HEAT TREATMENT OF METALS <ul><li>Annealing </li></ul><ul><li>Martensite Formation in Steel </li></ul><ul><li>Precipitation...
Heat Treatment  <ul><li>Various heating and cooling processes performed to effect structural changes in a material, which ...
Heat Treatment in Manufacturing <ul><li>Heat treatment operations are performed on metal workparts at various times during...
Principal Heat Treatments  <ul><li>Annealing </li></ul><ul><li>Martensite formation in steel </li></ul><ul><li>Tempering o...
Annealing <ul><li>Heating and soaking metal at suitable temperature for a certain time, and slowly cooling  </li></ul><ul>...
Annealing of Steel <ul><li>Full annealing  - heating and soaking the alloy in the austenite region, followed by slow cooli...
Annealing to Reduce Work Hardening <ul><li>Cold worked parts are often annealed to reduce strain hardening and increase du...
Annealing for Stress-Relief <ul><li>Annealing operations are sometimes performed solely to relieve residual stresses cause...
Martensite Formation in Steel <ul><li>The iron‑carbon phase diagram shows the phases of iron and iron carbide under equili...
©2007 John Wiley & Sons, Inc.  M P Groover,  Fundamentals of Modern Manufacturing  3/e Iron-Carbon Phase Diagram
<ul><li>Figure 27.1  The TTT curve, showing transformation of austenite into other phases as function of time and temperat...
Martensite  <ul><li>A unique phase consisting of an iron‑carbon solution whose composition is the same as the austenite fr...
<ul><li>Figure 27.2  Hardness of plain carbon steel as a function of carbon content in martensite and pearlite (annealed)....
Heat Treatment to Form Martensite  <ul><li>Consists of two steps: </li></ul><ul><li>Austenitizing - heating the steel to a...
©2007 John Wiley & Sons, Inc.  M P Groover,  Fundamentals of Modern Manufacturing  3/e TTT Curve
Quenching Media and Cooling Rate  <ul><li>Various quenching media are used to affect cooling rate  </li></ul><ul><ul><li>B...
Tempering of Martensite <ul><li>A heat treatment applied to martensite to reduce brittleness, increase toughness, and reli...
Hardenability  <ul><li>The relative capacity of a steel to be hardened by transformation to martensite  </li></ul><ul><li>...
Hardenability  <ul><li>Hardenability of steel is increased through alloying  </li></ul><ul><ul><li>Alloying elements havin...
<ul><li>Figure 27.4  Jominy end‑quench test: (a) setup, showing end quench of the test specimen; and (b) typical pattern o...
Precipitation Hardening <ul><li>Heat treatment that precipitates fine particles that block the movement of dislocations an...
Conditions for Precipitation Hardening <ul><li>The necessary condition for whether an alloy system can be strengthened by ...
<ul><li>Figure 27.5  Precipitation hardening: (a) phase diagram of an alloy system consisting of metals A and B that can b...
Sequence in Precipitation Hardening <ul><li>Solution treatment - alloy is heated to a temperature  T s  above the solvus l...
Surface Hardening <ul><li>Thermochemical treatments applied to steels in which the composition of the part surface is alte...
Carburizing <ul><li>Heating a part of low carbon steel in a carbon-rich environment so that C is diffused into surface  </...
Nitriding <ul><li>Treatment in which nitrogen is diffused into surface of special alloy steels to produce a thin hard casi...
Chromizing  <ul><li>Requires higher temperatures and longer treatment times than the preceding hardening treatments </li><...
Furnaces for Heat Treatment <ul><li>Fuel‑fired furnaces  </li></ul><ul><ul><li>Normally direct‑fired - the work is exposed...
Batch vs. Continuous Furnaces <ul><li>Batch furnaces  </li></ul><ul><ul><li>Heating system in an insulated chamber, with a...
Other Furnace Types <ul><li>Atmospheric control furnaces </li></ul><ul><ul><li>Desirable in conventional heat treatment to...
Selective Surface Hardening Methods <ul><li>These methods heat only the work surface, or local areas of the work surface  ...
Flame Hardening <ul><li>Heating of work surface by one or more torches followed by rapid quenching  </li></ul><ul><li>Appl...
Induction Heating <ul><li>Application of electromagnetically induced energy supplied by an induction coil to an electrical...
<ul><li>Figure 27.7  Typical induction heating setup. High frequency alternating current in a coil induces current in the ...
High‑frequency (HF) Resistance Heating <ul><li>Used to harden specific areas of steel work surfaces by application of loca...
<ul><li>Figure 27.8  Typical setup for high‑frequency resistance heating. </li></ul>©2007 John Wiley & Sons, Inc.  M P Gro...
Electron Beam (EB) Heating <ul><li>Electron beam focused onto small area, resulting in rapid heat buildup  </li></ul><ul><...
Laser Beam (LB) Heating <ul><li>High‑density beam of coherent light focused on a small area ‑ the beam is usually moved al...
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Ch27 1

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Ch27 1

  1. 1. HEAT TREATMENT OF METALS <ul><li>Annealing </li></ul><ul><li>Martensite Formation in Steel </li></ul><ul><li>Precipitation Hardening </li></ul><ul><li>Surface Hardening </li></ul><ul><li>Heat Treatment Methods and Facilities </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  2. 2. Heat Treatment <ul><li>Various heating and cooling processes performed to effect structural changes in a material, which in turn affect its mechanical properties </li></ul><ul><li>Most common applications are on </li></ul><ul><ul><li>Metals </li></ul></ul><ul><li>Similar treatments are performed on </li></ul><ul><ul><li>Glass‑ceramics </li></ul></ul><ul><ul><li>Tempered glass </li></ul></ul><ul><ul><li>Powder metals and ceramics </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  3. 3. Heat Treatment in Manufacturing <ul><li>Heat treatment operations are performed on metal workparts at various times during their manufacturing sequence </li></ul><ul><ul><li>To soften a metal for forming prior to shaping </li></ul></ul><ul><ul><li>To relieve strain hardening that occurs during forming </li></ul></ul><ul><ul><li>To strengthen and harden the metal near the end of the manufacturing sequence </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  4. 4. Principal Heat Treatments <ul><li>Annealing </li></ul><ul><li>Martensite formation in steel </li></ul><ul><li>Tempering of martensite </li></ul><ul><li>Precipitation hardening </li></ul><ul><li>Surface hardening </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  5. 5. Annealing <ul><li>Heating and soaking metal at suitable temperature for a certain time, and slowly cooling </li></ul><ul><li>Reasons for annealing: </li></ul><ul><ul><li>Reduce hardness and brittleness </li></ul></ul><ul><ul><li>Alter microstructure to obtain desirable mechanical properties </li></ul></ul><ul><ul><li>Soften metals to improve machinability or formability </li></ul></ul><ul><ul><li>Recrystallize cold worked metals </li></ul></ul><ul><ul><li>Relieve residual stresses induced by shaping </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  6. 6. Annealing of Steel <ul><li>Full annealing - heating and soaking the alloy in the austenite region, followed by slow cooling to produce coarse pearlite </li></ul><ul><ul><li>Usually associated with low and medium carbon steels </li></ul></ul><ul><li>Normalizing - similar heating and soaking cycle as in full annealing, but faster cooling rates, </li></ul><ul><ul><li>Results in fine pearlite, higher strength and hardness, but lower ductility </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  7. 7. Annealing to Reduce Work Hardening <ul><li>Cold worked parts are often annealed to reduce strain hardening and increase ductility by allowing strain‑hardened metal to recrystallize partially or completely </li></ul><ul><ul><li>When annealing is performed to allow for further cold working of the part, it is called a process anneal </li></ul></ul><ul><ul><li>When no subsequent deformation will be accomplished, it is simply called an anneal </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  8. 8. Annealing for Stress-Relief <ul><li>Annealing operations are sometimes performed solely to relieve residual stresses caused by prior shape processing or fusion welding </li></ul><ul><ul><li>Called stress‑relief annealing </li></ul></ul><ul><ul><li>They help to reduce distortion and dimensional variations that might otherwise result in the stressed parts </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  9. 9. Martensite Formation in Steel <ul><li>The iron‑carbon phase diagram shows the phases of iron and iron carbide under equilibrium conditions </li></ul><ul><ul><li>Assumes cooling from high temperature is slow enough to permit austenite to transform into ferrite and cementite (Fe 3 C) mixture </li></ul></ul><ul><li>However, under rapid cooling, so that equilibrium is prevented, austenite transforms into a nonequilibrium phase called martensite , which is hard and brittle </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  10. 10. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Iron-Carbon Phase Diagram
  11. 11. <ul><li>Figure 27.1 The TTT curve, showing transformation of austenite into other phases as function of time and temperature for a composition of about 0.80% C steel. Cooling trajectory shown yields martensite. </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Time-Temperature-Transformation Curve
  12. 12. Martensite <ul><li>A unique phase consisting of an iron‑carbon solution whose composition is the same as the austenite from which it was derived </li></ul><ul><li>Face‑centered cubic (FCC) structure of austenite is transformed into body‑centered tetragonal (BCT) structure of martensite </li></ul><ul><li>The extreme hardness of martensite results from the lattice strain created by carbon atoms trapped in the BCT structure, thus providing a barrier to slip </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  13. 13. <ul><li>Figure 27.2 Hardness of plain carbon steel as a function of carbon content in martensite and pearlite (annealed). </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Hardness of Plain Carbon Steel
  14. 14. Heat Treatment to Form Martensite <ul><li>Consists of two steps: </li></ul><ul><li>Austenitizing - heating the steel to a sufficiently high temperature for a long enough time to convert it entirely or partially to austenite </li></ul><ul><li>Quenching - cooling the austenite rapidly enough to avoid passing through the nose of the TTT curve </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  15. 15. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e TTT Curve
  16. 16. Quenching Media and Cooling Rate <ul><li>Various quenching media are used to affect cooling rate </li></ul><ul><ul><li>Brine -salt water, usually agitated (fastest cooling rate) </li></ul></ul><ul><ul><li>Still fresh water </li></ul></ul><ul><ul><li>Still oil </li></ul></ul><ul><ul><li>Air (slowest cooling rate) </li></ul></ul><ul><li>The faster the cooling, the more likely are internal stresses, distortion, and cracks in the product </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  17. 17. Tempering of Martensite <ul><li>A heat treatment applied to martensite to reduce brittleness, increase toughness, and relieve stresses </li></ul><ul><li>Treatment involves heating and soaking at a temperature below the eutectoid for about one hour, followed by slow cooling </li></ul><ul><li>Results in precipitation of very fine carbide particles from the martensite iron‑carbon solution, gradually transforming the crystal structure from BCT to BCC </li></ul><ul><li>New structure is called tempered martensite </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  18. 18. Hardenability <ul><li>The relative capacity of a steel to be hardened by transformation to martensite </li></ul><ul><li>It determines the depth below the quenched surface to which the steel is hardened </li></ul><ul><ul><li>Steels with good hardenability can be hardened more deeply below the surface and do not require high cooling rates </li></ul></ul><ul><li>Hardenability does not refer to the maximum hardness that can be attained </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  19. 19. Hardenability <ul><li>Hardenability of steel is increased through alloying </li></ul><ul><ul><li>Alloying elements having the greatest effect are chromium, manganese, molybdenum </li></ul></ul><ul><li>The mechanism by which these alloying elements work is to extend the time before the start of the austenite‑to‑pearlite transformation </li></ul><ul><ul><li>In effect, the TTT curve is moved to the right, thus permitting slower quenching rates </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  20. 20. <ul><li>Figure 27.4 Jominy end‑quench test: (a) setup, showing end quench of the test specimen; and (b) typical pattern of hardness readings as a function of distance from quenched end. </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Jominy End-Quench Test for Hardenability
  21. 21. Precipitation Hardening <ul><li>Heat treatment that precipitates fine particles that block the movement of dislocations and thus strengthen and harden the metal </li></ul><ul><li>Principal heat treatment for strengthening alloys of aluminum, copper, magnesium, nickel, and other nonferrous metals </li></ul><ul><li>Also utilized to strengthen a number of steel alloys that cannot form martensite by the usual heat treatment </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  22. 22. Conditions for Precipitation Hardening <ul><li>The necessary condition for whether an alloy system can be strengthened by precipitation hardening is the presence of sloping solvus line in the phase diagram </li></ul><ul><li>A composition in this system that can be precipitation hardened is one that contains two equilibrium phases at room temperature, but which can be heated to a temperature that dissolves the second phase </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  23. 23. <ul><li>Figure 27.5 Precipitation hardening: (a) phase diagram of an alloy system consisting of metals A and B that can be precipitation hardened; and (b) heat treatment: (1) solution treatment, (2) quenching, and (3) precipitation treatment . </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Precipitation Hardening
  24. 24. Sequence in Precipitation Hardening <ul><li>Solution treatment - alloy is heated to a temperature T s above the solvus line into the alpha phase region and held for a period sufficient to dissolve the beta phase </li></ul><ul><li>Quenching - to room temperature to create a supersaturated solid solution </li></ul><ul><li>Precipitation treatment - alloy is heated to a temperature T p , below T s , to cause precipitation of fine particles of the beta phase </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  25. 25. Surface Hardening <ul><li>Thermochemical treatments applied to steels in which the composition of the part surface is altered by adding various elements </li></ul><ul><li>Often called case hardening </li></ul><ul><li>Most common treatments are carburizing, nitriding, and carbonitriding </li></ul><ul><li>Commonly applied to low carbon steel parts to achieve a hard, wear‑resistant outer shell while retaining a tough inner core </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  26. 26. Carburizing <ul><li>Heating a part of low carbon steel in a carbon-rich environment so that C is diffused into surface </li></ul><ul><li>In effect the surface is converted to a high carbon steel, capable of higher hardness than the low‑C core </li></ul><ul><ul><li>Carburizing followed by quenching produces a case hardness of around HRC = 60 </li></ul></ul><ul><ul><li>Internal regions are low-C steel, with low hardenability, so it is unaffected by quench and remains relatively tough and ductile </li></ul></ul><ul><li>Most common surface hardening treatment </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  27. 27. Nitriding <ul><li>Treatment in which nitrogen is diffused into surface of special alloy steels to produce a thin hard casing without quenching </li></ul><ul><li>Carried out at around 500  C (950  F) </li></ul><ul><li>To be most effective, steel must have alloying ingredients such as aluminum or chromium to form nitride compounds that precipitate as very fine particles in the casing to harden the steel </li></ul><ul><li>Hardness up to HRC 70 </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  28. 28. Chromizing <ul><li>Requires higher temperatures and longer treatment times than the preceding hardening treatments </li></ul><ul><li>Usually applied to low carbon steels </li></ul><ul><li>Casing is not only hard and wear resistant; it is also heat and corrosion resistant </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  29. 29. Furnaces for Heat Treatment <ul><li>Fuel‑fired furnaces </li></ul><ul><ul><li>Normally direct‑fired - the work is exposed directly to combustion products </li></ul></ul><ul><ul><li>Fuels: natural gas or propane and fuel oils that can be atomized </li></ul></ul><ul><li>Electric furnaces </li></ul><ul><ul><li>Electric resistance for heating </li></ul></ul><ul><ul><li>Cleaner, quieter, and more uniform heating </li></ul></ul><ul><ul><li>More expensive to purchase and operate </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  30. 30. Batch vs. Continuous Furnaces <ul><li>Batch furnaces </li></ul><ul><ul><li>Heating system in an insulated chamber, with a door for loading and unloading </li></ul></ul><ul><ul><li>Production in batches </li></ul></ul><ul><li>Continuous furnaces </li></ul><ul><ul><li>Generally for higher production rates </li></ul></ul><ul><ul><li>Mechanisms for transporting work through furnace include rotating hearths and straight‑through conveyors </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  31. 31. Other Furnace Types <ul><li>Atmospheric control furnaces </li></ul><ul><ul><li>Desirable in conventional heat treatment to avoid excessive oxidation or decarburization </li></ul></ul><ul><ul><li>Include C and/or N rich environments for diffusion into work surface </li></ul></ul><ul><li>Vacuum furnaces </li></ul><ul><ul><li>Radiant energy is used to heat the parts </li></ul></ul><ul><ul><li>Disadvantage: time needed each cycle to draw vacuum </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  32. 32. Selective Surface Hardening Methods <ul><li>These methods heat only the work surface, or local areas of the work surface </li></ul><ul><li>They differ from surface hardening methods in that no chemical changes occur </li></ul><ul><li>Methods include: </li></ul><ul><ul><li>Flame hardening </li></ul></ul><ul><ul><li>Induction hardening </li></ul></ul><ul><ul><li>High‑frequency resistance heating </li></ul></ul><ul><ul><li>Electron beam heating </li></ul></ul><ul><ul><li>Laser beam heating </li></ul></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  33. 33. Flame Hardening <ul><li>Heating of work surface by one or more torches followed by rapid quenching </li></ul><ul><li>Applied to carbon and alloy steels, tool steels, and cast irons </li></ul><ul><li>Fuels include acetylene (C 2 H 2 ), propane (C 3 H 8 ), and other gases </li></ul><ul><li>Lends itself to high production as well as big components such as large gears that exceed the size capacity of furnaces </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  34. 34. Induction Heating <ul><li>Application of electromagnetically induced energy supplied by an induction coil to an electrically conductive workpart </li></ul><ul><li>Widely used for brazing, soldering, adhesive curing, and various heat treatments </li></ul><ul><li>When used for steel hardening treatments, quenching follows heating </li></ul><ul><li>Cycle times are short, so process lends itself to high production </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  35. 35. <ul><li>Figure 27.7 Typical induction heating setup. High frequency alternating current in a coil induces current in the workpart to effect heating. </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Induction Heating
  36. 36. High‑frequency (HF) Resistance Heating <ul><li>Used to harden specific areas of steel work surfaces by application of localized resistance heating at high frequency (400 kHz typical) </li></ul><ul><li>Contacts are attached to workpart at outer edges of the area </li></ul><ul><li>When HF current is applied, region under conductor is heated quickly to high temperature ‑ heating to austenite range typically takes less than a second </li></ul><ul><li>When power is turned off, area is quenched by heat transfer to the surrounding metal </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  37. 37. <ul><li>Figure 27.8 Typical setup for high‑frequency resistance heating. </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e High‑frequency Resistance Heating
  38. 38. Electron Beam (EB) Heating <ul><li>Electron beam focused onto small area, resulting in rapid heat buildup </li></ul><ul><li>Involves localized surface hardening of steel - high energy densities in a small region of part so that austenitizing temperatures can be achieved often in less than a second </li></ul><ul><li>When beam is removed, heated area is immediately quenched and hardened by heat transfer to surrounding metal </li></ul><ul><li>Disadvantage: best results are achieved when performed in a vacuum </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
  39. 39. Laser Beam (LB) Heating <ul><li>High‑density beam of coherent light focused on a small area ‑ the beam is usually moved along a defined path on the work surface </li></ul><ul><li>Laser - acronym for light amplification by stimulated emission of radiation </li></ul><ul><li>When beam is moved, area is immediately quenched by heat conduction to surrounding metal </li></ul><ul><li>Advantage of LB over EB heating is that laser beams do not require a vacuum </li></ul>©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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