材料成型及控制工程专业外语阅读

材料成形及控制工程

—专业英语阅读张枫129024032钟娜娜129024033周兵129024034祝龙飞129024035热加工—HotWorking在金属学中，把高于金属再结晶温度的加工叫热加工（T>T再）。热加工可分为金属铸造、热轧、锻造、焊接和金属热处理等工艺。有时也将热切割、热喷涂等工艺包括在内。热加工能使金属零件在成形的同时改善它的组织，或者使已成形的零件改变结晶状态以改善零件的机械性能。铸造、焊接是将金属熔化再凝固成型。热轧、锻造是将金属加热到塑性变形阶段，再进行成型加工，如合金钢需加热到形成均匀奥氏体后，进行热轧、锻造，温度低塑性不好，易产生裂纹，温度过高金属件易过分氧化，影响加工件质量。金属热处理只改变金属件的金相组织，它包括:退火、正火、淬火、回火等。物理冶金—Physical

Metallurgy通过非化学方法达到改变金属性能目的之冶金相关知识的学科，其主要为常见的主题包括退火、调幅分解、形核、长大和粒子粗化等冶金过程的原理。3.

1

Physical

Metallurgy

of

Hot

WorkingThe

principles

of

the

physical

metallurgy

of

hot

working

are

now

well

recognized.

During

the

deformation

process

itself,

e.g.

a

rolling

pass,

work

hardening

takes

place

but

is

balanced

by

the

dynamic

softening

processes

of

recovery

and

recrystallization.

These

processes,

which

are

thermally

activated,

lead

to

a

flow

stress

that

depends

on

strain

rate

and

temperature

as

well

as

on

strain.

The

structural

changes

taking

place

within

the

material

result

in

an

increase

in

dislocation

density

with

strain

until

in

austenitic

steels

and

nickel-

and

copper-base

alloys

a

critical

strain

（εc）is

reached

when

the

stored

energy

is

sufficiently

high

to

cause

dynamic

recrystallization

.

With

further

strain,

dynamic

recrystallization

takes

place

repeatedly

as

the

new

recrystallized

grains

are

themselves

work-hardened

to

the

critical

level

of

stored

energy.

热加工物理冶金的原理现在得到了充分的认识。在变形过程中，例如轧制过程，加工硬化发生，但会被回复和再结晶的动态软化过程所平衡。这些热激活的过程，产生一个取决于应变率、温度以及应变的流动应力。材料内部结构发生变化，产生应变，导致位错密度增加；在奥氏体钢和镍-铜合金中，当内部储存能足够高时会发生动态再结晶，材料达到一个临界应变（εc），此时位错密度不再增加。随着应变进一步发生，因为新的再结晶晶粒自身发生加工硬化，使内部储存能达到临界水平，动态再结晶会重复发生。PS:高温下金属材料的塑性变形过程可分为三个阶段：第一阶段，在变形过程中发生加工硬化和动态软化两个过程，这两个过程交替进行；第二阶段，随着变形量的增加，位错密度继续增加，内部储存能也继续增加，当变形达到一定程度，使奥氏体发生另一种转变，即发生动态再结晶。动态再结晶的发生与发展，使更多的位错消失，奥氏体的变形抗力下降，直到奥氏体全部发生动态再结晶，应力达到稳定值；第三阶段，奥氏体发生动态再结晶之后，变形量不断增加，应力值基本保持不变，呈现稳定状态。PS:动态再结晶要达到临界变形量和在较高的变形温度下才能（奥氏体发生另一种转变）发生。PS:最终晶粒的大小取决于成核和晶粒长大的相对速率。上述两者都与温度有关，提高再结晶温度，晶粒尺寸会增加。These

dynamic

structural

changes

leave

the

metal

in

an

unstable

state

and

provide

the

driving

force

for

static

recovery

and

static

recrystallization

to

take

place

after

the

deformation

pass.

Static

recrystallization

may

be

followed

by

grain

growth

if

the

temperature

is

sufficiently

high.这些动态结构变化会使材料处于不稳定的状态，并且为变形后的静态回复和静态再结晶的产生提供驱动力。如果温度足够高，静态再结晶之后可能会有晶粒长大。In

order

to

be

able

to

apply

these

principles

to

commercial

working

processes,

we

require

answers

to

two

main

questions:

(a)

how

long

does

recrystallization

take

place

after

a

deformation

pass？(b)

what

grain

size

is

produced

by

recrystallization

and

grain

growth?

The

answers

determine

the

structure

of

the

material

entering

the

next

and

subsequent

passes

and

hence

influence

the

flow

stress

of

the

material

and

the

working

forces

required.

Eventually

they

determine

the

structure

and

properties

of

the

hot

worked

products.为了在商业的生产过程中应用这些原理，我们需要去回答两个主要的问题：a.变形过程之后多长时间才会发生再结晶？b.再结晶和长大之后的晶粒度是多少？答案决定了材料进行下一步和之后过程的组织结构，也因此影响材料的流动应力和需要的工作应力。最终它们决定了热轧产品的结构和性能。Words

and

Terms

：physical

metallurgy

物理冶金

work

hardening

加工硬化

static

recovery静态回复

thermally

activated

热激活的

hot

working

热加工

dynamic

softening

动态软化

recrystallization

再结晶

dislocation

density

位错密度

critical

strain

临界应变Questions：

l)

When

does

dynamic

recrystallization

take

place

within

the

material

work

hardened?

2)

What

do

the

answers

to

the

two

questions

determine?3.

1.

1

Dynamic

Structural

Change

During

the

deformation

of

austenite

at

hot-workingtemperaturesand

constant

strain

rate,

the

characteristic

form

of

stress-straincurve

observed

is

illustrated

in

Fig.

3.

1.Thesecurvesareforlow-alloysteels,testedintorsion,butaresimilartothoseobtainedforothersteelsintheausteniticconditiontestedintorsion,tension,orcompression.Afterinitialrapidwork-hardeningthecurvesgothroughamaximumassociatedwiththeoccurrenceofdynamicrecrystallization.The

peak

in

flow

stress

occurs

after

some

low

fraction

of

recrystallization

has

taken

place

so

the

strain

to

the

peak(εp)isalwaysgreaterthanthecriticalstrainfordynamicrecrystallization(εc).在热加工温度和恒定的应变率下的奥氏体变形期间，得到的应力—应变特性曲线如图3.1所示。这些曲线是对测试转矩的低合金钢而言，但与那些处于奥氏体状态下的钢在测试转矩，拉伸或者压缩方面相似。在最初的快速加工硬化之后，曲线的应变达到最大值，这与动态再结晶有关系。流动应力的峰值产生在一些小部分的再结晶发生之后，峰值处的应变εp总是大于动态再结晶的临界应变εc。图3-1

The

relationship

between

the

two

strains

is

complex

,

but

it

hasbeen

suggested

thatεc=αεp(whereαisaconstant)isareasonableapproximationforconditionsofdeformationofinterestinhotworking.However,theproposedvaluesofαdiffer,being0.83,0.86,and0.67.ItcanbeseenfromFig.3.1thatεpincreasessystematicallywithZener-Hollomonparameter(Z),independentoftheparticularcombinationofstrainrate(ε)andtemperature(T/K)intherelationship Z=εexpQdef/RTWheretheactivationenergyQdefforthisalloysteelis314KJ/mol.Asimilarvalueof312KJ/molisappropriateforarangeofC-Mnsteelsbutlowervaluesof270and286KJ/molhavealsobeenobserved.这两种应变之间的关系复杂，但对于在热加工