Relation between existence of fatigue limit and

non-propagating micro-cracks

S. Nishida/N. Hattori/T. Shimada^

F̂aculty of Science and Engineering, Saga University,

Honjo-machi 1, Saga 840, Japan

^Yawata R&D Laboratory, Nippon Steel Corporation, Tobata 1-1,

Kitakyushu 804, Japan

Abstract

Fatigue tests had been performed by rotating bending fatigue testing machineabout HM steel (0.2%C) specimen with comparing that of SUS304 one usingsuccessive-taken replica method. It is clarified that the fatigue strength ofHM steel and SUS304 shows comparatively the same result, that is, the samefatigue limit and distinct knee point. In addition, some non-propagatingmicro-cracks are observed in the specimen's surface of HM steel subjected tothe stress of fatigue limit by ICT cycles, while no micro-cracks exist in thespecimen of SUS304 subjected to the same stress condition.

Following to the above tests, the authors have accomplished the same kindsof fatigue tests to investigate the effect of carbon content on the existence ofnon-propagating micro-cracks in the austenitic steels using the four kinds ofhigh manganese steels with changing and decreasing the carbon contentbetween 0.025%̂ 0.100%. It is clarified that the non-propagating micro-

cracks are not observed in the four kinds of HM steels (0.025%̂ 0.100%C)under the stress of each fatigue limit by ICT cycles. From this fact, the limitof the existence of non-propagating micro-cracks is concluded to be between0.1%̂ 0.2% of carbon content, and the characteristics for crack initiationunder the above stress conditions are founded to be predominantly influencedby its carbon content.

1. Introduction

It is said that high manganese non-magnetic austenitic steels (HM steels) hasbeen expected to be a promising component material of the track for a linearinduction motor train, nuclear fusion electric power systems etc., because itshows extra-stable non-magnetic properties, high tensile strength and ductilityeven in low temperature. This alloy steel contains much manganese, which

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46 Engineering Integrity

exists in large quantities in the ocean depth, instead of the expensive nickelcomposition. Therefore, there would be no problems for its as being rich innatural resource.As HM steel is a kind of new material and does not have come into wide use

yet, there are few reports about its mechanical properties, still less its fatiguestrength'"^Then, fatigue behavior of HM steel (0.2% C) have been investigated in this

paper, especially in view point of fatigue crack initiation by successive-takenreplica method under a cyclic stress with comparing that of the representativeaustenitic stainless steel (SUS304), which is also well-known as a non-magnetic one. In addition, the effect of carbon content on the existence ofnon-propagating micro-cracks in the austenitic steels has been researched usingfour kinds of HM steels with changing and decreasing the carbon content onlybetween 0.025%—0.100%.

2. Experimental procedure

First step ; the materials used in this test are HM steels (0.2%C) and typicalaustenitic steels (SUS304), which are both non-magnetic austenitic steels ofrolled thick plates for a market. Tables 1 and 2 list their chemicalcompositions and mechanical properties, respectively.Second step ; following to the above tests, the four kinds of HM steels with

changing and decreasing carbon content were used in this test. Tables 3 and 4list their chemical compositions and mechanical properties, respectively.

Figure 1 shows the dimensions of fatigue specimen, which was cut out bycoinciding the rolling direction with the specimen's axis and by doing a partialshallow notch with its rolling surface side'*. This shallow notch exists forlimiting fatigue-damaged part and does not affect for its fatigue strength at all.All specimens were annealed in vacuum for one hour at temperature of 600°Cafter polishing with fine emery paper to the longitudinal direction, andthereafter electro-polished to the depth of about 50 y. m

A rotating bending fatigue testing machine of uniform bending moment type(Ono-type fatigue tester, capacity; 98kN-m, frequency; 56.7Hz) had been used.The stress used in this paper is the nominal stress at the minimum defined byneglecting the existence of a partial notch; that is

a=32M/(7rd^) (I)where M is the bending moment and d is the specimen diameter.

In the case of fatigue test of SUS304, testing part of each specimen wascooled by pouring fresh water for prevention of exothermic fracture. It isconfirmed that there is no difference between the result under low frequency inair and that under high frequency with cooled by distilled water*. Theobservation of micro-crack initiation was adopted by successive-taken replicamethod in the circumferential direction at the specimen's surface and defined

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Engineering Integrity 47

Table 1 Chemical composition. (mass%)C Si Mn

HM 0.178 0.23 23.8SUS304 0.048 0.50 0.81

Table 2 Mechai

o oHM 251SUS304 253

Table 3 ChemicC Si Mn

HM-1 0.027 0.20 23.7HM-2 0.047 0.20 23.4HM-3 0.048 0.44 23.6HM-4 0.107 0.44 23.3

Table 4 Mecl

v oHM-1 196HM-2 164HM-3 181HM-4 219

LOCl -",- -e-

n *

/s

80

P S N0.024 0.001 0.20.026 0.007 8.4

lical properties. (

.2 &B

.9 751.7 5

.8 623.5 6

;al composition.P S N

0.016 0.006 0.20.017 0.006 0.30.015 0.004 0.30.016 0.004 0.3

lanical properties.

2 (?B5 783.0 46 766.9 48 746.3 45 770.3 4

™l <= r- 2 /-

^r- /. \y ,, ._/I Wi

"" st.

10 30

210

i Cr Nb N3 2.05 0.04 0.07 18.49 - 0.0

MPa,%)El

6.06.0

(imi Cr Nb N7 2.08 0.04 0.032 2.19 0.04 0.030 4.82 0.04 0.030 4.82 0.04 0.02

(MPa, %)El RA

1.6 62.65.7 66.06.5 68.87.5 70.5

Detail of A

A ~~

?10 80

Al43 0.00347

iss%)Al

2 0.0091 0.0100 0.0129 0.010

CNJ£/ o-^— f

Figure 1 Shape and dimensions of specimen.

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48 Engineering Integrity

as the length of one grain or so.

3. Result and discussion

3.1 Comparison of HM steel (0.2%) and SUS304

3.1.1 Fatigue strengthFigure 2 shows the S-N curves of HM steel and SUS304. As shown in thisfigure, there exists the same fatigue limit ( a wo=265MPa) about both materials,while the gradient of SUS304 becomes a little smaller than that of HM steel.When considering the relation between mechanical properties and its fatiguelimit, it is generally said that the fatigue limit of steel specimen a^o is closely

dependent on its tensile strength <JB. As HM steel shows higher tensilestrength and a little smaller proof stress than SUS304, that is, the former hassmaller yield ratio than the latter, the fatigue limit of both materials happenedto agree with each other and shows higher value than proof stress by about5%.

3.1.2 Fatigue crack initiationFigure 3 shows the representative successive observation result of fatigue crackinitiation of HM steel under cyclic stress amplitude a^ =320 MPa. Fatiguecracks initiate in grain boundary or in its neighborhood and propagate mainlyas a trans-crystalline crack. Figure 4 shows the representative successiveobservation result of fatigue crack initiation of SUS304 under cyclic stressamplitude a^ =280 MPa. Fatigue cracks also initiate and propagate in thesame manner as the above. Therefore, there is no difference about crackinitiation between both materials.On the other hand, Figure 5 show the surface state under the fatigue limit ( o^

=265 MPa) by 1X10^ cycles about both materials. The non-propagatingmicro-cracks are observed in the specimen's surface of HM steel and its lengthbecomes about 45 /z m, while the non-propagating micro-cracks are notobserved in the case of SUS304, though there appeared only slip bands. It isconsidered that the above phenomena would be caused by the difference ofcarbon content, corrosion resistance and yield ratio, etc.. In additon, the micro-Vickers hardness number, where slip bands are generated, becomes increase byabout 30% than that of the grains of before test or the grains where slip bandsare not generated under the above cyclic stress amplitude by 1.21 X 10^ cycles.

3.2 HM steels( carbon content; 0.025—0.20%)

3.2.1 Fatigue strengthFigure 6 shows the S-N curves of HM-1 to HM-4 with decreasing carboncontent than 0.2% C. When defined the fatigue limit as the fatigue strength

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Engineering Integrity 49

1x10" 1x10" 1x10"

Number of cycles to failure ,

Figure 2 S-N curves.

1x10

1x10

Number of cycles to failure ,

Figure 6 S-N curves.

1x10

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50 Engineering Integrity

N=0 1.5X10* 4.0X10* 5.0X10* 7.0X10* cycles

^ » Axial direction 20 n m

Figure 3 Successive observation of fatigue crack initiation.( HM steel, o . = 320 MPa, N ̂ = 1.21 X 10^ cycles)

N=0 7.0X10* 1.0X10^ 2.0X10^ 3.0X10^ cycles

4 + Axial direction . 20 jj. m

Figure 4 Successive observation of fatigue crack initiation.( SUS304, a , = 280 MPa, N ̂ = 7.33 X 10^ cycles)

•ft

(a) Non-propagating (b) Slip bandsmicro-crack of HM steel. of SUS304.

4 » Axial direction 20 fj. m

Figure 5 Specimen's surface state subjected to the stress amplitudeof fatigue limit by 1 X 10^ cycles.

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Engineering Integrity 51

of 10^ cycles, the fatigue limits of HM=1, HM-2, HM-3 and HM-4 become 245,248, 235 and 262 MPa, respectively. As shown in this figure, the four kindsof HM-1 to HM-4 steels do not show distinct knee point at all.

Figure 7 shows an illustration of S-N curves as shown in Figure 6. Each S-N curve in Figure 6 deviates from a linear line at A point and its gradientbecomes smaller according to approaching the stress level ( B point) of fatiguelimit. Table 5 lists the value of stress level at A and B point.

3.2.2 Fatigue crack initiationAs an example, Figure 8 shows the representative successive observation resultof fatigue crack initiation under cyclic stress amplitude between A point and Bpoint ( see in Figure?). Fatigue cracks initiate in grain and propagate accrossthe grain boundaty. Although not shown the results, it seemed that the fatiguecrack behavior under cyclic stress amplitude at above A point of each materialshown in Table 5 is different from the results under the above condition.Namely, the fatigue cracks propagation under the stress level at the above Apoint are affected grain structures, while those at between A point and B pointare not so affected.

3.3 Effect of carbon content on fatigue properties

Figure 9 shows the specimen's surface state subjected to the stress amplitude offatigue limit by by IX 10^ cycles. As shown in Fig.5, the specimen of HMsteel (0.20%) exists the non-propagating micro-cracks. On the other hand, itcan be seen that only slip bands exist in the specimen's surface of HM-1 toHM-4, though the non-propagating micro-cracks are not observed in it. It isconsidered that this phenomena would be caused by the difference of carboncontent.

Figure 10 shows the relation between carbon content and BC value (see inFigure 6). As shown in this figure, the carbon content becomes more, thevalue of BC becomes smaller, that is, the stress level of A point approaches thefatigue limit. Therefore, the S-N curve shows distinct knee point. Thecarbon content affect to the existence of non-propagating micro-cracks in thespecimen's surface of the HM steel specimen subjected to the stress of fatiguelimit by 10^ cycles.

4. Conclusions

Fatigue tests had been accomplished by rotating bending fatigue tests aboutplain specimen of HM steel with a partial shallow notch with comparing thatof SUS304. The main results obtained in this study are as follows;(1) The fatigue strength of the HM steel and SUS304 shows comparatively thesame result, e.g., the same fatigue limit and distinct knee point. In addition,the fatigue micro-cracks have initiated at one or two grains and propagated into

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52 Engineering Integrity

030.

Q.E

Knee point

Number of cycles to failure

Figure 7 Typical S-N curve.

Table 5 The stress value at point A and B. (MPa)

HM

HM-1HM-2HM-3HM-4

Stress of the A265278272264273

Stress of the B265

245248235262

A-B033242911

HM-1 HM-2 HM-3 HM-4—» Axial direction 20 ̂ m

Figure 9 Specimen's surface state subjected to the stress amplitudeof fatigue limit by 1 X 10^ cycles.

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Engineering Integrity 53

N=0 2.5X10^

(a)HM-l,

1.0X10^ 3.0X10^ 1.0X10^ cycles

, = 260 MPa,N; = 1.70X10* cycles)

N=0 6.5X10" 1.0X10' 5.5X10^ 7.0X10" cycles

(b) HM-2, a , = 250 MPa, N f = 8.15 X 10* cycles

N=0 2.0X10* 1.0X10^

• \ .2.0X10^ 4.0X10^ cycles

(c) HM-3, a , = 264 MPa, N , = 6.87 X 10" cycles

N=0 5.0X10^ 1.0X10^ 5.0X10^ 6.0X10^ cycles

(d) HM-4, a , = 265 MPa, N ̂ = 8.25 X 10^ cycles

^ ^ Axial direction 20 JJL m

Figure 8 Successive observation of fatigue crack initiation.

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54 Engineering Integrity

final fracture in both cases.(2) Some non-propagating micro-cracks (45 //m in length) are observed in thespecimen's surface of the HM steel specimen subjected to the stress of fatiguelimit by l(f cycles and no micro-cracks exist in the SUS304 specimensubjected to the same condition.(3) From the above fact, the fatigue limit of the HM steel is based on the limitof propagation of micro-crack, while that of SUS304 is based on the limit ofmicro-crack initiation.(4) The limit of the existence of non-propagating micro-cracks under the stressof fatigue limit by 1 X 10^ cycles is concluded to be between 0.1%—0.2% ofcarbon content. In addition, the characteristics for the above crack initiationare predominantly influenced by its carbon content.

References

(1)1. Maekawa, Y. Tanabe, S. Nishida, T. Ashizawa, H. Ogawa, JSME (inJapanese), Vol.56. No.225, 1990, pp. 1051-1057(2)S. Nishida, N. Hattori, T. Shimada, Proceeding of Fatigue'93, Vol. I , 1993,

pp.347-352(3)S. Nishida, N. Hattori, T. Shimada, S. Iwasaki, JSME(in Japanese), Vol.61,No.582, 1995,pp.239-244(4)e.g. H. Nisitani, Y. Hasuo, JSME (in Japanese), Vol.44, No.377, pp. 1-7(5)H.Nisitani, H. Noguchi, S. Nisida, K. Yamaguchi, JSME (in Japanese),Vol.56, No.525, 1990, pp. 1067-1073

0 0.05 0.1 0.15 0.2

Carbon content , %

Figure 10 Relation between carbon content and BC value.

0.25

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