Stainless steels are alloys containing at least 12% Cr. There are over 500 stainless steels. Corrosion resistance of stainless steel alloys is aided by the extent of chromium content. Another key element that is often added to the stainless steels is nickel, which is used to produce the widely known 18-8 alloy. The 18-8 alloy consists mainly of steel, containing 18 percent chromium and 8 percent nickel.

Stainless steels generally are divided into three classes: austenitic, ferritic, and martensitic.

By adding certain alloys, especially nickel, the high-temperature austenite can be retained at room temperature. Almost all of the austenitic steels in the AISI 3xx series are nonmagnetic. The most important austenitic alloy, AISI Type 302, contains 18 percent chromium and 8 percent nickel. Most other 3xx stainless steels are developed by adding more chromium and nickel and/or by adding/reducing other elements. This can be illustrated in Table 1. For example, small amounts of columbium (347), molybdenum (316), and titanium (321) are added to the austenitic base alloy to prevent carbide precipitation and to obtain other properties. Excessive carbide precipitation causes serious metal failure defects. Carbon content is reduced for some grades.

Table 1

A wide range of mechanical properties of austenitic stainless alloys is obtained by cold workhardening. During hot forming, the power requirement for austenitic alloys are higher than ferritic alloys. An alloy's strength and corrosion resistance depend upon the amount of chromium and nickel added. Austenitic stainless steel Type 309 (25% Cr, 12%Ni) yields high resistance to corrosion and oxidation. It also has excellent tensile strength and creep strength at high temperatures.

To be noted is that, when heated and slowly cooled within a 350°C to 900°C temperature range, austenitic alloys precipitate carbides of chromium at the grain boundaries. This chromium content seriously decreases corrosion resistance of the alloys. This cyclic or recurring heating and cooling action is called thermal cycle. In stainless steel Types 321 and 347, titanium or columbium are added to stabilize the carbides. Thus, grain boundary precipitation and corrosion are not a problem within this temperature range.

Carbide precipitation causes the 18-8 alloys to become subject to severe corrosion. The following two methods are used to reduce such corrosion:

Generally, corrosion resistance in austenitic alloys is higher than it is in the ferritic and martensitic alloys. However, in many chemicals, such as strong oxidizing acids and alkalis, austenitic stabilized types 316 and 317 have more resistance than the other austenitic carbide stabilizing elements. These two stabilized alloys have the addition of molybdenum to increase resistance to corrosion.
 

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