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Fractography of fatigue fractures in carburized steel
Materials Science and Engineering, 30 ( 1 9 7 7 ) 23 - 31
23
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
Fractography of Fatigue Fractures in Carburized Steel
N. L A Z A R I D I S
Steel Products Division, Inland Steel Research Laboratories, East Chicago, Indiana (U.S.A.) F. J. W O R Z A L A a n d B. I. S A N D O R
Engineering Department, University of Wisconsin, Madison, Wisconsin (U.S.A.) ( R e c e i v e d F e b r u a r y 7, 1 9 7 7 )
SUMMARY
Carburized specimens of AISI 4027 steel were tested in repeated bending at stress levels ranging from high-stress, low-cycle to lowstress, high-cycle, and the corresponding stress-life curve was determined. Considerable scanning electron microscopy was performed on fracture surfaces of failed specimens. Results indicate that a fatigue crack nucleates at the external surface of the carburized case, and propagates inward until it becomes large enough to cause rapid failure in bending. The crack surface appearance changes according to the growth rate of the crack and the hardness of the matrix material. A low-stress crack growth through the carburized case seemed to result in the well-established intergranular separation behavior. However, growth under high stress levels was rapid, causing a mixture of transgranular-intergranular fracture to result, the transgranular mode being more pronounced. The tough core, on the other hand, exhibited a more ductile mode of failure. The predominant mode consisted of microvoids, dimples of non-uniform size, and tear ridges with only occasional flat, quasicleavage step facets. Under no conditions did fatigue striations become apparent on either case or core material.
INTRODUCTION
The most significant goal of the present investigation was to establish differences in fracture surface appearance as a function of maximum stress level on specimen surface. It is hoped that this information can be utilized for failure analysis.
Scanning electron microscopy was used extensively to study, directly, the fracture surface of fatigue tested AISI 4027 carburized plates. LITERATURESURVEY
Basic fatigue background Repeated application of load or strain on a c o m p o n e n t can produce cyclic damage, called fatigue. When the accumulation of cyclic damage reaches a critical value, the material fails. This is a nucleation and growth process in the sense that a crack is nucleated as a result of extremely localized work hardening or softening, which then grows through the material in a stepwise fashion [1 - 3]. For the most part, the crack moves forward in the tensile (crack opening) half of the cycle. Though the crack-closing (compressive) half of the cycle does not usually contribute to crack propagation, it does damage the material in front of the crack tip to the point where it can propagate at the next half-cycle [4]. This sequential process of damage and growth leads to fatigue striations that are typical of many different steels and nonferrous metals and alloys [4, 5]. The coarseness or fineness of these striations depends entirely on the applied alternating stress level, which is proportional to the amount of plastic strain involved. The larger the a m o u n t of plastic strain or stress level involved, the coarser and wider the striations appear [4, 5]. In addition, each striation is the result of one stress cycle, allowing direct determination of the number of cycles present on the fracture surface. Thus, crack growth rates can be ascertained from microstructural features [4].
24
In general, the plastic strain or stress level involved is evidenced by the fineness or coarseness of characteristics of the fracture surface. Elastic stress levels related to long lives produce fine surface features, while gross plastic stresses result in tearing and a rough fracture surface. Fatigue behavior o f carburized steels Carburized materials can be considered as composites consisting of a ductile and tough core and a strong, b u t brittle, case. Fatigue behavior of such materials is the result of the interactions between the two. In a simplified b u t reasonable approach, the behavior of such a material can be approximated by studying each of the constituents individually, and then combining these results to predict the behavior of the composite. This procedure was adopted by Krotine et al. [6] who studied the monotonic properties of carburized steels. Also, Landgraf and gichman [71 studied the axial and repeated bending fatigue properties of AISI 4027 and similar 40xx-type steels. Their results indicate that, at long lives, high carbon steels are better than 4027, oil quenched and tempered. This they showed to be due to the transformation of retained austenite to martensite as a result of cyclic plasticity. The martensite produced by this process has been proven [7] to possess high strength and much greater toughness than thermally produced martensite. Thus, the cyclic stress-strain curve for the higher carbon content steels was found [8] to be higher than for AISI 4027, indicating that the former are stronger at long lives. On the other hand, the same investigators found that oil quenched and tempered 4027 offered superior low-cycle fatigue properties because of greater ductility. In this regime, cracks formed in the brittle case early in the life and then propagated through the softer core. Finally, the behavior of specimens with varying carburized case thicknesses seemed to depend on case thickness. As stress-strength distributions change across tbe specimen cross-section as a function of case thickness, they also affect the site of crack initiation. When the case is t o o thin, cracks start on the specimen surface. However, at a certain greater thickness, the crack will initiate under the surface, indicating a shift in the maximum of the
curve, oa/(o~ Oo), versus depth. In the above ratio, Oo is the value of the residual stresses at each particular location, and o~ is a material constant, termed the fatigue strength coefficient. Landgraf and Richman illustrated the shift in location of crack initiation with a light fractograph which shows a subsurface inclusion to have provided the crack initiation site [7]. -
-
EXPERIMENTAL PROCEDURES
Flat plates of AISI 4027 steel were machined from hot-forged stock. After machining, specimens (Fig. 1) were carburized for 6 - 8 h at 920 °(2 (1 690 °F) and a carbon potential of 1.5%, and oil quenched. No tempering was performed. The specimens were fatigue tested in repeated bending, using a BLH Sonntag fatigue machine operating at 1 800 rpm. Most of the specimens were tested as received, employing various stress levels on the surfaces. Some specimens were instrumented with strain gages to determine clamping stresses and to make certain that the load was alternating around zero. The stress was varied by changing a rotating eccentric weight according to the simple equation: 3PR O'max
bh 2 '
where P is the imposed load, R, the lever arm, b, the specimen width, and h, the specimen thickness. After fracture, thin slices containing the fracture surfaces were cut from specimens. .250mm~' "~i 6,35
I
I
I
7.000 i
2.125 _1_ 54 mm - I
1.500 38 mm/ 7~0 ~f19 ~m
1782.750 mm 70 mm
I
i
+ I
/1.250R 32 mm R PLACES
3.500 89 mm
7- -. . 12 m m
Fig. 1. Fatigue specimen shape and size.
25
After low-magnification macrophotographs of fracture surfaces were obtained, these specimens were examined by scanning electron microscopy. Cross sections of each were traversed, starting from the edge of apparent crack initiation. The mode of fracture at each point was studied, and representative photographs were taken at appropriate locations. Special attention was paid to differentiate fracture surfaces of specimens tested at highand low stress levels. Additional slices of material were also cut off, mounted, and polished. Microhardness traverses were again taken in the thickness direction. A limited amount of metallography was also performed. Finally, six specimens of 4027 steel were heat treated by austenitizing at 840 - 870 °C (1 550 - 1 600 °F) for 30 min, oil quenching,
800
z 700 u~ a
600
SO0
I
1'111'1211 10
30
50
I
I
I
3
I
]4
s
I
6
70 90100 150 200 DISTANCE FROM EDGE, m m IN x 10 3
mm
INxl03
Fig. 2. Microhardness traverse through thickness of typical specimen.
and tempering (OQ and T). Final hardness was approximately 40 Rc in the gage section. The specimens were similarly fatigue tested in repeated bending, and these results were compared with those obtained from carburized specimens. No scanning fractography was performed on the OQ and T specimens, however.
RESULTS AND DISCUSSION
Results of microhardness testing with the Knoop indenter are presented in Fig. 2. Figure 3 shows the stress-life (S-N) curve for carburized AISI 4027 specimens tested in repeated bending. For comparison purposes, the curve representing the results of the OQ and T specimens is also plotted. These results seem to verify the findings of Landgraf and Richman [ 7, 8] as seen in Fig. 3. In the lowcycle regime, the 4027 steel seems to be better than the carburized material due to its greater ductility. The two curves cross over at approximately 15 000 cycles, which agrees very well with the value reported by Landgraf and Richman. Above 15 000 cycles, the carburized material appears to be significantly better, which is due to the higher strength of the carburized case. The data then seem to suggest that, for low-cycle applications (below approximately 15 000 cycles or 30 000 reversals) it is preferable to use heat treated 4027 steel rather than carburized 4027. On the other hand, if the desired life exceeds this limit, it is definitely preferable to use carburized material for the particular application.
200
I. L O G I o N = 9 . 8 5 7 - 0 . 0 4 4 0 2. L O G t 0 N = 9 . 3 2 6 - 0 . 0 4 1 0 •y., 180 ~
~
3. L O G I o N
u~
Q
&
300
= 17.545-0.1150
160 Z 7,
1200
T
-~ 1100 ~
14o
,
~
1000
m
900 120
100
""~
700
\,, I
80 lO 2
lO 3
I lO 4 N U M B E R OF CYCLES, N
600
"'~ lO s
1° 6
Fig. 3. S-N curves for AISI 4027 carburized plates tested in repeated bending. Oil quenched and tempered (OQ & T) specimens are also included.
26
Fig. 4. Fracture surface of specimen 8-1 tested at a high stress level. (Approximately × 5)
because of the particular relationship between the selected stress levels. By far the greatest a m o u n t of work, however, has been centered around scanning electron fractography. Specimens tested at highand low stress levels were studied mainly to differentiate between the effects of high- and low stresses, or plastic strains, on fracture mode and surface appearance. Some specimens tested at intermediate stress levels were also examined to determine the change in fracture mode as the stress is gradually decreased from high to lower levels. In the following, the typical fracture surface appearance for each stress level and metallurgical constituent will be discussed initially. Comparisons will then be made, and conclusions will be drawn concerning the possibilities of reversing this technique for failure analysis purposes.
Low-stress, high-cycle fracture
Fig. 5. Fracture surface of specimen 3-1 tested at a low stress level. (Approximately × 7)
Initial fracture through the carburized case was predominantly intergranular (Figs. 6 - 8) for all low-stress-tested specimens, such as 3-1, 55-1, 66-1, 8-2, and 5-3. Usually taken at the very beginning of the crack near the surface of the specimen, the first photographs exhibit a certain amount of cleaved grains on the fracture surface. This effect is considered to be due to the large increase of stress above levels expected near the specimen surface.
Macro-examination of fracture surfaces with the naked eye or low magnification macrophotography can supply valuable information concerning the fatigue stress level and the resulting lifespan. Figures 4 and 5 show typical fracture surfaces of specimens tested at high and low stress levels. Specimens tested at a low stress level (~ 758.4 MPa or ~ 1 1 0 ksi max. stress), in general, exhibit a smoother, more granular appearance. High stress level (~1 170 MPa or ~ 1 7 0 ksi) specimens, on the other hand, show a darker, rougher fracture surface. The fatigue crack need only grow a much shorter distance through the specimen thickness before final failure occurs. Intermediate stress level ( ~ 8 9 5 MPa or 130 ksi) specimens exhibit an intermediate behavior, as would be expected. Their behavior, however, seems to follow that of low stress level specimens more closely, presumably
Fig. 6. The fracture surface is mostly intergranular. Some cleaved grains are also visible. Specimen 66-1. ( x l 084)
27
;
Fig. 7. Crack initiation through the carburized case. The mode is mostly intergranular. Specimen 5-3. (× 1 440)
Fig. 8. Moving away from the outer surface, the mode becomes entirely intergranular. Specimen 5-3. (× 1440)
Discovered by Sandor and Nakano [9], this effect can be very significant in determining small differences in stress or life levels for specimens within the long-life regime. Close examination of Figs. 6 and 7 can help clarify these effects of small stress variations, which seem to be accompanied, at least in this instance, by large differences in fatigue life (80 000 v s . 1.12 X 106 cycles, respectively). The described grain boundary separation is also d o c u m e n t e d in the literature, and is considered to occur along the prior austenite
q
Fig. 9. Typical appearance of the ductile core. Fracture is the result of dimple and void coalescence. Specimen 5-3. (× 1 430)
grain boundaries. (See ASM Metals Handbook, 8th Edition, Vol. 9, "Fractography and Atlas of Fractographs".) The reason for this effect is, however, not precisely known at the present time. This mode of intergranular separation gradually changes to a mixed mode, and finally to entirely ductile fracture. Here, the microstructural features are dimples, voids, and cavities with inclusions as shown in Fig. 9. Ductile tearing is also often observed. Occasionally, the fracture surface through the ductile core shows large, elongated cavities and tear ridges in a normal sequence, as Figs. 10 and 11 demonstrate. It is quite probable that these features are fatigue striations produced during application of cyclic loads. Ridges observed in Fig. 11 are spaced approximately three microns apart, which is reasonable for fatigue striation spacing. Beyond the specimen midsection, the fracture surface was mainly produced by large tearing loads, which presumably fracture the specimen in a short time. This effect is evidenced by the continuous coarsening of fracture surface features in the form of large cavities and extended areas of ductile tear and "stretch zones". Final fracture through the carburized case was always perfectly intergranular, the effect being observed even in highly stressed specimens. This is discussed in the appropriate Section of this paper.
28
Fig. 10. At the mid-thickness position, non-uniform size dimples and many tear ridges appear. Specimen 55-1. (×1 069)
For the record, specimens 8-1, 3-2, 6-2, 1-3, and 2-3 were tested at a load setting that corresponded to 1 170 MPa (170 ksi) maxim u m stress, and specimens 2-1 and 9-1 were tested at lower stress levels. Initial fracture through the carburized case is markedly different from that for the low stress specimens. Fracture starts transgranularly in the form of cleavage of martensitic platelets (Fig. 12). Occasionally, and mainly near the end of the carburized case, a small area of mixed intergranular-transgranular modes may appear with the occurrence of secondary cracks. This mode of fracture indicates that high stress and large plastic strain cause martensite plates and retained austenite to fracture primarily by overloading. Under these conditions, transgranular fracture occurs by cleavage of the brittle martensite or the weak austenite. At any rate, a narrow ribbon of mainly intergranular fracture is observed at the case-core interface. These features are either grains appearing on the surface or secondary intergranular microcracks. The width of this ribbon is inversely proportional to the prevailing stress level, cf. Figs. 13 and 14. The width of the intergranular ribbon increases as the stress is decreased. This observation is considered significant since it provides a further refinement of the method proposed for identifying the applied stress level within the high stress regime in failure analysis.
Fig. 11. Fracture mode becomes transgranular through the core. A mixture of dimples and tear ridges can be identified on the fracture surface. Specimen 3-1.
(x I ooo)
High-stress, low-cycle fracture Specimens in this category had a maximum total life of approximately 1 000 cycles. The exact number of cycles for each specimen could not be determined accurately due to the nature and characteristics of the fatigue machine. Nor could the precise stress level at which these specimens were tested be determined accurately, again for technical reasons.
Fig. 12. Initial crack surface through the case is almost entirely transgranular. Specimen 8-1. (X 1 560)
29
Fig. 13. Fracture surface of the case is still predominantly transgranular with only small indication of intergranular separation. Specimen 8-1. (x i 570)
Fig. 14. Last portion of the case; fracture is entirely intergranular. Specimen 9-1. (X1 089) The fracture surface of the core material is c o mp o s ed of dimples of non-uniform size, tear ridges, and large, fairly flat surfaces. The latter are considered to result from tearing of the ductile core material, which is restricted from flowing for mechanical reasons and, therefore, appears flat. B e y o n d the specimen midsection, fracture proceeded mostly by overload. This effect is evidenced in Fig. 15, which exhibits large cavities and elongated dimples with occasional appearances of inclusions. All of these features
Fig. 15. Near the end of the core material, the surface becomes very rough with large holes, inclusions, cavities, tear ridges, and dimples. Specimen 3-2. (x 1 630) indicate the increased tearing which occurs at this point. Finally, as the crack re-enters the carburized case material, the m ode changes, becoming entirely intergranular. This observation was universal, and may indicate that a small fatigue crack was nucleated on the opposite side of the specimen from where the main crack grew. If this were the situation, the secondary crack would have grown under a low stress level for only a short distance. Final fracture would simply involve joining the two cracks together. Examination of fracture surfaces of specimens tested at intermediate stress levels revealed, as expected, a m i xt ure of the features produced by the high- and low stress levels. In general, the transition from high- to low stress results in fracture surface characteristics which are continuous, varying with the stress level employed. Since " i n t e r m e d i a t e " stress used in this work was fairly close to the low stress levels, crack features resemble those produced by the low stresses. Comparing the fatigue fracture surfaces of specimens tested at the two distinct stress regimes reveals differences that can be used in failure analysis. The basic difference between low- and high stress fatigue fracture is the crack surface appearance of the first-to-fracture carburized case. Examination of all tested specimens revealed that, in low stress-high cycle fatigue
30 specimens the initial crack through the carburized case was entirely intergranular. Grain boundary separation and intergranular microcracks were the main fracture mode, fractured grains being present only occasionally. The number of such cleaved grains was found to increase with a rise in the stress level. The latter observation may be utilized further to refine the estimate of the approximate stress levels responsible for the failure. At the other extreme of the high stress-low cycle test, initial fracture through the carburized case was almost entirely transgranular. This behavior was quite different from that of the low stress-high cycle specimens, and is attributed to large overloads that tend to fracture the weak retained austenite and the brittle martensite. On the other hand, when the retained austenite is allowed to deform plastically to martensite, the grain boundaries become the weak link in the system, as compared with the strong, ductile, mechanicallyproduced martensite. Under these conditions, local deformations exceed the cohesive strength between grains, concentrating the fracture at these locations. Near the case-core interface, a narrow ribbon of intergranularly-fractured material is observed in the form of individual grains or intergranular secondary cracks. The width of this zone seemed to be inversely proportional to the (high) stress level. This observation, although only qualitative, affords refinements within the high stress regime. Examination of the core material, unfortunately, did not provide any similar criteria for differentiating between specimens tested at high- or low stress levels. Some indications concerning surface roughness and coarseness of the observed features did exist. However, utilization of such types of information is subject to local variations. Differences are t o o subtle to be employed easily in routine failure analysis. Macroscopic examination of the fracture surface, on the other hand, can provide more valuable information concerning the stress levels involved, and should n o t be overlooked.
CONCLUSIONS
The following conclusions can be drawn from the present investigation concerning
fatigue fracture behavior of carburized 4027 steel. (1) Fracture through the carburized case of specimens tested at a low stress level is predominantly intergranular. Some transgranular fracture occasionally appears in an amount that can be used to determine small differences in the stress level within the low-stress regime. (2) High stress-low cycle specimens exhibited a predominantly transgranular fracture through the carburized case at the crack-start side of the specimens. A narrow region of mixed intergranular-transgranular fracture was also observed near the case-core interface. Since this feature seemed to vary with small variations in the stress level, it can be used to determine small differences of stress in the high-stress region. (3) The ductile core material did n o t provide special features that would allow determination of precise stress levels. More information can be obtained from visual and low magnification study of fatigue fracture surfaces. (4) Oil quenched and tempered 4027 steel gave better fatigue resistance below 10 000 15 000 cycles than carburized 4027, due to its greater ductility. This can be a significant observation when a part is designed for low-cycle fatigue applications. At high stress levels, the carburized material has much better fatigue resistance, which has been attributed to the strength of martensite in the case. (5) An additional point of great significance is the behavior of retained austenite. During cyclic loading it transforms to martensite which possesses much greater ductility than thermally-transformed martensite. This implies that retained austenite is not as undesirable a constituent as it was previously considered to be. In fact, Landgraf and Richman showed that specimens containing large amounts of retained austenite, tested in fatigue, had a much better life than refrigerated ones which were fully transformed. ACKNOWLEDGEMENTS
The authors express their deep appreciation to the American Motors Corporation for supporting the present investigation. The interest and guidance of, and discussion with, Dr. John Grebetz and Mr. R o b e r t Koos of AMC are also appreciated.
31 REFERENCES
1 W. J. Plumbridge and D. A. Ryder, The Metallography of Fatigue, Metallurgical Reviews, The Metals and Metallurgy Trust, Review 136, 1969, pp. 119 142. 2 R. W. Hertzberg, Fatigue Fracture Surface Appearance, A S T M STP 415, p. 205. 3 Bela I. Sandor, Fundamentals of Cyclic Stress and Strain, The University of Wisconsin Press,Madison, Wisconsin, 1972. 4 J. C. McMillan and R. M. N. Pelloux, Fatigue Crack Propagation Under Program and R a n d o m Loads, A S T M STP 415, 1967.
5 C. Laird, The influence of Metallurgical Structure on the Mechanisms of Fatigue Crack Propagation, ASTM STP 415, 1967. 6 F. T. Krotine, M. F. McGuire, L. J. Ebert and A. R. Troiano, A. S~ M. Trans. Q., 62 (4) (1969) 829. 7 R. W. Landgraf and R. H. Richman, Fatigue Behavior of Carburized Steel, Fatigue of Composite Materials, ASTM STP 569, pp. 130 - 144. 8 R. H. Richman and R. W. Landgraf, Some effects of retained austenite on the fatigue resistance of carburized steel, Metall. Trans., 64 (1975) 955 964. 9 B. I. Sandor and Y. Nakano, Unpublished research, Univ. Wisconsin, Madison, Wisconsin.
23
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
Fractography of Fatigue Fractures in Carburized Steel
N. L A Z A R I D I S
Steel Products Division, Inland Steel Research Laboratories, East Chicago, Indiana (U.S.A.) F. J. W O R Z A L A a n d B. I. S A N D O R
Engineering Department, University of Wisconsin, Madison, Wisconsin (U.S.A.) ( R e c e i v e d F e b r u a r y 7, 1 9 7 7 )
SUMMARY
Carburized specimens of AISI 4027 steel were tested in repeated bending at stress levels ranging from high-stress, low-cycle to lowstress, high-cycle, and the corresponding stress-life curve was determined. Considerable scanning electron microscopy was performed on fracture surfaces of failed specimens. Results indicate that a fatigue crack nucleates at the external surface of the carburized case, and propagates inward until it becomes large enough to cause rapid failure in bending. The crack surface appearance changes according to the growth rate of the crack and the hardness of the matrix material. A low-stress crack growth through the carburized case seemed to result in the well-established intergranular separation behavior. However, growth under high stress levels was rapid, causing a mixture of transgranular-intergranular fracture to result, the transgranular mode being more pronounced. The tough core, on the other hand, exhibited a more ductile mode of failure. The predominant mode consisted of microvoids, dimples of non-uniform size, and tear ridges with only occasional flat, quasicleavage step facets. Under no conditions did fatigue striations become apparent on either case or core material.
INTRODUCTION
The most significant goal of the present investigation was to establish differences in fracture surface appearance as a function of maximum stress level on specimen surface. It is hoped that this information can be utilized for failure analysis.
Scanning electron microscopy was used extensively to study, directly, the fracture surface of fatigue tested AISI 4027 carburized plates. LITERATURESURVEY
Basic fatigue background Repeated application of load or strain on a c o m p o n e n t can produce cyclic damage, called fatigue. When the accumulation of cyclic damage reaches a critical value, the material fails. This is a nucleation and growth process in the sense that a crack is nucleated as a result of extremely localized work hardening or softening, which then grows through the material in a stepwise fashion [1 - 3]. For the most part, the crack moves forward in the tensile (crack opening) half of the cycle. Though the crack-closing (compressive) half of the cycle does not usually contribute to crack propagation, it does damage the material in front of the crack tip to the point where it can propagate at the next half-cycle [4]. This sequential process of damage and growth leads to fatigue striations that are typical of many different steels and nonferrous metals and alloys [4, 5]. The coarseness or fineness of these striations depends entirely on the applied alternating stress level, which is proportional to the amount of plastic strain involved. The larger the a m o u n t of plastic strain or stress level involved, the coarser and wider the striations appear [4, 5]. In addition, each striation is the result of one stress cycle, allowing direct determination of the number of cycles present on the fracture surface. Thus, crack growth rates can be ascertained from microstructural features [4].
24
In general, the plastic strain or stress level involved is evidenced by the fineness or coarseness of characteristics of the fracture surface. Elastic stress levels related to long lives produce fine surface features, while gross plastic stresses result in tearing and a rough fracture surface. Fatigue behavior o f carburized steels Carburized materials can be considered as composites consisting of a ductile and tough core and a strong, b u t brittle, case. Fatigue behavior of such materials is the result of the interactions between the two. In a simplified b u t reasonable approach, the behavior of such a material can be approximated by studying each of the constituents individually, and then combining these results to predict the behavior of the composite. This procedure was adopted by Krotine et al. [6] who studied the monotonic properties of carburized steels. Also, Landgraf and gichman [71 studied the axial and repeated bending fatigue properties of AISI 4027 and similar 40xx-type steels. Their results indicate that, at long lives, high carbon steels are better than 4027, oil quenched and tempered. This they showed to be due to the transformation of retained austenite to martensite as a result of cyclic plasticity. The martensite produced by this process has been proven [7] to possess high strength and much greater toughness than thermally produced martensite. Thus, the cyclic stress-strain curve for the higher carbon content steels was found [8] to be higher than for AISI 4027, indicating that the former are stronger at long lives. On the other hand, the same investigators found that oil quenched and tempered 4027 offered superior low-cycle fatigue properties because of greater ductility. In this regime, cracks formed in the brittle case early in the life and then propagated through the softer core. Finally, the behavior of specimens with varying carburized case thicknesses seemed to depend on case thickness. As stress-strength distributions change across tbe specimen cross-section as a function of case thickness, they also affect the site of crack initiation. When the case is t o o thin, cracks start on the specimen surface. However, at a certain greater thickness, the crack will initiate under the surface, indicating a shift in the maximum of the
curve, oa/(o~ Oo), versus depth. In the above ratio, Oo is the value of the residual stresses at each particular location, and o~ is a material constant, termed the fatigue strength coefficient. Landgraf and Richman illustrated the shift in location of crack initiation with a light fractograph which shows a subsurface inclusion to have provided the crack initiation site [7]. -
-
EXPERIMENTAL PROCEDURES
Flat plates of AISI 4027 steel were machined from hot-forged stock. After machining, specimens (Fig. 1) were carburized for 6 - 8 h at 920 °(2 (1 690 °F) and a carbon potential of 1.5%, and oil quenched. No tempering was performed. The specimens were fatigue tested in repeated bending, using a BLH Sonntag fatigue machine operating at 1 800 rpm. Most of the specimens were tested as received, employing various stress levels on the surfaces. Some specimens were instrumented with strain gages to determine clamping stresses and to make certain that the load was alternating around zero. The stress was varied by changing a rotating eccentric weight according to the simple equation: 3PR O'max
bh 2 '
where P is the imposed load, R, the lever arm, b, the specimen width, and h, the specimen thickness. After fracture, thin slices containing the fracture surfaces were cut from specimens. .250mm~' "~i 6,35
I
I
I
7.000 i
2.125 _1_ 54 mm - I
1.500 38 mm/ 7~0 ~f19 ~m
1782.750 mm 70 mm
I
i
+ I
/1.250R 32 mm R PLACES
3.500 89 mm
7- -. . 12 m m
Fig. 1. Fatigue specimen shape and size.
25
After low-magnification macrophotographs of fracture surfaces were obtained, these specimens were examined by scanning electron microscopy. Cross sections of each were traversed, starting from the edge of apparent crack initiation. The mode of fracture at each point was studied, and representative photographs were taken at appropriate locations. Special attention was paid to differentiate fracture surfaces of specimens tested at highand low stress levels. Additional slices of material were also cut off, mounted, and polished. Microhardness traverses were again taken in the thickness direction. A limited amount of metallography was also performed. Finally, six specimens of 4027 steel were heat treated by austenitizing at 840 - 870 °C (1 550 - 1 600 °F) for 30 min, oil quenching,
800
z 700 u~ a
600
SO0
I
1'111'1211 10
30
50
I
I
I
3
I
]4
s
I
6
70 90100 150 200 DISTANCE FROM EDGE, m m IN x 10 3
mm
INxl03
Fig. 2. Microhardness traverse through thickness of typical specimen.
and tempering (OQ and T). Final hardness was approximately 40 Rc in the gage section. The specimens were similarly fatigue tested in repeated bending, and these results were compared with those obtained from carburized specimens. No scanning fractography was performed on the OQ and T specimens, however.
RESULTS AND DISCUSSION
Results of microhardness testing with the Knoop indenter are presented in Fig. 2. Figure 3 shows the stress-life (S-N) curve for carburized AISI 4027 specimens tested in repeated bending. For comparison purposes, the curve representing the results of the OQ and T specimens is also plotted. These results seem to verify the findings of Landgraf and Richman [ 7, 8] as seen in Fig. 3. In the lowcycle regime, the 4027 steel seems to be better than the carburized material due to its greater ductility. The two curves cross over at approximately 15 000 cycles, which agrees very well with the value reported by Landgraf and Richman. Above 15 000 cycles, the carburized material appears to be significantly better, which is due to the higher strength of the carburized case. The data then seem to suggest that, for low-cycle applications (below approximately 15 000 cycles or 30 000 reversals) it is preferable to use heat treated 4027 steel rather than carburized 4027. On the other hand, if the desired life exceeds this limit, it is definitely preferable to use carburized material for the particular application.
200
I. L O G I o N = 9 . 8 5 7 - 0 . 0 4 4 0 2. L O G t 0 N = 9 . 3 2 6 - 0 . 0 4 1 0 •y., 180 ~
~
3. L O G I o N
u~
Q
&
300
= 17.545-0.1150
160 Z 7,
1200
T
-~ 1100 ~
14o
,
~
1000
m
900 120
100
""~
700
\,, I
80 lO 2
lO 3
I lO 4 N U M B E R OF CYCLES, N
600
"'~ lO s
1° 6
Fig. 3. S-N curves for AISI 4027 carburized plates tested in repeated bending. Oil quenched and tempered (OQ & T) specimens are also included.
26
Fig. 4. Fracture surface of specimen 8-1 tested at a high stress level. (Approximately × 5)
because of the particular relationship between the selected stress levels. By far the greatest a m o u n t of work, however, has been centered around scanning electron fractography. Specimens tested at highand low stress levels were studied mainly to differentiate between the effects of high- and low stresses, or plastic strains, on fracture mode and surface appearance. Some specimens tested at intermediate stress levels were also examined to determine the change in fracture mode as the stress is gradually decreased from high to lower levels. In the following, the typical fracture surface appearance for each stress level and metallurgical constituent will be discussed initially. Comparisons will then be made, and conclusions will be drawn concerning the possibilities of reversing this technique for failure analysis purposes.
Low-stress, high-cycle fracture
Fig. 5. Fracture surface of specimen 3-1 tested at a low stress level. (Approximately × 7)
Initial fracture through the carburized case was predominantly intergranular (Figs. 6 - 8) for all low-stress-tested specimens, such as 3-1, 55-1, 66-1, 8-2, and 5-3. Usually taken at the very beginning of the crack near the surface of the specimen, the first photographs exhibit a certain amount of cleaved grains on the fracture surface. This effect is considered to be due to the large increase of stress above levels expected near the specimen surface.
Macro-examination of fracture surfaces with the naked eye or low magnification macrophotography can supply valuable information concerning the fatigue stress level and the resulting lifespan. Figures 4 and 5 show typical fracture surfaces of specimens tested at high and low stress levels. Specimens tested at a low stress level (~ 758.4 MPa or ~ 1 1 0 ksi max. stress), in general, exhibit a smoother, more granular appearance. High stress level (~1 170 MPa or ~ 1 7 0 ksi) specimens, on the other hand, show a darker, rougher fracture surface. The fatigue crack need only grow a much shorter distance through the specimen thickness before final failure occurs. Intermediate stress level ( ~ 8 9 5 MPa or 130 ksi) specimens exhibit an intermediate behavior, as would be expected. Their behavior, however, seems to follow that of low stress level specimens more closely, presumably
Fig. 6. The fracture surface is mostly intergranular. Some cleaved grains are also visible. Specimen 66-1. ( x l 084)
27
;
Fig. 7. Crack initiation through the carburized case. The mode is mostly intergranular. Specimen 5-3. (× 1 440)
Fig. 8. Moving away from the outer surface, the mode becomes entirely intergranular. Specimen 5-3. (× 1440)
Discovered by Sandor and Nakano [9], this effect can be very significant in determining small differences in stress or life levels for specimens within the long-life regime. Close examination of Figs. 6 and 7 can help clarify these effects of small stress variations, which seem to be accompanied, at least in this instance, by large differences in fatigue life (80 000 v s . 1.12 X 106 cycles, respectively). The described grain boundary separation is also d o c u m e n t e d in the literature, and is considered to occur along the prior austenite
q
Fig. 9. Typical appearance of the ductile core. Fracture is the result of dimple and void coalescence. Specimen 5-3. (× 1 430)
grain boundaries. (See ASM Metals Handbook, 8th Edition, Vol. 9, "Fractography and Atlas of Fractographs".) The reason for this effect is, however, not precisely known at the present time. This mode of intergranular separation gradually changes to a mixed mode, and finally to entirely ductile fracture. Here, the microstructural features are dimples, voids, and cavities with inclusions as shown in Fig. 9. Ductile tearing is also often observed. Occasionally, the fracture surface through the ductile core shows large, elongated cavities and tear ridges in a normal sequence, as Figs. 10 and 11 demonstrate. It is quite probable that these features are fatigue striations produced during application of cyclic loads. Ridges observed in Fig. 11 are spaced approximately three microns apart, which is reasonable for fatigue striation spacing. Beyond the specimen midsection, the fracture surface was mainly produced by large tearing loads, which presumably fracture the specimen in a short time. This effect is evidenced by the continuous coarsening of fracture surface features in the form of large cavities and extended areas of ductile tear and "stretch zones". Final fracture through the carburized case was always perfectly intergranular, the effect being observed even in highly stressed specimens. This is discussed in the appropriate Section of this paper.
28
Fig. 10. At the mid-thickness position, non-uniform size dimples and many tear ridges appear. Specimen 55-1. (×1 069)
For the record, specimens 8-1, 3-2, 6-2, 1-3, and 2-3 were tested at a load setting that corresponded to 1 170 MPa (170 ksi) maxim u m stress, and specimens 2-1 and 9-1 were tested at lower stress levels. Initial fracture through the carburized case is markedly different from that for the low stress specimens. Fracture starts transgranularly in the form of cleavage of martensitic platelets (Fig. 12). Occasionally, and mainly near the end of the carburized case, a small area of mixed intergranular-transgranular modes may appear with the occurrence of secondary cracks. This mode of fracture indicates that high stress and large plastic strain cause martensite plates and retained austenite to fracture primarily by overloading. Under these conditions, transgranular fracture occurs by cleavage of the brittle martensite or the weak austenite. At any rate, a narrow ribbon of mainly intergranular fracture is observed at the case-core interface. These features are either grains appearing on the surface or secondary intergranular microcracks. The width of this ribbon is inversely proportional to the prevailing stress level, cf. Figs. 13 and 14. The width of the intergranular ribbon increases as the stress is decreased. This observation is considered significant since it provides a further refinement of the method proposed for identifying the applied stress level within the high stress regime in failure analysis.
Fig. 11. Fracture mode becomes transgranular through the core. A mixture of dimples and tear ridges can be identified on the fracture surface. Specimen 3-1.
(x I ooo)
High-stress, low-cycle fracture Specimens in this category had a maximum total life of approximately 1 000 cycles. The exact number of cycles for each specimen could not be determined accurately due to the nature and characteristics of the fatigue machine. Nor could the precise stress level at which these specimens were tested be determined accurately, again for technical reasons.
Fig. 12. Initial crack surface through the case is almost entirely transgranular. Specimen 8-1. (X 1 560)
29
Fig. 13. Fracture surface of the case is still predominantly transgranular with only small indication of intergranular separation. Specimen 8-1. (x i 570)
Fig. 14. Last portion of the case; fracture is entirely intergranular. Specimen 9-1. (X1 089) The fracture surface of the core material is c o mp o s ed of dimples of non-uniform size, tear ridges, and large, fairly flat surfaces. The latter are considered to result from tearing of the ductile core material, which is restricted from flowing for mechanical reasons and, therefore, appears flat. B e y o n d the specimen midsection, fracture proceeded mostly by overload. This effect is evidenced in Fig. 15, which exhibits large cavities and elongated dimples with occasional appearances of inclusions. All of these features
Fig. 15. Near the end of the core material, the surface becomes very rough with large holes, inclusions, cavities, tear ridges, and dimples. Specimen 3-2. (x 1 630) indicate the increased tearing which occurs at this point. Finally, as the crack re-enters the carburized case material, the m ode changes, becoming entirely intergranular. This observation was universal, and may indicate that a small fatigue crack was nucleated on the opposite side of the specimen from where the main crack grew. If this were the situation, the secondary crack would have grown under a low stress level for only a short distance. Final fracture would simply involve joining the two cracks together. Examination of fracture surfaces of specimens tested at intermediate stress levels revealed, as expected, a m i xt ure of the features produced by the high- and low stress levels. In general, the transition from high- to low stress results in fracture surface characteristics which are continuous, varying with the stress level employed. Since " i n t e r m e d i a t e " stress used in this work was fairly close to the low stress levels, crack features resemble those produced by the low stresses. Comparing the fatigue fracture surfaces of specimens tested at the two distinct stress regimes reveals differences that can be used in failure analysis. The basic difference between low- and high stress fatigue fracture is the crack surface appearance of the first-to-fracture carburized case. Examination of all tested specimens revealed that, in low stress-high cycle fatigue
30 specimens the initial crack through the carburized case was entirely intergranular. Grain boundary separation and intergranular microcracks were the main fracture mode, fractured grains being present only occasionally. The number of such cleaved grains was found to increase with a rise in the stress level. The latter observation may be utilized further to refine the estimate of the approximate stress levels responsible for the failure. At the other extreme of the high stress-low cycle test, initial fracture through the carburized case was almost entirely transgranular. This behavior was quite different from that of the low stress-high cycle specimens, and is attributed to large overloads that tend to fracture the weak retained austenite and the brittle martensite. On the other hand, when the retained austenite is allowed to deform plastically to martensite, the grain boundaries become the weak link in the system, as compared with the strong, ductile, mechanicallyproduced martensite. Under these conditions, local deformations exceed the cohesive strength between grains, concentrating the fracture at these locations. Near the case-core interface, a narrow ribbon of intergranularly-fractured material is observed in the form of individual grains or intergranular secondary cracks. The width of this zone seemed to be inversely proportional to the (high) stress level. This observation, although only qualitative, affords refinements within the high stress regime. Examination of the core material, unfortunately, did not provide any similar criteria for differentiating between specimens tested at high- or low stress levels. Some indications concerning surface roughness and coarseness of the observed features did exist. However, utilization of such types of information is subject to local variations. Differences are t o o subtle to be employed easily in routine failure analysis. Macroscopic examination of the fracture surface, on the other hand, can provide more valuable information concerning the stress levels involved, and should n o t be overlooked.
CONCLUSIONS
The following conclusions can be drawn from the present investigation concerning
fatigue fracture behavior of carburized 4027 steel. (1) Fracture through the carburized case of specimens tested at a low stress level is predominantly intergranular. Some transgranular fracture occasionally appears in an amount that can be used to determine small differences in the stress level within the low-stress regime. (2) High stress-low cycle specimens exhibited a predominantly transgranular fracture through the carburized case at the crack-start side of the specimens. A narrow region of mixed intergranular-transgranular fracture was also observed near the case-core interface. Since this feature seemed to vary with small variations in the stress level, it can be used to determine small differences of stress in the high-stress region. (3) The ductile core material did n o t provide special features that would allow determination of precise stress levels. More information can be obtained from visual and low magnification study of fatigue fracture surfaces. (4) Oil quenched and tempered 4027 steel gave better fatigue resistance below 10 000 15 000 cycles than carburized 4027, due to its greater ductility. This can be a significant observation when a part is designed for low-cycle fatigue applications. At high stress levels, the carburized material has much better fatigue resistance, which has been attributed to the strength of martensite in the case. (5) An additional point of great significance is the behavior of retained austenite. During cyclic loading it transforms to martensite which possesses much greater ductility than thermally-transformed martensite. This implies that retained austenite is not as undesirable a constituent as it was previously considered to be. In fact, Landgraf and Richman showed that specimens containing large amounts of retained austenite, tested in fatigue, had a much better life than refrigerated ones which were fully transformed. ACKNOWLEDGEMENTS
The authors express their deep appreciation to the American Motors Corporation for supporting the present investigation. The interest and guidance of, and discussion with, Dr. John Grebetz and Mr. R o b e r t Koos of AMC are also appreciated.
31 REFERENCES
1 W. J. Plumbridge and D. A. Ryder, The Metallography of Fatigue, Metallurgical Reviews, The Metals and Metallurgy Trust, Review 136, 1969, pp. 119 142. 2 R. W. Hertzberg, Fatigue Fracture Surface Appearance, A S T M STP 415, p. 205. 3 Bela I. Sandor, Fundamentals of Cyclic Stress and Strain, The University of Wisconsin Press,Madison, Wisconsin, 1972. 4 J. C. McMillan and R. M. N. Pelloux, Fatigue Crack Propagation Under Program and R a n d o m Loads, A S T M STP 415, 1967.
5 C. Laird, The influence of Metallurgical Structure on the Mechanisms of Fatigue Crack Propagation, ASTM STP 415, 1967. 6 F. T. Krotine, M. F. McGuire, L. J. Ebert and A. R. Troiano, A. S~ M. Trans. Q., 62 (4) (1969) 829. 7 R. W. Landgraf and R. H. Richman, Fatigue Behavior of Carburized Steel, Fatigue of Composite Materials, ASTM STP 569, pp. 130 - 144. 8 R. H. Richman and R. W. Landgraf, Some effects of retained austenite on the fatigue resistance of carburized steel, Metall. Trans., 64 (1975) 955 964. 9 B. I. Sandor and Y. Nakano, Unpublished research, Univ. Wisconsin, Madison, Wisconsin.