Steel solid solution

Fig. 7. Micrographs of representative grades of cemented carbides for steels containing different amounts of solid-solution multicarbides in a Co binder (9).

Strong materials either have a high intrinsic strength, /, (like diamond), or they rely on the superposition of. solid solution strengthening obstacles fo and work-hardening f i, (like high-tensile steels). But before we can use this information, one problem... 

The most successful way of combating exhaust-system corrosion is, in fact, stainless steel. This is a good example of how - just as with dry oxidation - the addition of foreign atoms to a metal can produce stable oxide films that act as barriers to corrosion. In the case of stainless steel, Cr is dissolved in the steel in solid solution, and Cr203 forms on the surface of the steel to act as a corrosion barrier. 

As you can see from the tables in Chapter 1, few metals are used in their pure state -they nearly always have other elements added to them which turn them into alloys and give them better mechanical properties. The alloying elements will always dissolve in the basic metal to form solid solutions, although the solubility can vary between <0.01% and 100% depending on the combinations of elements we choose. As examples, the iron in a carbon steel can only dissolve 0.007% carbon at room temperature the copper in brass can dissolve more than 30% zinc and the copper-nickel system - the basis of the monels and the cupronickels - has complete solid solubility. 

Austenitic steels of the 304S15 type are normally heat treated at 1 050°C and cooled at a fairly rapid rate to remove the effects of cold or hot working, and in this state much of the carbon is in supersaturated solid solution. Reheating to temperatures below the solution treatment temperature leads to the formation of chromium-rich MjjCj precipitates predominantly at the grain boundaries with the production of chromium gradients and reduced corrosion resistance as is the case with the martensitic steels. Any attack is... 

Iron Low-carbon steels Low-carbon alloy steels Decarburised malleable Chromium 980-1 050 (Halide) 1. Gaseous 2. Semi- gaseous 3. Pack 25-75 urn Solid-solution (ferritic) 20-25% Cr Ductile (200-300 HV) Weldable Heat treatment acceptable ... 

There are a number of differences between interstitial and substitutional solid solutions, one of the most important of which is the mechanism by which diffusion occurs. In substitutional solid solutions diffusion occurs by the vacancy mechanism already discussed. Since the vacancy concentration and the frequency of vacancy jumps are very low at ambient temperatures, diffusion in substitutional solid solutions is usually negligible at room temperature and only becomes appreciable at temperatures above about 0.5T where is the melting point of the solvent metal (K). In interstitial solid solutions, however, diffusion of the solute atoms occurs by jumps between adjacent interstitial positions. This is a much lower energy process which does not involve vacancies and it therefore occurs at much lower temperatures. Thus hydrogen is mobile in steel at room temperature, while carbon diffuses quite rapidly in steel at temperatures above about 370 K. 

But in metals it is relatively common for solid solutions to form. The atoms of one element may enter the crystal of another element if their atoms are of similar size. Gold and copper form such solid solutions. The gold atoms can replace copper atoms in the copper crystal and, in the same way, copper atoms can replace gold atoms in the gold crystal. Such solid solutions are called alloys. Some solid metals dissolve hydrogen or carbon atoms—steel is iron containing a small amount of dissolved carbon. 

NOTE Steel consists of ferrite and pearlite phases. Ferrite is a solid solution of carbon in iron, whereas pearlite is a mixture of ferrite and laminar cementite. Cementite is a carbide, such as ferric carbide (Fe3C), although part of the iron content may be replaced by other metals including molybdenum. 

Describes certain types of stainless steels and other metals that consist of nonmagnetic iron containing face-centered cubes of carbon or other elements in solid solution. 

A type of iron or steel exhibiting a needle-like microstructure of solid solution of supersaturated carbon. 

A solution is a homogeneous mixture of two or more substances. As described in Chapter 3, a solution contains a solvent and one or more solutes. The solvent determines the state of the solution, and normally the solvent is the component present in the greatest quantity. The most common solutions are liquids with water as solvent, but solutions exist in all three states of matter. The atmosphere of our planet, air, is a gaseous solution with molecular nitrogen as the solvent. Steel is a solid solution containing solutes such as chromium and carbon that add strength to the solvent, iron. 

Solid solutions generally form in one of two ways, both of which involve forming the solid flxim the liquid phase. One way is to heat the solid solvent until it melts, add the solutes into the molten material, and then cool the melt until it solidifies. Solid solutions of one metal in another, such as brass and steel, are prepared in this way. A second method is to dissolve the solid solvent and solutes in an appropriate liquid, then cool or evaporate the liquid until a solid precipitates. Solid solutions of organic substances can form in this manner. 

A second way for a solid to accommodate a solute is interstitially, with solute atoms fitting in between solute atoms in the crystal stmcture. An important alloy of this type is carbon steel, a solid solution of carbon in iron, also shown in Figure 12-4. Steels actually are both substitutional and interstitial alloys. Iron is the solvent and carbon is present as an interstitial solute, but varying amounts of manganese, chromium, and nickel are also present and can be in substitutional positions. 

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