Strengthening and Thermal Processing of Metals

1. Strengthening and Thermal Processing of Metals IMEMATS Section EC AY 2011 � 2012

2. Strengthening of Metals Materials engineers are often called on to design alloys having high strengths with some ductility and toughness. Ordinarily, ductility is sacrificed when an alloy is strengthened. Fortunately, several strengthening techniques are available.

3. Strengthening of Metals Important in understanding the strengthening mechanisms of metals is the relation between dislocation motion and mechanical behavior. The ability of a metal to plastically deform (e.g., become more ductile) depends on the ability of dislocations to move.

4. Strengthening of Metals Dislocation motion is analogous to the mode of locomotion employed by a caterpillar.

5. Representation of the analogy between caterpillar and dislocation motion

6. Strengthening of Metals By reducing the mobility of dislocations, mechanical strength may be enhanced. i.e., the greater the mechanical forces required to initiate plastic deformation. This makes the metal hard and strong.

7. Strengthening of Metals In contrast, the more unconstrained the dislocation motion, the greater the ease with which a metal may deform. This results in a softer and weaker, but more ductile, material.

8. Strengthening of Metals Virtually all strengthening techniques rely on a simple principle: Restricting or hindering dislocation motion renders a material harder and stronger.

9. Strengthening of Single-Phase Metal Alloys Grain size reduction Solid-solution alloying Strain hardening

10. Grain Size Reduction The size of the grains, or average grain diameter, in a polycrystalline metal, influences the mechanical properties of the material. Adjacent grains usually have different crystallographic orientations and, of course, a common grain boundary.

12. Grain Size Reduction During plastic deformation, slip or dislocation motion must take place across this common boundary. A fine-grained material (one that has small grains) is harder and stronger than one that is coarse grained, because the former has a greater total grain boundary area that impedes dislocation motion.

13. Grain Size versus Yield Strength For many materials, the yield strength ?y varies with grain size according to ?y = ?0 + kyd-1/2 In this expression, termed the Hall-Petch equation, d is the average grain diameter, and ?0 and ky are constants for a particular material.

15. Grain Size Grain size may be regulated by the rate of solidification from the liquid phase, and also by plastic deformation followed by an appropriate heat treatment, as will be discussed later. It should be noted that grain size reduction improves not only strength, but also the toughness of many alloys.

16. Solid-Solution Strengthening Another technique to strengthen and harden metals is alloying with impurity atoms that go into either substitutional or interstitial solid solution. Aptly, this is called solid-solution strengthening.

17. Solid-Solution Strengthening A solid solution is formed when atoms or ions of a guest element or compound are assimilated completely into the crystal structure of the host material. This is similar to the way salt or sugar in small concentrations dissolves in water.

18. Solid-Solution Strengthening High-purity metals are almost always softer and weaker than alloys composed of the same base metal. Increasing the concentration of the impurity results in an increase in tensile and yield strengths and a decrease in ductility.

22. Solid Solution Strengthening Alloys are stronger than pure metals because impurity atoms that go into solid solution ordinarily impose lattice strains on the surrounding host atoms. As a result, dislocation movement is restricted.

23. Solid Solution Strengthening For instance, an impurity atom smaller than a host atom for which it substitutes exerts tensile strains on the surrounding crystal lattice. Conversely, a larger substitutional atom imposes compressive strains in its vicinity.

26. Solid Solution Strengthening Solid solution strengthening explains why plain carbon steel is stronger than pure iron (Fe), and why alloys of copper containing small concentrations of Be are much stronger than pure Cu.

27. Strain Hardening Strain hardening is mechanism of increasing the strength of a material by deformation. Deliberately deforming a material increases the number of dislocations in it, thereby increasing the strength, since increasing the dislocation density increases the number of �stop signs� for dislocation motion.

28. Strain Hardening Strain hardening is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. Sometimes it is also called work hardening or, because the temperature at which deformation takes place is �cold� relative to the absolute melting temperature of the metal, cold working.

29. Strain Hardening By applying a stress that exceeds the original yield strength of the metallic material, we have strain hardened or cold worked the metallic material, while simultaneously deforming it.

30. Strain Hardening It is sometimes convenient to express the degree of plastic deformation as percent cold work rather than as strain: where A0 is the original area of the cross section that experiences deformation and Ad is the area after deformation.

31. Strain Hardening The yield and tensile strength of metals increase with increasing cold work. However, the price for this enhancement of hardness and strength is in the ductility of the material.

36. Strain Hardening Strain hardening is demonstrated with a stress-strain diagram: Initially, a metal with yield strength ?y0 is plastically deformed to point D. The stress is released, then reapplied, leading to a resultant new yield strength ?yi. The metal has thus become stronger during the process because ?yi is greater than ?y0.

38. Strain Hardening Thus, the imposed stress necessary to deform a metal increases with increasing cold work. Strain hardening is often utilized commercially to enhance the mechanical properties of metals during fabrication procedures. The effects of strain hardening may be removed by an annealing heat treatment.

39. Effect of heat on strengthening It should be noted that the strengthening effects due to grain size reduction and strain hardening can be eliminated or at least reduced by an elevated temperature treatment. Conversely, solid-solution strengthening is unaffected by heat treatment.

40. Recovery, Recrystallization and Grain Growth The microstructural and property changes due to plastically deforming a polycrystalline metal specimen can be reversed by appropriate heat treatments. Such restoration results from two different processes that occur at elevated temperatures: recovery and recrystallization, which may be followed by grain growth.

41. Recovery During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at an elevated temperature. In addition, physical properties like electrical and thermal conductivities are recovered to their precold-worked states.

42. Recrystallization Even after recovery is complete, the grains are still in a relatively high strain energy state. Recrystallization is the formation of a new set of strain-free and equiaxed grains (i.e., having approximately equal dimensions in all directions) that have low dislocation densities and are characteristic of the pre-cold worked condition.

46. Recrystallization In addition, the mechanical properties that were changed as a result of cold working are restored to their precold-worked values. That is, the metal becomes softer and weaker, yet more ductile.

47. Recrystallization The degree or fraction of recrystallization increases with time. Temperature also affects the recrystallization process.

49. Recrystallization The recrystallization behavior of a particular metal alloy is sometimes specified in terms of a recrystallization temperature, the temperature at which recrystallization just reaches completion in 1 hour.

50. Recrystallization The recrystallization temperature is typically between one-third and one-half of the absolute melting temperature of a metal or alloy, and depends on several factors, like the amount of prior cold work and the purity of the alloy.

51. Recrystallization Increasing the percentage cold work enhances the rate of recystallization, such that the recrystallization temperature is lowered, and approaches a constant or limiting value at high deformations.

53. Grain Growth After recrystallization, the strain-free grains will continue to grow if the metal specimen is left at the elevated temperature. This phenomenon is called grain growth.

54. Grain Growth An energy is associated with grain boundaries. As grains increase in size, the total boundary area decreases, yielding an attendant reduction in the total energy. This is the driving force for grain growth.

55. Grain Growth Grain growth occurs by the migration of grain boundaries. Large grains grow at the expense of small ones that shrink. Grain boundary motion is the short-range diffusion of atoms from one side of the boundary to the other. Grain boundary movement and atomic motion are opposite to each other.

57. Grain Growth Grain size depends on both time and temperature. Grain growth proceeds more rapidly as the temperature increases. This is due to the enhancement of diffusion rate with rising temperature.

59. Grain Growth The mechanical properties at room temperature of a fine-grained metal are usually superior (i.e., higher strength and toughness) to coarse-grained metals. If the grain structure of a single-phase alloy is coarser than that desired, refinement may be accomplished by plastically deforming the material, then subjecting it to a recystallization heat treatment.

60. Grain Growth Remember: The driving force for grain growth is the reduction in grain boundary area. Grain boundaries are defects and their presence causes the free energy of crystalline solids to increase. Thus, the thermodynamic tendency of polycrystalline materials is to transform into materials that have a larger average grain size.

61. Thermal Processing of Metals Thermal processing is commonly practiced for altering mechanical properties of metals. Annealing Heat treatment Precipitation hardening

62. Annealing Annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended period of time and then slowly cooled.

63. Purpose of Annealing Relieve stresses Increase softness, ductility and toughness Produce a specific microstructure

64. Stage of Annealing Heating to the desired temperature Holding or �soaking� at that temperature Cooling, usually to room temperature

65. Effect of Time on Annealing During annealing, temperature gradients exist between the outside and inside portions of the material. If the rate of temperature change is too great, temperature gradients and internal stresses may become significant and lead to warping or cracking.

66. Effect of Time on Annealing If annealing time is too short, the transformation reactions are not given enough time to go to completion.

67. Effect of Temperature on Annealing Annealing may be accelerated by increasing the temperature, because diffusional processes are normally involved.

68. Types of Annealing Process Annealing Stress Relief Normalizing Full Annealing Spheroidizing

69. Process Annealing It is used to soften and increase the ductility of a previously strain-hardened material. It is commonly used during fabrication procedures that require extensive plastic deformation, to allow a continuation of deformation without fracture or excessive energy consumption.

70. Internal Residual Stresses Internal residual stresses may develop in metal pieces due to Plastic deformation processes such as machining or grinding Non-uniform cooling of a piece that was fabricated or processed at an elevated temperature, such as a weld or a casting A phase transformation induced upon cooling wherein parent and product phases have different densities

71. Residual Stresses Distortion and warping may occur if residual stresses are not removed.

72. Stress Relief Internal residual stresses are relieved by �stress relief annealing� at a relatively low temperature such that effects from cold working and other heat treatments are not affected. The metal is held long enough to attain a uniform temperature, and finally cooled to room temperature in air.

73. Annealing of Ferrous Alloys Several annealing procedures are employed to enhance the properties of steel alloys. Refer to the phase diagram for steel on the next slide during the discussion.

76. Normalizing Steels that have been plastically deformed on purpose, say by a rolling operation, consist of pearlite and most likely a pro-eutectoid phase; the grains are irregularly shaped, relatively large, and vary substantially in size.

77. Normalizing Normalizing is used to refine the grains (i.e., decrease the average grain size) and produce a more uniform size distribution. Fine-grained pearlitic steels are tougher than coarse-grained ones.

78. Normalizing Normalizing is done by heating to at least 55oC above the upper critical temperature. After sufficient time, the alloy is completely transformed to austenite, a procedure termed austenizing. The treatment is terminated by cooling in air.

79. Full Annealing Full annealing is done by heating to at least 55oC above the upper critical temperature. The alloy is then �furnace cooled�: the furnace is turned off, and both furnace and steel cool to room temperature at the same rate, which takes several hours.

80. Full Annealing The microstructural product of this annealing is coarse pearlite (in addition to any proeutectoid phase) and is relatively soft and ductile. The full-anneal cooling procedure is time consuming; however, a microstructure having small grains and a uniform grain structure results.

81. Spheroidizing Medium- and high-carbon steels having a microstructure containing coarse pearlite may still be too hard to machine or plastically deform. In the spheroidizing of steels, there is a coalescence of Fe3C to form spheroid particles. Spheroidized steels have a maximum softness and ductility.

82. Ways of Spheroidizing Heating the alloy to just below the eutectoid (?700oC). If the precursor microstructure contains pearlite, spheroidizing times will ordinarily range between 15 and 25 h. Heating to just above the eutectoid, then either cooling very slowly in the furnace or holding at a temperature just below the eutectoid.

83. Ways of Spheroidizing Heating and cooling alternately within ?50oC of the eutectoid.

84. Heat Treatment of Steels The goal of heat treating steels is to produce martensitic steels. After heating, the specimen is quenched in either water, oil, or air. During quenching, the surface will cool more rapidly than interior regions. Thus, the austenite will transform over a range of temperatures, yielding a possible variation of microstructure and properties with position within a specimen.

85. Factors Affecting Heat Treatment Composition of the alloy Type and character of the quenching medium Size and shape of the specimen

86. Hardenability Hardenability is a term that describes the ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment. It is a qualitative measure of the rate at which hardness drops off with distance into the interior of a specimen as a result of diminished martensite content.

87. Hardenability A high hardenability means the steel alloy forms martensite not only at the surface but to a large degree throughout the entire interior.

88. Jominy End-Quench Test A cylindrical specimen 25.4 mm in diameter and 100 mm long is austenitized at a prescribed temperature for a prescribed time. After removal from a furnace, the specimen is quickly mounted in a fixture as shown in the next slide.

90. Jominy End-Quench Test The lower end is quenched by a jet of water of specified flow rate and temperature. Thus, the cooling rate is a maximum at the quenched end and diminishes with position from this point along the length of the specimen.

91. Jominy End-Quench Test After the piece has cooled to room temperature, shallow flats 0.4 mm deep are ground along the specimen length and Rockwell hardness measurements are made for the first 50 mm along each flat. A hardenability curve is produced when hardness is plotted as a function of position from the quenched end.

92. Hardenability Curves A typical hardenability curve shows the following characteristics: The quenched end cools most rapidly and exhibits maximum hardness (100% martensite) With diminishing cooling rate, more time is allowed for carbon diffusion and other microstructures are present in greater quantities, such as pearlite and bainite.

94. Hardenability and Continuous Cooling A correlation between position along the Jominy specimen and continuous cooling transformations can also be presented, as shown in the next slide.

96. Effect of Composition on Hardenability The effect of composition, in terms of the presence in steel of elements like nickel, chromium and molybdenum, is shown in the next slide. Alloying elements delay the austenite-to-pearlite and/or bainite reactions, thus permitting more martensite to form for a particular cooling rate, yielding greater hardness.

98. Effect of Composition on Hardenability Hardenability curves also depend on carbon content, as shown in the next slide. The hardness at any Jominy position increases with the concentration of carbon.

100. Batch Variability in Hardness During steel production, there is always a slight, unavoidable variation in composition and average grain size from one batch to another. To account for this, hardenability data is plotted as a band representing the maximum and minimum values that would be expected for a particular alloy.

102. Influence of Quenching Medium Water produces the most severe or rapid quenching, followed by oil, which is more effective than air. The degree of agitation of each medium also influences the rate of heat removal. Increasing the velocity of the quenching medium across the specimen surface enhances the quenching effectiveness.

103. Influence of Quenching Medium Oil quenches are suitable for heat treating of many alloy steels. In fact, for higher carbon steels, a water quench is too severe because cracking and warping may be produced. On the other hand, air cooling may be too slow. Air cooling of austenitized plain carbon steels ordinarily produces an almost totally pearlitic structure.

104. Influence of Specimen Geometry During quenching, heat energy must be transported to the surface of a specimen before it can be dissipated into the quenching medium. As a result, the cooling rate within and throughout the interior of a steel structure varies not only with position, but also with geometry and size.

107. Influence of Specimen Geometry The cooling rate (oC/s) is also expressed as equivalent Jominy distance (mm), because the data is often used in conjunction with hardenability curves.

108. Hardness Distribution Knowing the cooling rate as a function of geometry enables the prediction of the hardness distribution across the specimen in the transverse direction.

110. Surface Area to Mass Ratio Because heat energy is dissipated to the quenching medium at the specimen surface, the rate of cooling for a particular quenching treatment depends on the ratio of surface area to the mass of the specimen.

111. Surface Area to Mass Ratio The larger this ratio, the more rapid the cooling rate, and the consequently, the deeper the hardening effect. Irregular shapes with edges and corners have larger surface-to-mass ratios than regular and rounded shapes (e.g., spheres and cylinders) and are thus more amenable to hardening by quenching.

112. Effect of Geometry on Other Mechanical Properties The mechanical properties of steel specimens that have been quenched and subsequently tempered will also be a function of specimen diameter. This phenomenon is illustrated in the figure on the next slide.

116. Precipitation Hardening The formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix may occur by phase transformation induced by (or as a result of) heat treatment. Such a process is called precipitation hardening or age hardening or dispersion strengthening. The small particles of the new phase are termed precipitates.

117. Precipitation Hardening Precipitation hardening is accomplished by two different heat treatments: Solution heat treatment Precipitation heat treatment

118. Solution Heat Treatment In this stage, all solute atoms are dissolved to form a single-phase solid solution. The alloy (Co) is heated to a temperature (To) within a single-phase field. Then, the alloy is rapidly cooled (T1). The single-phase nature is retained since cooling was done quickly, resulting to a �supersaturated� solid solution that is metastable. At this point, the alloy is relatively soft and weak.

120. Precipitation Heat Treatment The supersaturated single-phase solid solution is heated again, this time to an intermediate temperature (T2). A precipitate phase then begins to form as finely dispersed particles. After this �aging� period, the alloy is cooled to room temperature.

122. Precipitation Heat Treatment The strength and hardness of the alloy, after this process, depends on both the precipitation temperature and the aging time at this temperature. For some alloys, aging occurs spontaneously at room temperature over extended periods of time.

123. Overaging With increasing time, the strength or hardness increases, reaches a maximum, and finally diminishes. This reduction in strength and hardness that occurs after long periods is known as overaging.

125. Mechanism of Aging The mechanism of aging is best illustrated by the aluminum-copper system.

127. Mechanism of Aging During the initial hardening stage, copper atoms cluster together in very small and thin discs. These clusters, called zones, in time and because of subsequent diffusion of copper atoms, become particles as they increase in size.

128. Mechanism of Aging These precipitate particles then pass through two transition phases (denoted as ?�� and ?�) before the formation of the equilibrium ? phase. Maximum strengthening and hardening coincide with the ?�� phase, which may be preserved upon cooling the alloy to room temperature. Overaging results from continued particle growth and the development of ?� and ? phases.

131. Mechanism of Aging The strengthening process is accelerated as the temperature is increased. Of course, associated with the increase in strength is a reduction in ductility.

134. Natural and Artificial Aging Some alloys experience appreciable hardening at room temperature and after relatively short periods of time. For example, several aluminum alloys used for rivets exhibit this behavior. These rivets are driven while soft, then allowed to harden at the normal ambient temperature. This is termed natural aging. Artificial aging, in contrast, is carried out at elevated temperatures.

135. Miscellaneous Considerations The usual sequence of strengthening processes is as follows: heat treatment ? cold working ? precipitation hardening

136. Miscellaneous Considerations If the alloy is precipitation hardened before cold working, more energy must be expended in its deformation; in addition, cracking may also result because of the reduction in ductility that accompanies the precipitation hardening.

137. Miscellaneous Considerations Most precipitation hardened alloys are limited in their maximum service temperatures. Exposure to temperatures at which aging occurs may lead to a loss of strength due to overaging.