Page 42 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
20
During annealing treatment, the material is heated above the upper critical
temperature and is held for a designated time until the composition becomes uniform.
It is then cooled slowly in a furnace to a temperature below the lower critical tempera-
ture and continues to be cooled to room temperature outside the furnace [9]. Annealing
produces a soft, ductile, low-strength material, free of significant internal stresses with
a refined grain microstructure.
Normalizing is similar to annealing, except the material is heated above the upper
critical temperature and cooled in still air to room temperature. Compared with
annealing, the rapid cooling in normalizing produces harder steel with higher strength
and a coarser grain structure. Normalizing is often used as a final treatment [8].
Through-hardening is accomplished by heating elements above the transformation
temperature, followed by rapidly cooling within a quenching medium such as water or
oil [9]. In the controlled cooling rate process or quenching, austenite formed above the
transformation temperature is transformed into martensite; the hardest, strongest form
of steel. The rate of cooling determines the amount of transformation and thus the hard-
ness and strength.
Tempering is usually executed immediately after quenching. It involves reheating the
hardened steel to a temperature below the transformation temperature range followed
by a desired cooling to ambient temperature. The selection of tempering temperate
depends upon the composition and the degree of hardness or toughness required. With
increasing tempering temperature, tensile strength and yield strength decrease, whereas
ductility improves [9]. The tempering process modifies the steel properties and relieves
residual stresses.
Case hardening hardens material surfaces only. Case hardening includes carburiz-
ing, nitriding, cyaniding, carbonitriding, induction hardening and flame hardening
processes. Carburizing or nitriding is based on the diffusion of additional carbon or
nitrogen to some depth on the surface of already machined and heat-treated elements
to achieve a high surface hardness. Induction hardening and flame hardening rapidly
heat medium carbon or alloy steel element surfaces for a limited time so that a small,
controlled depth of material reaches the transformation range then, upon immediate
quenching and tempering, only the area above the transformation range produces the
high level of martensite required for high hardness. Case hardening produces a great
hardness on the outer surface, while retaining ductility and toughness in the core [8, 10].

1.4.4 Material Selection

The selection of materials is an important decision in machine element design. Satis-
factory performance of machine elements and machines depends greatly on materials
and their properties. Materials selection starts by referring to previous applications or
similar experiences, the desirable material properties and the knowledge of physical,
economical and processing properties of materials. Here are main points for material
selection. A more detailed systemic approach can be found in reference by Ashby [18].
The required material properties are determined by service conditions, potential
failure modes and so on. Service conditions could be fluctuating loads, corrosive
environments or high temperatures. The required service performance relates to the
corresponding material properties. Therefore, an element subjected to a fluctuating
load should have high fatigue strength; an element working in a corrosive environment