Introduction to the Fundamentals of Corrosion

 Introduction to the Fundamentals of Corrosion

THE SIGNIFICANT TECHNICAL CHALLENGES and the high cost directly related to corrosion provide
strong incentives for engineers and other technical personnel to develop a firm grasp on the fundamental bases
of corrosion. Understanding the fundamentals of corrosion is necessary not only for identifying corrosion
mechanisms (a significant achievement by itself), but also for preventing corrosion by appropriate corrosion
protection means and for predicting the corrosion behavior of metallic materials in service conditions.
Understanding the mechanisms of corrosion is the key to the development of a knowledge-based design of
corrosion resistant alloys and to the prediction of the long-term behavior of metallic materials in corrosive
environments.
Two major areas are usually distinguished in the corrosion of metals and alloys. The first area is where the
metal or alloy is exposed to a liquid electrolyte, usually water, and thus typically called aqueous corrosion. The
second area is where corrosion takes place in a gaseous environment, often called oxidation, high- temperature
oxidation, or high-temperature corrosion, and called gaseous corrosion here. These two areas have been (and
still are sometimes) referred to as wet corrosion and dry corrosion. This distinction finds its origin (and its
justification) in some fundamental differences in the mechanisms, in particular the electrochemical nature of
reactions occurring in aqueous solution (or in a nonaqueous electrolyte), as compared to the formation of thick
oxide layers in air or other oxidizing atmospheres, at high temperature with fast transport processes by solidstate
diffusion through a growing oxide. The separation between the two areas, however, should not be
overemphasized, because there are also similarities and analogies, for example:
· The initial stages of reaction involve the adsorption of chemical species on the metal surface that can be
described by the Gibbs equation for both liquid and gaseous environments.
· The nucleation and growth phenomena of oxide layers and other compounds
· The use of surface analytical techniques
The fundamental aspects of aqueous and gaseous corrosion are addressed in this first Section of the Handbook.
Corrosion of metallic materials is generally detrimental and must be prevented, but if it is well understood and
controlled, it can also be used in a powerful and constructive manner for electrochemical production of fine
patterns on metal as well as on semiconductor surfaces. These constructive purposes also include
electrochemical machining (down to the micro- or even the nanoscale), electrochemical and chemicalmechanical
polishing, and anodes for batteries and fuel cells. These topics are also addressed in this Section.

THE DRIVING FORCE of corrosion is the lowering of energy associated with the oxidation of a metal.

Thermodynamics examines and quantifies this driving force. It predicts if reactions can or cannot occur (i.e., if

the metal will corrode or be stable). It does not predict at what rate these changes can or will occur: this is the

area of kinetics. However, knowing from thermodynamics what reactions are possible is a necessary step in the

attempt to understand, predict, and control corrosion.

Ions that are present in the solution are charged because of the loss or gain of electrons. The positive charged

ions (cations) and negative charged ions (anions) also have an electric field associated with them. The solvent

(water) molecules act as small dipoles; therefore, they are also attracted to the charged ions and align

themselves in the electric field established by the charge of the ion. Because the electric field is strongest close

to the ion, some water molecules reside very close to an ionic species in solution. The attraction is great enough

that these water molecules travel with the ion as it moves through the solvent. The tightly bound water

molecules are referred to as the primary water sheath of the ion. The electric field is weaker at distances outside

the primary water sheath, but it still disturbs the polar water molecules as the ion passes through the solution.

The water molecules that are disturbed as the ion passes, but do not move with the ion, are usually referred to as

the secondary water sheath. Figure 1 shows a representation of the primary and secondary solvent molecules for

a cation in water. Because of their smaller size relative to anions, cations have a stronger electric field close to

the ion and more water molecules are associated in their primary water sheath. Anions have few, if any,

primary water molecules. A detailed description of the hydration of ions in solution is given in Ref 1

ONE OF THE IMPORTANT FEATURES of the electrified interface between the electrode and the electrolyte

in the aqueous corrosion of metals is the existence of a potential difference across the double layer, which leads

to the definition of the electrode potential. The electrode potential is one of the most important parameters in

both the thermodynamics and the kinetics of corrosion. The fundamentals of electrode potentials are discussed

in this article. Examples of the calculations of the potential at equilibrium are given in the article “Potential

versus pH (Pourbaix) Diagrams” in this Section of the Volume.

electrodes are left unchanged. Hence, the magnitude and the sign of the cell voltage at equilibrium (emf)

depend only on the couple of half-reactions involved. From the thermodynamic convention, the free energy

change of a spontaneous cell reaction, which liberates energy, is negative. If the emf is the potential of

electrode 2 minus the potential of electrode 1 (ΔE = E2 - E1), and if ΔrG designates the free energy of the cell

reaction written in the sense of Eq 13, that is, reduction at electrode 2 (Eq 12) and oxidation at electrode 1 (Eq

11), the relation between ΔrG and ΔE is:

ΔrG = -nFΔE (Eq 14)

where n is the number of electrons exchanged in both half-reactions, and F is the Faraday constant (96,487

coulombs) equal to the charge of 1 mole of electrons.

Using Eq 3 and 4, Eq 14 may be rewritten as:

(Eq 15)

Compared to a chemical equilibrium where, at a given temperature, the ratio is equal to a

constant Keq (Eq 6), the electrochemical cell at equilibrium is a system with one more degree of freedom,

because either the ratio of activities or the cell emf can impose on it.

If the cell reaction occurs under conditions in which the reactants and products are in their standard states, the

equation becomes:

(Eq 16)

where ΔE0 is the standard cell emf.

The Hydrogen Potential Scale. The absolute potential of an electrode, or even the potential difference between a

metal electrode and the surrounding solution, cannot be determined experimentally. It is only possible to

measure the voltage across an electrochemical cell, that is, the difference of potential between two identical

wires connected to two electrodes. A potential scale may be defined by measuring all electrode potentials with

respect to an electrode of constant potential, called the reference electrode. The reference electrode arbitrarily

chosen to establish a universal potential scale is the standard hydrogen electrode (SHE). It consists of a

platinized platinum electrode (wire or sheet) immersed in an aqueous solution of unit activity of protons,

saturated with hydrogen gas at a fugacity of 1 bar. The half-cell reaction is the equilibrium:

H+(aq) + e- H2(g) (Eq 17)

The SHE possesses the advantages of achieving its equilibrium potential quickly and reproducibly and

maintaining it very stable with time (see comparison with other reference electrodes in the article “Potential

Measurements with Reference Electrodes” in this Volume). From the convention, the SHE potential is taken as

zero. The potential of any electrode can then be determined with respect to this zero reference and is called the

potential of the electrode on the standard hydrogen scale, denoted E(SHE).

The Potential Sign Convention (Reduction Convention). Before establishing tables of standard potentials for

various electrodes on the standard hydrogen scale, it is necessary to fix the convention for the sign of a standard

potential value (E0). The potential of any electrode is expressed with respect to the SHE by building (really or

virtually) a cell in which the other electrode is a SHE. Consider a typical half-cell reaction with an oxidationreduction

(O/R) couple, where O represents the oxidized species and R the reduced species. The cell is

represented as:

Pt, H2(g)(f = 1 bar)|H+(aq)(a = 1) || O/R (Eq 18)

Depending on the position of the O/R electrode on the hydrogen scale, the spontaneous single electrode or halfcell

reaction will proceed in one direction or the other: R oxidation: cR → bO + ne- combined with proton

reduction: H+(aq) + e- → H2(g); or, O reduction: bO + ne- → cR combined with hydrogen oxidation: H2(g)

→ H+(aq) + e-.

For example, the coupling of the oxidation/ reduction couple Fe2+/Fe with the H+/H2 couple brings about the

spontaneous oxidation of iron (the free energy of the cell reaction is negative, the Fe2+/Fe electrode is

negative). The situation is entirely different with a Cu2+/Cu system. If coupled with the H+/H2 couple, the

Potential Measurements with Reference Electrodes

ELECTRODE POTENTIAL MEASUREMENT is an important aspect of corrosion studies and corrosion

prevention. It is included in any determination of the corrosion rate of metals and alloys in various

environments and in the control of the potential in cathodic and anodic protection. The potential of an electrode

can be determined only by measuring the voltage in an electrochemical cell between this electrode and an

electrode of constant potential, called the reference electrode. Many errors and problems can be avoided by

careful selection of the best reference electrode for a specific case and by knowledge of the electrochemical

principles that control the potential measurements in order to obtain meaningful measurements. A reference

electrode, once selected, must be properly used, taking into account the stability of its potential and the problem

of ohmic (IR) drop. Many different reference electrodes are available, and others can be designed by the users

themselves for particular situations. Each electrode has its characteristic rest potential value, which is used to

convert potential values measured with respect to this reference into values expressed with respect to other

references. In particular, the conversion of the potentials from or to the hydrogen scale is frequently required

for use of potential- pH diagrams, which are discussed in the article “Potential versus pH (Pourbaix) Diagrams”

in this Section of the volume.

The Three-Electrode Device

When a system is at rest and no significant current is flowing, the use of only one other electrode as a reference

is sufficient to measure the test (or working) electrode potential versus the reference potential. When a current

is flowing spontaneously in a galvanic cell or is imposed on an electrolytic cell, reactions at both electrodes are

not at equilibrium, and there is consequently an overpotential on each of them. The potential difference

measured between the two electrodes then includes the value of the two overpotentials, and it is not possible to

determine the potential of the test electrode. To obtain this value, a third electrode, the auxiliary or

counterelectrode, must be used (Fig. 1). In this arrangement, current flows only between the test and the

auxiliary electrodes. A high-impedance voltmeter placed between the test and the reference electrodes prevents

any significant current flow through the reference electrode, which then shows a negligible overpotential and

remains very close to its rest potential. Most reference electrodes can be damaged by current flow. The test

electrode potential and its changes under electric current flow can then be measured with respect to a fixed

reference potential. The three- electrode system is widely used in laboratory and field potential measurement.

Operating Conditions for Reference Electrodes

When a reference electrode has been selected for a particular application, its proper use requires caution and

specific measurement conditions. When measuring the potential of a polarized test electrode versus a reference

electrode, it is important not to polarize or damage the latter by applying a significant current density. Also, the

ohmic (IR) drop must be minimized.

Very Low Current Density. It is important to use a reference electrode that operates at its known open-circuit

potential and thus avoid applying any significant overpotential to it. This is achieved by using a highimpedance

voltmeter that has a negligible input current and, for test electrode polarization measurements, by

using an auxiliary electrode in a three-electrode system (Fig. 1).

The value tolerated for the maximum overpotential on the reference electrode, at the condition that it stays

under the limit over which the electrode suffers irreversible damages (like the calomel electrode), is a matter of

judgment that depends on the accepted magnitude of error in the particular case under investigation. The use of

an electrometer or a high-impedance voltmeter (1012 ohms) fulfills the usual requirements. When a lower

impedance instrument is used, an unacceptable overpotential could result if the electrode is too polarizable.

The IR Drop and Its Mitigation. The IR drop is an ohmic voltage that results from electric current flow in ionic

solutions. Electrolytes have an ohmic resistance; when a current passes through them, an IR voltage can be

observed between two distinct points. When the reference electrode is immersed at some distance from a

working or test electrode, it is in the electric field somewhere along the current path. An electrolyte resistance

exists along the path between the test and the reference electrodes. As current flows through that path, an IR

voltage appears in the potential measurement according to:

V = VT - VR + IR (Eq 13)

where VT is the test potential to be measured, VR is the reference electrode potential, and IR is the ohmic drop.

In this case, the liquid junction potential has been neglected. The IR drop constitutes a second unknown value in

a single equation. It must be eliminated or minimized.

The Luggin capillary is a tube, usually made of glass, that has been narrowed by elongation at one end. The

narrow end is placed as close as possible to the test electrode surface (Fig. 1), and the other end of the tube goes

to the reference electrode compartment. The Luggin capillary is filled with cell electrolyte, which provides an

electric link between the reference and the test electrode. The use of a high-impedance voltmeter prevents

significant current flow into the reference electrode and into the capillary tube between the test electrode and

the reference electrode compartment (Fig. 1). This absence of current eliminates the IR drop, and the

measurement of VT is then possible. A residual IR drop may, however, exist between the tip of the Luggin

capillary and the test electrode. This is usually negligible, however, especially in high-conductivity media.

The remote electrode technique can be used only for measurement in an electrolyte with very low resistivity,

usually in the laboratory. It is applicable, for example, in a molten salt solution, in which the ohmic resistance R

is very small. In such a case, the reference electrode can be placed a few centimeters away from the test

electrode, because the IR drop remains negligible. In other electrolytes (for example, in measurements in soils),

the ohmic resistance is rather large, and the IR drop cannot be eliminated in this manner.

The Current Interruption Technique. In this case, when the current is flowing, the IR drop is included in the

measurement. A recording of the potential is shown in Fig. 5. At time t1, the current is interrupted so that I = 0

and IR = 0.

At the moment of the interruption, however, the electrode is still polarized, as can be seen at point P in Fig. 5.

The progressive capacitance discharge and depolarization of the test electrode take some time. The potential

measured at the instant of interruption then represents the test electrode potential corrected for the IR drop.

Precise measurements of this potential are obtained with an oscilloscope.

Potential Conversion Between Reference Electrodes. Due to the number of different reference electrodes used,

each potential measurement must be accompanied by a clear statement of the reference used. It is often needed

to express electrode potentials versus a particular reference, regardless of the actual reference used in the

measurement. The procedure is illustrated in the following example. The electrode potential of a buried steel

pipe is measured with respect to a CuSO4/Cu electrode, and the value is -650 mV for a pH 4 environment. If

that value is mistakenly placed in the iron E-pH (Pourbaix) diagram (Fig. 1 in the article “Potential versus pH

(Pourbaix) Diagrams” in this Volume), it could be concluded that corrosion will not occur. This conclusion,

however, would be incorrect, because the E-pH (Pourbaix) diagrams are always computed with respect to the

SHE. It is then necessary to express the measured electrode potential with respect to the SHE before consulting

the E-pH (Pourbaix) diagram. The CuSO4/Cu electrode potential is +310 mV versus SHE, so this value must be

added to the measured potential: ESHE = -650 + 310 = -340 mV.

Calculation and Construction of E-pH Diagrams

Potential-pH diagrams are based on thermodynamic calculations. The equilibrium lines that set the limits

between the various stability domains are calculated for the various electrochemical or chemical equilibria

between the chemical species considered. There are three types of reactions to be considered:

· Electrochemical reactions of pure charge (electron) transfer

· Electrochemical reactions involving both electron and solvated proton (H+) transfer

· Acid-base reactions of pure H+ transfer (no electrons involved)

Pure Charge Transfer Reactions. These electrochemical reactions involve only a reduced species on one side

and an oxidized species and electrons on the other side. They have no solvated protons (H+) as reacting

particles; consequently, they are not influenced by pH. An example of a reaction of this type is the oxidation/

reduction of Ni/Ni2+: Ni Ni2+ + 2e-. From the Nernst equation (Eq 1), the equilibrium potential for the

couple Ni2+/Ni can be written

Molten Salt Corrosion Thermodynamics

MOLTEN SALTS—in contrast to aqueous solutions in which an electrolyte (acid, base, salt) is dissolved in a

molecular solvent—are essentially completely ionic. Thus the terms solute and solvent can be defined only in

quantitative terms. For example, the terms lose their meaning in a NaCl-AgCl melt where composition can vary

continuously from pure NaCl to pure AgCl. This is true even when the electrodes immersed in the melt are

reversible only to some of the ions in the melt. For example, in the cell:

Ag|AgCl|1NaCl|Cl2 (Eq 1)

the chlorine electrode is reversible to Cl- and the silver electrode is reversible to Ag+. When this cell is used to

obtain thermodynamic data, it is assumed that the cell is stable; that is, its composition does not change with

time. However, when the concentration of AgCl is very low, this will not be the case, since the silver electrode

will react spontaneously with the melt:

Ag + NaCl = AgCl + Na (Eq 2)

Thus the concentration of AgCl will spontaneously increase in the melt, and the electromotive forces (emf)

measured for the preceding cell will not be stable but will change in the direction indicating increasing Ag

concentration. The point at which this happens depends on the system.

Thermodynamics of Cells

One major use of electrochemical cells is to obtain thermodynamic data for salts. The basic thermodynamics

applicable to galvanic cells for aqueous solutions is discussed elsewhere in this Volume. Only those aspects that

are different for molten salts are emphasized in this article. Thus, for the cell given in Eq 1, the cell reactions

Methods for Determining Aqueous Corrosion
Reaction Rates.

CORROSION OF MATERIALS IN AQUEOUS SOLUTIONS is often thermodynamically possible but

kinetically limited. Therefore, it is important to determine the rates of corrosion processes. Corrosion rate

determination can serve many engineering and scientific purposes. For example, it can be used to:

· Screen available materials to find the most resistant material for a given application.

· Determine operating conditions where corrosion rates are low versus those where rates are high, by

varying conditions.

· Determine probable service lifetimes of materials forming components, equipment, and processes.

· Evaluate new alloys or treatments or existing alloys in new environments.

· Evaluate lots, heats, or treatments of materials to ensure that specified quality is achieved before release,

shipment, or acceptance.

· Evaluate environmental conditions such as new chemical species, inhibitors, or plant- operation

conditions such as temperature excursions.

· Determine the most economical means of reducing corrosion through use of inhibitors, pretreatments,

coatings, or cathodic protection.

· Determine the relative corrosivity of one environment compared to another.

· Study corrosion mechanisms.

Methods for determination of corrosion rates can be differentiated between those that measure the cumulative

results of corrosion over some period of time and those that provide instantaneous rate information. Corrosion

rates do not often increase monotonically with environmental conditions but exhibit sharp thresholds that

distinguish regions of low corrosion rates from other regions where corrosion rates are dangerously high. It is

sometimes of greater interest to define these thresholds than it is to determine the rates in the regions where

corrosion rates are high. Examples of the latter are pitting or crevice corrosion where passive films are broken

down and local corrosion rates can be extremely high. This article addresses electrochemical methods for

instantaneous rate determination and threshold determination as well as nonelectrochemical methods that can

determine incremental or cumulative rates of corrosion.

Fundamentals of Corrosion in Gases..

ENGINEERING MATERIALS are subject to deterioration when exposed to high-temperature environments.

Whether they survive or not in technological applications depends on how fast they react. The rate of corrosion

varies widely; some intermetallics (β-NiAl) react extremely slowly. Some metals (Fe) oxidize very rapidly,

whereas other metals (Cr, Co) react relatively slowly. From the chemical point of view, the gas- metal reactions

represent a broad class of heterogeneous reactions. The composition and structure of the scales produced on

metals are a key factor in their behavior in technical applications.

Historically, corrosion in gases has been primarily a problem in combustion systems. Thus, the gas-metal

reactions are usually referred to as oxidation in its broad chemical sense, whether the reaction is with pure

oxygen, water, sulfur dioxide (SO2), or whatever the gas might be. The corrosion product (oxide layer) is

termed scale. Being the corrosion product, the protective properties of scale decrease the reaction rate. The

concepts and methods developed to understand gas-metal reactions can be used to describe any arbitrary gas solid reaction at high temperature; for example, oxidation of the silicone carbide. The high temperature

corrosion is a highly technical challenge; the reason for this is that the efficiency of thermal processes and

engines increases with operating temperature. Such high- temperature service is especially damaging to most

metals because of the exponential increase of reaction rate with temperature. In most cases, corrosion resistance

at high temperatures does not accompany the good mechanical properties of structural materials; therefore,

protective coatings must be applied.

Electrochemical principles are insufficient to understand the mechanism of oxidation. For gaseous reactions, a

basic understanding of the diffusion processes is much more profitable. The first results of a high-temperature

corrosion study (not yet defined as corrosion and even diffusion) were published in 1684 by Boyle in

Experiments and Considerations about the Porosity of Bodies in Two Essays. In studying reactive diffusion in

the Cu-S system, Boyle reported the observation of interaction between copper and sulfur through examination

of metallographic cross sections. Electrochemistry and aqueous corrosion principles were developed at the

beginning of the 19th century. In 1855 Fick formulated the basic principles of diffusion in solids. The

systematic study of high-temperature oxidation began in the 1920s. In 1933 Wagner published his pioneering

paper on gas corrosion of metals. The first journal devoted to corrosion in gases, Oxidation of Metals, was

published in the 1960s.

The following articles introduce the subject of gas corrosion to professional engineers and students. A brief

summary of thermodynamic concepts is followed by an explanation of the defect structure of solid oxides and

the effect of these defects on the rate of mass transport. Commonly observed kinetics of oxidation are described

and related to the observed corrosion mechanisms, as illustrated in Fig. 1 of the next article, “Thermodynamics

of Gaseous Corrosion.”

In high temperature gaseous corrosion, the oxidant first adsorbs on the metal surface in molecular (physical

adsorption) and ionic form (chemical adsorption), and it may also dissolve in metal. Oxide nucleates at

favorable sites and most commonly grows laterally, due to surface diffusion, to form a complete thin film

(scale). As the scale thickens, it provides a protective barrier to shield the metal from the gas. For scale growth,

electrons must move through the film to reach the oxidant atoms adsorbed on the surface, and oxidant ions

and/or metal ions must move through the scale barrier. Diffusion of the oxidant into the metal may result in

internal oxidation.

Growth and thermal stresses in the oxide scale may create microcracks and/or delaminate scale from the

underlying metal. Stresses affect the diffusion process and modify the oxidation mechanism and very often

cause scale spallation. Improved oxidation resistance can be achieved by developing better alloys, by applying

protective coatings, and by altering the composition of the gas phase.

Thermodynamics of Gaseous Corrosion

METALS can react chemically when exposed to air or to other more aggressive gases. The reaction rate of

some metals is so slow that they are virtually unattacked, but for others, the reaction can be violent. As with

most chemical processes, elevated-temperature service is more severe because of the exponential increase in

reaction rate with temperature.

The most common reactant is oxygen in the air; therefore, all gas-metal reactions are usually referred to as

oxidation, using the term in its broad chemical sense whether the reaction is with oxygen, water vapor,

hydrogen sulfide (H2S), or whatever the gas might be. Throughout this article, the process is called oxidation,

and the corrosion product is termed an oxide.

Corrosion in gases differs from aqueous corrosion in that electrochemical principles do not help greatly in

understanding the mechanism of oxidation. For gaseous reactions, a fundamental knowledge of the diffusion

processes involved is much more useful. The principles of high-temperature oxidation began to be understood

in the 1920s, whereas electrochemistry and aqueous corrosion principles were developed approximately 100

years earlier. The first journal devoted to corrosion in gases, Oxidation of Metals, began publication in 1970.

This article addresses thermodynamic concepts; the commonly observed kinetics of oxidation are described in

the article “Kinetics of Gaseous Corrosion Processes” in this Volume.

The mechanisms of oxidation are shown schematically in Fig. 1. The gas is first adsorbed on the metal surface

as atomic oxygen. Oxide nucleates at favorable sites and most commonly grows laterally to form a complete

thin film. As the layer thickens, it provides a protective scale barrier to shield the metal from the gas. For scale

growth, electrons must move through the oxide to reach the oxygen atoms adsorbed on the surface, and oxygen

ions, metal ions, or both must move through the oxide barrier. Oxygen may also diffuse into the metal.

Growth stresses in the scale may create cavities and microcracks in the scale, modifying the oxidation

mechanism or even causing the oxide to fail to protect the metal from the gas. Improved oxidation resistance

can be achieved by selection of suitable alloys for the given environment and by application of protective

coatings.

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 History of Casting

THE CASTING OF METAL is a prehistoric technology, but one that appears relatively late in the archaeological record.
There were many earlier fire-using technologies, collectively called by Wertime pyrotechnology, which provided a basis
for the development of metal casting.
 Among these were the heat treatment of stone to make it more workable, the
burning of lime to make plaster, and the firing of clay to produce ceramics. At first, it did not include smelting, for the
metal of the earliest castings appears to have been native.
The earliest objects now known to have been have of metal are more than 10,000 years old (see Table 1) and were
wrought, not cast. They are small, decorative pendants and beads, which were hammered to shape from nuggets of native
copper and required no joining. The copper was beaten flat into the shape of leaves or was rolled to form small tubular
beads. The archaeological period in which this metalworking took place was the Neolithic, beginning some time during
the Aceramic Neolithic, before the appearance of pottery in the archaeological record.


9000 B.C. Earliest metal objects of wrought native copper Near East
6500 B.C. Earliest life-size statues, of plaster Jordan
5000-3000 B.C. Chalcolithic period: melting of copper; experimentation with smelting Near East
3000-1500 B.C. Bronze Age: arsenical copper and tin bronze alloys Near East
3000-2500 B.C. Lost wax casting of small objects Near East
2500 B.C. Granulation of gold and silver and their alloys Near East
2400-2200 B.C. Copper statue of Pharoah Pepi I Egypt
2000 B.C. Bronze Age Far East
1500 B.C. Iron Age (wrought iron) Near East
700-600 B.C. Etruscan dust granulation Italy
600 B.C. Cast iron China
224 B.C. Colossus of Rhodes destroyed Greece
200-300 A.D. Use of mercury in gilding (amalgam gilding) Roman world
1200-1450 A.D. Introduction of cast iron (exact date and place unknown) Europe
Circa 1122 A.D. Theophilus's On Divers Arts, the first monograph on metalworking written by a craftsman Germany
1252 A.D. Diabutsu (Great Buddha) cast at Kamakura Japan
Circa 1400 A.D. Great Bell of Beijing cast China
16th century Sand introduced as mold material France
1709 Cast iron produced with coke as fuel, Coalbrookdale England
1715 Boring mill or cannon developed Switzerland
1735 Great Bell of the Kremlin cast Russia
1740 Cast steel developed by Benjamin Huntsman England
1779 Cast iron used as architectural material, Ironbridge Gorge England
1826 Zinc statuary France
1838 Electrodeposition of copper Russia, England
1884 Electrolytic refining of aluminum United States, France

Native metals were then perhaps considered simply another kind of stone, and the methods that had been found useful in
shaping stone were attempted with metal nuggets. It seems likely that the copper being worked was also being annealed,
because this was a treatment that already being given store. Proof of annealing could be obtained from the microstructures
of these early copper artifacts were it not for their generally corroded condition (some are totally mineralized) and the
natural reluctance to use destructive methods in studying very rare objects.
The appearance of plasters and ceramics in the Neolithic period is evidence that the use of fire was being extended to
materials other than stone. Exactly when the casting of metals began is not known. Archaeologists give the name
Chalcolithic to the period in which metals were first being mastered and the date this period, which immediately preceded
the Bronze Age, very approximately to between 5000 and 3000 B.C. Analyses of early cast axes and other objects give
chemical compositions consistent with their having been cast from native copper and are the basis for the conclusion that
the melting of metals had been mastered before smelting was developed. The furnaces were rudimentary. It has been
shown by experiment that it was possible to smelt copper, for example, in a crucible. Nevertheless, the evidence for
casting demonstrates an increasing ability to manage and direct fire in order to achieve the required melting temperatures.
The fuel employed was charcoal, which tended to supply a reducing atmosphere where the fire was enclosed in an effort
to reduce the loss of heat. Smelting followed.
The molds were of stone The tradition of stone carving was longer than any of the pyrotechnologies, and the
level of skill allowed very finely detailed work. The stone carved was usually of a smooth texture such as steatite or
andesite, and the molds produced are themselves often very fine objects, which can be viewed in museums and
archaeological exhibitions. Many are open molds, although they were not necessarily intended for flat objects. Elaborate
filigree for jewelry was cast in open molds and then shaped by bending into bracelets and headpieces, or cast in parts and
then assembled. Certain molds, described by the archaeologist as multifaceted, have cavities carved in each side of a
rectangular block of stone. Such multifaceted molds would have been more portable than separate ones and suggest
itinerant founding, but they may simply represent economy in the use of a suitable piece of stone.

The Bronze Age

The Bronze Age began in the Near East before 3000 B.C. The first bronze that could be called a standard alloy was

arsenical copper, usually containing up to 4% As, although a few objects contain 12% or more. This alloy was in

widespread use and occurs in objects from Europe and the British Isles (Fig. 2) as well as the Near East. The metal can

sometimes be recognized as arsenical copper by the silvery appearance of the surface, which occurred as a result of

inverse segregation of the arsenic-rich low-melting phase to the surface. This is the same phenomenon that produces tin

sweat on tin bronzes, and it led earlier excavators to describe these artifacts as silver plated. A few examples of arsenic

plating on tin bronze can be seen on objects from Anatolia and Egypt, but the plating method is not known.

The use of 5 to 10% Sn as an alloying element for copper has the obvious advantages of lowering the melting point,

deoxidizing the melt, improving strength, and producing a beautiful, easily polished cast surface that reproduces the

features of the mold with exceptional fidelity--vitality important properties for art castings (Fig. 3). There are several

hypotheses to explain the development of tin bronze. One is that of the so-called natural alloy, that is, metal smelted from

a mixed ore of copper and tin. Another suggests the stream tin (tin ore in the form of cassiterite) may have been added

directly to molten copper. The more vexing question has been the sources of the tin, copper, and silver that have been

excavated from sites in such areas as Mesopotamia, which lack local metal resources. Cornwall or Afghanistan was long

thought to have been the source of this early tin, but more recent investigations have located stream tin in the Eastern

Desert of Egypt and sources of copper and silver as well as tin in the Taurus mountains of south central Anatolia in

modern Turkey.

Recent experiments have shown that metal cast into an open mold is sounder if the open face is covered after the mold

has been filled. This observation may have led to the use of bivalve (permanent two-part) molds. They were in common

use for objects having bilateral symmetry, such as axes of various designs and swords. The molds were made such that

the flash occurred at the edge, which required finishing to sharpen (Fig. 4). These edges are often harder than the body of

the object, evidence of deliberate work hardening. There is also evidence in the third millennium B.C. for the lost wax

casting of small objects of bronze and silver, such as the stag from Alaça Hoyük, now in Ankara. This small object is also

of interest because the casting sprues were left in place attached to the feet, clearly showing how the object was cast.

Although there is abundant evidence from such objects that lost wax casting was employed early in the Bronze Age, the

remnants of the process, such as broken investment and master molds, have eluded researchers. Wax may well have been

the material of the model; other material may have been used, but no surviving evidence of any of these materials has

been recognized. Similarly, the mold dressings used then and later remain unknown. Nevertheless, discoveries are

occasionally made that greatly enlarge the geographical area in which lost wax casting in thought to have taken place.

One of these discoveries occurred in 1972 at a site in England called Gussage All Saints.

At Gussage, an Iron Age (first century B.C.) factory was excavated. The lost wax process was used in this factory for the

mass production of bronze bridle bits and other metal fittings for harnesses and chariots. More than 7000 fragments of

clay investment molds were recovered (Fig. 5), along with crucible fragments, charcoal slag, and other debris thought to

represent the output of single season. The bronze was leaded and in one case had been used to bronze plate a ring of

carbon steel by dipping. This is the first site in Great Britain where direct evidence of lost wax casting has been found, yet

the maturity of the industry suggests that earlier sites remain to be located.

The Far East

The Bronze Age in the Far East began in about 2000 B.C. more than a millennium after its origin in the Near East. It is

not yet clear whether this occurred in China or elsewhere in southeast Asia, and there are vigorous efforts underway to

discover and interpret early metallurgical sites in Thailand. The later date for the development of metallurgy in the Far

East let to an obvious assumption that the knowledge of metal smelting and working had entered the area by diffusion

from the West. This assumption was countered by mapping the geographical distribution of dated metallurgical sites in

China, which indicates development in a generally east-to-west direction. The question of independent origin for the

metallurgy of southeast Asia remains open.

Casting was the predominant forming method in the Far East. There is little evidence of other methods of metalworking

in China before about 500 B.C. Antique Chinese cast bronze ritual vessels were of such complexity that it was the opinion

until recently that these must have been cast by the lost wax method. This had also been the opinion of Chinese scholars

in recent centuries. In the 1920s, however, a number of mold fragments were unearthed at Anyang, prompting

reevaluation of the lost wax hypothesis. The molds were ceramic, and they were piece molds.

Very early Bronze Age sites, approximately 2000 B.C., in Thailand present similar evidence. At one of these sites a burial

was unearthed that contained the broken pieces of an apparently unused ceramic bivalve mold. The bronze founder had

been buried with a piece of the mold in each hand.

The Chinese mold was a ceramic piece mold, typically of many separate parts. The wall sections of the vessels cast in

these molds are quite thin and testify to very fine control over the design of the molds and pouring of the metal. The

metal, usually a leaded tin bronze, was used to great effect but also in an economical manner. Parts, such as legs, which

could have been cast solid, were instead cast around a ceramic core held in position in the mold by chaplets. The chaplets

took several forms; some were cross shaped, others square. They were of the same alloy as the vessel but can clearly be

seen in radiographs. They have occasionally become visible on the surface because their patina appears slightly different

from that of the rest of the vessel.

Metal parts that in the Western tradition would have been made separately and then joined by soldering or welding were

incorporated into Chinese vessels by a sequence of casting on. Handles and legs might be cast first, the finished parts set

in the mold, and the body of the vessel then poured Elaborate designs demanded several such steps. An unusual

feature of this way of thinking about mold making and casting metal is the deliberate incorporation of flash into the

design elements.

The surface decoration of the vessels sometimes employed inlay or gilding, but even in these examples much of the

decoration is cast in. Various decorative elements may have been molded from a master model, impressed into the mold

with loose pieces, or incorporated by casting on metal elements. By using a leaded tin bronze, the founder increased the

fluidity of the melt and consequently the soundness of the casting even in the usual thin sections. However, such a fluid

melt also has a greater tendency to penetrate the joints between the pieces of the mold so as to produce flash. If the

surface of the bronze is meant to be smooth, the flash must be trimmed away. The Chinese founders eventually took this

casting flaw and made it a deliberate element of their design. The joints of the mold were placed in relation to the rest of

the surface decoration such that the flash needed only to be trimmed to an even height to be accepted as part of the cast-in

decoration.

Cast Iron

Cast iron appeared in China in about 600 B.C. Its use was not limited to strictly practical applications, and there are many

examples of Chinese cast iron statuary. Most Chinese cast irons were unusually high in phosphorus, and, because coal

was often used in smelting, high in sulfur as well. These irons, therefore, have melting points that are similar to those of

bronze and when molten are unusually fluid. The iron castings, like the Chinese cast bronzes, are often remarked upon for

the thinness of their wall sections.

There is some dispute concerning the date of the introduction of cast iron into Europe and the route by which it came.

There is less disagreement about the assumption that it was brought from the East. The generally agreed upon date for the

introduction of cast iron smelting into Europe is the 15th century A.D.; it may have been earlier. At this time, cast iron

was less appreciated as a casting alloy than as the raw material needed for "fining" to wrought iron, the form in which

iron could be used by the local blacksmith.

The mass production of cast iron in the West, as well as its subsequent use as an important structural material, began in

the 18th century at Coalbrookdale in England. Here Abraham Darby devised a method of smelting iron with coal by first

coking the coal. He was successful because the local ores fortuitously contained enough manganese to scavenge the sulfur

that the coke contributed to the iron. The vastly greater amounts of cast iron that could be produced by using coke rather

than charcoal from dwindling supplies of timber were eventually put to use nearby in erecting the famous Iron Bridge

The dome of the United States Capitol Building is an example, as is the staircase designed by Louis Sullivan for the

Chicago Stock Exchange now at the Metropolitan Museum in New York City. Cast iron architectural elements were

usually painted; the Capitol dome is painted to resemble the masonry of the rest of the building. Finishes other than paint

were also used. The Sullivan staircase was copper plated and then patinated to give it the appearance of having been cast

in bronze. Another method suitable for interior iron work was the treatment of the surface by deliberate light rusting,

followed by hydrogen reduction of the rust. This produced a velvety black adherent layer of magnetite (Fe3O4) that was

both attractive and durable.

Granulation

Not all casting requires a shaped mold. The exploitation of surface tension led to granulation. The tiny spheres produced

when small amounts of molten metal solidified without restraint were being used as decoration in gold jewelry by 2500

B.C. Granulation was primarily done in gold, silver, or the native alloy of gold and silver called electrum. Some granules

were attached to copper or gilt-silver substrates. The finest work in granulation was done by the Etruscans in about the

seventh century B.C. Its fineness has given it the name "dust granulation," the granules being less than 0.2 mm (0.008 in.)

in diameter. Many thousands of granules were used to create the design on a single object. The Etruscan alloy was gold

with about 30% Ag and a few percent of copper. The method of joining the granules varied. Sweating or soldering have

both been observed, but the exact method used is often still a matter of dispute.

Tumbaga

New World metallurgy is a metallurgy almost without iron. The exception was the use of meteoric iron, which was most

important among the Eskimos, who traded it all across the North. Copper-using cultures flourished further south until the

sources of native copper were exhausted. There is no evidence of smelting among the native population of what is now

the United States until the arrival of the Europeans.

In South America, however, the story is quite different. Early European explorers were overwhelmed by the amount of

gold and silver objects they found. Many of these objects were of sheet gold or its alloys, and it has been suggested that

sheet metal was viewed then as a kind of textile, as textiles in these cultures were not limited to clothing and were used

for weapons and armor. The most interesting castings are of an alloy called tumbaga, which contained gold, silver, and

copper in various proportions. Molds have been found (some never used) that were made by the lost wax process. After

an object had been cast in tumbaga, it was pickled in a corrosive solution that attacked the silver and especially the copper

and, when rinsed off, left a surface layer enriched in gold. This method of gilding is called mise-en-couleur, or "depletion

gilding."

Africa

Africa, where sculpture is often the province of the blacksmith, presents several interesting traditions of casting. Among

them are the famous Benin bronzes of Nigeria and the gold weights of Ghana, formerly the Gold Coast. Both of these

traditions produced castings in brass, with the brass having a high enough zinc content to appear golden. The source of

the brass, or at least that of the zinc, may well be indicated by the portrait of a Portuguese trader in a Benin bronze (Fig.

8). Recent discoveries of zinc furnaces and distillation retorts at Zawar, near Udaipur in India, as well as the very long

trade routes that were opened in the 17th century, suggest the possibility that the metal may have been traded from India.

The Benin bronzes were cast by the lost wax process, and the traditional method has been recorded on film.

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Nondestructive Inspection of Boilers and Pressure Vessels

 Nondestructive Inspection of Boilers and Pressure Vessels

DURING THE FABRICATION of a boiler, pressure vessel, and such related components as boiling water reactor piping
or steam generator tubes, various types of nondestructive inspection (NDI) are performed at several stages of processing,
mainly for the purpose of controlling the quality of fabrication. In-service inspection is used to detect the growth of
existing flaws or the formation of new flaws. This can be done while the vessel is in operation or down for servicing. The
inspection methods used include visual, radiographic, ultrasonic, liquid penetrant, magnetic particle, eddy current, and
acoustic emission inspection, as well as replication microscopy and leak testing. The assurance of component quality
depends largely on the adequacy of NDI equipment and procedures and on the qualification of personnel conducting the
inspection. In many cases, nondestructive inspection, both prior to and during fabrication, must be done to sensitivities
more stringent than those required by specifications. The use of timely inspection and rigid construction standards results
in the reduction of both the costs and delays due to rework.
Quality planning starts during the design stage. For inspections to be meaningful, consideration must be given to the
condition of the material, the location and shape of welded joints, and the stages of production at which the inspection is
to be conducted. During fabrication, quality plans must be integrated with the manufacturing sequence to ensure that the
inspections are performed at the proper time and to the requirements of the applicable standard. In the newest nuclear
plants, quality design planning includes:
· Avoidance of complex weld geometries to facilitate attachment of ultrasonic transducers to the surface
at the best positions
· The increased use of ring forgings for pressure vessel components; this means that there are no
longitudinal welds that have to be inspected in service. The result is a reduction in the amount of inservice
inspection and man-rem exposures
· Incorporating large numbers of access points for introducing mechanized inspection equipment, which
can be operated remotely, thus avoiding exposures to operators and enabling more accurate processing
than is possible with handheld inspection equipment
· The elimination of welds between cast austenitic components; inspection of welds through cast welds is
difficult because they are opaque to ultrasonic inspection to a large degree.

Boiler and Pressure Vessel Code and Inspection Methods

Pressure vessels--both fossil fuel and nuclear--are manufactured in accordance with the rules of the applicable American

Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Ref 2). For nuclear vessels, section XI of the

ASME code establishes rules for continued nondestructive inspections at periodic intervals during the life of the vessel.

One feature of the rules in section XI is the mandatory requirement that the vessel be designed so as to allow for adequate

inspection of material and welds in difficult-to-reach areas. Section III of the code describes the material permitted and

gives rules for design of the vessel, allowable stresses, fabrication procedure, inspection procedure, and acceptance

standards for the inspections.

Pressure vessels are constructed in various sizes and shapes, and some of the largest are those manufactured for the

nuclear power industry. Some pressure vessels are more than 6 m (20 ft) in diameter and 20 m (70 ft) in length and weigh

almost 900 Mg (1000 tons). Thickness of the steel in the walls of these vessels ranges from about 150 mm (6 in.) to more

than 400 mm (15 in.), although many pressure vessels and components are fabricated from much thinner material. Joining

of the many vessel sections is accomplished by welding. Welders of pressure vessels are qualified according to section IX

of the ASME Boiler and Pressure Vessel Code, and welding is done in accordance with qualified welding procedures.

Nondestructive inspection of welds is only a part of the inspection requirements; the materials themselves must be

inspected prior to welding. For pressure vessels that are not constructed according to the ASME code, it is a matter of

agreement between the manufacturer and the user as to whether NDI methods are to be employed and which method or

methods are to be used.

Nondestructive Inspection Methods. An appendix to each section of the ASME code establishes the methods for

performing nondestructive inspection to detect surface and internal discontinuities in materials. Four inspection methods

are acceptable: radiographic, magnetic particle, liquid penetrant, and ultrasonic inspection. All these methods are

mandatory for nuclear vessels, for section III, and for division 2 of section VIII of the code. Ultrasonic inspection is listed

in division 1 of section VIII of the code as nonmandatory. Leak testing, eddy current inspection, acoustic emission

inspection, and visual inspection are included in section V. Details as to which method is to be used and the required

acceptance standards are specified in the appropriate articles on materials and fabrication. All NDI personnel must be

qualified and certified to SNT-TC-1A procedures (Ref 3).

Radiographic Inspection. Methods of radiographic inspection are extensively detailed in the ASME codes;

radiography using either x-rays or radioisotopes as the radiation source is permitted. Radiography is the oldest inspection

method detailed in the codes and is probably the most understood and the most widely accepted. A principal reason for its

wide use is that radiography provides a permanent record of the results of the inspection. This record is important because

the inspector can review the radiographs at any time to ensure that federal, state, or insurance requirements have been

met.

Acceptance standards were developed according to the limits of radiography (what can or cannot be detected by the

method) and by the quality level obtainable by the manufacturing practices used to produce the vessels. Essentially, the

acceptance standards do not permit the existence of indications of the following types of flaws: cracks, incomplete fusion,

incomplete penetration, slag inclusions exceeding a given size that is not related to the thickness of the part, and porosity

that exceeds that presented in illustrated charts provided in the codes. These standards result from the ability to

distinguish among porosity, slag, and incomplete fusion in the radiograph; more important, they also mean that no

indications of cracks or of incomplete fusion are permitted.

Magnetic Particle Inspection. The procedures for magnetic particle inspection reference ASTM E 709 (Ref 4) or

section V of the ASME code for the method. Acceptance standards permit no cracks, but rounded indications of

discontinuities are permitted provided they do not exceed a certain size or number in a specified area. Magnetic particle

inspection is universally used on ferromagnetic parts, on weld preparation edges of ferromagnetic materials, and on the

final welds after the vessel has been subjected to the hydrostatic test. A magnetic particle inspection must be conducted

twice on each area, with the lines of magnetic flux during the second application at approximately 90° to the lines of

magnetic flux in the first application. Depending on the shape of the part and its location at the time of inspection,

magnetization can be done by passing a current through the part or by an encircling coil and sometimes by a magnetic

yoke. The acceptance level is judged by a qualified operator and is subject to review by an authorized code inspector.

Liquid penetrant inspection is usually employed on nonferromagnetic alloys, such as some stainless steels and highnickel

alloys. The acceptance standards are the same as for magnetic particle inspection and are also judged by an

operator, subject to review by a code inspector. The methods are specified to those contained in ASTM E 165 (Ref 5) or

section V of the ASME code. Water-washable, postemulsifiable, or solvent-removable penetrants can be used. A waterwashable

color-contrast penetrant is usually employed because it is easy to handle, requires no special ventilation, and is

nontoxic. Sometimes, special requirements dictate the use of either a solvent-removable color-contrast penetrant or a

fluorescent penetrant.

Ultrasonic inspection is used to inspect piping, pressure vessels, turbine rotors, and reactor coolant pump shafts.

Straight-beam ultrasonic inspection is specified to detect laminations in plates and to detect discontinuities in welds and

forgings. This technique is described in general and specific terms in section XI of the ASME code, in the United States

Nuclear Regulatory Commission Regulatory Guide 1.150 (Ultrasonic Testing of Reactor Vessel Welds During Preservice

and Inservice Examinations), and in companion reports written by utility ad hoc committees (Ref, 1). Angle-beam

inspection is specified for welds, and a more detailed procedure is presented, including reporting requirements, It is

mandatory, however, that ultrasonic inspection, either by straight beam or angle beam, be conducted to a detailed written

procedure. These procedures are usually developed by the manufacturer. Specifications and standards for steel pressure

vessels are given in ASTM A 577 (Ref 6), A 578 (Ref 7), and A 435 (Ref 8). Acceptance standards for the inspection of

welds by ultrasonics closely parallel the acceptance standards for radiography. Cracks, incomplete fusion, and incomplete

penetration are not permitted. The size permitted for other linear indications is the same for the slag permitted by

radiography. However, ultrasonic inspection can detect cracks better than radiography, but it is sometimes difficult to

separate cracks from other linear indications by ultrasonics. Furthermore, ultrasonic inspection procedures refer to the

amplitude of the signal obtained from a calibration notch, hole, or reflector placed in a standard reference block, but not

all slag inclusions or cracks in an actual workpiece present a similar response to that obtained from the artificial

calibrator.

Advanced ultrasonic systems (see the section "In-Service Quantitative Evaluation" in this article) and the improvements

in codes and regulations have combined to make ultrasonic inspection one of the most commonly used nondestructive

methods in the power industry. Advanced ultrasonic methods are intended to ensure that the vessel remains fit for

continued service by detecting and sizing defects that could degrade structural integrity.

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Introduction to Machining Processes

Introduction to Machining Processes

MACHINING is a term that covers a large collection of manufacturing processes designed to remove unwanted material,
usually in the form of chips, from a workpiece. Machining is used to convert castings, forgings, or preformed blocks of
metal into desired shapes, with size and finish specified to fulfill design requirements. Almost every manufactured
product has components that require machining, often to great precision. Therefore, this collection of processes is one of
the most important of the basic manufacturing processes because of the value added to the final product. By the same
token, machining processes are often the most expensive.
The majority of industrial applications of machining are in metals. Although the metal cutting process has resisted
theoretical analysis because of its complexity, the application of these processes in the industrial world is widespread.
Machining processes are performed on a wide variety of machine tools.
dual-turret numerically controlled (NC) lathe. Workpieces are held in workholding devices, such as a three-jaw chuck.
The tools used to cut metal are in the turrets. Other examples of basic machine tools are milling machines, drill presses,
grinders, shapers, broaching machines, and saws.

Each of the basic machine tool types has many different configurations. Lathes, for example, may be engine lathes, turret

lathes, tracer lathes, or automatic-screw machines. Lathes have followed the trend of other machine tools, and NC lathes

can now be routinely purchased.

The primary chip formation processes are listed below, with alternative versions in parentheses. Each process is

performed on one or more of the basic machine tools. For example, drilling can be performed on drill presses, milling

machines, lathes, and some boring machines:

· Turning (boring, facing, cutoff, taper turning, form cutting, chamfering, recessing, thread cutting).

· Shaping (planing, vertical shaping)

· Milling (hobbing, generating, thread milling)

· Drilling (reaming, tapping, spot facing, counterboring, countersinking)

· Sawing (filing)

· Abrasive machining (grinding, honing, lapping)

For each of the basic machine tool types, there are many different kinds of workholders, cutting tools, and cutting tool

holders, resulting in a rather formidable list of equipment and processes. In this Volume, a Section entitled "Fundamentals

of the Machining Process" is presented first, with the intent of putting these processes into perspective and helping the

reader to understand the problems associated with using machining processes in the manufacture of products.

Overview of Machining Process Variables

Metal cutting processes can be viewed as consisting of independent (input) variables, dependent variables, and

independent-dependent interactions or relationships. The engineer or machine tool operator has direct control over the

input variables and can specify or select them when setting up the machining process. Several input variables are

described below. Figure 3 summarizes the input/output relationships associated with metal cutting.

Workpiece Material. The metallurgy and chemistry of the workpiece can either be specified or is already known.

Quite often, a material is selected for a particular application chiefly because it machines well. Cast iron and aluminum,

for example, are known to machine easily. Other metals, such as stainless steel or titanium, are difficult to machine. They

often have large cutting forces or poor surface finishes, which can result in short cutting tool life, yet these metals are

selected to meet other functional design criteria. Machining practice for specific workpiece materials are reviewed in the

Section "Machining of Specific Metals and Alloys" in this Volume.

Starting Geometry. The size and shape of the workpiece may be dictated by preceding processes (casting, forging,

forming, and so forth) or may be selected from standard machining stock (for example, bar stock for screw machines).

Usually this variable directly influences the machining process or processes that are selected, as well as the depths of cut.

Specific Machining Processes. The selection of machining processes required to convert the raw material into a

finished product must be based on the geometry of the part (size and shape, rotational or non-rotational), the required

finishes and tolerances, and the quantity of the product to be made. Machining processes can be grouped into three broad

categories. These include traditional chip formation processes, abrasive machining processes, and nontraditional

machining processes.

Chip Formation Processes. As described earlier, there are seven basic chip formation processes: turning, shaping,

milling, drilling, sawing, broaching, and abrasive machining. The equipment and principles of operation associated with

each of these processes (with the exception of abrasive machining, which is treated separately) are described in the

Section titled "Traditional Machining Processes" in this Volume.

Abrasive machining is the basic process by which chips are formed by very small cutting edges that are integral parts

of abrasive particles. The principles of abrasive machining, the fundamental differences between metal cutting and

grinding, and the abrasives and equipment used for abrasive machining operations are described in the Section "Grinding,

Honing, and Lapping" in this Volume.

Nontraditional Machining Processes. Machining processes that involve compression/shear chip formation have a

number of inherent disadvantages. These include:

· High costs incurred with chip formation (high energy output and chip removal, disposal, and/or

recycling)

· Heat buildup that often results in workpiece distortion

· High forces that create problems in holding the workpiece and which can also cause distortion

· Undesirable cold working and residual stresses in the workpiece that often necessitate further processing

to remove the harmful effects

· Limitations as to the size and delicacy of the workpiece

Tool Materials. 

The three most common cutting tool materials currently in use for production machining operations are

high-speed steel (HSS), both in wrought and powder metallurgy (P/M) form; carbides; and coated tools. Cubic boron

nitride (CBN), ceramics, and diamonds are also being widely employed. Generally speaking, HSS is used for general-purpose

tools, for tools of complex design or for tools used when cutting speeds are more modest. Carbide and ceramic

tool materials, which can operate at faster cutting speeds, come in a wide variety of grades and geometries. Titanium

nitride and titanium carbide coatings for HSS and carbides are now commonplace. Selection of a tool material that

provides reliable service while fulfilling the functional requirements is still an art. The harder the tool material, the better

it can resist wear at faster cutting speeds. The faster the cutting speed, the higher the cutting temperature and the shorter

the tool life. Retention of hardness at elevated temperatures as well as long tool life are desirable characteristics in cutting

tools. See the Section "Cutting Tool Materials" in this Volume for descriptions of the processing, properties, and

applications associated with the aforementioned materials.

Cutting Parameters. 

For every machining operation, it is necessary to select a cutting speed, a feed, and a depth of

cut. Many factors impinge on these decisions because all of the dependent variables are influenced by them. Proper

selection of variables also depends on the other input variables that have been selected; that is, the total amount of

material to be removed, the workpiece and tool materials, and the machining process or processes. These need to be

selected before preliminary choices for speed, feed, and depth of cut can be made.

Tool Geometry. 

Cutting tools are usually designed to accomplish specific operations, and thus the tool geometry

(angles) is selected to accomplish specific machining functions. Generally speaking, large rake and clearance angles are

preferred, but they are possible only on HSS tools. Tools made from carbides, ceramics, and other very hard materials

must be given small tool angles, which keep the tool material in compression during machining and thereby avoid tensile

failure and brittle fractures of the tool. The greater the precision required of the process, the better the geometry of the

cutting edge itself must be.

Dependent Variables

Dependent variables are determined by the process based on the prior selection of the input or independent variables.

Thus, the manufacturing engineer's control over these is usually indirect. The important dependent variables are cutting

force and power, size and properties of the finished product, surface finish, and tool wear and tool failure.

Cutting Force and Power. To machine metal at a specified speed, feed, and depth of cut, with a specified lubricant,

cutting tool material, and geometry, generates cutting forces and consumes power. A change in any of the variables alters

the forces, but the change is indirect in that the engineer does not specify the forces, only the parameters that generate

those forces. Forces are important in that they influence the deflections in the tools, the workpieces, and the workholders,

which in turn affect the final part size. Forces also play a roll in chatter and vibration phenomena common in machining.

Obviously, the manufacturing engineer would like to be able to predict forces (and power) so that he can safely specify

the equipment for a manufacturing operation, including the machine tool, cutting tool, and workholding devices. The

basic concepts associated with the modeling and understanding of cutting forces and power are explained in the article

"Forces, Power, and Stresses in Machining" in this Volume.

Size and Properties of the Finished Product. Ultimately, the objective of machining is to obtain a machined

surface of desired size and geometry with the desired mechanical properties. Because machining is a localized, plastic

deformation process, every machined surface will have some residual deformation (stresses) left in it. These residual

stresses are usually tensile in nature and can interact with surface flaws to produce part failure from fatigue or to cause

corrosion. In addition, every process has some inherent process variability (variations about average size) that changes

with almost all of the input variables. Thus, the manufacturing engineer must try to select the proper levels of input

variables to produce a product that is within the tolerance specified by the designer and has satisfactory surface properties.

Surface Finish. The final finish on a machined surface is a function of tool geometry, tool material, workpiece material,

machining process, speed, feed, depth of cut, and cutting fluid. Surface finish is also related to the process variability.

Rough surfaces have more variability than smooth surfaces. Often it is necessary to specify multiple cuts, that is, roughing

and finish cuts, to achieve the desired surface finish, or it may be necessary to specify multiple processes, such as

following turning with cylindrical grinding, in order to obtain the desired finish. The effect of various machining

processes on surface finish and on the properties of the final products are described in the article "Surface Finish and

Surface Integrity" in this Volume.

Tool Wear and Tool Failure.

 The plastic deformation and friction inherent in machining generate considerable heat,

which raises the temperature of the tool and lowers its wear resistance. The problem is subtle, but significant. As the tool

wears, it changes in both geometry and size. A dull cutting edge and change in geometry can result in increased cutting

forces that in turn increase deflections in the workpiece and may create a chatter condition. The increased power

consumption causes increased heat generation in the operation, which accelerates the wear rate. The change in the size of

the tool changes the size of the workpiece. Again, the engineer has only indirect control over these variables. He can

select slow speeds, which produce less heat and lower wear rates, but which decrease the production rates because the

metal removal rate is decreased. Alternatively, the feed or depth of cut can be increased to maintain the metal removal

rate while reducing the speed. Increasing either the feed or depth of cut directly increases the cutting forces. Therefore,

while tool life may be gained, some precision may be lost due to increased deflection and chatter. Wear mechanisms,

determination of modes of tool failure, and tool life testing are examined in the article "Tool Wear and Tool Life" in this

Volume.

Shear Front-Lamella Structure.

 The shear process itself is a nonhomogeneous (discontinuous) series of shear fronts

(or narrow bands) that produce a lamellar structure in the chips. This fundamental structure occurs on the microscale in all

metals when they are machined and accounts for the unique behavior of the machining process.

Individual shear fronts (Fig. 2) coalesce into narrow shear bands. The shear bands are very narrow (20 to 200 nm)

compared to the thickness of a lamella (2 to 4 m) and account for the large strain and high strain rates that typify this

process.

These fundamental structures are difficult to observe in normal metal cutting, but can be readily observed in a scanning

electron microscope with specially prepared workpieces. Figure 3 shows micrographs from an orthogonal machining

experiment performed inside a scanning electron microscope. The fundamental shear front-lamella structure is readily

observed. The side of the workpiece has been given a mirror polish so that the shear fronts can be observed. The shear

fronts are produced by the activation of many dislocations traveling in waves from the tool tip to the free surface. The

lamella represents heavily deformed material that has been segmented by the shear fronts. When machined, all metals

deform by this basic mechanism. The shear fronts relieve the applied stress.

Mechanics of Chip Formation

Introduction

THE BASIC MECHANISM involved in metal cutting is that of a localized shear deformation on the work material

immediately ahead of the cutting edge of the tool. The relative motion between the tool and the workpiece during cutting

compresses the work material near the tool and induces a shear deformation (called the primary deformation), which

forms the chip. The chip passes over the rake face of the cutting tool and receives additional deformation (called the

secondary deformation) because of the shearing and sliding of the chip against the tool.

These two plastic deformation processes have a mutual dependence. The material element that rubs the rake face has been

heated and plastically deformed during its passage through the primary shear process; therefore, the secondary process is

influenced by the phenomena on the shear plane. At the same time, the shear direction is directly influenced by the rake

face deformation and friction processes. The shear direction influences the heating and straining of the chip in the primary

process. In terms of metal cutting theory, this means that shear stress and shear direction must be determined

simultaneously. Such theoretical analyses are usually based on the mechanics of the process.

This article will review the following:

· The fundamental nature of the deformation process associated with machining

· The principles of the orthogonal cutting model

· The effect of workpiece properties on chip formation

· The mechanics of the machining process

Additional information on the modeling and analysis of chip formation can be found in the article "Forces, Power, and

Stresses in Machining," which immediately follows in this Section.

Forces, Power, and Stresses in Machining

THE MODELING AND ANALYSIS of chip formation has been a continuing exercise over the past century. The metal

cutting process is a unique and complex production process distinguished by:

· Large shear strains, usually of the order of 2 to 5 (Ref 1)

· Exceptionally high shear strain rates, typically from 103 to 105 s-1 with local variations as high as 107 s-1

(Ref 2, 3)

· The rubbing of the tool flank over a freshly cut surface that is chemically clean and active

· Many process and tooling parameters with a wide range of settings that can drastically alter the cutting

process

· A large number of metallurgical parameters in the workpiece that can influence its response to the

cutting tool

These factors and others make the modeling of metal cutting a difficult task that continues to evolve over time. The

models and the discussion presented in this article will attempt to explain the basic concepts of the many complex factors

that influence the forces, power, and stresses in machining.

Forces and Energy in Orthogonal Machining

Although most production machining processes are oblique (that is, having three component forces), models of the

orthogonal (that is, two force) machining of metals are useful for understanding the basic mechanics of machining and

can be extended for modeling of the production processes.

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