鼎湖山-

2021年2月9日发(作者：大棚建设)

The

understanding

of

heat

treatment

is

embraced

by

the

broader

study

of

metallurgy.

Metallurgy is the physics, chemistry, and engineering related to metals from ore extraction to the

final product.

Heat treatment is the operation of heating and cooling a metal in its solid state to change its

physical properties. According to the procedure used, steel can be hardened to resist cutting action

and abrasion, or it can be softened to permit machining.

With the proper heat treatment internal stresses

may be removed, grain size reduced, toughness

increased,

or

a

hard

surface

produced

on

a

ductile

interior.

The

analysis

of

the

steel

must

be

known

because

small

percentages

of

certain

elements,

notably

carbon,

greatly

affect

the

physical properties.

Alloy steel owe their properties to the presence of one or more elements other than carbon,

namely

nickel,

chromium,

manganese,

molybdenum,

tungsten,

silicon,

vanadium,

and

copper.

Because

of

their

improved

physical

properties

they

are

used

commercially

in

many

ways

not

possible with carbon steels.

The

following

discussion

applies

principally

to

the

heat

treatment

of

ordinary

commercial

steels known as plain carbon steels. With this process the rate of

cooling is the controlling factor,

rapid cooling from above the critical range

results in hard structure, whereas very

slow cooling

produces the opposite effect.

•

A Simplified Iron-carbon Diagram

If we focus only on the materials

normally known as steels, a simplified

diagram is often

used.

Those portions of the iron-carbon diagram near the delta region and those above 2% carbon

content are of little importance to the engineer and are deleted. A simplified diagram, such as the

one in Fig.2.1, focuses on the eutectoid region and is quite useful in understanding the properties

and processing of steel.

The key transition described in this diagram is the decomposition of single-

phase austenite(γ)

to the two-phase

ferrite plus carbide structure as

temperature drops.

Control

of

this

reaction,

which

arises

due

to

the

drastically

different

carbon

solubility

of

austenite and ferrite, enables a wide

range of properties to be achieved through heat treatment.

To

begin

to

understand

these

processes,

consider

a

steel

of

the

eutectoid

composition,

0.77%

carbon,

being

slow

cooled

along

line

x-

x’

in

Fig

.2.1.

At

the

upper

temperatures,

only

austenite is present, the 0.77% carbon being dissolved in solid solution with the iron. When the

steel cools to 727

℃

(1341

℉

), several changes occur simultaneously.

The iron wants to change from the FCC austenite structure to the BCC ferrite structure, but

the ferrite can only contain 0.02% carbon in solid solution.

The rejected carbon forms the carbon- rich cementite intermetallic with composition

Fe3C.

In

essence,

the

net

reaction

at

the

eutectoid

is

austen

ite

0.77%C→ferrite

0.02%C+cementite

6.67%C.

Since this chemical separation of the

carbon component occurs entirely in the

solid state,

the resulting structure is a fine mechanical mixture of ferrite and cementite. Specimens prepared

by polishing and etching in a weak solution of nitric acid and alcohol reveal the lamellar structure

of alternating plates that forms on slow cooling.

This

structure

is

composed

of

two

distinct

phases,

but

has

its

own

set

of

characteristic

properties and goes by the name pearlite, because of its resemblance to mother- of- pearl at low

magnification.

Steels

having

less

than

the

eutectoid

amount

of

carbon

(less

than

0.77%)

are

known

as

hypo-eutectoid steels. Consider now the transformation of such a material represented by cooling

along line y-

y’ in

Fig.2.1.

At

high

temperatures,

the

material

is

entirely

austenite,

but

upon

cooling

enters

a

region

where

the

stable

phases

are

ferrite

and

austenite.

Tie-line

and

level-law

calculations

show

that

low-carbon ferrite nucleates and grows, leaving the remaining austenite richer in carbon.

At 727

< p>
℃

(1341

℉

), the austenite is of eutectoid composition (0.77% carbon)

and further cooling

transforms the remaining austenite to pearlite. The resulting structure is a mixture of primary or

pro-eutectoid

ferrite (ferrite that formed

above the eutectoid reaction) and regions

of pearlite.

Hypereutectoid

steels

are

steels

that

contain

greater

than

the

eutectoid

amount

of

carbon.

When such steel cools, as shown in z-

z’ of Fig.2.1 the process is similar to the hypo

-eutectoid case,

except that the primary or pro-eutectoid phase is now cementite instead of ferrite.

As

the

carbon-rich

phase

forms,

the

remaining

austenite

decreases

in

carbon

content,

reaching

the

eutectoid

composition

at

727

℃

(1341

℉

).

As

before,

any

remaining

austenite

transforms to pearlite upon slow cooling through this temperature.

t should be remembered that the transitions that have been described by the phase diagrams

are for equilibrium conditions, which

can be approximated by slow cooling. With slow

heating,

these transitions occur in the reverse

manner.

However,

when

alloys

are

cooled

rapidly,

entirely

different

results

may

be

obtained,

because sufficient time is not provided for the normal phase reactions to occur, in

such cases, the

phase diagram is no

longer a useful tool for engineering

analysis.

•

Hardening

Hardening

is

the

process

of

heating

a

piece

of

steel

to

a

temperature

within

or

above

its

critical range and then cooling it rapidly.

If the carbon content of the steel is known, the proper temperature to which the steel should

be heated may be obtained by reference to the iron-iron carbide phase diagram. However, if the

composition

of

the

steel

is

unknown,

a

little

preliminary

experimentation

may

be

necessary

to

determine the range.

A good procedure to follow is to heat-quench a number of small specimens of the steel at various

temperatures

and

observe

the

result,

either

by

hardness

testing

or

by

microscopic

examination.

When the correct temperature is obtained, there will be a marked change in hardness and other

properties.

In any heat-treating operation the rate of heating is important. Heat flows from the exterior to

the interior of steel at a definite rate. If the steel is heated too fast, the outside becomes hotter than

the interior and uniform structure cannot be obtained.

If a piece is irregular in shape, a slow rate is all the more essential to eliminate warping and

cracking. The heavier the section, the longer must be the heating time to achieve uniform results.

Even after the correct temperature has been reached, the piece should be held at that temperature

for a sufficient period of time to permit its thickest section to attain a uniform temperature.

he

hardness

obtained

from

a

given

treatment

depends

on

the

quenching

rate,

the

carbon

content, and the work size. In alloy steels the kind and amount of alloying element influences only

the hardenability (the ability of the workpiece to be hardened to depths) of the steel and does not

affect the hardness except in unhardened or partially hardened steels.

Steel

with

low

carbon content

will

not

respond appreciably

to

hardening

treatment.

As

the

carbon

content

in

steel

increases

up

to

around

0.60%,

the

possible

hardness

obtainable

also

increases.

Above

this

point

the

hardness

can

be

increased

only

slightly,

because

steels

above

the

eutectoid

point

are

made

up

entirely

of

pearlite

and

cementite

in

the

annealed

state.

Pearlite

responds

best

to

heat- treating

operations;

and

steel

composed

mostly

of

pearlite

can

be

transformed into a hard steel.

As the size of parts to be hardened increases, the surface hardness decreases somewhat even

though

all

other

conditions

have

remained

the

same.

There

is

a

limit

to

the

rate

of

heat

flow

through steel.

No matter how cool the quenching medium may be, if the heat inside a large piece cannot

escape faster than a certain critical rate, there is a definite limit to the inside hardness. However,

brine or water quenching is capable of rapidly bringing the surface of the quenched part to its own

temperature and maintaining it at or close to this temperature.

Under

these

circumstances

there

would

always

be

some

finite

depth

of

surface

hardening

regardless of size. This is not true in oil quenching, when the surface temperature may be high

during the critical stages of quenching.

•

Tempering

Steel that has been hardened by rapid quenching is brittle and not suitable for most uses. By

tempering or drawing, the hardness and brittleness may be reduced to the desired point for service

conditions

．

As these properties are reduced there is also a decrease in tensile strength and an increase in

the ductility and toughness of the steel. The operation consists of reheating quench-hardened steel

to some temperature below the critical range followed by any rate of cooling.

Although this process softens steel, it differs considerably from annealing in that the process lends

itself to close control of the physical properties and in most cases does not soften the steel to the

extent that annealing would. The final structure obtained from tempering a fully hardened steel is

called tempered martensite.

Tempering is possible because of the instability of the martensite, the principal constituent of

hardened steel. Low- temperature draws, from 300

℉

to 400

℉

(150

℃

~205

℃

), do not cause much

decrease in hardness and are used principally to relieve internal strains.

As the tempering temperatures are increased, the breakdown of the martensite takes place at a

faster rate, and at about 600

℉

(315< /p>

℃

) the change to a structure called tempered martensite is very

rapid.

The

tempering

operation

may

be

described

as

one

of

precipitation

and

agglomeration

or

coalescence of

cementite.

A substantial precipitation of cementite begins at 6 00

℉

(315

℃

), which produces a decrease in

hardness. Increasing the temperature causes coalescence of the carbides with continued decrease

in hardness.

In

the

process

of

tempering,

some

consideration

should

be

given

to

time

as

well

as

to

temperature.

Although

most

of

the

softening

action

occurs

in

the

first

few

minutes

after

the

temperature

is

reached,

there

is

some

additional

reduction

in

hardness

if

the

temperature

is

maintained for a prolonged time.

Usual

practice

is

to

heat

the

steel

to

the

desired

temperature

and

hold

it

there

only

long

enough to have it uniformly heated.

Two

special

processes

using

interrupted

quenching

are

a

form

of

tempering.

In

both,

the

hardened steel is quenched in a salt bath held at a selected lower temperature before being allowed

to

cool.

These

processes,

known

as

austempering

and

martempering,

result

in

products

having

certain desirable physical properties.

•

Annealing

The primary purpose of annealing is to soften hard steel so that it may be machined or cold

worked.

This is usually accomplished by heating the steel too slightly above the critical temperature,

holding it there until the

temperature of the piece is uniform

throughout, and then cooling at a

slowly controlled rate so that the temperature of the surface and that of the center of the piece are

approximately the same.

This process is known as full annealing because it wipes out all trace of previous structure,

refines the crystalline structure, and softens the metal. Annealing also

relieves internal stresses

previously set up in the metal.

The

temperature

to

which

a

given

steel

should

be

heated

in

annealing

depends

on

its

composition;

for

carbon

steels

it

can

be

obtained

readily

from

the

partial

iron-iron

carbide

equilibrium diagram. When the annealing temperature has been reached, the steel should be held

there until it is uniform throughout.

This usually takes about 45min for each inch(25mm) of thickness of the largest section. For

maximum softness and ductility the cooling rate should be very slow, such as allowing the parts to

cool down with the furnace. The higher the

carbon content, the slower this rate must be.

The heating rate should be consistent with the size and uniformity of sections, so that the entire

part is brought up to temperature as uniformly as possible.

•

Normalizing and Spheroidizing

The

process

of

normalizing

consists

of

heating

the

steel

about

50

℉

to

100

℉

(10

℃

~40

℃

)

above the upper critical range and cooling in still air to room temperature.

This

process

is

principally

used

with

low-

and

medium-carbon

steels

as

well

as

alloy

steels

to

make the grain structure more uniform, to relieve internal stresses, or to achieve desired results in

physical properties. Most commercial steels are normalized after being rolled or cast.

Spheroidizing is the process of producing a structure in which the cementite is in a spheroidal

distribution. If steel is heated slowly to a temperature just below the critical range and held there

for a prolonged period of time, this structure will be obtained.

The globular structure obtained gives improved machinability to the steel. This treatment is

particularly useful for hypereutectoid steels that must be machined.

•

Surface Hardening

Carburizing

The

oldest

known

method

of

producing

a

hard

surface

on

steel

is

case

hardening

or

carburizing.

Iron

at

temperatures

close

to

and

above

its

critical

temperature

has

an

affinity

for

carbon.

The carbon is absorbed into the metal to form a solid solution with iron and converts the outer

surface

into

high-carbon

steel.

The

carbon

is

gradually

diffused

to

the

interior

of

the

part.

The

depth of the case depends on the time and temperature of the treatment.

Pack

carburizing

consists

of

placing

the

parts

to

be

treated

in

a

closed

container

with

some

carbonaceous material such

as charcoal or coke. It is a

long process and used to produce fairly

thick cases of from 0.03 to 0.16 in.(0.76~4.06mm) in depth.

Steel

for

carburizing

is usually

a

low-carbon

steel of

about

0.15% carbon

that

would not

in

itself

responds

appreciably

to

heat

treatment.

In

the

course

of

the

process

the

outer

layer

is

converted into high- carbon steel with a content ranging from 0.9% to 1.2% carbon.

A steel with varying carbon content and, consequently, different critical temperatures requires

a special heat treatment.

Because there is some grain growth in the steel during the prolonged carburizing treatment,

the work should be heated to the critical temperature of

the core and then cooled, thus refining

the core structure. The steel should then be reheated to a point above the transformation range of

the case and

quenched to produce a hard, fine structure.

The

lower

heat-treating

temperature

of

the

case

results

from

the

fact

that

hypereutectoid

steels are normally austenitized for hardening just above the lower critical point. A third tempering

treatment may be used to reduce strains.

Carbonitriding

Carbonitriding,

sometimes

known

as

dry

cyaniding

or

nicarbing,

is

a

case-hardening

process in which the steel is held at a temperature above the critical range in a gaseous atmosphere

from which it absorbs carbon and nitrogen.

Any carbon-rich gas with ammonia can be used. The wear-resistant case produced

ranges from

0.003

to

0.030

inch(0.08~

0.76mm)

in

thickness.

An

advantage

of

carbonitriding

is

that

the

hardenability of the case is significantly increased when nitrogen is added, permitting the use of

low-cost steels.

Cyaniding

Cyaniding, or liquid carbonitriding as it is sometimes called, is also a process that combines

the absorption of carbon and nitrogen to obtain surface hardness in low-carbon steels that do not

respond to ordinary heat treatment.

The

part

to

be

case

hardened

is

immersed

in

a

bath

of

fused

sodium

cyanide

salts

at

a

temperature slightly above the Ac1 range, the duration of soaking depending on the depth of the

case. The part is then quenched in water or oil to obtain a hard surface.

Case

depths

of

0.005

to

0.015in.

(0.13~0.38mm)

may

be

readily

obtained

by

this

process.

Cyaniding is used principally for the treatment of small parts.

Nitriding

Nitriding is somewhat similar to ordinary case hardening, but it uses a different material and

treatment to create the hard surface constituents.

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