机械专业英语金属热处理

2021年1月22日发(作者：席潮海)
Heat treatment of metal
In industry today there are more than a thousand different metals being used
to manufacture products. The modem automobile has more than one hundred different
metals
used in
its
construction. An attempt will
be made in
this
passage
to
give an
understanding of the basic classification of metals.
Metals were formerly thought to be those elements that had a metallic luster
and
were
good
conductor
of
heat
and
electricity.
Actually,
metals
are
generally
defined
as
those
elements
whose
hydroxides
from
bases
(such
as
sodium
or
potassium).the nonmetals* hydroxides from acids (such as sulphur). Metals may exist
as
pure
elements.
When
two
or
more
metallic
elements
are
combined,they
form
a
mixture called an alloy
The term alloy is used to identify any metallic system. In metallurgy it is a
substance,
with
metallic
properties,
that
is
composed
of
two
or
more
elements,
in
timately
mixed.
Of
these
elements
one
must
be
a
metal.
Plain
carbon
steel,
in
the
sense, is basically an alloy of iron and carbon. Other elements are present in the form
of impurities. However, for commercial purposes, plain carbon steel is not classified
as an alloy steel.
Alloy
maybe
further
classified
as
ferrous
and
nonferrous.
Ferrous
alloys
contain iron. Nonferrous alloys do
not contain commercial varieties of iron
and steel are alloys. The ordinary steels are thought of as iron-carbon alloys. However,
practically all contain silicon and manganese as well. In addition, there are thousands
of
recognized
alloy
steels.
Examples
are
special
tool
steels,
steels
for
castings,
forgings, and rolled shapes. The base metal for all these is iron.
Steels arc often called by the principal alloying clement present. Examples
arc
silicon
steel,
manganese
steel,
nickel
steel,
and
tungsten
steel.
Even
nonferrous
alloys may contain iron in a




small amount, as impurities. Some of the nonferrous
alloys are bronze, brass, and monel.
Although pure metals solidify at a constant temperature, alloys do not. The
first
nuclei
have
a
tendency
to
form
at
a
higher
temperature
than
that
at
which
complete
solidification
occurs.
Each
element
in
an
alloy
has
its
own
peculiarities
relative to temperature. Thus, the change in temperature as solidification progresses
causes the solid being formed to change in chemical composition.
Many alloying elements dissolve in the base metal in different proportions
in liquefied and solidified steels. The proportion of the alloying clement that remains
in solid solutions has a tendency to vary with the temperature and grain structure of
the allov that is formed.
Nonferrous
metals
are
seldom
formed
in
the
pure
state.
They
must
be
separated from the gangue before the ore can be reduced. Thus, a process known as
ore-dressing is performed. Metals and metal compounds arc heavier than the ganguc.
They settle to the bottom if such a mixture has been agitated in water. This process is
similar
to
the
method
used
by
the
early
miners
who
panned
for
gold.
However,
refinements have been developed to speed up the accumulation of metal compound of
metal compounds by using this principal.
The
reverberatory
furnace
is
the
type
most
often
used
in
the
smelting
of
nonferrous metals. This furnace is constructed of refractory brick with a steel structure
on the outside. The charge is placed in the furnace and heated indirectly by the flame.
Slag inducers or fluxes are added to the charge to reduce oxidation.

Properties of metals
Metals have properties that distinguish them from other materials. The most
important
of
these
properties
is
strength,
or
the
ability
to
support
weight
without
bending
or
breaking.
This
property
combined
with
toughness,
or
ability
to
bend
without
breaking,
is
important.
Metals
also
have
advantages
regarding
resistance
to
corrosion. They arc responsive to heat treatment.
Metals
can
be
cast
into
many
shapes
and
sizes.
They
can
be
welded,
hardened,and softened. Metals also possess another important property-recycling and
reuse. When a particular product is discarded, it can be cut into convenient sections.
These sections can be put into a furnace, remelted, and used in another product.
The
properties
of
metals
may
be
classified
in
three
categories:
chemical
properties,
mechanical
properties,
and
physical
properties.
Here
we
will
emphasize
the
primary
mechanical
properties
of
metals.
In
understanding
the
related
areas
of
metal working and methods used today, the mechanical properties of metals are of the
utmost importance.
The
hardness
of
metals
varies
greatly.
Some,
like
lead,
can
be
indented
easily.
Others
like
tungsten
carbide,
approach
diamond
hardness.
They
arc
of
great
value as dies for cutting tools of various types. Heat treatment causes changes in the
hardness. Annealed tool steel can readily be machined. Often, this is difficult after it
has
been
hardened
and
tempered.
Annealed
brass
is
comparatively
soft.
When
cold- worked the hardness is greatly increased.
A
tough
metal
possesses
very
high
strength.
It
also
has
the
capability
to
deform permanently and resist rupture. Toughness enables the metal to survive shock
or impact without fracture.
The
strength
of
a
metal
is
its
ability
to
resist
deformation
or
rupture.
In
certain items, a combination of strength and plasticity is desirable. Machine tools are
an example.
Annealing
The
word
anneal
has
been
used
before
to
describe
heat-treating
processes
for softening and regaining ductility in connection with cold working of material. It
has
a
similar
meaning
when
used
in
connection
with
the
heat
treating
of
allotropic
materials. The purpose of full annealing is to decrease hardness, increase ductility, and
sometimes
improve
machinability
of
highcarbon
steels
that
might
otherwise
be
difficult
to
cut.
The
treatment
is
also
used to
relieve
stresses,
refine
grain
size,
and
promote uniformity of structure throughout the material.
Machinability
is
not
always
improved
by
annealing.
The
word
machinability is used to describe several interrelated factors, including the ability of a
material
to
be
cut
with
a
good
surface
finish.
Plain
low
carbon
steel,
when
fully
annealed, are soft and relatively weak, offering litter resistance to cutting, but usually
having
sufficient
ductility
and
toughness
that
a
cut
chip
tends
to
pull
and
tear
the
surface from which it is removed, leaving a comparatively poor quality surface, which
results
in
a
poor
machinablity
of
many
of
the
higher
plain
carbon
and
most
of
the
alloy steels can usually be greatly improved by annealing, as they are often too hard
and strong to be easily cut at any but their softest condition.
The
procedure
for
annealing
hypoeutectoid
steel
is
to
heat
slowly
to
approximately 60

soak for a long enough period that the
temperature equalizes throughout the material and homogeneous austenite is formed,
and then to allow the steel to cool very slowly by cooling it in the furnace or burying
it in the maximum ferrite and the coarsest pearlite to place the steel in its softest, most
ductile, and least strained condition.
Normalizing
The purpose of normalizing is somewhat similar to that of annealing with
the
Exceptions that the steel is not to its softest condition and the pearlite is left
rather fine instead of coarse. Refinement of grain size, relief of internal
stresses, and improvement of structural uniformity together with recovery of
some
ductility provide high toughness qualities in normalized steel. The process
is frequently used for improvement of machinability and for stress relief to reduce

distortion that might occur with partial machining or aging.
The
procedure
for
normalizing
is
to
austenitize
by
slowly
heating
to
approximate
80c
above
the
AC3
or
Accm3
temperature
for
hypoeutectoid
or
hypereutectoid.
Steels, respectively; providing soaking time for the formation of austcnitc;
and cooling slowly in still air. Note that the steels with more carbon than the eutectoid
composition
arc
heated
above
the
Accm
instead
of
the
Ac
13
used
for
annealing.
The
purpose
of
normalizing
is
to
attempt
to
dissolve
all
the
cementite
during austenitization to eliminate, as for as possible, the settling of hard, brittle
iron
carbide
in
the
grain
boundaries.
The
desired
decomposition
products
are smallgrained, fine pearlite with a minimum of free ferrite and free cementite.
Sphcroidizing
Minimum
hardness
and
maximum
ductility of steel
can be produced by
a
process called spheroidizing, which causes the iron carbide to form in small spheres
or nodules in a ferrite matrix. In order to start with small grains that spheroidize more
readily,
the
process
is
usually
performed
on
normalized
steel.
Several
variations
of
processing are used, but all require the holding of the steel near the Al temperature
(usually slightly bclow)for a number of hours to allow the iron carbide to form on its
more stable and lower energy state of small, rounded globules.
The
main
need
for
the
process
is
to
improve
the
machinability
quality
of
high
carbon
steel
and
to
pretreat
hardened
steel
to
help
produce
greater
structural
uniformity after quenching because of the lengthy treatment time and therefore rather
high
cost,
sphcroidizing
is
not
performed
nearly
as
annealing
or
normalizing.
Hardening of steel
Most of the heat treatment hardening processes for the steel is the based on
the production of high percentages of martensite .The first step, therefore, is that
Used
for
most
of
the
other
heat-treating
processes-treatment
to
produce
austenite.
Hypoeutectoid
steels
are
heated
to
approximately
60t:
above
the
Ac3
temperature
and
allowed
to
soak
to
obtain
temperature
uniformity
and
austcnitc