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MAGNETIC MATERIALS are broadly classified into two groups with either hard or soft magnetic characteristics. Hard magnetic materials are characterized by retaining a large amount of residual magnetism after exposure to a strong magnetic field. These materials typically have coercive force, Hc, values of several hundred to several thousand oersteds (Oe) and are considered to be permanent magnets. The coercive force is a measure of the magnetizing force required to reduce the magnetic induction to zero after the material has been magnetized. In contrast, soft magnetic materials become magnetized by relatively low-strength magnetic fields, and when the applied field is removed, they return to a state of relatively low residual magnetism. Soft magnetic materials typically exhibit coercive force values of approximately 400 A · m-1 (5 Oe) to as low as 0.16 A · m-1 (0.002 Oe). Soft magnetic behavior is essential in any application involving changing electromagnetic induction such as solenoids, relays, motors, generators, transformers, magnetic shielding, and so on. Important characteristics of magnetically soft materials also include:

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Continue reading "Magnetically Soft Materials"

Tinplate. The largest single application of tin worldwide is in the manufacture of tinplate (steel sheet coated with tin), which accounts for about 40% of total world tin consumption. Since 1940, the traditional hot dip method of making tinplate has been largely replaced by electrodeposition of tin on continuous strips of rolled steel. Electrolytic tinplate can be produced with either equal or unequal amounts of tin on the two surfaces of the steel base metal. Nominal coating thicknesses for equally coated tinplate range from 0.38 to 1.5 μm (15 to 60 μin.) on each surface. The thicker coating on tinplate with unequal coatings (differential tinplate) rarely exceeds 2.0 μm (80 μin.). Tinplate is produced in thicknesses from 0.15 to 0.60 mm (0.006 to 0.024 in.).

Over 90% of world production of tinplate is used for containers (tin cans). Traditional tinplate cans are made of three pieces of tin-coated steel: two ends and a body with a soldered side seam. Innovations in can manufacture have produced two-piece cans made by drawing and ironing. Tinplate cans find their most important use in the packaging of food products, beer, and soft drinks, but they are also used for holding paint, motor oil, disinfectants, detergents, and polishes. Other applications of tinplate include signs, filters, batteries, toys, and gaskets, and containers for pharmaceuticals, cosmetics, fuels, tobacco, and numerous other commodities. Continue reading "Tin in Coatings"

Introduction TIN was one of the first metals known to man. Throughout ancient history, various cultures recognized the virtues of tin in coatings, alloys, and compounds, and the use of the metal increased with advancing technology. Today, tin is an important metal in industry even though the annual tonnage used is much smaller than those of many other metals. One reason for the small tonnage is that, in most applications, only very small amounts of tin are used at a time.

Tin Production and Consumption Tin is produced from both primary and secondary sources. Secondary tin is produced from recycled materials (see the article "Recycling of Nonferrous Alloys" in this Volume). Figure 1 shows the consumption of primary and secondary tin in the United States during recent years. Figure 2 shows 1988 data for the relative consumption of tin in the United States by application.

Continue reading "Tin and Tin Alloys"

GAS-METAL ARC WELDING (GMAW) is an arc welding process that joins metals together by heating them with an electric arc that is established between a consumable electrode (wire) and the workpiece. An externally supplied gas or gas mixture acts to shield the arc and molten weld pool.
 
 Although the basic GMAW concept was introduced in the 1920s, it was not commercially available until 1948. At first, it was considered to be fundamentally a high-current-density, small-diameter, bare-metal electrode process using an inert
 
 
 gas for arc shielding. Its primary application was aluminum welding. As a result, it became known as metal-inert gas (MIG) welding, which is still common nomenclature. Subsequent process developments included operation at low current densities and pulsed direct current, application to a broader range of materials, and the use of reactive gases (particularly carbon dioxide) and gas mixtures. The latter development, in which both inert and reactive gases are used, led to the formal acceptance of the term gas-metal arc welding.
 
 The GMAW process can be operated in semi-automatic and automatic modes. All commercially important metals, such as carbon steel, high-strength low-alloy steel, stainless steel, aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen. Advantages. The applications of the process are dictated by its advantages, the most important of which are: Continue reading "Gas-Metal Arc Welding"

THE GAS-TUNGSTEN ARC WELDING (GTAW) process is performed using a welding arc between a nonconsumable tungsten-base electrode and the workpieces to be joined. C.E. Jackson defined a welding arc as "a sustained electrical discharge through a high-temperature conducting plasma producing sufficient thermal energy so as to be useful for the joining of metals by fusion." This definition is a good foundation for the discussion that follows.
 
 The physics of GTAW are fundamental to all arc processes and are more straightforward, because the complications of materials (for example, filler and flux) transferred through and interacting with the arc can be avoided. Geometrically, the arc discharge in GTAW is between a rod-shaped tungsten electrode and a planar-shaped electrode, that is, the workpiece. Pure tungsten electrodes are less expensive and, possibly, more environmentally compatible than those with rare earth or other oxide additions. They are used for lower-specification welds, where tungsten contamination that is caused by the molten electrode surface can be tolerated. They are also used for alternating current (ac) welding of aluminum, copper, magnesium, and thin sections of low-alloy and stainless steels.
  Continue reading "Arc Physics of Gas-Tungsten Arc Welding"

DURING FUSION WELDING, the thermal cycles produced by the moving heat source cause physical state changes, metallurgical phase transformation, and transient thermal stress and metal movement. After welding is completed, the finished product may contain physical discontinuities that are due to excessively rapid solidification, or adverse microstructures that are due to inappropriate cooling, or residual stress and distortion that are due to the existence of incompatible plastic strains.

In order to analyze these problems, this article presents an analysis of welding heat flow, focusing on the heat flow in the fusion welding process. The primary objective of welding heat flow modeling is to provide a mathematical tool for thermal data analysis, design iterations, or the systematic investigation of the thermal characteristics of any welding parameters. Exact comparisons with experimental measurements may not be feasible, unless some calibration through the experimental verification procedure is conducted.

Welding Thermal Process. A physical model of the welding system is shown in Fig. 1. The welding heat source moves at a constant speed along a straight path. The end result, after either initiating or terminating the heat source, is the formation of a transient thermal state in the weldment. At some point after heat-source initiation but before termination, the temperature distribution is stationary, or in thermal equilibrium, with respect to the moving coordinates. The origin of the moving coordinates coincides with the center of the heat source. The intense welding heat melts the metal and forms a molten pool.

Some of the heat is conducted into the base metal and some is lost from either the arc column or the metal surface to the environment surrounding the plate. Three metallurgical zones are formed in the plate upon completion of the thermal cycle: the weld-metal (WM) zone, the heated-affected zone (HAZ), and the base-metal (BM) zone. The peak temperature and the subsequent cooling rates determine the HAZ structures, whereas the thermal gradients, the solidification rates, and the cooling rates at the liquid-solid pool boundary determine the solidification structure of the WM zone. The size and flow direction of the pool determines the amount of dilution and weld penetration. The material response in the temperature range near melting temperatures is primarily responsible for the metallurgical changes. Continue reading "Heat Flow in Fusion Welding"

WELDING AND JOINING processes are essential for the development of virtually every manufactured product. However, these processes often appear to consume greater fractions of the product cost and to create more of the production difficulties than might be expected. There are a number of reasons that explain this situation.

First, welding and joining are multifaceted, both in terms of process variations (such as fastening, adhesive bonding, soldering, brazing, arc welding, diffusion bonding, and resistance welding) and in the disciplines needed for problem solving (such as mechanics, materials science, physics, chemistry, and electronics). An engineer with unusually broad and deep training is required to bring these disciplines together and to apply them effectively to a variety of processes.

Second, welding or joining difficulties usually occur far into the manufacturing process, where the relative value of scrapped parts is high.

Third, a very large percentage of product failures occur at joints because they are usually located at the highest stress points of an assembly and are therefore the weakest parts of that assembly. Careful attention to the joining processes can produce great rewards in manufacturing economy and product reliability. The Section "Fusion Welding Processes" in this Volume provides details about equipment and systems for the major fusion welding processes.

The purpose of this Section of the Volume is to discuss the fundamentals of fusion welding processes, with an emphasis on the underlying scientific principles. Because there are many fusion welding processes, one of the greatest difficulties for the manufacturing engineer is to determine which process will produce acceptable properties at the lowest cost. There are no simple answers. Any change in the part geometry, material, value of the end product, or size of the production run, as well as the availability of joining equipment, can influence the choice of joining method. For small lots of complex parts, fastening may be preferable to welding, whereas for long production runs, welds can be stronger and less expensive. Continue reading "Energy Sources Used for Fusion Welding"

FATIGUE is the progressive, localized, and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that have maximum values less than (and often much less than) the static yield strength of the material. Fatigue may culminate into cracks and cause fracture after a sufficient number of fluctuations. Fatigue damage is caused by the simultaneous action of cyclic stress, tensile stress, and plastic strain. If any one of these three is not present, a fatigue crack will not initiate and propagate. The plastic strain resulting from cyclic stress initiates the crack; the tensile stress promotes crack growth (propagation). Although compressive stresses will not cause fatigue, compressive loads may result in local tensile stresses. Microscopic plastic strains also can be present at low levels of stress where the strain might otherwise appear to be totally elastic.

During fatigue failure in a metal free of cracklike flaws, microcracks form, coalesce, or grow to macrocracks that propagate until the fracture toughness of the material is exceeded and final fracture occurs. Under usual loading conditions, fatigue cracks initiate near or at singularities that lie on or just below the surface, such as scratches, sharp changes in cross section, pits, inclusions, or embrittled grain boundaries. Continue reading "Fatigue Failure in Metals"

FRACTURE MECHANICS is a very powerful tool for predicting the loads and crack lengths at which fracture can occur. Broken down to its essential form, it allows an engineer to predict the onset of fracture if the following information is available:

· Load/crack geometry (usually available from NDI)

· A formula for the so-called stress-intensity parameter, K, for the load/crack geometry of interest (the result of sophisticated mathematical analysis but available in handbooks, such as Ref 1)

· The numerical value of the fracture toughness (generally denoted KIc), which is determined experimentally through well-defined procedures (Ref 2)

· For fatigue crack propagation, knowledge of the crack growth rate as a function of the stress-intensity parameter With this information and the use of fracture mechanic methods (as briefly described in the next section for monotonic and fatigue fracture), it is possible to compute the life without any consideration of those processes that determine the values of the fracture toughness or the crack growth rates. Such procedures as are outlined below are obviously of great value in carrying out engineering calculations for existing or contemplated components.

However, in some instances the properties are insufficient to meet the engineering requirements. In such cases it is necessary to consider alternate materials or, in some instances, to develop alternate heat treatments and compositions that yield properties that allow the requirements to be met. Continue reading "Alloy Design for Fatigue and Fracture"

FATIGUE PROPERTIES are an integral part of materials comparison activities and offer information for structural life
estimation in many engineering applications. They are a critical element in the path relating the materials of construction
to the components and must take into account as many influences as possible to reflect the actual product situation. In
application, fatigue is a detail analysis, trying to assess what will occur at a particular location of a component or
assembly under cyclic loading.


The topic of fatigue properties is very broad and is typically based on testing coupons. To be applicable, determined
properties must support one of the fatigue design philosophies that may be applied to the part. In this article the three
general approaches to fatigue design are stated, with discussion of their respective attributes, and their individual property
requirements are described. The intent here is not to present a comprehensive catalog of properties; that would take many
volumes this size. Instead, the purpose is to provide the basic insights necessary to examine those properties that can be
found, review some of the common presentation formats, and recognize their inherent characteristics. It is important to
review information critically for any use, to know when a direct "apples to apples" comparison can be made, and
potentially to know how to manipulate some of the data to put it on equal footing with information gathered from diverse
sources. The susceptibility of mechanical properties to variation through microstructural manipulation and structural
consideration can be substantial. Continue reading "Fatigue Properties in Engineering"

IT IS DIFFICULT to identify exactly when the problems of failure of structural and mechanical equipment became of critical importance; however, it is clear that failures that cause loss of life have occurred for over 100 years (Ref 1, 2). Throughout the 1800s bridges fell and pressure vessels blew up, and in the late 1800s railroad accidents in the United Kingdom were continually reported as "The most serious railroad accident of the week"! Those in the United States also have heard the hair-raising stories of the Liberty ships built during World War II. Of 4694 ships considered in the final investigation, 24 sustained complete fracture of the strength deck, and 12 ships were either lost or broke in two. In this case, the need for tougher structural steel was even more critical because welded construction was used in shipbuilding instead of riveted plate. In riveted plate construction, a running crack must reinitiate every time it runs out of a plate. In contrast, a continuous path is available for brittle cracking in a welded structure, which is why low notch toughness is a more critical factor for long brittle cracks in welded ships. Similar long brittle cracks are less likely or rare in riveted ships, which were predominant prior to welded construction.

Nonetheless, even riveted ships have provided historical examples of long brittle fracture due, in part, from low toughness. In early 1995, for example, the material world was given the answer to an old question, "What was the ultimate cause of the sinking of the Titanic?" True, the ship hit an iceberg, but it now seems clear that because of brittle steel, "high in sulfur content even for its time" (Ref 3), an impact which would clearly have caused damage, perhaps would not have resulted in the ultimate separation of the Titanic in two pieces where it was found in 1985 by oceanographer Bob Ballard. During the undersea survey of the sunken vessel with Soviet Mir submersibles, a small piece of plate was retrieved from 12,612 feet below the ocean's surface. Examination by spectroscopy revealed a high sulfur content, and a Charpy impact test revealed the very brittle nature of the steel (Ref 3). However, there was some concern that the high sulfur content was, in some way, the result of eighty years on the ocean floor at 6,000 psi pressures.

Subsequently, the son of a 1911 shipyard worker remembered a rivet hole plug which his father had saved as a memento of his work on the Titanic. Analysis of the plug revealed the same level of sulfur exibited by the plate from the ocean floor. In the years following the loss of the Titanic metallurgists have become well aware of the detrimental effect of high sulfur content on fracture. Continue reading "Fracture and Structure"

Fatigueis a technical term that elicits a degree of curiosity. When citizens read or hear in their media of another fatigue failure, they wonder whether this has something to do with getting tired or "fatigued" as they know it. Such is not the case.

One way to explain fatigue is to refer to the ASTM standard definitions on fatigue, contained in ASTM E 1150. It is difficult, if not impossible, to carry on intelligent conversations if discussions on fatigue do not use a set of standard definitions such as E 1150. Within E 1150, there are over 75 terms defined, including the term fatigue: "fatigue (Note 1): the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations (Note 2). Note 1--In glass technology static tests of considerable duration are called `static fatigue' tests, a type of test generally designated as stress-rupture. Note 2--Fluctuations may occur both in load and with time (frequency) as in the case of `random vibration'." (Ref 2). The words in italics (emphasis added) are viewed as key words in the definition. These words are important perspectives on the phenomenon of fatigue: Continue reading "What is Fatigue ?"

THE DISCOVERY of fatigue occurred in the 1800s when several investigators in Europe observed that bridge and railroad components were cracking when subjected to repeated loading. As the century progressed and the use of metals expanded with the increasing use of machines, more and more failures of components subjected to repeated loads were recorded. By the mid 1800s A. Wohler (Ref 1) had proposed a method by which the failure of components from repeated loads could be mitigated, and in some cases eliminated. This method resulted in the stress-life response diagram approach and the component test model approach to fatigue design.

Undoubtedly, earlier failures from repeated loads had resulted in failures of components such as clay pipes, concrete structures, and wood structures, but the requirement for more machines made from metallic components in the late 1800s stimulated the need to develop design procedures that would prevent failures from repeated loads of all types of equipment. This activity was intensive from the mid-1800s and is still underway today. Even though much progress has been made, developing design procedures to prevent failure from the application of repeated loads is still a daunting task. It involves the interplay of several fields of knowledge, namely materials engineering, manufacturing engineering, structural analysis (including loads, stress, strain, and fracture mechanics analysis), nondestructive inspection and evaluation, reliability engineering, testing technology, field repair and maintenance, and holistic design procedures. All of these must be placed in a consistent design activity that may be referred to as a fatigue design policy. Obviously, if other time-related failure modes occur concomitantly with repeated loads and interact synergistically, then the task becomes even more challenging. Inasmuch as humans always desire to use more goods and place more demands on the things we can design and produce, the challenge of fatigue is always going to be with us. Continue reading "Industrial Significance of Fatigue Problems"

Introduction

Metal composite materials have found application in many areas of daily life for quite some time. Often it is not realized that the application makes use of composite materials. These materials are produced in situ from the conventional production and processing of metals. Here, the Dalmatian sword with its meander structure, which results from welding two types of steel by repeated forging, can be mentioned. Continue reading "Basics of Metal Matrix Composites"

Thermoplastic rubber
 
 One of the more interesting developments in the past 30 years has been that of thermoplastic rubbers. One of the problems with conventional vulcanised rubber is that, once cross-linking or vulcanisation has taken place, the material is, like a thermosetting plastic. ‘set’ and cannot be melted and reprocessed. Thus material used in a process but not incorporated into the final product or defective products cannot be re-used as can a thermoplastic material. Thus many attempts have been made to produce a rubbery material in which effective cross-links exist at normal use temperature but which disappear (become heatfugitive) at elevated temperatures. To some extent entanglements and crystalline zones fulfil this role and rubbers have been used that are simply very high molecular weight polymers (but thus difficult to process) or slightly crystalline. Polymers have also been made in which ionic cross-links exist at low temperatures, but which lose their force at elevated ones. A number of true covalent-bonded systems have been devised in which these bonds break down at elevated temperature, but which reform at low temperatures. Such systems have been of limited use. Far more successful has been the use of block copolymers.
 
 Such block copolymers differ from the more common random copolymers in that the monomers of each type are grouped together in one chain. One such material consists of a block of butadiene molecules (forming a segment of a rubbery polybutadiene block) set between two blocks of styrene molecules (forming glassy polystyrene blocks). Such as system is known as a styrene-butadiene-styrene (SBS) triblock copolymer. At room temperature the polystyrene ends congregate into domains effectively forming cross-links between many triblock molecules at the chain ends. However, above Tg these domains tend to break up and, because the overall molecular weight is quite low, the whole system melts and is capable of flow. When the melt is cooled the domain structures and thus the cross-links reform. Slightly different are the polyetherpolyester block copolymers in which amorphous polyether zones are separated by crystallisable polyester blocks. At room temperature these latter blocks do crystallise together to produce small crystal structures which act as cross-links. These latter materials are available in a variety of polyether/polyester ratios and thus vary in stiffness and rubberiness. Because of the high melting point of the polyester blocks these materials have good heat resistance, and because of their chemical nature they have good oil resistance. They have become important engineering rubbers. 
  Continue reading "Thermoplastic rubber Natural rubber Styrene-butadiene rubber"

The first case study shows how a knowledge of steel microstructures can help us trace the chain of events that led to a damaging engineering failure. The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel – the mud drum – which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the “hot” tubes moves upwards from the mud drum to the steam drum, and the water in the “cool” tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe–0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C.
 

Continue reading "Metallurgical detective work after a boiler explosion"

We saw in the last chapter that carbon steels could be strengthened by quenching and tempering. To get the best properties we must quench the steel past the nose of the Ccurve. The cooling rate that just misses the nose is called the critical cooling rate (CCR). If we cool at the critical rate, or faster, the steel will transform to 100% martensite.* The CCR for a plain carbon steel depends on two factors – carbon content and grain size. We have already seen (in Chapter that adding carbon decreases the rate of the diffusive transformation by orders of magnitude: the CCR decreases from ≈105°C s−1 for pure iron to ≈200°C s−1 for 0.8% carbon steel (see Fig. 12.1). We also saw in Chapter 8 that the rate of a diffusive transformation was proportional to the number of nuclei forming per m3 per second. Since grain boundaries are favourite nucleation sites, a fine-grained steel should produce more nuclei than a coarse-grained one. The fine-grained steel will therefore transform more rapidly than the coarse-grained steel, and will have a higher CCR (Fig. 12.1).
 
 Quenching and tempering is usually limited to steels containing more than about 0.1% carbon. Figure 12.1 shows that these must be cooled at rates ranging from 100 to 2000°C s−1 if 100% martensite is to be produced. There is no difficulty in transforming the surface of a component to martensite – we simply quench the red-hot steel into a bath of cold water or oil. But if the component is at all large, the surface layers will tend to insulate the bulk of the component from the quenching fluid. The bulk will cool more slowly than the CCR and will not harden properly. Worse, a rapid quench can create shrinkage stresses which are quite capable of cracking brittle, untempered martensite. These problems are overcome by alloying. The entire TTT curve is shifted to the right by adding a small percentage of the right alloying element to the steel – usually molybdenum (Mo), manganese (Mn), chromium (Cr) or nickel (Ni) (Fig. 12.2). Numerous low-alloy steels have been developed with superior hardenability – the ability to form martensite in thick sections when quenched. This is one of the reasons for adding the 2–7% of alloying elements (together with 0.2–0.6% C) to steels used for things like crankshafts, high-tensile bolts, springs, connecting rods, and spanners. Alloys with lower alloy contents give martensite when quenched into oil (a moderately rapid quench); the more heavily alloyed give martensite even when cooled in air. Having formed martensite, the component is tempered to give the desired combination of strength and toughness. Continue reading "Steel Hardenability"

We saw in Chapter 8 that, if we cool eutectoid γ to 500°C at about 200°C s−1, we will miss the nose of the C-curve. If we continue to cool below 280°C the unstable γ will begin to transform to martensite. At 220°C half the γ will have transformed to martensite. And at –50°C the steel will have become completely martensitic. Hypoeutectoid and hypereutectoid steels can be quenched to give martensite in exactly the same way (although, as Fig. 11.8 shows, their C-curves are slightly different).

Continue reading "Quenched and tempered carbon steels"

Introduction
 
 Whenever you have to report on the structure of an alloy – because it is a possible design choice, or because it has mysteriously failed in service – the first thing you should do is reach for its phase diagram. It tells you what, at equilibrium, the constitution of the alloy should be. The real constitution may not be the equilibrium one, but the equilibrium constitution gives a base line from which other non-equilibrium constitutions can be inferred.
 
 Using phase diagrams is like reading a map. We can explain how they work, but you will not feel confident until you have used them. Hands-on experience is essential. So, although this chapter introduces you to phase diagrams, it is important for you to work through the “Teaching Yourself Phase Diagrams” section at the end of the book. This includes many short examples which give you direct experience of using the diagrams. The whole thing will only take you about four hours and we have tried to make it interesting, even entertaining. But first, a reminder of some essential definitions.
  Continue reading "Equilibrium constitution and phase diagrams"

Intrloduction
 
 The status of steel as the raw material of choice for the manufacture of car bodies rests priracipally on its price. It has always been the cheapest material which meets the necessary strength, stiffness, formability and weldability requirements of Barge-scale car body production. Until recently, the fact that steel has a density two-thirds that of lead was accorded little significance. But increasing environmental awareness and tightening legislative requirements concerning damaging emissions and fuel efficiency are now changing that view. Car makers are looking hard at alternatives to steel.
 
 Energy And Cars
 
 Energy is used to build a car, and energy is used to run it. Rising oil prices mean that, since about 1980, the cost of the petrol consumed during the average life of a car is comparable with the cost of the car itself. Consumers, therefore, now want more fuelefficient cars, and more fuel-efficient cars pollute less. A different perspective on the problem is shown in Table 27.1: 35% of all the energy used in a developed country is consumed by private cars. The dependence of most developed countries on imported oil is such a liability that they are sleeking ways of reducing it. Private transport is an attractive target because to trim its energy consumption does not necessarily depress the economy. In the US, for example, legiskition is in place requiring that the average fleet-mileage of a manufacturer increase from 22.5 to 34.5 miles per gallon; and in certain cities (Los Angeles, for example) a target of zero emission has been set. How can this be achieved?

 

Continue reading "Materials and energy in car design"

Introduction In this chapter we examine three quite different problems involving friction and wear. The first involves most of the factors that appeared in Chapter 25: it is that of a round shaft or journal rotating in a cylindrical bearing. This type of journal bearing is common in all types of rotating or reciprocating machinery: the crankshaft bearings of an automobile are good examples. The second is quite different: it involves the frictional properties of ice in the design of skis and sledge runners. The third case study introduces us to some of the frictional properties of polymers: the selection of rubbers for anti-skid tyres.
 
 CASE STUDY 1 : THE DESIGN OF JOURNAL BEARINGS
 
 In the proper functioning of a well-lubricated journal bearing, the frictional and wear properties of the materials are, suprisingly, irrelevant. This is because the mating surfaces never touch: they are kept apart by a thin pressurised film of oil formed under conditions of hydrodynamic lubrication. Figure 26.1 shows a cross-section of a bearing operating hydrodynamically. The load on the journal pushes the shaft to one side of the bearing, so that the working clearance is almost all concentrated on one side. Because oil is viscous, the revolving shaft drags oil around with it. The convergence of the oil stream towards the region of nearest approach of the mating surfaces causes an increase in the pressure of the oil film, and this pressure lifts the shaft away from the bearing surface. Pressures of 10 to 100 atmospheres are common under such conditions. Provided the oil is sufficiently viscous, the film at its thinnest region is still thick enough to cause complete separation of the mating surfaces. Under ideal hydrodynamic conditions there is no asperity contact and no wear. Sliding of the mating surfaces takes place by shear in the liquid oil itself, giving coefficients of friction in the range 0.001 to 0.005.
 

Continue reading "Case studies in friction and wear"

Introduction
 
 In the last chapter we said that one of the requirements of a high-temperature material - in a turbine blade, or a super-heater tube, for example - was that it should resist attack by gases at high temperatures and, in particular, that it should resist oxidation. Turbine blades do oxidise in service, and react with H2S, SO2 and other combustion products. Excessive attack of this sort is obviously undesirable in such a highly stressed component. Which materials best resist oxidation, and how can the resistance to gas attack be improved?
 
 Well, the earth's atmosphere is oxidising. We can get some idea of oxidationresistance by using the earth as a laboratory, and looking for materials which survive well in its atmosphere. All around us we see ceramics: the earth's crust (Chapter 2) is almost entirely made of oxides, silicates, aluminates and other compounds of oxygen; and being oxides already, they are completely stable. Alkali halides, too, are stable: NaC1, KC1, NaBr - all are widely found in nature. By contrast, metals are not stable: only gold is found in 'native' form under normal circumstances (it is completely resistant to oxidation at all temperatures); all the others in our data sheets will oxidise in contact with air. Polymers are not stable either: most will burn if ignited, meaning that they oxidise readily. Coal and oil (the raw materials for polymers), it is true, are found in nature, but that is only because geological accidents have sealed them off from all contact with air. A few polymers, among them PTFE (a polymer based on -CF2-), are so stable that they survive long periods at high temperatures, but they are the exceptions. And polymer-based composites, of course, are just the same: wood is not noted for its high-temperature oxidation resistance.
 
 How can we categorise in a more precise way the oxidation-resistance of materials? If we can do so for oxidation, we can obviously follow a similar method for sulphidation or nitrogenation.
  Continue reading "Oxidation of materials"

Introduction
 
 We now come to the final properties that we shall be looking at in this book on engineering materials: the frictional properties of materials in contact, and the wear that results when such contacts slide. This is of considerable importance in mechanical design. Frictional forces are undesirable in bearings because of the power they waste; and wear is bad because it leads to poor working tolerances, and, ultimately, to failure. On the other hand, when selecting materials for clutch and brake linings - or even for the soles of shoes - we aim to maximise friction but still to minimise wear, for obvious reasons. But wear is not always bad: in operations such as grinding and polishing, we try to achieve maximum wear with the minimum of energy expended in friction; and without wear you couldn’t write with chalk on a blackboard, or with a pencil on paper. In this chapter and the next we shall examine the origins of friction and wear and then explore case studies which illustrate the influence of friction and wear on component design.
 
 Friction between materials
 
 As you know, when two materials are placed in contact, any attempt to cause one of the materials to slide over the other is resisted by a friction force (Fig. 25.1). The force that will just cause sliding to start, F,, is related to the force P acting normal to the contact surface by 
 

Continue reading "Friction and wear"

For most rotating machines used in the process industries, the trend is toward higher speeds, higher horsepowers per machine, and less sparing. The first of these factors increases the need for precise balancing and alignment. This is necessary to minimize vibration and premature wear of bearings, couplings, and shaft seals. The latter two factors increase the economic importance of high machine reliability, which is directly dependent on minimizing premature wear and breakdown of key components.
 
 Balancing, deservedly, has long received attention from machinery manufacturers and users as a way to minimize vibration and wear. Many shop and field balancing machines, instruments, and methods have become available over the years. Alignment, which is equally important, has received proportionately less notice than its importance justifies. Any kind of alignment, even straightedge alignment, is better than no alignment at all. Precise, two-indicator alignment is better than rough alignment, particularly for machines 3600 RPM and higher. It can give greatly improved bearing and seal life, lower vibration, and better overall reliability. It does take longer, however, especially the first time it is done to a particular machine, or when done by inexperienced personnel. The process operators and mechanical supervisors must be made aware of this time requirement. If they insist on having the job done in a hurry. they should do so with full knowledge of the likelihood of poor alignment and reduced machine reliability. Figure 5-1 shows a serious machinery failure which started with piping-induced misalignment, progressed to coupling distress, bearing failure, and finally, total wreck.
  Continue reading "Alignment and Balancing"

In hydrocarbon processing plants, mechanical seals for pumps and compressors, tube fittings and pipe flanges often use O-rings to prevent fluid flow or leakage. According to application, O-rings can be categorized as static (seal between flange facings) and dynamic (subjected to movement or wobble). Table 10-13 lists the commonly available O-ring materials in decreasing order of preference based on an overall desirability for O-ring sealing service, with cost and availability considered secondary. When following the design steps results in several candidate elastomers for a specific application, this table may be used for final selection. (Letter suffixes identify elastomers compound designations.
 
 Next, the user has to consider temperature limitations of the elastomers. Here Table 10-14 will be helpful.
 
 Chemical compatibility of O-rings with a process fluid and temperature limits will define the method of O-ring production, using full-circle molding, ambient adhesive bonding and hot bonding or vulcanizing. Having no joint and hence no weak point, full-circle molded O-rings are the most common for reliability in operation. Available in a wide range of stock sizes and materials, O-rings of this type also can be custommolded. Ambient adhesive-bonded O-rings of any diameter can be quickly and easily made, using cord stock of most materials except silicone rubber. A simple jig used for cutting square ends and aligning them for bonding gives a smooth joint, which can sometimes be made in place without machine disassembly. Vulcanizing is considered to be an intermediate method in terms of nonstock O-ring delivery, chemical, and temperature resistance. Thermal and chemical resistance of the hot, bonded O-rings is superior to the adhesive-bonded, but inferior to the molded ones. Continue reading "Selection and Application of 0-Rings"

In trying to achieve improved wear resistance it would be well not to neglect proven traditional steel-hardening methods. In surface hardening of alloy steels the core of a machinery part may bc trcated to produce a desired structure for machinability or a strength level of service, whereas the surface may be subsequently hardened for high strength and wear resistance.
 
 Flame hardening involves very rapid surface heating with a direct high temperature flame, followed by cooling at a suitable rate for hardening. The process utilizes a fuel gas plus air or oxygen for heating. Steels commonly flame hardened are of the medium, 0.30 to 0.60 percent carbon range with alloy suitable for the application. The quenching medium may be caustic, brine, water, oil, or air, as required. Normally quenchants are sprayed, but immersion quenching is used in some instances. To maintain uniformity of hardening, it is necessary to use mechanical equipment to locate and time the application of heat, and to control the quench.
 
 As with conventional hardening, residual stresses may cause cracking if they are not immediately relieved by tempering. In some instances residual heat after quenching may be sufficient to satisfactorily relieve hardcning stresses. As size dictates, either conventional furnace tempering or flame tempering may be used. With flame tempering, the heat is applied in a manner similar to that used for hardening but utilizing smaller flame heads with less heat output.
 
 Carburizing is one of the oldest heat treating processes. Evidence exists that in ancient times sword blades and primitive tools were made by carburization of low carbon wrought irons. Today, the process is a science whereby carbon is added to steel within desired limitations to a controlled amount and depth. Carburizing is usually, but not necessarily, performed on steels initially low in carbon.

Continue reading "Hardening of Machinery Components"

Another process that will restore worn or corroded machinery surfaces is industrial plating, usually electroplating. This process is not normally applied on-site but parts in need of restoration have to be shipped to a company specializing in this type of work.
 
 Surface preparation for plating is usually achieved by smooth machining or grinding. In some cases, shot or grit blasting may be suitable. A very rough surface before plating is neither necessary nor desirable. Unless a greater thickness of deposit is required for wear, corrosion allowance, or for bearing material compatibility, there is no need to remove more metal than required to clean up the surface. Sharp corners and edges should be given as large a radius or diameter as possible. Areas not requiring resurfacing will be protected by the plating shop.
 
 Materials that can be repaired belong to the majority of metals used in normal design practice. It is, however, very important that the plating company be informed of the composition or specification.
  Continue reading "Industrial Plating Chrome-Plating of Cylinder Liners"

Many times metals are surfaced with austenitic stainless steels or soft nickel-chromium alloys for the sole purpose of corrosion resistance. For some applications, costly metals such as tantalum, silver or gold are used as surfacings. If a particular application requires a very special material, a surfacing technique probably can be used to put this special metal on only the functional surfaces, with a reduction in cost. In an effort to come up with a viable hard surfacing selection system, a series of wear tests was conducted on fusion surfacing materials from each of the classifications detailed in the preceding pages. Several vendors’ products in each classification were tested, and the welding characteristics of each material determined. Ceramics, tool steels, and special purpose materials were not tested.
 
 The specific procedure for evaluating the fusion surfacings was to make multilayer test coupons, determine the welding characteristics, and run metal-to-metal and abrasive wear tests on the materials that performed satisfactorily in the welding tests. The compositions of the hard surfacing alloys tested are shown in Table 10-1.
 

Continue reading "Hard Surfacing Techniques II - Special Purpose Materials"

     Almost every welding technique can be used to apply a hard-surfacing material. Referring to the definition of hard surfacing-applying by welding or spraying techniques a material with properties superior to those of the basis metal-it can readily be seen that this can be accomplished in many ways. Figure 10-2 illustrates most of the methods used. Each has advantages and disadvantages. Shielded metal-arc welding is the most common and versatile welding technique, but many of the hardsurfacing alloys have not been available in a coated electrode form.
 
 Oxyacetylene welding is the preferred method for applying bare filler metals, in that it minimizes “dilution” or mixing of the filler metal with the basis metal, so that the hardest deposit is achieved with this mode of deposition. Gas welding is slow, and it is difficult to control the deposit profile. Gas tungsten-arc welding is also slow, but provides the most accurate deposit profile of any of the fusion processes; it has the major disadvantage of significant dilution, with a corresponding loss in deposit hardness. Gas metal-arc welding is one of the fastest processes for applying hard surfacing; however (once again) not all surfacing alloys are available as wire that can be roll-driven through the welding gun. (When used for hard surfacing, the gas metal-arc process is often used without a shielding gas, and then is referred to as the “open arc” process.)
 

Continue reading "Hard Surfacing Techniques"

Many repairs of worn machinery surfaces can be achieved by hard surfacing. By definition, hard surfacing is the process of applying, by specialized welding techniques, a material with properties superior to the basis metal.
 
 Perhaps the chief factor limiting wide acceptance of this process today is the aura of mystery surrounding the properties of the various hardsurfacing alloys. There are literally hundreds of hard-surfacing alloys commercially available, each with a strange sounding name and a vendor’s claim that it is the ultimate material for this or that application. Rather than sift through the chaff to determine which should be used on an urgent problem, many machinery maintenance people drop the idea of hard surfacing and rely on more familiar techniques. This section will discuss the various forms of wear, and show how a few hard-surfacing materials can solve most wear problems. The following information is intended to simplify the field of hard surfacing so that maintenance and design engineers can effectively use the process to reduce maintenance and fabrication costs.

Continue reading "Protecting Machinery Parts Against Loss Of Surface"