Page 41 - Analysis and Design of Machine Elements

Figure 1.3 Hardness conversions [17]. An Overview of Machine Design 19

performance and failure modes have close relations to mechanical properties. For
example, wear relates closely to the hardness of a material. The higher the carbon con-
tent, the greater the hardness of steels. Commonly used hardness definitions in machine
elements are Brinell hardness (HBW or HB), Rockwell hardness (HRA/HRB/HRC,
where A, B, C represent different scales) [8] and Vickers hardness (HV). The conversion
from HRC to HV and HBW (300 kg load, 10 mm ball) is presented in Figure 1.3.
Physical properties refer to density, electrical, magnetic and optical properties, as well
as thermal properties, such as thermal conductivity, thermal expansion, specific heat,
melting point, flammability and so on. Chemical properties include composition, cor-
rosion resistance, degradation, oxidation, embrittlement and so on. The consideration of
chemical properties closely related to the operating environments of machine elements.
Dimensional properties refer to the size, shape, surface finish, tolerances and appearance
of raw materials or blank materials, which affect manufacturing costs. Processing prop-
erties refer to the materials’ manufacturability; that is, capability for machining, casting,
forming, welding and so on. Usually, ductile materials can be easily forged, rolled or
drawn, while brittle materials should be shaped by other methods.

1.4.3 Heat Treatments
The final material properties are dramatically affected by heat treatments. Heat treat-
ment refers to time-controlled or temperature-controlled heating and cooling processes
with an aim to modify materials’ mechanical properties. Commonly used heat treat-
ments for steel machine elements include annealing, normalizing, quenching, temper-
ing and case hardening.