Chp 09 - Chapter 9 Modern alloy developments 9.2 Commercial...

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9.1 Introduction In this chapter we will outline some of the devel- opments and properties of modern metallic alloys. Crucial to these materials have been the significant developments that have taken place in manufacturing, made possible by a more detailed understanding of the manufacturing process itself and of the behaviour of the material during both processing and in-service performance. Casting techniques in particular have advanced much over the past decade and now pro- vide reliable clean material with precision. Process modelling is developing to the extent that the process designer is able to take the microstructural specifi- cation for a given composition, which controls the properties of the material, and define an optimum man- ufacturing route to provide the desired material and performance. Modern alloys therefore depend on the proper integration of alloy composition and structure with processing to produce the desired properties and performance. 9.2 Commercial steels 9.2.1 Plain carbon steels Carbon is an effective, cheap, hardening element for iron and hence a large tonnage of commercial steels contains very little alloying element. They may be divided conveniently into low-carbon (<0.3% C), medium-carbon (0.3-0.7% C) and high- carbon (0.7-1.7% C). Figure 9.1 shows the effect of carbon on the strength and ductility. The low- carbon steels combine moderate strength with excellent ductility and are used extensively for their fabrication properties in the annealed or normalized condition for structural purposes, i.e. bridges, buildings, cars and ships. Even above about 0.2% C, however, the ductility is limiting for deep-drawing operations, and brittle fracture becomes a problem, particularly for Figure 9.1 Influence of carbon content on the strength and ductility of steel. welded thick sections. Improved low-carbon steels (<0.2% C) are produced by deoxidizing or 'killing' the steel with Al or Si, or by adding Mn to refine the grain size. It is now more common, however, to add small amounts (<0.1%) of Nb which reduces the carbon content by forming NbC particles. These particles not only restrict grain growth but also give rise to strengthening by precipitation-hardening within the ferrite grains. Other carbide formers, such as Ti, may be used but because Nb does not deoxidize, it is possible to produce a semi-killed steel ingot which, because of its reduced ingot pipe, gives increased tonnage yield per ingot cast. Medium-carbon steels are capable of being quenched to form maitensite and tempered to develop toughness with good strength. Tempering in higher-temperature regions (i.e. 350-55O 0 C) produces a spheroidized car- bide which toughens the steel sufficiently for use as Chapter 9 Modern alloy developments Strengt h M N m- * Elongatio n (•/. )
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axles, shafts, gears and rails. The process of ausform- ing can be applied to steels with this carbon content to produce even higher strengths without significantly reducing the ductility. The high-carbon steels are usu-
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Chp 09 - Chapter 9 Modern alloy developments 9.2 Commercial...

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