Heat treatmentANNEALING PROCESSESannealing refers to a heat
treatment in which a material is exposed to an elevated temperature
for an extended time period and then slowly cooled.Purposerelieve
stresses; increase softness, ductility,and toughness; and/or
produce a specific microstructurethree stagesheating to the desired
temperature,holding or soaking at that temperature, and cooling,
usually to room temperature.Process Annealingsoften and increase
the ductility of a previously strain-hardened metal. (Recovery and
recrystallization)Stress Relief(1)plastic deformation processes
such as machining and grinding; (2) nonuniform cooling of a piece
that was processed or fabricated at an elevated temperature, such
as a weld or a casting; and (3) a phase transformation that is
induced upon coolingwherein parent and product phases have
different densities(heated-long -cooled)Heat-treating temperature
ranges for plain carbonsteels
NormalizingAn annealing heat treatment called normalizing is
used to refine the grains (i.e., to decrease the average grain
size) and produce a more uniform and desirable size distribution;
fine-grained pearlitic steels are tougher than coarse-grained
ones.Normalizing is accomplished by heating at least 55 above the
upper critical temperatureAustenitizingAfter sufficient time has
been allowed for the alloy to completely transform to
austenite.Full Anneal: often utilized in low- and mediumcarbon
steels that will be machined or will experience extensive plastic
deformation during a forming operation.
The alloy is then furnace cooled; that is, the heat-treating
furnace is turned off and both furnace and steel cool to room
temperature at the same rate, which takes several hours.(coarse
pearlite- soft and ductile)SpheroidizingMCS and HCS coarse
pearlite-hard to machine. Heating the alloy at a temperature just
below the eutectoid [line A1, 700C]If the precursor microstructure
contains pearlite, spheroidizing times will ordinarily range
between 15 and 25 h. Heating to a temperature just above the
eutectoid temperature, and then either cooling very slowly in the
furnace, or holding at a temperature just below theeutectoid
temperature. Heating and cooling alternately within about+/-50C of
the linePRECIPITATION HARDENINGstrength and hardness of some metal
alloys may be enhanced by the formation of extremely small
uniformly dispersed particles of a second phase within the original
phase matrixAge hardening is also used to designate this procedure
because the strength develops with time, or as the alloy ages.phase
diagram for aprecipitation-hardenablealloy of composition Co
aluminumcopper, copperberyllium, coppertin, and
magnesiumaluminum; some ferrous alloys are also precipitation
hardenableOveragingreduction in strength and hardness that occurs
after long time periods is known as overaging.
TemperingTempering is a process of heat treating, which is used
to increase the toughness of iron-based alloys. It is also a
technique used to increase the toughness of glass.tempering is
usually performed after hardening, to reduce some of the excess
hardness, and is done by heating the metal to a much lower
temperature than was used for hardening.Case hardeningFlame and
induction hardeningCarburizing-0.1 and 0.3 wt% CNitriding-heats the
steel part to 482621CCyaniding-fast and efficient; it is mainly
used on low carbon steels-871-954C-NaCNCarbonitriding-a gaseous
atmosphere of ammonia and hydrocarbons is usedFerritic
nitrocarburizing- N2 difuusion
