第一工業大学研究報告第25号（2013）pp.59－67

59

Effectof humidity
on initiationand propagation
properties propertiesof a fatigue crack
Effectof humidity
on initiationand propagationof maraging steel

of a fatigue crack of maraging steel
Kohji Kariya and Norio Kawagoishi
Kohji KARIYA and Norio KAWAGOISHI

Abstract Effect of humidity on fatigue properties of 18% Ni maraging steels was investigated using thesteels with different hardness and aging structure under rotating bending by varying the relative humidity(RH) from 25% to 95%. Both of a crack initiation and its propagation in the early stage of fatigue process
were accelerated by high humidity, causing the large decrease in fatigue strength. The fatigue strength at 107
cycles in RH85% decreased to less than 50% of the one in RH25%. By successive observation of thespecimen surface at fatigue process and fracotgraphic analysis, it was concluded that the promotion of crack
initiation was due to anodic dissolution and the acceleration of crack propagation was caused by the
propagation of a brittle crack which was assisted by hydrogen atom generated in accompany of cathodereaction. The acceleration of crack propagation was suppressed by formation of reverted austenite.
Keywords

Fatigue, Maraging steel, Humidity, Crack initiation, Crack propagation

1. Introduction
Maraging steel is an ultra high strength steel which is strengthened by means of many strengthening
mechanisms such as precipitation strengthening, solution hardening, grain refinement strengthening
and so on [1]. However, the fatigue strength is very lower than expected from its static strength
similar to other high strength steels. One of the reasons for low resistance to fatigue is due to its
high susceptibility for humidity [2]. Moreover, the effect of humidity on fatigue strength largely
depends on hardness, microstructure and so on. While, studies on effect of humidity on fatigue
properties of maraging steel were very limited in comparison with those on the resistance to stresscorrosion cracking of the steel [3].
In the present study, in order to investigate the effect of humidity on crack initiation and its
propagation behavior of 18%Ni maraging steels with different hardness and aging structure, fatigue
tests were carried out in various relative humidity under rotating bending.

2. Experimental procedures
The materials used were a 300-grade and a 350-grade of 18% Ni maraging steels. The chemicalcompositions in mass % of the steels were shown in Table 1.
The steels were solution treated for 5.4ks at 1123K in vacuum, followed by air cooling and age
hardened at different conditions in a salt bath. Mean grain sizes of a prior austenite were about 20
μm in both steels.
Figure 1 shows aging curves. Aging conditions examined were under-aging and over-one whichshowed nearly the same hardness of 550HV in 300-grade steel and peak-aging of HV705
in350-grade as indicated by circles in Fig. 1. These aging conditions were selected to investigate theeffects of hardness and reverted austenite on fatigue properties in high humidity. That is, it was
confirmed that the over-aged steel contained 10% of reverted austenite but no reverted austenite

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第25号（2013）

Table 1. Chemical composition (mass %)
Grade

C

Si

Mn

Ni

Mo

Co

Ti

Al

Fe

300G

0.005

0.05

0.01

18.47

5.14

9.09

0.89

0.11

Bal.

350G

0.001

0.01

0.01

17.89

4.27

12.36

1.3

0.08

Bal.

was in other steels by using X ray diffraction. In the following, these steels will be denoted as SteelA, Steel B and Steel C, respectively as shown in Fig. 1 and Table 2.
Table 2 shows mechanical properties of the steels.
Figure 2 shows shape and dimensions of specimens. Fatigue strength was investigated by using
plain specimens, and successive observation of specimen surface was carried out by using partiallynotched specimens which was localized the crack initiation site. Specimens were machined after
solution treatment, and then aged at the conditions shown in Fig. 1. Prior to fatigue testing, all of
the specimens were paper and electro polished to remove the work affected layer and make theobservation easier. Fatigue tests were carried out using a rotating bending fatigue testing machinewith a capacity of 15 N·m operating at about 50Hz in relative humidity (RH) of 25%, 45%, 65%,85% and 95%. The accuracy of humidity was RH±5%. The temperature in atmosphere was not
750
Steel C : 753K,150ks

Hardness, HV

700
650
600
Steel B : 813K,150ks

550
Steel A : 753K,2.8ks

500
450
0.1

1

10

100

1000

Aging time, t ks

Figure 1. Aging curves
Table 2. Mechanical properties
Vickers
Steel

Aging condition

hardness
HV

0.2% proof stressTensile strength

σ0.2 (MPa)σB (MPa)

Revertedaustenite

γ(%)

A

753K,2.8ks

550

1730

1833

0

B

813K,150ks

550

1634

1798

10

C

753K,150ks

705

2300

2370

0

KARIYA and KAWAGOISHI：Effect of humidity on initiation and propagation properties of a fatigue crack of maraging steel 61(a) Plain specimen(b) Partially notched specimen

Figure 2. Shape and dimensions of specimens
controlled, but the temperature range was 298±3K. Observation and measurement of the changes inthe surface state of a specimen and crack length due to stress repetitions were carried out by plastic
replication technique. Crack length a was defined as the surface length along to the circumferential
direction on the specimen surface. Fracture surfaces were observed by using scanning electron
microscope (SEM).

3. Result and discussion

800

800

700

700

Stress amplitude, σa MPa

Stress amplitude, σa MPa

3.1 Effect of humidity on fatigue properties
Figure 3 shows S-N curves of Steel A and Steel C in various environments showing the effect ofhardness on humidity dependence of fatigue strength. All of the fracture occurred from thespecimen surface in both humidity. Moreover, in low humidity, any surface cracks were notobserved in non-fractured specimen at the fatigue limit σw defined as fatigue strength at 107 cycles,
meaning that fatigue limit was mainly controlled by the resistance to a crack initiation.

600
500
400
300
200
100
0

Steel A
RH25%
RH45%
RH65%
RH85%
RH95%
5

10

6

10

7

10

Number of cycles to failure, Nf cycle

600
500
400
300
200
100
0

Steel C
RH25%
RH45%
RH65%
RH85%
RH95%
5

10

6

10

7

10

Number of cycles to failure, Nf cycle

Figure 3. S-N curves of Steel A and Steel C in various humidity
Figure 4 shows humidity dependence on fatigue limit. Effect of humidity on fatigue strength
was very small below about humidity of RH50%, and over the humidity, fatigue strength waslargely decreased with increasing in humidity. The decrease in fatigue strength was larger in highstrength steel. For example, fatigue limit of Steel C in RH85% was decreased to below a half of the第一工業大学研究報告

62第25号（2013）

one in RH25%. Such humidity conditions are not special cases in daily service of machines andstructures. Therefore, the marked decrease in fatigue strength is very important result to beconsidered in practical application.
Figure 5 shows crack growth curves and the crack length against to relative number of cycles tofailure N/Nf in RH25% and RH85%. Figure 6 shows relation between crack growth rate and stressintensity factor range. As seen from these figures, the initiation of a crack and its early propagation
are accelerated by high humidity and most of fatigue life is occupied by the growth life of a crack inhigh humidity. On the other hand, the growth rate of crack longer than about a few of grain sizes ofa prior austenite are not influenced by humidity in both steels. In addition, the difference in the

800
700

σw (MPa)

600
500
400
300
200

Steel A (HV550)
Steel C (HV705)

100
0

0

20

40

60

80

100

Humidity (%)

Figure 4. Humidity dependence of fatigue limit in Steel A and Steel C

10

Crack length, a mm

Crack length, a mm

10

1

0.1
RH 25% 85%

0.01
0

5

1x10

5

2x10

SteelA (σa=590MPa)
SteelC (σa=750MPa)5

3x10

Number of cycles, N cycle(a) a-N curves

5

4x10

1

0.1

RH 25% 85%

0.01
0.0

0.2

0.4

0.6

N / Nf(b) a-N/Nf curves

Figure 5. Crack growth curves of Steel A and Steel C

0.8

SteelA (σa=590MPa)
SteelC (σa=750MPa)

1.0

KARIYA and KAWAGOISHI：Effect of humidity on initiation and propagation properties of a fatigue crack of maraging steel 63

crack
crack propagation
propagation rate
rate byby hardness
hardness isis very
very small.
small. These
These results
results indicate
indicate thatthat thethe decrease
decrease inin fatigue
fatiguestrength
strength was
was mainly
mainly caused
caused byby thethe accelerations
accelerations ofof thethe initiation
initiation ofof aa crack
crack andand thethe crackcrack
propagation
propagation inin thethe early
early stagestage ofof fatigue
fatigue processprocess inin bothboth steels.
steels.

-5-5

Crack growth rate, da/dN m/cycle

10
10

-6
-6

10
10

-7
-7

10
10

-8
-8

10
10

-9
-9

10
10

RH
RH 25%
25% 85%
85%

-10
-10

10
10

Steel
Steel AA
Steel C
C
Steel

-11
-11

10
10

10
10

100
100
1/2
1/2

StressStress intensity
intensity factor
factor range,
range, ΔΔKK MPa
MPa･･mm

FigureFigure 6.
6. Crack
Crack growth
growth rate
rate against
against toto stress
stress intensity
intensity factor
factor range
range inin Steel
Steel AA andand Steel
Steel CC
FigureFigure 77 shows
shows examples
examples ofof cracks
cracks inin thethe early
early propagation
propagation processprocess ofof specimen
specimen ofof Steel
Steel C
C in
in
RH25%
RH25% andand RH85%.
RH85%. AA pit
pit atat crack
crack initiation
initiation site
site inin RH25%
RH25% waswas generated
generated byby electro-polishingelectro-polishing
beforebefore fatigue
fatigue test.
test. AA crack
crack inin RH85%
RH85% isis relatively
relatively wide
wide inin comparison
comparison with
with thethe oneone inin RH25%,
RH25%,
suggestingsuggesting thatthat aa crack
crack initiation
initiation was
was promoted
promoted byby humidity
humidity throughthrough anodic
anodic dissolution.
dissolution. This
This maymay
bebe aa main
main reason
reason forfor thethe promotion
promotion ofof aa crack
crack initiation
initiation by
by highhigh humidity.
humidity.

(a)(a) RH25%
RH25%

(b)
(b) RH85%
RH85%

FigureFigure 7.
7. Feature
Feature ofof crack
crack ofof Steel
Steel CC (←→(←→ Axial
Axial direction)
direction)
FigureFigure 88 shows
shows crack
crack morphologies
morphologies ofof Steel
Steel AA andand Steel
Steel CC inin bothboth humidity.
humidity. In
In Steel
Steel A,A, aa crack
crack
propagates
propagates inin zigzag
zigzag mannermanner along
along grain
grain boundaries
boundaries inin highhigh humidity,
humidity, thoughthough itit isis aa straightstraight crack
crack inin
low
low humidity.
humidity. That
That is,is, thethe growth
growth inin highhigh humidity
humidity waswas caused
caused byby an
an intergranularintergranular crack
crack andand thatthat inin
low
low humidity
humidity was
was aa transgranulartransgranular crack.
crack. On
On thethe otherother hand,
hand, inin casecase ofof Steel
Steel C,
C, itit isis notnot confirmed
confirmed

第一工業大学研究報告

64第25号（2013）

the influence of humidity on crack morphologies in so far as the surface observation. However,even in Steel C as seen from Fig. 9, brittle facets are also observed near the crack initiation site in
RH85% as shown by mark ⇒, though they are not in RH25%. Therefore, the acceleration of crack
propagation was caused by hydrogen embrittlement in both steels.

(a-1) RH25% (a-2) RH85%(a) Steel A((b-1) RH25%(b-2) RH85%(b) Steel C：Crack initiation site,：Crack tip)

Figure 8. Crack morphologies of Steel A and Steel C (←→ axial direction)(a) RH25%(b) RH85%
Figure 9. Fracture surface of Steel C

3.2 Effect of reverted austenite on fatigue properties in high humidity
Figure 10 shows S-N curves of Steel B with reverted austenite in RH25%, RH65% and RH85%. In
the figure, results of Steel A which contains no reverted austenite shown in Figure 3 are also
indicated by lines only. The effect of humidity is very small in the Steel B in comparison with SteelA, though these steels have the same hardness.
Figures 11 and 12 show crack growth curves and relation between crack growth rate and stressintensity factor range of Steel B in humidity of RH25% and RH85%. In Fig. 12, the results of
Steel A are also indicated. As seen from these figures, the acceleration of the propagation of a small
crack is hardly observed in Steel B, though the crack initiation is promoted by high humidity.

KARIYA and KAWAGOISHI：Effect of humidity on initiation and propagation properties of a fatigue crack of maraging steel 65

Stress amplitude, σa MPa

800
700
600
500
400
300
200
100
0

Steel A (γ= 0%)
RH25%
RH65%
RH85%
Steel B (γ= 10%)
RH25%
RH65%
RH85%
5

6

10

7

10

10

Number of cycles to failure, Nf cycle
Figure 10. S-N curves of Steel B
Moreover, not only the effect of humidity but also the one of aging structure on the crack growth
rates are not observed.
Figure 13 shows crack morphology of the Steel B in humidity of RH25% and RH85%. There isno or little difference in the morphologies in both humidity and also in comparison with Steel A in
25% similar to crack growth rate mentioned above. This may be explained from that reverted
austenite widely distributed in the matrix [4] becomes trap sites of hydrogen [5, 6] and suppresses
the acceleration of crack propagation assisted by hydrogen yielded in Steel A stated above.

Crack length, a mm

10

1

0.1

0.01

RH 25% 85%

0

5

1x10

SteelB (σa=650MPa)

5

2x10

Number of cycles, N cycle
Figure 11. Crack growth curves of Steel B

5

3x10

第一工業大学研究報告

66第25号（2013）

-5
-5

Crack
growthrate,rate,
da/dN
m/cycleCrackgrowthda/dNm/cycle

10
10

-5

10
-6
-6
10
10
-6

10
-7
-7
10
10
-7

10
-8
-8
10
10
-8

10
-9
-9
10
10

RH
RH 25%
25% 85%
85%

-9

10
-10
-10
10
10
-10

10
-11
-11
10
10

10
10

-11

10

Steel AA
RH 25% 85% Steel
Steel
B
Steel
B
Steel A
Steel B

100
100

1/2
1/2
10factorStressStress intensity
intensity
factor range,
range, ΔΔKK MPa
MPa･･mm 100
1/2

Stressrate
intensity
factorrange,intensity
ΔK MPafactor･m range
FigureFigure 12.
12. Crack
Crack growth
growthrate
againstagainsttoto stress
stressintensity
factorrange inin Steel
Steel BB
Figure 12. Crack growth rate against to stress intensity factor range in Steel B(a) Plain specimen(b) Partially notched specimen

Figure 2. Shape and dimensions of specimens

600
500
400

litude, σa MPa

litude, σa MPa

controlled, but the temperature range was 298±3K. Observation and measurement of the changes inthe surface state of a specimen(a)andcrack length due(b)toRH85%stress repetitions were carried out by plastic
(a)
RH25%
RH25%
(b)
RH85%
replication technique. Crack length a was defined as the surface length along to the circumferential(( (a) :Crack:Crack initiation
initiation site,
site,:Crack:Crack tip)
tip)
RH25%
(b) RH85%
direction on the specimen surface.
Fracture surfaces
were observed by using scanning electron(13. Crack:Crackinitiationof
site,:Cracktip)direction)
Figure 13.
Crackmorphology
morphology
of
Steel
Steel B
B (←→
(←→
axial
axial
direction)
microscope (SEM).Figure
Figure 13. Crack morphology of Steel B (←→ axial direction)
3.
Resultand discussion
44.. Conclusions
Conclusions
The
effects
effects ofof humidity
humidity andand aging
aging structure
structure on
on initiation
initiation andand propagation
propagation ofof aa fatigue
fatigue crack
crack ofof
4The
. Conclusions
3.1
Effectofof
humidityonand
fatiguemaraging
maraging
steelswere
were investigatedinvestigatedin
instructure
variousvarious relative
relative
humidity
humidity(RH).
Fatigue
Fatigueofstrength
strengthwas
wascracklargelylargelyTheeffectssteels
humidity
agingproperties
on initiationand (RH).
propagationa fatigue
ofFigure3 shows
S-N
curvesof
Steelinthe
AvariousandSteel
C fatigue
inhumidity
variousenvironments
showingtheeffectofdecreased
decreasedbyby highhighhumidity,
humidity,
thoughthoughthedecreasedecreaseininfatiguestrength
strength
was
was
very
very small
small
below
below
humidity
maraging
steelswere
investigatedrelative(RH).
Fatiguestrengthwashumidity
largely
hardnessonhumidity
dependence
of decrease
fatiguestrength.
All
of
the
fracture
occurred
from
the
of
of RH50%.
RH50%.
The
main
main
reasonreasonforfor thethe decrease
decreaseinin fatigue
fatiguestrength
strengthwas
was
accelerations
accelerations
ofofbelow
both
both ofofaa crack
crack
decreased
byThe
high
humidity,
thoughthein fatigue
strengthwasvery small
humidity
specimensurfacein reason
both humidity.
Moreover,in low
humidity,any
surface of
crackswere
notinitiation
initiation
and
and
its
itsmainpropagation
propagationinin thethe early
early
stage
stage
of
fatiguefatigueprocess.
process.
The
The
propagation
propagationofofbothaa larger
largercrackof
RH50%.
The
for
decreaseinof
fatiguestrengthwas
accelerationsofa crack
7
observed
in non-fractured
specimenatsuccessive
thestagefatiguelimit σwprocess.
definedas fatigue
strength
10 cycles,waswas not
not influenced
influenced
byby humidity.
humidity.
By
successive
observationofof fatigue
fatigueprocess
processandandfracotgraphic
fracotgraphic
initiation
andits propagationin theBy
early
of observation
fatigueThe
propagationof aatlargercrack
meaningthatfatiguelimit
wasmainlycontrolledbytheresistance
to
a
crack
initiation.
analysis,analysis,thethe promotion
promotionofof crack
crack initiation
initiation
was
was dueduetoto anodic
anodicofdissolution
dissolution
andand thetheacceleration
of
of
was
not influenced
by humidity.
By
successive
observation
fatigue process
andacceleration
fracotgraphic
crack
crack propagation
propagation
was
was ofembrittlement
embrittlement
due
due
to
todue
hydrogenhydrogengenerated
generatedinin andcathodecathodereaction.
reaction. The
The
analysis,the promotion
crack initiationwasto anodic
dissolution
the accelerationofacceleration
accelerationofof crack
crackwaspropagation
propagationwas
was suppressed
suppressedbyformationformationofof reverted
revertedaustenite.
crack
embrittlement
due to byhydrogengeneratedin austenite.
cathode reaction. The
800propagation
800
accelerationof crack propagation was suppressed by formationof reverted austenite.
700
700
600
500
400

KARIYA and KAWAGOISHI：Effect of humidity on initiation and propagation properties of a fatigue crack of maraging steel 67

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Journal of Metals, (1967)60-66.
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Transactions of the Japan Society of Mechanical Engineers, Series A, 49 (1983)1029-1035.
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70 (1984) 896-903.
[4] M. Moriyama, S. Takaki, N. Kawagoishi, Influence of reversion austenite on fatigue property of
350 ksi grade 18% Ni maraging steel, Heat Treatment, 41(2001)266-271.
[5] P.J. Ferreira, I.M. Robertson, H.K. Birnbaum, Hydrogen effects on the interaction between
dislocations, Acta Materialia, 46 (1998) 1749-1757.
[6] R.O. Ritchie, M.H. Cantro Cedemo, V. H. Zackey, E.R. Parker, Effects of Silicon additions andretained austenite on stress corrosion cracking in ultra strength steels, Metallurgical Transaction,
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