material & processes for ndt part 2

7/25/2019 Material & Processes for NDT Part 2

1/11

THE

NATURE

OF

MATERIALS

AND

SOLID STATE

CHANGES

IN METALS

31

The

Effect

of Energy on the Atom. Metallic

Structure.

Solidification.

Grain

Size. SOLID

STATE

CHANGES

IN

METALS: Work

Har-

dening. Plastic

Deformation.

Cold

Work.

RE-

CRYSTALLIZATION:

Recovery.

Recrystalli-

zation.

Grain

Growth. AGE HARDENING.

ALLOTROPIC CHANGES.

HEAT

TREAT.

MENT

OF

STEEL:

Approximate

Equilibrium

Heat-Treatment

Processes. Austenitization.

Annealing.

Normalizing.

Spheroidizing.

Har-

dening

of

Steel.

Tempering.

CORROSION:

Di

rect

Chemical Action.

Electrolytic

(Electro-

chemical)

Reaction.

Corrosion

Rate Dependent

on

Several

Factors.

Types of

Corrosion.

Corro-

sion Frotection.

FERROUSMETALS

45

Choosing Metals

and

Alloys.

Ferrous

Raw

Ma-

terials.

CAST IRONS.

STEEL: Wrought

lron.

Steel

Making.

Plain Carbon

Steel.

Alloy

Steels.

Low

Alloy

Structural Steels. Low

Alloy

AISI

Steels. Stainless

Steels.

Tool

and

Die

Steels.

Cast

Steels. MATERIAL IDENTIFICATION

SYSTEMS.

NONFERROUS

METALS AND

PLASTICS

57

ALUMINUM ALLOYS:

General

Properties.

Wrought

Aluminum Alloys. Property

Changes.

Cast

Aluminum

Alloys.

COPPER

ALLOYS:

General Properties.

Brasses

and

Bronzes.

NICKEL

ALLOYS.

MAGNESIUM ALLOYS.

ZINC

ALLOYS. SPECIAL

GROUPS

OF

NONFERROUS

ALLOYS:

Heat-

and Corro-

sion-Resistant

Alloys.

Other

Nonferrous

Metals.

NON-METALS:

Plastics. Plastic

Ma-

terials.

Types

of Plastics.

Characteristics

of

Plastics.

THE

NATURE

OF MANUFACTURING 73

MODERN

MANUFACTURING:

Markets. De-

sign. Processing.

States

of Matter.

Shape-

Changing

Processes.

Summary.

THE

CASTING PROCESS

79

The

Process.

SOLIDIFICATION

OF

METALS:

Solidification. Shrinkage.

POUR-

ING

AND

FEEDING

CASTINGS: Casting

Design.

Pouring. The

Gating System. Risers.

Chills. FOUNDRY

TECHNOLOGY.

SAND

MOLDING:

Green

Sand. Patterns. Flasks.

Sand Compaction.

Cores. Green Sand Advan-

tages

and

Limitations.

Dry

Sand Molds.

Floor

and Pit

Molds.

Shell Molds.

METAL

MOLD

AND

SPECIAL

PROCESSES:

Permanent

Mold

Casting.

Die

Casting.

Investment

Cast-

ing.

Plaster Mold

Casting.

Centrifugal

Cast-

ings.

Continuous

Casting.

MELTING

EQUIP-

MENT:

Cupola.

Crucible

Furnaces. Pot

Fur-

naces.

Reverberatory

Furnaces. Electric

Arc

Furnaces.

Induction

Furnaces. FOUNDRY

MECHANIZATION.

THE

WELDING

PROCESS

95

BONDS: Nature of

Bonding. Fusion

Bonding.

Pressure

Bonding. Flow Bonding. CoId

Bond-

ing.

WELDING

METALLURGY:

Composi-

tion

Efects.

Effects on Grain Size and Struc-

ture.

Effects

of

Welding

on Properties.

DIS-

TORTIONS

AND

STRESSES.

WELDING

PROCESSES

AND

DESIGN

105

HEAT

FOR

WELDING:

Chemical

Reactions.

The Electric Arc. Welding

Equipment and Pro-

cedures.

Arc Welding

Electrodes. Modification

of Arc

Welding

for

Special

Purposes. Automa-

tic

Welding.

Electric Resistance

Heating.

SPECIAL WELDING

PROCESSES: Elec-

tron-Beam Welding.

Plasma Arc.

Ultrasonic

Welding.

Friction

Welding.

Electroslag

Welding.

Explosion

Welding.

Diffusion

Weld-

ing.

WELDING

DESIGN:

Joints.

Design

Con-

siderations.

Weldability.

WELD

DEFECTS:

Fusion

Welding.

Dimensional Defects.

Struc-

tural

Discontinuities.

Weld Metal and

Base

Metal

Properties. Basic

Symbols

for

NDT.

11

PLASTIC FLOW

r27

EFFECTS

OF

DEFORMATION:

Work

Har-

dening

and

Recrystallization. Effects of

Flow

Rate.

Direction Effects.

Temperature

and

Load-

ing

Systems

Effects. Grain

Size.

RELATIVE

EFFECTS

OF

HOT

AND

COLD

WORKING:

Mechanical Properties.

Finish

and Accuracy.

Process

Requirements.

MILLWORK, FORGING, AND POWDER

METALLURGY

I27

MILLWORK:

Hot

Rolling.

Cold

Finishing.

T\rbe

and

Pipe Making. Extrusion. FORGING

AND ALLIED

OPERATIONS:

NDT of Forg-

ings.

Open

Die

Forging.

Closed

Die

Forging.

Forging

with

Progressive Application of Pres-

sure. Powder

Metallurry. Pressing.

Sintering.

Sizing and Postsintering Treatments.

Applica-

tion

for

Powdered

Metal

Products.

PRESSWORKING

OF SHEET

METAL

L4I

Shearing.

Bending.

Drawing. New Develop-

ments in

Sheet

Metal Forming.

14

MACHININGFUNDAMENTALS

r47

The Machining Process.

Chip

Formation.

Cut-

ting

Tool

Materials.

Abrasives.

MACHINE

TOOLS: Machinabilitv. Finish.

NUMERICAL

CONTROL.

10

I2

13


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5/11

2

Materials

and

Processes

for

NDT

Technology

Decisions

to

accept

or reject

following a

test

result

must

be based on a

thorough

knowledge

of

materials

and

the

properties,

processes

and

their effect

on

pro-

perties,

test

techniques, design

requirements,

product

applications, service

conditions,

and suitable

life ex-

pectancy.

Clearly

this

much knowledge

is seldom

lo-

cated

in

a single

individual, and

group

decisions

or con-

sultations

may

be

necessary.

NDT

correlation

may

require

the

cooperation

be-

tween test supervisors, designers,

metallurgists,

man-

ufacturing

personnel,

customer

personnel,

and

test

personnel.

REQUTREMENTS

FOR

NDT

SUPERVISORY

PERSONNEL

From the

above

it

can

be seen

that

supervisory

per-

sonnel

in

charge

of

nondestructive

testing

operations

must have

adequate background

knowledge

for

resolu-

tion of

complex

problems

in

establishing

tests and

in'

terpreting results.

Background

Knowledge.

It

is

important

that

a

NDT

supervisor be

well

versed

in

all

the

available

NDT methods, their

applications

and

limitations.

In

addition, the reliability of the

methods

and

their corre-

lation

with

desired material and

product

characteris-

tics

are

very

important.

Knowledge

of the

product

design,

purpose,

and

func-

tion together

with

process

details

may enhance appli'

cation of test methods

by

supplying

information

re-

garding

the

importance

of the

test interpretations

and

possible

sources

of

discontinuities,

faults, and/or

de-

fects

that

could cause

product

failure.

Familiarity

with

all

policies,

local,

industry-wide,

governmental,

or safety and

environmental

agencies

may

affect the inspection methods chosen.

Although

these

policies

may be seemihgly

unimportant

at

times,

they

can

have

very strong

implications and

influences.

Ability

To

Communicate

Is Always

Important.

The

ability

to

communicate

in

both

written

and

oral

manner

cannot

be

overemphasized in

consideration

of success-

ful

job

accomplishment,

particularly

in supervisory

positions.

It

is

necessary

for

a

NDT

supervisor

to

direct,

in-

struct,

and

manage

the

personnel

of that

group.

It is essential that

cooperation be

maintained

with

other manufacturing

or

working

personnel

with

whom

the

projects

are

associated.

There is need for

concise

accurate

reports to

higher

rranagement.

These reports,

by

the

way,

particularly

when

written,

are the

principal

evidence

by which

repu-

tation

and advancement

are developed.

In addition,

NDT

supervisory

personnel

are

often

called

upon

to

interface

with

customer

personnel in

solving

proble'nq

and interpreting tst results.

QUALIFICATION

AND

CERTIFICATION

OF

NDT

LEVEL III

TESTING PERSONNEL

The American Society

for Nondestructive

Testing,

in

its Recommended

Practice

No.

SNT-TC-IA

entitled

"Personnel

Qualification

and

Certification

in

Nondes-

tructive Testing"

indicated

the

responsibilities

and

capabilities of

Level III

nondestructive

test

personnel

in

the

following words:*

"An

NDT

Level

III

individual should be capable

of

establishing

techniques

and

procedures;

interpreting

codes,

standards,

specifications, and

procedures;

and

designating the

particular

test methods, techniques,

and

procedures

to

be used.

The NDT Level III should

be

responsible

for

the

NDT

operations

for

which

quali

fied

and

to

which

assigned,

and should be capable

of in-

terpreting and evaluating

results

in

terms of existing

codes,

standards, and specifications.

The NDT Level

III should

have

sufficient

practical

background

in

ap-

plicable

materials,

fabrication,

and

product

technology

to establish techniques and to assist

in

establishing ac-

ceptance

criteria

where none

are otherwise

available.

The

NDT

Level III should

have

general

familiarity

with

other appropriate NDT

methods,

and should

be

qualified

to

train

and examine

NDT

Level

I

and

Level

II

personnel

for

certification."

SOURCES OF

TECHNICAL

INFORMATION

AVAILABLE TO NDT

PERSONNEL

Regardless

of the sources

of technical

knowledge

by

which

NDT

personnel

obtain their

positions, perpetual

updating

and extension

of

that

knowledge is essential.

Continual

Improvement

of

Entire Knowledge

Base

Needed.

Additional

knowledge of NDT

techniques,

par'

ticularly

new developments,

is very important.

Accom-

panying

this,

however, should be broadening

of

knowl-

edge,

as

well as

keeping

up

with

new

developments,

in

the

field

where

the NDT techniques are

applied.

Greater

familarity

with engineering

materials, manu-

facturing

processes,

changing

designs and

service

re'

quirements,

management

policies, government

re'

quirements,

environmental

impacts,

personal

safety

and other areas can be

vital

to successful

application

of

NDT.

Many

Sources

of

NDT Information

Available.

Per-

sonal ambition and

expenditure

of

time

permit

informa'

tion to

be

gained

in

a

variety

of

ways.

Activity

(not

simply attendance)in

suitable

technical

societies can be a

good

source of

new methods and

ap-

plications

of

both

testing and

manufacturing

tech-

niques.

*The

statement

of

NDT Level

III

requirements is taken

from

the

June

1980 Edition

of

ASNT

Recommended

Practice No. SNT-TC-1A,

as illustrative of

general

requirements. Note

that future changes

may

occur in SNT-TC-IA, and the current document should

be

re-

ferred

to in

a.ll cases.


6/11

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7/11

4

Materials and

Processes

for NDT

Technology

may require

inspection to be certain

that

they

will

main-

tain their

as-manufactured

qualities

such as

color,

polish,

stability,

etc.

Products more likely to

need

careful

testing and

eval-

uations are those

used

in load

carrying

applications

where failure may involve

loss

of use,

expensive

repair,

or

danger

to

other

products,

structures,

and

even

life.

Although the manufactured item

is

a

product,

it is

the

material

of

that

product

that

may

fail,

so

material fail-

ure

types

and

causes

are of

interest.

Material

Failure Definition. The simplest

definition

of failure is that the item of interest becomes

unusable,

but

there

are

several ways

in

which

a

product may

be'

come

unusable.

It is

usually

important to

know the

type

of failure that might

be

expected

in

order

to know

for

what to inspect, how to inspect,

how

to

eliminate

the

fault,

and

how

to

assess

the risk of

failure.

Complex units

with

moveable

parts

may

become

in-

operable

because

of failure of

some

minor element.

An

automobile, for

example,

may not

run

for

lack of

fuel,

a

tire

goes

flat, or the ignition is out of adjustment.

There

are

two

generally

accepted

types

of

material

failure:

one is the easily recognized

fracture

or separa-

tion into two or more

parts;

the second

is the

less

easily

recognized

permanent

deformation

or change

of shape

and/or

position.

Although

complete

fracture is

unmistakable,

an

inci'

pient

type

which

will

be discussed

in

connection

with

"fatigue failure" with suitable inspection

methods

can

be determined before complete failure

occurs.

Fracture

failure

in some

complex structures

may

also

become

progressive.

An example of

progressive

failure would

be

the release of load

by

some weak

component

in a struc-

tural

configuration

such

as

a

large bridge.

The released

load

must

be

absorbed

by

neighboring structural

ele-

ments.

Unless

these neighbors can spread

the

new

load

and become stabilized,

they

will become

overloaded

and, if

stressed above their elastic

limit, will

deform,

crack, or fracture,

causing

additional

load to

be

passed

along in

a

way that

causes

the entire structure

to

col-

lapse

almost

immediately.

A

simple example of

progressive

failure

would be

the

breaking

of

a

gear

tooth

in

a

mechanical

power

system.

The following

gear

tooth is

then subjected

to shock

(im-

pact)

loads

which

increase

the

stress

levels

so

that

fail-

ure

is

more likely. In this

case also

it is

probable

that

if

power

is

maintained, all

the gear

teeth

will

be

broken

off

in

a short

period

of time.

Material

Failure

Causes.

Products and structures

may

be subject to a

number

of service conditions.

Im-

posed

loads

may

be static

(stationary

or fixed) or

dyna-

mic

(varying).

The

use

environment

may

contribute

cor-

rosion,

vibration,

or temperatures and

pressures higher

or

lower

than normal.

The

product

may

also be

subject

to

abuse. Mechanical

failure

is

always

a

result

of

stresses.

above

some

critical

value

for

each

material

that

cause

deformation

or

fracture. Such

excessive

stresses are

set

up

by

some

combination

of material

de-

fect,

excess

load,

improper type load, or design error.

1.

As far

as

failure is

concerned.

static

loads

some-

times include

dynamic

loads

that

are

slowly

applied.

The

principal

reasons

for failure

under

static

loads in-

clude large

discontinuities

(both

internal

and

external),

poor

dimensional

control during

manufacturing,

mas-

sive overloading

during use, and

unsatisfactory

ori-

ginal

design or

combination

of

these

factors.

2.

Dynamic

loads

are

varying

loads

that can

be

single-directional or

multi-directional

with multidirec-

tional loading

being

more

serious

as a cause

for failure.

When the

cycles

of loading

become

high

(usually

mil-

lions

but dependent

on

the

material),

failure

can

occur

at

stress levels far

below those determined

by

static

load

tests. Although millions

of cycles seems high, there

are

many

applications, such as

a

rotating

shaft

under

bend-

ing load,

where

millions

of

loading

cycles

can

be

reached

in

a

relatively

short time.

As

pointed

out, slow or low frequency

dynamic

load-

ing

is

similar

to

static

loading

except that

even

low fre-

quency

loads

applied suddenly

create

a

condition

of

shock which can

cause

failure

at a

level lower

than nor-

mally

expected.

3.

Service at

high temperature

reduces

most

of

the

desirable

material

properties

of

metals

including the

ability

to support

load. The tendency

for creep also

in-

creases

with

increased temperature.

The temperatures

at

which

property

values

become critical depend upon

the

particular

material

and the

previous

treatment

it

has received.

Most

metallic materials

also

exhibit

a brittle charac-

teristic

(much

like

cold

glass)

through

a transition tem-

perature

range

usually at

lower

temperatures.

4.

Pressure creating stress

above a

material's

elastic

limit

may

cause

material flow

(plastic

flow),

distortion,

and

cross-sectional weakening,

effects that would

be

in-

tensified

at

elevated temperatures.

Fluctuating

pres-

sures

of high

frequency

create

dynamic

loading

that

may

decrease

safe

operatinglevels. Itwas

reported that

several

early day

pressurized

cabin

aircraft

failed by

fuselage

skin

failure

due

to this

cause.

5.

Corrosive environments or

a

combination of

mate-

rials

that

cause

corrosion can

produce

failure in two

ways.

The

corrosion

may

actually

reduce

the amount of

material

available to

carry

load,

but

even

more

impor-

tant

in

many

cases

is

that the

corrosion

may

create

small

discontinuities which serve

as stress

risers that

become

the nucleus

for fatigue failure.

6.

Many

structures and systems

are

subjected to vi-

bration during

service. Included

are

transportation

equipment, machines,

and

devices

that

have moving

parts.

In

addition, some structures may

vibrate be-

cause

they

are excited by some outside influence.

Stresses

from

vibration may

be superimposed

on

stresses

from other loading

sources. The

principal

prob-

lem

created

by

vibration is

the introduction

of

cyclic

loading

leading

toward fatigue.failure.

7.

Excess

loading

from

abuse

may

be accidental

but

nearly

always has the human

element

as a

source.

Con-

trol is

attempted

by use

of

design

factors

of safety,


8/11

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6

\Iaterials

and

Processes

ior

NDT

Technology

rial

is

the

same as though the

procedure

were carried

on

rg6

shnnge the material

properties

To

some degree,

knowledge

of the

processing

is

there-

fore necessary

in

order to understand

the effect on

the

noaterial.

to evaluate

the

material

properties,

and

to

crace

down the

source of

problems.

PURPOSE

FOR

USE OF

NDT

A

critical

task for

persons

responsible for nondestruc-

tive testing

operations

is

often

the

determination

of the

true

reasons

why

nondestructive

tests

have been

re-

quested,

specified, or

needed.

The

interpretation

of

test

indications

depends

critically upon

the

purpose

of the

tests,

and

this often

determines

the

stage

of

manufac-

turing or assembly

at which

tests

should

be done.

Ultimate

Purpose-Reliability,

Serviceability.

In

the

preceding paragraphs,

the common

assumption

has

been made that the tests

are

to

be used

to assure

relia-

bility

and to

prevent premature

failures

of

materials,

parts,

or

assemblies

during their

intended service.

In

general,

this

serviceability

is

the

ultimate

purpose

of

most

nondestructive tests.

However,

it is often the

case

that

the test

itself

does

little

to

predict

the serviceabil-

ity

or

safety

of the

final

product,

assembly,

structure,

or

system. In this

case, there

is no

way to determine

that a

discontinuity or

material condition constitutes a

dan-

gerous

"defect" which may lead to

premature

failure

in

service. This by

no

means

invalidates

or countermands

the

need

for

the

nondestructive

test. Other

valid

rea-

sons

for nondestructive

testing

may include the

follow-

ing.

Specific Purposes for

NDT.

1.

Identification or sorting

of

material.

2.

Identification

of material

properties

and

the relia-

bility associated with their existence.

3.

Indication of

proper

material

and suitable

quality

control during

processing

in

order

to

prevent

further

costly

processing.

4. Tests to

assure completeness,

proper

dimensions

and

geometry,

and

proper

relationships among

as-

sembled components.

5.

Tests

during

service

to

discover

initiation

of

pos-

sible

failure

before

it

actually occurs.

6. Diagnostic tests after failure to determine

the

fail-

ure

reason.

This knowledge might

be

useful

for

product

design

change,

test

method

change,

quality

control

re-

cords,

and

for

records

to combat

possible

product liabil-

ity suits.

NDT

IN

FRACTURE

CONTROL

Fracture

Control

Design

Philosophy.

When using

factors

of safety in

design,

it is

assumed

that competent

judgement

in

providing

a safety factor can

provide high

assurance of the safe life of

a

product

or structure.

At

the same

time, consideration must

be

given

to conserva-

tion of material.

Historically, many

complex

structures

and machines

have

survived admirably under such

de-

sign concepts. However, others

have failed from seem-

ingly inexplicable

causes,

some

with

costly and

disas-

trous

consequences.

Most

design

procedures

still

are based

upon

over

two

and a

half

centuries of using

Hooke's

law

which

relates

stress and strain

in

elastic bodies.

The

safety factor

is

applied to a value of strength

that the material used

in

a

design

is

presumed to

possess.

The

value

of

.strength is

that which

is hoped to

be

representative

of

the

material

used under

the

assumption

that the

material

is

continu-

ous

and

has

uniform

properties

throughout.

In this

de-

sign

process,

discontinuities can

be

accounted

for

,

if it

is

h,nown

they exist, by their effect

on reduction of cross-

sectional area available

to

sustain the applied

loads, or

their

effect on the

local

volumetric strength

of the

mate-

rial.

Conventional

design

practices

incorporating safety

factors ordinarily

will

succeed

if:

1.

the

material

used

will

not

be

accidentally

or

other-

wise overloaded,

2.

the

variations

in ordinary

loads

will

be as

in-

tended,

3.

the

environmental

factors are

properly

antici-

pated,

4.

unknown

or

undetected

discontinuities will

not

grow

to

a

critical

size.

Notable

catastrophic

failures have

been analyzed and

invariably

found to involve

an

oversight in

one or a com-

bination of the

above

factors

that caused

failure.

Sud-

den, complete, and unsuspected fracture failures occur

in components

and

structures

when

a

crack

or

other

de-

fect reaches

some

critical

size

and

rapidly

propogates.

Where the

consequences

of

unexpected

failure

are un-

usually

expensive in

terms

of

public

safety

and/or

money,

more

and

more designs of

such critical

nature re-

quire

consideration

of

fracture-resistant

qualities

in

materials

selection

and usage.

Implicit in frature

con-

trol

design

criteria

is

the

need,

by some means, to assure

that

unexpected

flaws

of some specific critical size are

not

present

when the

product

is

introduced into

service.

Nondestructive

testing and

proof

testing

are

the

princi-

pal

means

upon which such assurance is based. Proof

testing

always

involves

some

risk

that

the test

itself

will

either

cause

immediate

failure during

the

test,

or

cause

flaw

growth

to

a

point

below

actual

failure

but

be-

yond

the

point

where additional

flaw

growth

can be

safely sustained

during

the

service

life

of the

compon-

ent

in

question.

Nondestructive testing is

heavily relied

upon as

the

basis of assurance against the

presence

of

flaws large

enough

to

either

cause

immediate fracture or

to

grow

large

enough

to

cause

later

premature

fracture.

Often,

the successful implementation

of'a fracture control

de-

sign

depends

solely upon the

reliability

of

nondestruc-

tive testing to

detect

flaws in

otherwise sound com-


10/11

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11/11

8

Materials

and

Processes

for NDT

Technology

NDT

Demonstration

Programs.

Pioneering

efforts

to include

meaningful

fracture

control

criteria

into

structural

design

first appeared

in the

specifications

for

military

aircraft

in the

late

1960s.

Since

then,

fracture

control

criteria

have been

applied

to

aircraft,

space-

craft,

nuclear

components,

pipelines,

and

pressure

ves-

sels.

The

most ambitious

programs to

date

have

in-

cluded

several

military

aircraft

and

NASA's

Space

Shuttle.

Typically,

the

overall

vehicle

specifications

have

required the contractors

involved

to

demonstrate

inspection

capabilities

to

detect

certain

size

flaws

in

components

designated

fracture

critical

at90Vo

proba-

bility

of detection,

957o

confidence

level.

Several

approaches

have been

used

by

the contrac'

tors

to

provide

the

required

demonstration.

Generally,

some

economically

practical numbers

of

fatigue

cracked

specimens

are

prepared

and

intermingled

with

unflawed

specimens.

Well-defined

procedures

are

used

by

inspection

personnel under

production

conditions.

The

parts

used

in

the

demonstration

are

usually

entered

into

the

normal

sequence

of

inspection

so

that

the

in-

spection

personnel

are

not

biased

toward

particular

awareness

that

a

demonstration

is being

conducted.

In

other

cases,

only

certain

inspection

personnel have been

classified

as

being

qualified to

inspect

fracture

critical

components.

Such

components

are clearly

marked

and

channeled

specifically

to

their

selected

inspectors.

In

any

event,

through

experiment

design

and

statistical

analysis,

the outcome

of

NDT demonstration

programs

is

intended

to

provide

a workable

interface

between

the

needs of

fracture

mechanics

and

the

realities

of

the ap-

plications

and

limitations

of NDT.

material & processes for ndt part 2

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