Technology & Engineering
Strengthening of Metals
Strengthening of metals refers to the process of improving the mechanical properties of metals, such as hardness and toughness, to make them more suitable for specific applications. This can be achieved through various methods, including alloying, heat treatment, and cold working, which help to enhance the strength and performance of the metal without compromising its other properties.
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9 Key excerpts on "Strengthening of Metals"
- R. E. Smallman, R J Bishop(Authors)
- 1999(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 8Strengthening and toughening
8.1 Introduction
The production of materials which possess considerable strength at both room and elevated temperatures is of great practical importance. We have already seen how alloying, solute-dislocation interaction, grain size control and cold-working can give rise to an increased yield stress. Of these methods, refining the grain size is of universal application to materials in which the yield stress has a significant dependence upon grain size. In certain alloy systems, it is possible to produce an additional increase in strength and hardness by heat-treatment alone. Such a method has many advantages, since the required strength can be induced at the most convenient stage of production or fabrication; moreover, the component is not sent into service in a highly stressed, plastically deformed state. The basic requirement for such a special alloy is that it should undergo a phase transformation in the solid state. One type of alloy satisfying this requirement, already considered, is that which can undergo an order–disorder reaction; the hardening accompanying this process (similar in many ways to precipitation-hardening) is termed order-hardening. However, conditions for this form of hardening are quite stringent, so that the two principal hardening methods, commonly used for alloys, are based upon (1) precipitation from a supersaturated solid solution and (2) eutectoid decomposition.In engineering applications, strength is, without doubt, an important parameter. However, it is by no means the only important one and usually a material must provide a combination of properties. Some ductility is generally essential, enabling the material to relieve stress concentrations by plastic deformation and to resist fracture. The ability of materials to resist crack propagation and fracture, known generally as toughness, will be discussed in this chapter. Fracture can take many forms; some special forms, such as brittle fracture by cleavage, ductile fracture by microvoid coalescence, creep fracture by triple-point cracking and fatigue cracking, will be examined.
eBook - ePubPhysical Metallurgy
Principles and Design
- Gregory N. Haidemenopoulos(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
8Strengthening mechanisms
8.1 Introduction
Pure metals are soft materials. For most engineering applications they cannot provide the required strength. This is the reason alloys were invented. The strength of metals can be increased by alloying and by certain thermal, mechanical or thermomechanical processing. In the previous chapter we have seen that the plastic deformation of metals proceeds by the glide of dislocations, a process we called slip. Strengthening can be achieved by creating obstacles to dislocation glide. It is these obstacles that are created with the alloying and suitable processing discussed above. The strengthening mechanisms, which is the subject of the present chapter, are simply mechanisms of interaction between dislocations and various obstacles. The major mechanisms are: lattice resistance, strain hardening, solid solution strengthening, grain boundary strengthening and precipitation strengthening. In lattice resistance, the main obstacle to dislocation glide is the crystal lattice, the atomic bonds in particular. In strain hardening the main obstacle impeding dislocation glide is other dislocations. The interaction between dislocations forms sessile dislocations, which cannot contribute to plastic deformation. Strain hardening has been discussed in detail in the previous chapter and will not be considered further. In solid solution strengthening the obstacles are solute atoms, either substitutional or interstitial, the strain field of which interacts with dislocations. In grain boundary strengthening, the obstacles are high-angle grain boundaries, but also sub-boundaries and interfaces as the interfaces between ferrite and cementite in pearlite or the interfaces between martensite laths. Finally, in precipitation strengthening, the obstacles are precipitates, second-phase particles or intermetallic compounds formed during thermal processing. In most alloys the yield strength is the result of a superposition of strengthening mechanisms. The effectiveness of each mechanism is characterized by the specific obstacle strength, which is related to the stress required to overcome the obstacles at T = 0K
eBook - ePub- Adrian P Mouritz(Author)
- 2012(Publication Date)
- Woodhead Publishing(Publisher)
The chapter explains the fundamental engineering science behind the development of high-strength metals for use in weight-efficient aircraft structural components. To understand the Strengthening of Metals, it is necessary to have a basic understanding of the arrangement of atoms in metals. A brief description of the crystal structures of the metals used in aircraft is provided. Following this is an overview of the various imperfections in the crystal structure which affect the strength of metals. The mechanisms by which these imperfections increase the strength of metals are outlined. The Strengthening of Metals by dislocations is explained. Dislocations are central to all the main strengthening mechanisms in metals, and what they are and how they increase strength is described. The Strengthening of Metals by precipitation hardening, intermetallic compounds, and control of the grain structure is explained. The information in this chapter provides a basic understanding of the various ways that aerospace engineers can increase the strength of metal alloys for structural applications. 4.2 Crystal structure of metals The atoms in solid metals are arranged in an ordered and repeating lattice pattern called the crystal structure. A crystalline material consists of a regular array of atoms that is repeated over a long distance compared with the atomic size. A simple analogy is the stacking of oranges in a grocery store, with each orange representing a single atom and each layer of oranges being a lattice plane (Fig. 4.2). The arrangement pattern of the atoms is defined by the unit cell of the crystal. The unit cell is the basic building block, having the smallest repeatable structure of the crystal, and it contains a full description of the lattice structure
- J. T. Black, Ronald A. Kohser(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
Castings that have been cooled too rapidly can possess a segregated solidification structure known as coring (discussed more fully in Chapter 4). Homogeniza- tion can be achieved by heating to moderate temperatures and then holding for a sufficient time to allow thorough diffusion to take place. Similarly, heating for several hours at relatively low temperatures can reduce the internal stresses that are often produced by forming, welding, or brazing. Recrystallization (dis- cussed in Chapter 3) is a function of the particular metal, the amount of prior deformation, and the desired recrystallization time. In general, the more a metal has been strained, the lower the recrystallization temperature or the shorter the time. With- out prior straining, however, recrystallization will not occur, and heating will only produce undesirable grain growth. 5.3 Heat Treatments Used to Increase Strength Six major mechanisms are available to increase the strength of metals: 1. Solid-solution strengthening 2. Strain hardening 3. Grain-size refinement °C °F 1000 900 800 700 600 500 Temperature 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Weight percent carbon 1800 1600 1400 1200 1000 Process anneal Spheroidizing anneal Normalizing Full annealing and hardening A 1 A 3 A cm FIGURE 5.2 Graphical summary of the process heat treatments for steels on an equilibrium diagram. 70 CHAPTER 5 Heat Treatment 4. Precipitation hardening 5. Dispersion hardening 6. Phase transformations Although all of these might not be applicable to a given metal or alloy, these heat treatments can often play a significant role in inducing or altering the final properties of a product. In solid-solution strengthening, a base metal dissolves other atoms, either as substitutional solutions, where the new atoms occupy sites in the host crystal lattice, or as interstitial solutions, where the new atoms squeeze into “holes” between the atoms of the base lattice.
- J. T. Black, Ronald A. Kohser(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
In solid-solution strengthening, a base metal dissolves other atoms, either as substitutional solutions, where the new atoms occupy sites in the host crystal lattice, or as interstitial solutions, where the new atoms squeeze into “holes” between the atoms of the base lattice. The amount of strengthening depends on the amount of dissolved solute and the size difference of the atoms involved. Because distor- tion of the host structure makes dislocation movement more difficult, the greater the size difference, the more effective the addition. Strain hardening (discussed in Chapter 3) produces an increase in strength by means of plastic deformation under cold-working conditions. Because grain boundaries act as barriers to dislocation motion, a metal with small grains tends to be stronger than the same metal with larger grains. Thus grain-size refinement can be used to increase strength, except at elevated tempera- tures, where grain growth can occur and grain boundary dif- fusion contributes to creep and failure. It is important to note that grain-size refinement is one of the few processes that can improve strength without a companion loss of ductility and toughness. In precipitation hardening, or age hardening, strength is obtained from a nonequilibrium structure that is produced by a three-step heat treatment. Details of this method will be provided in the following section. Strength obtained by dispersing second-phase particles throughout a base material is known as dispersion harden- ing. To be effective, the dispersed particles should be stronger than the matrix, adding strength through both their reinforcing action and the additional interfacial surfaces that present bar- riers to dislocation movement. Phase transformation strengthening involves those alloys that can be heated to form a single phase at elevated temperature and subsequently transform to one or more low temperature phases on cooling.
eBook - PDF- Mikell P. Groover(Author)
- 2012(Publication Date)
- Wiley(Publisher)
Part VI Property Enhancing and Surface Processing Operations 20 HEAT TREATMENT OF METALS Chapter Contents 20.1 Annealing 20.2 Martensite Formation in Steel 20.2.1 The Time-Temperature- Transformation Curve 20.2.2 The Heat Treatment Process 20.2.3 Hardenability 20.3 Precipitation Hardening 20.4 Surface Hardening The manufacturing processes covered in the preceding chap- ters involve the creation of part geometry. We now consider processes that either enhance the properties of the workpart (Chapter 20) or apply some surface treatment to it, such as cleaning or coating (Chapter 21). Property-enhancing oper- ations are performed to improve mechanical or physical properties of the work material. They do not alter part geometry, at least not intentionally. The most important property-enhancing operations are heat treatments. Heat treatment involves various heating and cooling procedures performed to effect microstructural changes in a material, which in turn affect its mechanical properties. Its most common applications are on metals, discussed in this chapter. Similar treatments are performed on glass-ceramics (Section 2.2.3), tempered glass (Section 7.3.1), and powder metals and ceramics (Sections 10.2.3 and 11.2.3). Heat treatment operations can be performed on a metallic workpart at various times during its manufacturing sequence. In some cases, the treatment is applied before shaping (e.g., to soften the metal so that it can be more easily formed while hot). In other cases, heat treatment is 480 used to relieve the effects of strain hardening that occur during forming, so that the material can be subjected to further deformation. Heat treatment can also be accom- plished at or near the end of the sequence to achieve the final strength and hardness required in the finished product. The principal heat treatments are annealing, martensite formation in steel, precipitation hardening, and surface hardening.
eBook - PDF- Cemil Hakan Gur, Jiansheng Pan, Cemil Hakan Gur, Jiansheng Pan(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
The process of annealing can be divided into recovery, recrystallization, and grain growth. Recovery is usually de fi ned as the restoration of the mechanical properties of the cold-worked metal without observable change in microstructure. Recrystallization is the replacement of the cold-worked structure by a new set of strain-free grains. Recrystallization is evidenced by a decrease in strength and an increase in ductility. The density of dislocations decreases considerably on recrystallization, and all effects of strain hardening are eliminated. Some of the grains of a fi ne-grained recrystallized metal can begin to grow rapidly at a higher temperature. It is called the secondary recrystallization [5]. Very good results in metal strengthening can be achieved by the combination of metal strengthening by microstructural changes in conjunction with strain hardening. Very high strengths can be achieved by ausforming, that is, thermal-mechanical processes in which martensite is formed from an austenitic matrix, which had been previously strengthened by plastic deformation. The dislocation density of ausformed martensite is very high (10 13 cm 2 ), and the dislocations are usually uniformly distributed and the in fl uence of precipitation contribution to the strength of ausformed martensite is more intensive than in ordinary quenched martensite. Strong alloys are those in which the particles are formed in dense dislocation cell structures of the deformed matrix. Very strong alloys are produced by combining the effects of dispersion and strain hardening [4,5]. 4.4.8 T OUGHENING M ECHANISMS Generally, an inverse relation exists between strength and toughness, but some micromechanisms exist for simultaneously increasing strength and toughness. It could be achieved by processes such as proper alloy chemistry, melting practice, suitable microstructure and phase distribution, and microstructure re fi nement.
eBook - PDFFundamentals of Modern Manufacturing
Materials, Processes, and Systems
- Mikell P. Groover(Author)
- 2019(Publication Date)
- Wiley(Publisher)
592 P A R T VII 26 PROPERTY ENHANCING AND SURFACE PROCESSING OPERATIONS The manufacturing processes in the preceding chapters involve the creation of workpiece geometry. This part of the book covers processes that either enhance the properties of a work part (this chapter) or apply some surface treatment to it, such as clean- ing or coating (Chapter 27). Property-enhancing operations are performed to improve mechanical or physical properties of the work material. They do not alter part geometry, at least not inten- tionally. The most important property-enhancing operations are heat treatments. Heat treatment consists of various heating and cooling procedures performed to effect microstructural changes in a material, which in turn affect its mechanical properties. Its most common applications are on metals, discussed in this chapter. Similar treatments are performed on glass-ceramics (Section 7.4.3), tempered glass (Section 12.3.1), and powder metals and ceramics (Sections 15.3.3 and 16.2.3). Heat treatment operations can be performed on a metallic work part at various times during its manufacturing sequence. In some cases, the treatment is applied before shaping (e.g., to soften the metal so that it can be more easily formed while hot). In other cases, heat treatment is used to relieve the effects of strain hardening that occur during forming, so that the material can be sub- jected to further deformation. Heat treatment can also be accomplished at or near the end of the sequence to achieve the final strength and hardness required in the finished product. The principal heat treatments are annealing, martensite formation in steel, precipitation hardening, and surface hardening. 26.1 Annealing Annealing consists of heating the metal to a suitable temperature, holding at that temperature for a certain time (called soaking), and slowly cooling.
eBook - PDFFundamentals of Modern Manufacturing
Materials, Processes, and Systems
- Mikell P. Groover(Author)
- 2016(Publication Date)
- Wiley(Publisher)
VII P A R T 595 26 PROPERTY ENHANCING AND SURFACE PROCESSING OPERATIONS Heat Treatment of Metals The manufacturing processes in the preceding chapters involve the creation of workpiece geometry. This part of the book covers processes that either enhance the properties of a work part (this chapter) or apply some surface treatment to it, such as cleaning or coating (Chapter 27). Property-enhancing oper- ations are performed to improve mechanical or physical prop- erties of the work material. They do not alter part geometry, at least not intentionally. The most important property-enhancing operations are heat treatments. Heat treatment involves various heating and cooling procedures performed to effect microstruc- tural changes in a material, which in turn affect its mechan- ical properties. Its most common applications are on metals, discussed in this chapter. Similar treatments are performed on glass-ceramics (Section 7.4.3), tempered glass (Section 12.3.1), and powder metals and ceramics (Sections 15.3.3 and 16.2.3). Heat treatment operations can be performed on a metallic work part at various times during its manufacturing sequence. In some cases, the treatment is applied before shaping (e.g., to soften the metal so that it can be more easily formed while hot). In other cases, heat treatment is used to relieve the effects of strain hardening that occur during forming, so that the material can be subjected to further deformation. Heat treatment can also be accomplished at or near the end of the sequence to achieve the final strength and hardness required in the finished product. The principal heat treat- ments are annealing, martensite formation in steel, precipitation hardening, and surface hardening.
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