Page 78 - Shigley's Mechanical Engineering Design

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Materials 53
Molybdenum
While molybdenum is used alone in a few steels, it finds its greatest use when combined
with other alloying elements, such as nickel, chromium, or both. Molybdenum forms
carbides and also dissolves in ferrite to some extent, so that it adds both hardness and
toughness. Molybdenum increases the critical range of temperatures and substantially
lowers the transformation point. Because of this lowering of the transformation point,
molybdenum is most effective in producing desirable oil-hardening and air-hardening
properties. Except for carbon, it has the greatest hardening effect, and because it also
contributes to a fine grain size, this results in the retention of a great deal of toughness.
Vanadium
Vanadium has a very strong tendency to form carbides; hence it is used only in small
amounts. It is a strong deoxidizing agent and promotes a fine grain size. Since some vana-
dium is dissolved in the ferrite, it also toughens the steel. Vanadium gives a wide harden-
ing range to steel, and the alloy can be hardened from a higher temperature. It is very
difficult to soften vanadium steel by tempering; hence, it is widely used in tool steels.

Tungsten
Tungsten is widely used in tool steels because the tool will maintain its hardness even
at red heat. Tungsten produces a fine, dense structure and adds both toughness and hard-
ness. Its effect is similar to that of molybdenum, except that it must be added in greater
quantities.

2–16 Corrosion-Resistant Steels
Iron-base alloys containing at least 12 percent chromium are called stainless steels.
The most important characteristic of these steels is their resistance to many, but not all,
corrosive conditions. The four types available are the ferritic chromium steels, the
austenitic chromium-nickel steels, and the martensitic and precipitation-hardenable
stainless steels.
The ferritic chromium steels have a chromium content ranging from 12 to 27 per-
cent. Their corrosion resistance is a function of the chromium content, so that alloys
containing less than 12 percent still exhibit some corrosion resistance, although they
may rust. The quench-hardenability of these steels is a function of both the chromium
and the carbon content. The very high carbon steels have good quench hardenability up
to about 18 percent chromium, while in the lower carbon ranges it ceases at about
13 percent. If a little nickel is added, these steels retain some degree of hardenability up
to 20 percent chromium. If the chromium content exceeds 18 percent, they become dif-
ficult to weld, and at the very high chromium levels the hardness becomes so great that
very careful attention must be paid to the service conditions. Since chromium is expen-
sive, the designer will choose the lowest chromium content consistent with the corro-
sive conditions.
The chromium-nickel stainless steels retain the austenitic structure at room tem-
perature; hence, they are not amenable to heat treatment. The strength of these steels
can be greatly improved by cold working. They are not magnetic unless cold-worked.
Their work hardenability properties also cause them to be difficult to machine. All
the chromium-nickel steels may be welded. They have greater corrosion-resistant prop-
erties than the plain chromium steels. When more chromium is added for greater cor-
rosion resistance, more nickel must also be added if the austenitic properties are to be
retained.