金属热处理中英文对照外文翻译文献

transformation

curves

can

be

plotted

as

shown

in

Fig.1

for

eutectoid

steel

．

The

curve

at

extreme

left

represents

the

time

required

for

the

transformation

of

austenite

to

pearlite to start at any given temperat ure

．

Similarly

，

the curve at extreme right represents the time

required for completing the transformation

．

Between the two curves are the points representing partial

transformation.

The

horizontal

lines

Ms

and

Mf

represent

the

start

and

finish

of

martensitic

transformation.

Classification of Heat Treating Processes

In

some

instances

，

heat

treatment

procedures

are

clear-cut

in

terms

of

technique

and

app lication

．

whereas in other instances

，

descriptions or simple explanations are insufficient because

the

same

technique

frequently

may

be

used

to

obtain

different

objectives

．

For

example,

stress

relieving and tempering are often accomplished with the same equipment and by use of identical time

and temperature cycles

．

The objectives

，

however

，

are different for the two processes

．

The

following

descriptions

of

the

principal

heat

treating

processes

are

generally

arranged

according to their interrelationships

．

Normalizing consists of heating a ferrous alloy to a suitable temperature (usually 50

°

F to 100

°

F or 28

℃

to 56

℃

) above its specific upper transformation temperature

．

This is followed by cooling

in

still

air

to

at

least

some

temperature

well

below

its

transformation

temperature

range

．

For

low-carbon

steels,

the

resulting

structure

and

properties

are

the

same

as

those

achieved

by

full

annealing

；

for most ferrous alloys, normalizing and annealing are not synonymous.

Normalizing usually is used as a conditioning treatment, notably for refining the grains of steels

that

have

been

subjected

to

high

temperatures

for

forging

or

other

hot

working

operations.

The

normalizing process usually is succeeded by another heat treating operation such as austenitizing for

hardening, annealing, or tempering.

Annealing is a generic term denoting a heat treatment that consists of heating to and holding at a

suitable

temperature

followed

by

cooling

at

a

suitable

rate.

It

is

used

primarily

to

soften

metallic

materials, but also to simultaneously produce desired changes in other properties or in microstructure.

The

purpose

of

such

changes

may

be,

but

is

not

confined

to,

improvement

of

machinability,

facilitation of cold work (known as in-process annealing), improvement of

mechanical or electrical

properties, or to increase dimensional stability. When applied solely to relive stresses, it commonly is

called stress-relief annealing, synonymous with stress relieving.

When the term

“

an nealing

”

is applied to ferrous alloys without qualification, full annealing is

applied. This is achieved by heating above the alloy

’

s transformation temperature, then applying a

cooling

cycle

which

provides

maximum

softness.

This

cycle

may

vary

widely,

depending

on

composition and characteristics of the specific alloy.

Quenching is a rapid cooling of a steel or alloy from the austenitizing temperature by immersing

the workpiece in a liquid or gaseous medium. Quenching medium commonly used include water, 5%

brine, 5% caustic in an aqueous solution, oil, polymer solutions, or gas (usually air or nitrogen).

Selection of a quenching medium depends largely on the hardenability of material and the mass

of the material being treating (principally section thickness).

The

cooling

capabilities

of

the

above- listed

quenching

media

vary

greatly.

In

selecting

a

quenching

medium,

it

is

best

to

avoid

a

solution

that

has

more

cooling

power

than

is

needed

to

achieve the results, thus minimizing the possibility of cracking and warp of the parts being treated.

Modifications

of

the

term

quenching

include

direct

quenching,

fog

quenching,

hot

quenching,

interrupted quenching, selective quenching, spray quenching, and time quenching.

Tempering. In heat treating of ferrous alloys, tempering consists of reheating the austenitized and

quench-hardened steel or iron to some preselected temperature that is below the lower transformation

temperature (generally below 1300

℃

or 705

℃

). Tempering offers a

means of

obtaining various

combinations of mechanical properties. Tempering temperatures used for hardened steels are often no

higher

than

300

℃

(150

℃

).

The

term

“

tempering

”

should

not

be

confused

with

either

process

annealing or stress relieving. Even though time and temperature cycles for the three processes may be

the same, the conditions of the materials being processed and the objectives may be different.

Stress relieving. Like tempering, stress relieving is always done by heating to some temperature

below

the

lower

transformation

temperature

for

steels

and

irons.

For

nonferrous

metals,

the

temperature may vary from slightly above room temperature to several hundred degrees, depending

on the alloy and the amount of stress relief that is desired.

The

primary

purpose

of

stress

relieving

is

to

relieve

stresses

that

have

been

imparted

to

the

workpiece from such processes as forming, rolling, machining or welding. The usual procedure is to

heat workpiece to the pre-established temperature long enough to reduce the residual stresses (this is a

time- and

temperature-dependent

operation)

to

an

acceptable

level;

this

is

followed

by

cooling

at

a

relatively slow rate to avoid creation of new stresses.

The

generally

accepted

definition

for

heat

treating

metals

and

metal

alloys

is

“

heating

and

cooling a solid metal or alloy in a way so as to obtain specific conditions or properties.

”

Heating for

the sole purpose of hot working (as in forging operations) is excluded from this definition

．

Likewise

，

the

types

of

heat

treatment

that

are

sometimes

used

for

products

such

as

glass

or

plastics

are

also

excluded from coverage by this definition

．

Transformation Curves

The basis for heat treatment is the time-temperature- transformation curves or TTT curves where

，

in a single diagram all the three parameters are plotted

．

Because of the shape of the curves

，

they are

also sometimes called C-curves or S-curves

．

To

plot

TTT

curves

，

the

particular

steel

is

held

at

a

given

temperature

and

the

structure

is

examined at predetermined intervals to record the amount of transformation taken place

．

It is known

that

the

eutectoid

steel

(T80)

under

equilibrium

conditions

contains

，

all

austenite

above

723

℃，

whereas below

，

it is pearlite

．

To form pearlite

，

the carbon atoms should diffuse to form cementite

．

The

diffusion being a rate process

，

would require sufficient time for complete transformation of austenite

to pearlite

．

From different samples

，

it is possible to note the amount of the transformation taking place