Page 39 - Analysis and Design of Machine Elements

1.4.1.2 Cast Irons and Cast Steels An Overview of Machine Design 17
Cast irons contain iron, carbon (between 2∼4%), silicon and manganese [10]. Additional
alloying elements are sometimes added to improve material properties. Cast irons have
excellent machinability, hardness, wear resistance and damping capability, yet lower ten-
sile strength.
The commonly used cast irons are white cast iron, grey cast iron, ductile and nodu-
lar cast iron, malleable cast iron and alloy cast iron. White cast iron is extremely hard,
wear-resistant, brittle and hard to machine. Typical applications are found in extrusion
dies and railway brake shoes. Grey cast iron has high compressive strength and good
wear resistance, yet it is brittle and weak under tension and has poor weldability. Due
to low cost and convenient batch casting, grey cast iron is widely used in engine blocks,
machine bases and frames. Ductile cast iron, or nodular cast iron, has substantial duc-
tility, improved tensile strength, stiffness and impact resistance. Typical applications
include engine crankshafts and heavy-duty gears. Malleable cast iron is annealed white
cast irons with high strength and elastic modulus, good machinability and wear resis-
tance. Typical applications are railway equipment and construction machinery. Nickel,
chromium and molybdenum are the common elements added in alloy cast iron. The
addition of these elements increases strength, hardness, wear resistance and impact
resistance [8–10].
Cast steels have the same alloying elements as wrought steels. Since the mechanical
properties can be modified by heat treatment, cast steels are used for complex shaped
elements requiring a high strength.

1.4.1.3 Nonferrous Alloys
Commonly used nonferrous alloys in machine elements include aluminium alloys, mag-
nesium alloys, nickel alloys, titanium alloys, zinc alloys and copper alloys.
Aluminium alloys have both wrought and cast forms. The attractive properties of
aluminium alloys are lightweight, good corrosion resistance, relative ease of forming
and machining. They are widely used in structural and mechanical applications without
strength requirements. Magnesium alloys are the lightest engineering metals, have a
high strength-to-weight ratio and are widely used in aircraft and automotive industries.
Nickel alloys are used in applications requiring corrosion and oxidation resistance,
∘
high strength and toughness at temperature extremes as high as 1093 Coraslow as
∘
−240 C [10]. Examples are turbine engine components, chemical processing equipment
and marine components. Commercially available nickel alloys include Monel, Inconel
and Hastelloy.
Titanium alloys have excellent corrosion resistance, outstanding strength-to-weight
ratios and high-temperature strength. However, titanium alloys are expensive and have
poor machinability. Typical applications of titanium alloys include aerospace and air-
craft structures, marine components and human internal replacement devices.
Zinc alloys are inexpensive with moderate strength. They have low melting tempera-
ture and widely used in castings and zinc galvanizing. Typical zinc die-castings include
automotive parts, building hardware and so on.
Copper alloys include brasses and bronzes. When copper is alloyed with zinc, it is
called brass. Brass has excellent machinability and corrosion resistance and is used in
tubing or piping, screw connections, locks, watches and so on. Bronze refers to cop-
per alloyed with metals such as tin, lead, phosphor, aluminium, silicon, nickel. Wrought