Fatigue Resistance of Oxygen Cut Steel

8/10/2019 Fatigue Resistance of Oxygen Cut Steel

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•

WELDING

RESEARCH

SUPPLEMENT

TO

THE WELDINC JOURNAL, AUCUST 1977

Sponsored

by the merican

Welding

Society

and the Welding

Research

Council

Fatigue Resistance

o

Oxygen ut Steel

atigue

life

s

strongly

influenced

by

oxygen

cut

surface

roughness and fractography indica tes that resolidified

metal deposits at the torch edge of a cut may often be

the si te

of fatigue crack

initiation

BY

R.

PLECKI

R. YESKE

C.

ALTSTETTER AND F V.

LAWRENCE

JR.

Oxygen cut surfaces of

roughnesses were pre

Fatigue specimens of high

trength, low

alloy

ASTM A572) and

, quenched-and-tem

ASTM A514) grades

of

steel ·

prepared

with four

controlled

Load-controlled

were performed using a

stress cycle. The sur

face roughness was quantified by the

MS deviation from

flatness , and

the

microstructures were

metallographically and

microhardness measurements.

Fatigue

life

was

found

to be strongly

influenced by roughness, and to differ

by

as

much

as

an

order

of magnitude

etween rough and smooth surfaces at

the lowest stress leve

ls.

Fractography

indicated

that resolidified metal de

osits at the torch edge of the cut were

often the site

of

fatigue crack

initia

tion .

The effect of oxygen cutting on

fatigue

life

could be qualitatively

understood on the basis of the stress

oncentration resulting from an array

of

surface notches.

Introduction

The oxygen

cutting of

thi ck steel

sect ions is

an economical alternative

to other cutting and shaping

methods

,

particularly if little further machining

or

grinding of the cut edge is required.

In fatigue applications, the surface and

sub-surface regions exert a strong

influence

on behavior; thu s, it is

necessary to

assess

the i m pact

of

oxygen cutting in these regions. In

addition to the modification of surface

properties

due

to chemical changes,

microstructural

changes and changes

in the state of residual stress, changes

in geometrical features such as rough

ness, gouges, drag lines,

melted

beads

and cracks may be introduced by

oxygen

cutting.

The effect of oxygen cutting

on

fatigue life has been previously stud

ied in

mi

Id steel

having

a ferrite

pearlite microstructure,

and results

of

different investigators vary widely.'-

Processing variables have been found

to significantly modify the steel hard

ness and microstructure.

However

,

these

va

riables have

not

always been

sys tematically controlled, and the state

of the metal has been

quite

variable.

R. PLECKI is Researeh Assistant R. YESKE is

Assistant Prol essor C

ALTSTETTER

is

Pro-

lessor Department 01 Metallurgy nd

Minin g and

F V. LA

WRENCE

IR

is Asso

eiate Prolessor Departmenls 01 Civil Engi-

neering and Metallurgy nd Mining Uni

versily

01

lIIinois at Urbana-Champaign

Urbana /llin ois.

For example, Koenigsberger

2

and co

workers have reported both the pres

ence and the absence

of

significant

surface

hardening

, sub -surface

soften

ing, and through-thickness softening

32 mm) for different suppliers of

oxyge n cut specimens. The

Nether

lands

group'

has reported surface

hardnesses ranging

from 245

VHN

to

well

over

double

that value

as

a result

of changes in preheating procedure,

nozzle type, gas type and pressu

re s,

and cutting speed. Even Goldberg,'

who

defined surface quality and care

fully quantified

the

surface roughness,

found

wide variability in hardness

changes.

Despite

these changes in

the

mate

rial, the generally held view is that

excessive surface rough ness decreases

the fatigue

life

, and surface

rough

ening is the most important result of

oxygen cutting.

The

principal

objective of the pres

ent work was to extend the current

knowledge

of

fatigue

behavior of

oxygen cut surfaces to a

higher

stress

regime. Thus high strength , low alloy

ASTM A572) and quenched-and

tempered steels ASTM A514) were

investigated at short lifetimes . Surface

roughness was varied in a

controlled

manner. The

microstructure

and hard

ness profiles were

determined

for each

roughness and steel type. The fatigue

WELDINC RESEARCH SUPPLEMENT 225-s



Table

I-Mechanical

Properties

Material

A572

1

A5'14

1

A514 (normalized)lb '

Yield

Ultimate

stra n

strength

strength at fracture

5)',

ksi

5 , ksi

e

r

,

%

55

79 23

108.3

119 .0 15.2

52.0

o

( ISupplied y manufacturero .

( Ove(all spec imen length 12 in. 1 in . gage lenglh, 0.5 in. d a meler

redLJ

ced sectlon.

results could be rationalized by the

stress concentration e ff ects of a peri

odic

array of notc hes .

Experimental Procedure

Two structural steels were obtained

as 1 in . (25

mm) th i

ck

pl

at

e. One

wa s

co mparable

to

ASTM A572 grade 42

0.22% ma x. ca rb on,

42

ksi (290 MPa)

minimum yield

s

trength, and the

second

was

a

quenched-and-tem

pered steel

which

conformed to

ASTM

A514 grade e (0.2 max. carbon ,

0.005% max.

boron

, 100 k

si

(609

MPa)

minimum yield

strength).

The

tensile

properties

of these

mate

rials are given

in

Table 1.

The

pl ates

were cut

into

12 in. (305 mm) sq uares

and

mill

ed to the cross-sec tion shown

in

Fig

1.

Speci

mens

of a varying

rough

ne

ss

were

ox

yge n cut from these

blanks by moving

them

beneath a

cutting torch whi ch was oscillated at

different amplitudes

and

frequencies.

The cutting

parameters were

as

follows 1 ti p , propylene gas at 5 psig

(34 KP a), preheat oxygen at 7 psig (48

KP

a), c

utting

o xyge n at 50 psig (345

KPa), average

cutting

speed 9 ipm (3.8

mm /

s)

and

torch

tip

to

metal separa

tion of 1/ 4 in. (6 mm ) . After cutting,

one side of the specimen was milled

parallel

to the

opposite

oxygen cut

surface

to

give a

3/4

in .

sq uare

test

sect ion. AII spe ci mens

were

wire

brushed by

hand

before testing . Spec

imens were

mounted in

an MTS

machine using se lf-alignin g grips

and

were fatigue tested using a ze ro-to

tension

stress cycle

(R

= O

under load

co

nt r

o l at 5-'15 Hz .

The

contour

of the oxygen cut

surface of each spec imen wa s m ea

sured using an

LVDT

profilom eter with

a tungsten ca

rbide

stylus inc

lined

30

degrees

from

the surface normal.

Profile traces were reco

rded

at

the

midpoint and

at the melted

(upper)

edge.

The

RMS

amplitude

of

deviation

from

flatness

was

used

to characterize

the

notch-p

eak di str

ibution

.

Figure 2 sho

ws

traces

and photo

graphs of the four rou g

hn

ess levels

used in

this

s

tudy:

0.001 , 0.002, 0.007,

and

0.011

in

. (25

,50,180,

and 280

,um .

Triplicate

fatigue te sts were run at

each of

two

stress levels

and

four

roughness leve ls for each steel. Spec

imens of th e quenched-and-tempered

steel were also

normali

zed afte r oxy

gen c

uttin

g and then tes

ted

. A l imited

number of tests wer e also performed

on

machined

specimens.

Followi

ng fatigu e testi ng, selec

ted

spec imens were sec tioned and

mounted fo r op ti ca l

metallograph

y

and SEM fra c tograph

y. Microhardness

profil

es

n

orm

al

to th

e oxyge n cut

surface

were determin ed using a

Fleme Cul

Surfece

7

Radius (Machined)

6

1

- 12 ~

Flame Cul Surface

Machined

Surfaces

Fig. 1 Specimen

geo

metr

y

226 s I

AUGUST

19

77

Kno

op

indenter

and 100 gram (gm)

load.

Results

Metallography and Microhardness

O xygen

cutt

ing produces a var ie ty

of

micro

struc tures and several

kind

s of

hardness

profile.

At

many po in ts along

the surface of either stee l,

th

ere

was

a

decarburized laye r, p

ar t

icularly where

oxide droplet

s

had

so

lidified

on the

s

urface.

Th ese nodules

of resolidifi

ed

metal were invariabl y

de

ca

rburized

and

were light etched

with

3% nital

Fig . J

Microhardness

mea surements

verifi ed that these region s were much

so fter than the adJacent metal. Furth er

ev

idence

of deca rburi za tion

wa

s th e

absence of pea rlit e

in

these re g ions

after spec

imen

s

were normalized

(in

vacuum).

Below

th e

decarburized zone was

a

relati

ve

ly

hard

zone in both steels. In

A572 steel, the mi c ro

co

nst i tuent wa s

ac icular,

and it

s cha

racter

chan ged

from coarse

\lVidmanstatten

ferrite ju

st

under the

surface to the

appearan

ce

of

bainite

a

nd

th en marten site . In the

A514 steel, j ust under the decarburized

zone was a

dark

etching,

fine

grained,

m art ensitic zone extending

to

about

0.04 in. (1.0 mm)

below the

surface.

Beyond this zo ne was the intercriti ca l

heat affected zone

(HAZ),

con sis ting

of coarse

fe

rrit

e

grains with

som e

very

fine

pear

li t

e

colonies

at th e

grain

boundaries.

In

one s

pecimen

of A514,

there

was

a second zone of coa rse ferrite and

pearlite

at

0.

14-0.18

in. (3.6

to

4

.6

mm) .

Other

specimens of A514 showed only

a single suc h zone. The intercr it ica l

HAZ

in th e A572 steel extended from

about 0.06

to

0.10

in

. (1.5

to

2.5 mm)

below

the surface. In both stee ls, the

HAZ ex tended as mu ch as 0.2 in. (5

mm) below th e surface.

Figures 4

and

5 are microhardn

ess

traverses across

th

e HAZ

of

a sa

mple

of

each type o f stee l.

There

wa s a mo re

distinct tend enc y for the A514 to

show

plateaus of hardness which co uld be

co

rre

lated to the different

microstruc

tur

es.

In

A572

under

the martensiti

c

zone,

there

was a

general decrea se

in

hardness which fol lowed the observed

modificati o ns o f

pearlite

in a

ferrit

e

matrix.

Fatigue

Test

Results

The

res

ul t

s

of th

e

fatigue te

sts for

the two

steels are plotted

in

Fi gs . 6 and

7. The RMS rough ness was meas ured

for

eac h fa tigue specim en and the

po

in t

s in Figs. 6

and

7 a re

co

rre s

pond

ingl

y

identified.

A

numb

er

of

spec

imens

broke

in the grips, thereby

decreasing

th

e numb er of data points

somewhat. Fo r this reason , sca tt er

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Pii'U

" t i

1 ' MX I «

=§ f

1

in

the SEM

at

up

to X 1000,

the

oxygen

cut surfaees were smoothl y eo ntoured

in some plaees and dendritie in others.

The fatigue craeks

were often

seen to

:§f

1.0

l

have

not

been

ass igned

in a

s way. The lines in Figs.

6

and

7

least-squa res fit s for the roughest

smoot

he

st

spee

imen

s.

of the

re

sul ts for the

cut surfaee (Figs.

6

7)

shows that the

higher st

rength

has a higher fatigue

istanee than the A572 at 10 eyeles.

lO

eyele

s t

he fatigue strengths of

and A572 steels are

for

all

identi

ea l.

The A514 steel

a greater sensitivity to rough

cut surfaees in that for a given

life the

allowable

stress level

more

severely by a

oxygen-eut in A514 than in A572

is

partly

due to

the greater

of the S-N plot

for

A514.

fatigue failures almost always

at the upper edge, elosest to

to r

eh.

At

thi s edge there was

more reso lidified metal. At

s

point

al

so

, there is a le

ss

regular

aph y with somewhat greater

S roughn ess- Fig. 8.

gure 9 shows a crack in A514 steel

at ing at the edge of a deearburized

on the surfaee. When viewed

Fig

J- Oxide layer A) al oxygen cul

surface

01

A574 sleel specimen adjacenl to

underl

ying

dec

a

rburized zone

(8)

follow the interdendriti c spaces and

the boundary between

smooth

and

dendritie areas. The fracture surfaees

them se

lve s s

howed

largel y transgran

ular craeks with so me crack braneh

ing.

Discussion

Stress Concentration al Nolches

The simplest

treatment

of the effect

of oxygen eutting on fatigue is to

disregard all

but

the geometrieal

features of the oxygen

cut

surfaee. A

model for the effeet

of

oxygen

eutting

was

developed

by assuming that th e

surfaee co ntour can be approximated

as

a se rie s of notehes.

Assuming

elastie

behavior,

the

theoretieal stress eon

een tration factor, K at a single noteh

tip is

defined simply

as

the

ratio of

the

maximum

longitudinal

stress at

the tip,

to the average, remote stress, S

K,

= oi

1 )

For an elliptieal surfaee noteh

of

depth

, t, and

root

radius, r (see Fig. 10)

l. 0.10 .1

.

2 Prol

ilomele

r lraces a

nd

ma

crograp

hs 01

lhe tour oxygen cul

surfaces s

ludied

600

500

.

...

.

e e

Z

I

>

400

300

.......

. .

200

0

Dislance Below Surface. mill imelers

Fig 4- 0 xygen cul sur lace 01 A514 slee l

specimen

: lop micros

lru

clural modilication s;

bOllom microhardness variation



b.

In

the

limi

t

of

large spaci ng, K,

for

the array approaches

that

for

a s

ingle

no t

ch. For the c

ase

of

se

mi

c irc

ular

no t

ches, Fi

g.

12 illu strat

E:s

that

for a

finite

arra y )

the

stress conce

nt r

ation is

interm

ed iate

between

that

o f a s

in

gle

no tch and an

infin

ite array Further

more

, K decreases w

ith the num ber of

notches

and is greater for

the

end

notch

than for the

center notch.

For

the more

realistic case

of ellip

tical

notches,

it wa s

assu

med

that the

percent

reduction in

K for an

array

will

be

roughly

the same as

that found

for semicircul

ar

notche

s.

Effect of

Notches

on Fatigue

The elastic st

re

ss co nce

ntr

ation fac

tor

is

appropriate onl

y

under

co

mplet

ely elast ic

conditions

.

Ho

w

ever, th ere is always so me small

amount of no t

ch

root plasticity

, espe

cially in fati gue l

oad

ing. The refore, the

K

va

lues given

by

eqs.

1

and

2

and

graphically

shown

in Figs.

11

and

12

must be

modified for

fatigue. The

fatigue

notch

factor, K is

defined as

the ratio

of

fatigu e stre

ngth

s

of

unnotched to notch

ed specim ens at

long I ves and

K - S

(sm

oo

th)

f -

S

notched)

(3)

is usually le

ss

than K because

of

pl

as

ticit

y eHects. Peterso n

G

has

found

that

these eHects can be empirically

acco

unted

for

by

a

mat

e

ri

al

property,

a, w here

K, - 1

K

f

1 +

(4)

1 + al

r

For steels

the

value of (a) can be re

lated to the

ultimate

ten sil e strength

Su

lOO/:

6

5

Z

I

>

4

3

Partially Pearl,te

ntercritical

arlensi

le

Mod. Pearlile

Spheroid

ized

and Ferrite

+

Pearlite and

Fine Ferrite

Ferrite

OiSfance 8elow Surfoce

mi

llimelers

Fig

5 Microh

ardne

ss va

riation

in A572

oxygen cut

steel

1

'

5Or

o

a:

Roughne

IRMSI

0 .001

in

.

A 514

o 0 .002 in .

25 r-

HiQh

Strength Sleel

v

0 .

007 in .

Vi

l 0 .0 .

RoO

, 1

1

10 •

10

10

la·

iO

Cyc les lo Failure

Fi

g

6- S- N diagram lo r A514 steel

specimens

100

Vi 25

Fig

. 7

N

diagram lor

A572 stee l s

pe

c

imens

Roug hness (R MSI

o 0.

001

in.

- ¿ s ~ e e l

o 0.

002

in.

. 0.007

in

l

0.

011 in .

RoO

, 1

lO

Cyc les lo Foilure

th e elast ic stress

concentr

ation

factor

fo r a sing le

notch is

approxim ately

For

the

case

of

a

semicircular not

ch,

this expression reduces to K, = 3. At

hi gher

va

lues

of notch

acuity,

t

the

theo ret ical elastie-stress

concentration

may reach

quite hi

gh

va

lue

s,

as

s

ho

w n

in

Fig. 11

.

A

periodic

,

infinite

array

of

no tc

he

s

is far l

ess detrimental than

a s

in

gle

notch.

The

value

of

K

for

such

an

array

depends not

only

upon th

e

no

tc h

ac uity, but

also

on the notch

spacing,

Fi

g. 8- Topograph y

oi

oxygen

cut

sur face .

top at

torch e

dge

, botlom

a t

midsectibn

228 5 IAUGUST 1977

Fig 9- Fatigue crack

originating

at

de

c

arburiz

ed, reso lidified metal



3.5

End Nolch of The Array

Middle Naleh of The Array

2.5

2.0

1 Y ./

~ ~ ~ l s ~ ~ ?TI-

a =

.

001

300 ) J.S

(5)

Su

e Su isgiven in ksi,and (a) is in

ch

es.

Fatigue Resistance 01 Oxygen Cut Surlace

Asseen in Figs.6 and 7 the fatigue

res istance

of

spec im ens with oxygen

ut surfac

es

decreased

with

increasing

roughness, particularly at longer lives,

the state

of

stress at the

notch

tip

s approaches elastic

conditions.

s

is

in agreeme

nt

with

previously

ted relationships.

In

orde r to place this trend on a

more

quantitative

basis, the

geomet-

i cal parameters, r, t, b and c were

lor

lhe different

roughness

levels tested. K, values

were

then

for both semicircular

and

l l ipti

ca l notches. Using the base

et

al

Su values for each steel, the

factorK,wascalculated

for both

semi

ircular and e

l l iptical notch

arrays

for

rough es t surfaces tested.

The calculated

va

lues are

shown

in

ab le 2 and

compared with

experi-

e

nt

all y

determined va

lues.The

latter

rees

timat

edusingeq. (3).

To mini-

z

e the

effect of micro

s

tructural

lteration due to oxygen cutting

,

the

Table 2-Comparison

01 Calculated and Measured

Fatigue Stress Concentration Factor,

K

,

( a l

Calc

ul

ated K,

Material

a

(in .)

Semi

c

ir

c

ul

ar E

lIipti

ca

l

.

Measured

K,

AS72

0.0'10

-130

-

1.83

1.28

AS

14

0.

0053

1.48

2. -

16

H8

AS

14

normalized)

0.012

1.

24

1.80

-1.53

tn)

K

r

a l u e ~

Jre l

or

roUglW ";l spec

im

ensl

es

led

(0.011

in .RMS) co

mpar

ed 10 smoo

the

sl specimens

lested (OJ)(JI

in .RMS).

200,

1 I

i

1 1 ' 1 1

i i

100f

"'

'"

<>

""

o::

50f

o __

"'

"'

"

Vi

25

f-

Roughness (RMS)

o 0.001 in.

A

514

Normolized Sleel

o 0.002 in .

o

0.007

in.

o 0.011

in

.

10

I 1

I

1 1 I

I I I I

!

I

lO·

10

5

10

6

Id

Cycles 10 Fai lure

Fig 13 S N di agram lo r no rmaliz ed A514 steel specimens

smoothes t oxygen c

ut

spec imen re

lhe

stresses

which

caused failure in

lO'

sul tswere taken for th e

numerator

and

cycles. Thesestresses

were

taken

from

the roug

he

st s

pe

c

imen

results

for the

the curves

of Figs.

6and 7. The close

denominator

.The

va lue

s

of

stre

ss were

agreement in Table 2

between

mea-

WELDING RESEARCH SUPPLEMENT I229-s

60

1 111111

b

l.

-1

K

,

2

l

' 1 1 1 11111 1

" 1

! ! ! t

1 2 4 6 8 10 20

40

~ ~ ~ "

tlr

Fig . 11 Elastic

s

tress concentration la cto r lor sing le sllrlace

notch

e

r

- - - - - , - - - - - , - - - - , - -1

b

l.

.1

K

,

S

~ 2 r

1.5

Infinile Array

S.

1.0':-'

_____- '-_____

-

_____L-____ J

o 34

b ~

b/c

1O

ellla l (lOp) and idealized oxygen cllt surlace topogra - Fig 12 Ela stic str

ess

co nc entration laclOr lor periodic arrays 01

ies

semicircul

ar surlac e no tch es

40

20

M i n ~ r T O )

, ,

r-.c in;

r

s-+ t.? $+

r : l ~

CT

max

CT

max



2 0 0 r - - - - - - - - - - - - - - - - - - - - - - - - - . - - - - - - - - - - - - ~

2 0 0 r . r ~

High Strength ~ t e e (A 514)

Mild Steel

(A

572)

"'

'

100

1001

---..

.

........

e

g 50f-

-

.

o

50 -

-

o

--- -- -

----.----

-

o;:

<f)

"'

25f

2 5 -

-

Rough

Flame

Cut

Surface

_ .

Raugh

Flame Cut Surfaee

f)

Smoo

th

Flame Cut

Sur

fa

ce

Smaoth Flame Cut Surface

Machined

Surfaee

f)

Maehined Su rfaee

I I

I

10

5

10

6

Id

10

5

10

6

Id

Cyeles

Cyeles

Fig .

14

Com

parison

01

machined and

ox

ygen cut

surlace

lati

gue

Fig.

75 C

omparison 01 machined and oxygen cut s

urf

ace fatigue

resiSlance - A514 sleel specimens

su red values of K, and th ose ca lculated

f

or an

array of semicircu lar notches

indicates tha t the

model

is va

lid

and

that th e approx

imations

are reason

able; however, further data wo uld be

useflll.

Accord in gly, spec imens

of

the A514

ste

el

we

re

norm alized after oxygen

cuttin g an d tested subsequen t to wire

brushing to remo

ve

th

e scale

from

the

n

ormaliz

in g treatments. Hard n

es

s

pro

fil es demonstrated

that

aside from

some decarbu ri zat i

on

at the sll rface

the hardn

ess

was uniformo The ten sil e

properties are

show

n in Table 1.

Fatigue results are shown in

Fig

. 13 and

experi men tal and calcul ated K¡ valu es

are co mp ared in Table

2.

Although the

agreement

is

not

as

good

as

for the

as

cut

s

ur f

ace

s,

it is qualitatively

cor

ree .

A

furthe

r

refinement

o f

the model

would be to take

into account the

layer of altered

microstructure

at the

surface of the flame

cut

spec imen

s.

Her e the hardness and S wou ld be

higher, a

nd

the

(a

) valu e

low

er. This

woul d have the effect of in cre asing

the calcu lat ed

K, va

lue s. It is felt that

such ref in ements are unjustified in th e

prese

nt wo

rk,

considering

the

approxi

mations which

have been used. The

si

tu ation

is

further complicat

ed by

the

chang es in geo

metr

y as

the

fat i

gue

crack propagates, by residu

al st

resses

and by simultaneo us changes in

microstructur

é' be

low

the surface.

Comparíson of Oxygen Cut

and

Machíned Surfaces

The ultimate

obje

ct ive of oxygen

cu

ttin

g is to achieve an economy in

fabrication; however, this mu st be

done with out un wa rranted degrada

tion in pro pert ies. In orde r to eva lu ate

the quality of

the trade-off between

economy

and pe

rforman

ce, co mp ari

sons we re made between oxygen cu t

an

d machined specimen s

for

the same

sp

ecim en geome try and nom in al

st

ress

230-5 I

AUGUS

T1 977

res is

t3nce A572

steel s

pe

c

im

ens

leve ls. A rough indication co uld be

obtained

by noting that

thr

ee of

the

lateral surfaces of the fatigue spec

imens we re machined and one was in

the oxygen cut co ndi t i

on

, yet nearly all

fat igue cracks initi ated

at

the oxygen

cut sur face. Further verifi

ca

t ion o f the

sup er iority of the mac hin ed surface

was

obta

in ed by performing a l imited

num

ber

of

fat igue tests on fu ll y

machin

ed spec

im

ens of

the

same

geometry, base metal and stress leve l.

Figures 14 and 15 show the re sul ts

for the machined spec imen s in com

parison w ith the lives for the smoot h

est and roughest oxygen

cut

spec

imens. From the limited amount of

data it is impossible to draw unequiv

oca l conc lu sions , yet it is elear that at

high stress leve ls, machined

surfaces

are s

uperior

to even

the

s

moothe

st

oxygen

cut

sur face.

At

a l

ower

stress

leve l, it appea rs

that

a

we

ll-made

oxygen cut sur face may result in

no

degradati

on

of fatigue life.

E

xtrapo

l

ation to

l

ower

stre

sses

in

Figs. 14 and 15 indicates th at machined

surfaces may in

fa

ct be

in f

erio r to a

smooth oxygen cut. Thi s may be a case

of

th e benefits of residual stress and

microstructural alteration

as a resll l t of

oxygen cu t ting more than com pensat

ing

for

the

geometric

al

effect

s of a

rough surface, resulting in an overa ll

improvement in fat igue li fe.

Conclusions

1. Th e fatigue resistance of the

smoothest oxygen cut s

ur f

ace is

greatest for th e high strength steel

A514. However, at l ives equal to or

grea ter than 10

G

cyeles, the difference

between A514 and th e A572

re

sul ts

dimini

shes such

that

th e tw o steels

give app rox

imately the

sa

me

fatigue

stren

gt

h.

2. Increasing surface roughness has

the greatest eff ect on stress level in the

AUTHORS

.

..

See

page

244-5.

A514 steel and on ly a

moderate

effect

on the A572 stee l.

3. Oxygen cut surfaces o f both

stee ls have fatigue resistances inferior

to mac hin ed surfaces in the l ife range

10' to 10

G

cyc les. Extrapo lati on of the

test results to longer lives

wo

uld indi

cate the po ss

ibility

th at smooth oxy

gen

cut

su rfaces may

out-pe rform

machined

surfaces at lives greater th an

lO cycles.

4. The

influen

ce of surfa ce rough

ness on fatigue life can be qualitatively

under

stood by considering th e su rf ace

roughne ss to be a

periodic

array of

surface

not

ches.

Ackn

ow l

edgments

This work was sponsored

by

the

Caterpillar Tractor

Company

, Peoria ,

Ill inoi

s. The suggestions and coop era-

tion

of Mr. W ill s Fildes and

Mr

. Cal

Loyd of that compa ny

were imp

o rt

ant

contributions to th is work. The use of

the testing fac

iliti

es

of

the

Civil Engi

nee ring

Departme

nt of the University

o f Illi nois are also appreciated.

References

1.

Koenigsberger, F., and Garcia -

Ma r

tin ,

Z, Fatigu e Strengt h o f Flame-Cut Sp ec

imens in Br ig

ht

Mi

ld Stee l, British We lding

¡oumal

, january 1955,

pp

. 37-41.

2.

Koenigsberger, F. , and Green, H. W.,

" Fatigue Stren gth of Fl am e-Cut Specim ens

in Black Mild Steel," British Welding ¡our-

nal, Jul y 1955, pp. 313-321.

3. " The Properties

oi

Flame-Cu t

Ed

ges,"

Netherlands Institute of Welding, Final

Rep

ort

of

W

orkin

g

Group

1913, M ay 1973.

4. Goldberg, F., " Influenc e of Thermal

Cutt

ing and It s

Qu a

l

ity

on the Fatigue

Strength

of

Steel, Welding ¡

oumal

, 52 9),

Sept. 1973, Res. Sup p., pp. 392 -s to 404-s.

5. Peterson,

R. E.,

Stress Conce

nt r

ation

Factors W iley and Sons, Ine. , New York,

1974.

6. Peterson, R. E., No

tch

Sensitivity,

Metal

Fatigue, Cha

pter

13, S

ine

s and Wais

man, E

ditors

,

McGraw-Hill

Book CO., In

e.

,

1959.

Documents

Fatigue Resistance of Oxygen Cut Steel