Page 38 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Table 1.2 Alloying elements and their effects on microstructure and properties [8–10].
Added elements Effects by adding alloying elements
Chromium Forms various carbides of chromium with refined grain structure
Improves hardness, toughness and wear resistance
Increases strength at elevated temperatures
Nickel Dissolved in ferrite without forming carbides or oxides
Increases strength without decrease ductility
Improves toughness, hardness and corrosion resistance
Manganese Added in steels as a deoxidizing and desulfurizing agent
Dissolved in ferrite and forms carbides
Increases strength and hardness at proper amount
Silicon Added in steels as a deoxidizing agent
Used with manganese, chromium and vanadium to stabilize carbides
Improves strength, hardness and wear resistance
Molybdenum Dissolved in ferrite partially and form carbides, contributes to a fine grain size
Improves hardenability and high-temperature strength
Increases hardness and toughness
Vanadium Dissolved in ferrite and easy to form carbides, hence is used in small amounts
Strong deoxidizing agent and promotes a fine grain size
Improves strength, toughness, keeps hardness at high temperature
Tungsten Maintains hardness even at high temperature, widely used in tool steels
Produces a fine, dense structure
Similar effect to that of molybdenum, but needs greater quantities
Increases both toughness and hardness

have low-carbon steels with less than 0.3% carbon, medium-carbon steels with 0.3–0.5%
carbon and high-carbon steels with more than 0.5% carbon [9]. Carbon has a great effect
on strength, hardness and ductility of any steel. Alloy steels have sufficient quantities of
one or more elements other than carbon introduced to modify steel properties. Com-
monly added alloying elements include chromium, nickel, manganese, molybdenum,
vanadium and many others. Their effects on the microstructures and steel properties
are listed in Table 1.2.
Alloy steels have two types for special applications, that is, stainless steels and
superalloys. Stainless steels contain a minimum of 10% chromium in three main types;
austenitic, ferritic and martensitic stainless steels [9]. The most important characteristic
of stainless steels is their resistance to corrosive environments.
Superalloys are primarily used for elevated temperature applications, such as in gas
turbines, jet engines and heat exchangers. The most attractive properties of superalloys
are the high temperature strength and resistance to creep, oxidation and corrosion [10].
Other types of steels are also available, such as structural steels and tool steels. Struc-
tural steels are basically low-carbon, hot-rolled steels in the form of sheet, plate and
bar, and are usually used in construction and machines. Tool steels, as the designation
implies, serve for making tools in manufacturing engineering.