Page 147 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

As a general guideline for part design for heat treating, sharp internal or exter-
nal corners should be avoided; otherwise, stress concentrations at these corners may
raise the level of stresses high enough to cause cracking. The part should have its
thicknesses as nearly uniform as possible; also, the transition between regions of dif-
ferent thicknesses should be made smooth. Parts with holes, grooves, keyways,
splines, and asymmetrical shapes may likewise be difficult to heat treat, because they
may crack during quenching. Large surfaces with thin cross sections are likely to
warp. Hot forgings and hot steel-mill products may have a decarburized skin (a
layer that has lost its carbon, Section 4.10); as a result, they may not respond suc-
cessfully to heat treatment.

SUMMARY

° Commercially pure metals generally do not have sufficient strength for most en-
gineering applications; consequently, they must be alloyed with various elements
which alter their structures and properties. Important concepts in alloying are the
solubility of alloying elements in a host metal and the phases present at various
ranges of temperature and composition.
° Alloys basically have two forms: solid solutions and intermetallic compounds.
Solid solutions may be substitutional or interstitial. There are certain conditions
pertaining to the crystal structure and atomic radii that have to be met in order to
develop these structures.
° Phase diagrams show the relationships among the temperature, composition,
and phases present in a particular alloy system. As temperature is decreased at
various rates, correspondingly various transformations take place, resulting in
microstructures that have widely different characteristics and properties. Among
the binary systems, the most important is the iron-carbon system, which in-
cludes a wide range of steels and cast irons. Important components in this sys-
tem are ferrite, austenite, and cementite. The basic types of cast irons are gray
iron, ductile (nodular) iron, white iron, malleable iron, and compacted-graphite
iron.
° The mechanisms for hardening and strengthening metal alloys involve heating
the alloy and subsequently quenching it at varying cooling rates. As a result,
important phase transformations take place, producing structures such as
s heroidite, bainite and martensite. Heat treatin
P earlite fine or coarse » P » g of
nonferrous alloys and stainless steels involves solution treatment and precipita-
tion hardening.
° The control of the furnace atmosphere, the quenchants used, the characteristics of
the equipment, and the shape of the parts to be heat treated are important heat-
treatment considerations. Hardenability is the capability of an alloy to be hard-
ened by heat treatment. The Jominy end-quench hardenability test is a method
commonly used to determine hardenability bands for alloys.
° Case hardening is an important process for improving the wear and fatigue resist-
ance of parts. Several methods are available, among them carburizing, nitriding,
induction hardening, and laser-beam hardening.
° Annealing includes several alternative processes (normalizing, process annealing,
stress relieving, tempering, austempering, and martempering), each having the
purpose of enhancing the ductility and toughness of heat-treated parts.