Properties of High Strength Steels

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1984

Properties of High Strength SteelsB. Johansson

H. Nordberg

J. M. Tullen

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J33, B.; N2%#&2, H.; % 5&, J. M., "P20&2*&3 ' H* S2& S&&3" (1984). International Compressor Engineering

Conference. P0&2 474.0://%$3.*#.052%5&.&%5/*$&$/474

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358

F

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ELEVATED TEMPERATURE DATA

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C .



The deformation of the valve across the discharge

opening

has

been

discussed in terms of

low tempera

ture creep. (Ref. 1) To make such

an

analysis possi

ble creep 5upture data

in

the

temperature

range

250 to 400

C

has

been

collected,

The resul ts

are

given in

Figure

4 and

5.

Rupture

stress

MPa

AISI

1095

Stainless

716

AISI

420

5 0 0 ~ - - ~ - - - - - - - ~ - - - - , _ - - ~

KSI

300

240

180

120

60

o L ~ ~ ~ ~ ~ ~

100

10

1

10

2

10

3

10

4

10

5

h

Time

Figure 4.

Creep rupture

for the 1 carbon steel

and

the

12 chromium

steels

Rupture stress

MPa

KSI

A I S I

301

17 7 PH

240

1500

>or

350 °C

180

•

400 °C

~ : a : .

~ : : : : : :

~ - - - - -

~ - _ . . . 300

°C

---

350 °C

000

120

500

60

Figure 5

Creep rupture for the austenit ic

steels

359

The austenit ic steels AISI 301 and

17-7

PH, show

almost no decrease in

rupture

strength with time

whereas the martensitic steels UH SS 716

and

AISI 420, lose some strength but

only

to such an

extent

that they

always remain superior to

the

austenit ic grades,

The

AISI

1095

loses

i t s

strength

so rapidly

that

after less

than

100 hours

i t has

lower rupture

strength

than

the austenit ic grades.

FRACTURE TOUGHNESS

The toughness test ing was made on a center-cracked

tension specimen with the principal dimensions as

in Figure 6. Correction of K for the plast ic

deformation in

the crack p l a N ~ x h a s

been carried out

according

to the

procedure given in Reference

(2).

-0-

I

-0-

1

75

0

0

..

Figure

6.

Center-cracked

tension specimen for

fracture

toughness

test ing

Stress Intensity

KSivm

Kmax

MN m

3

'

2

200

160

120

80

40

0

AISI301

17 7

PH

0

0

AJSI420

0

200.

160

~

r---

~ t i s 7 1 6

__...

120

r .

~ 9 5

~

80

...........

40

1400 1600

1800

260

2000 M

Pa

200 220 240

280 300 KSI

Ultimate Tensile

Strength

Figure

7. Fracture

toughness, K ax

as

a

function

of ultimate tensi le s ~ r e n g t h



The general

trend

in the

toughness

resul ts are

given

in Figure 7. Increasing toughness from plain

carbon steel to

the

martensitic 12

%

Cr-steels and

to the

austeni t ic grades

301 and 17-7

PH is

expect

ed. A surprisingly

large difference in toughness

is

noted between

the

two

12

%Cr-steels. A part

of

this

difference is due to the larger amounts of

chromium

carbides

in

the

UHB

SS

716.

When

the

material

deforms

plastically, voids are ini t ia ted

in the

carbide-matrix

interface giving

rise

to a

more favorable path for crack propagation. This

effect is

also

seen

in

plane strain fracture tough

ness of tool steels with similar chemical

composi

tions.

FATIGUE

From each material some 25 canti lever bend specimen

were blanked and iso-finished according to

the

same

procedure

as for

compressor

valves. The specimens

were

fatigue

tested

in

plane

reversed

(R- -1)

bend

ing in a Sonntag SF-2U machine at a frequency

of

30Hz.

The

fatigue

l imit

was

evaluated

a t

2•10

6

cycles

of l i fe

by

means of

the

stair-case method

(Ref.

3).

The results are given

in

Table 4.

TABLE 4.

Plane bending,

fatigue properties

G rade

Fatigue

l imit

Standard

Ratio

deviation

Fatigue l imit /

W/mm'

\{ 1

MN/IIUI1'

KSI Tensile atrens;th

AISI 1095

+

- 750

+

- 109

10 1.5

0.39

AISI

420

: :

774

:':

112 15

2.2

0.45

UHB

SS 716

+

- 820

: :

119

15

2.2 0.44

AISI

301

: :

580

:':

84 23

3.3 0.45

17-7

: :

600

: :

87 24

3.5 0.42

The austenitic

grades,

301

and

17-7

PH,

show sub

stantial ly lower

fatigue

l imits owing to their

lower tensi le strength. These grades also show

dist inctly

higher standard

deviations

giving

rise

to even lower design

stresses for high

survival

rates. This

is

shown in Figure 8 where the stress

for 95

percent

survival at a confidence

level of

97.5

percent

is given

for the investigated

mate

r ia ls .

The stair-case method for

the

evaluation of fa t i -

gue properties was

designed to

give the best

est i-

mate for 50

percent

survival i .e .

the

mean

fatigue

l imit.

I t

is

well

known

to

give

less

accurate

estimates of standard deviations

and

thus

a high

degree of uncertainty when extrapolated t high

rates

of specimen survival.

In

an

earl ier study, Ref

4, the

results from a

stair-case evaluation were compared with the more

accurate, and

much more test-piece

consuming,

pro

bi t method. This study was done on AISI 1095 in

pulsating loading. Mean values

as

well as

95

per

cent survival

a t a 97.5

percent

confidence level are

shown in Figure 9. The difference

between

the two

· methods is at 50

percent

survival less than one

percent and

has grown to some 8 - 9

percent

a t

360

95

percent

survival. If

the

distr ibut ion

function

is

known, or

postulated,

fatigue stresses for even

higher survival

rates could be calculated

with an

ever - increasing

difference

between

the

two eva

luation

methods.

Stress Amplitude

MPa

StainlesG

KSI

716

A SI

A

-

120

: :800

B

...

AISI

420

1095

A

-

c

-

"" ":":"'

B

---

c

-

D

--

110

D

--

: :700

D

--

100

A

SI

17-7 P

301

A

r

A

...--

90

600

B

--

c

c

D

---

80

D

---

500

: :

70

: :400

60

Figure 8.

Mean fatigue

l imit

(A) and fatigue stress

for

95

%

survival

(C) at

97.5

%

confi-

dence level

(B)

respectively

(D)

for

the steels

investigated.

Stress amplitude

MPa

KSI

540±540

500:t5oO

460

: :460

420: :420

STAIR CASE

A 1 -

B

c f

D

PROBIT

A

B

o ---

ao

tao

.75: :75

70: :70

65: :65

60: :60

Figure 9.

Mean

fatigue l imit

(A)

and fatigue stress

for 95 % survival (C) a t 97,.5 % confi

dence

level , (B) respectively

(D),

estimated

from two different testing

procedures.





REF

EREN

CES

1)

P

Mad

se n:

Pl

asti

c de

fo rm

at io

ns of

dis

ch a

rge

val v

es

i

n

he rm

et i

c

com

pre

ssor

s. Pr

oc

197

6 P

urd u

e

Camp

T

ec h

Co nf

We

st

La

faye

tte I

nd

3

02.

2) R

Dus il

B

Appell :

Fa t

igue

a

nd

fra

ctur

e me

chan

ics

pr o

per

tie s

of

v

alve

s

tee

ls

i

bid pp

82

.

3

)

W J

Dix

on

A M

Moo

d:

A m e

th od

for

o

bt ai

nin

g and

ana l

yzi n

g

se

nsi-

t

v t

y d

at a.

J A

m

Stat

is ti

cal

A ss.

4

19

48)

10 9.

4

)

B

Joh

anss

on:

Int

erna

l r

epo

rt. U

ddeh

olm

Res

ea rc

h.

5

)

R

Dus

il B

J

ohan

sso

n:

F

atigu

e

f

ract

ure beh

avio

ur

of

im

pact

l

oade

d

com

pre

ssor

va

lv es

. Proc

1

978

Purd

ue

Cam

p Te

ch

C

onf

, W

es t

La

fa y

ett e

Ind.

124

.

6

)

R

Dus

il B

J

oha

nsso

n:

Mat

er ia

ls

as pe

ct s o

f im

pac

t

fat

igu

e

of

va

lv e

ste

els

ib id

pp

116

.

362

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Properties of High Strength Steels