li></ul>Hydrogen Cyclic stress Fatique crack