Journal of the Society of Materials Science, Japan

DIVISION AND COMMITTEE

1990 Volume 39 Issue 440 Pages 476-482
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.476

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Creep Damage Evaluation of Boiler Tubes

Takashi SATO, Teruo KOYAMA, Masataka NOMURA, Shigekatsu SUGAI

1990 Volume 39 Issue 440 Pages 483-488
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.483

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Nondestructive and destructive residual life evaluation techniques for high temperature component materials have been studied by many researchers, and recently some of these techniques have been applied to actual power plants. But in the case of creep damage evaluation of boiler tubes, these techniques have been applied only at typical locations because of a huge number of tubes to be inspected and the unreliability of metal temperature assessments.
For developing creep damage evaluation techniques for boiler tubes, 3 tubes (JIS STB52D) were taken from an actual power plant after a prolonged service and then examined. These tubes were metallographically observed, and several kinds of creep tests were carried out to evaluate the metal temperature in service and the extent of creep damage. The relationships of the area fraction of cavities and the diametral strain to the creep damage were also discussed. The results obtained are summarised as follows:
1) The metal temperature in service and the creep damage of each tube could be evaluated properly by using the results obtained from creep rupture tests.
2) The area fraction of cavities observed near the tube surface increased with creep damage, and this behaviour was similar to that of SUS321HTB.
3) The diametral strain ε^d showed a good correlation with the creep damage, and ε^d increased rapidly at the later stage of the creep life. The diametral strain can be an effective parameter to evaluate the creep damage of tubes.

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Sintering of Creep-Induced Grain Boundary Cavities in 316 Stainless Steel

Masaharu MURATA, Hideo TANAKA, Norio SHINYA, Ryo HORIUCHI

1990 Volume 39 Issue 440 Pages 489-495
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.489

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Creep cavities associated with sigma phase particles appeared at grain boundaries in 316 stainless steel specimens crept under the stress condition of 37MPa and 53MPa at 750°C. In order to sinter the cavities formed, the specimens were annealed, hot-isostatically pressured and compressively crept at 750°C. Precise density measurements and SEM observations on these specimens were made to study the rate of sintering.
The results obtained were as follows;
(1) Small cavities (<2μm) disappeared by annealing, whereas large crack-like cavities (>2μm) still remained after annealing. The sintering rate calculated by the grain boundary diffusion model was very much higher than the experimental one. This showed that sintering of cavities was not controlled by grain boundary diffusion.
(2) The rate of sintering was considerably accelerated under the hot-isostatic pressure condition of 37MPa.
(3) The effect of compressive creep (20 to 53MPa) on sintering was most marked. Very large crack-like cavities (≥10μm) disappeared quickly by compressive creep. The rate of sintering in compressive creep was more than double as quick as that in hot-isostatic pressure. It was considered that the compressive strain removed the constraint to sintering of cavities and activated the atomic diffusion from grain boundaries to cavity surface.

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Cavitation during Creep in Notched Bar of CrMoV-Steel

Katsuhiko TAKITA, Yorimasa TAKEDA, Makoto NAKAMURA, Kenzaburo TAKEZOE

1990 Volume 39 Issue 440 Pages 496-502
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.496

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In actual components, almost all creep failures occur at a stress concentrated parts. Therefore, the cavitation during creep in a notched bar was examined both by using FEM and actual testing on notched CrMoV-steel bar specimens, which were heat-treated in advance so as to get the creep-embrittlement in a rather short time. The change of creep voids in their number, distribution, orientation and coarescing direction in the notch root region was made clear by using SEM and an image processor. It was found that the cavitation behavior corresponded well with the creep damage based on the maximum principal stress.

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Creep-Fatigue Strength of SUS304 Welded Joints at 550°C and the Evaluation on the Basis of Strain Concentration

Hiroyuki KAGAWA, Shinichi HASEBE, Ryuuji KOMINE, Syouichi KATOH, Kazum ...

1990 Volume 39 Issue 440 Pages 503-508
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.503

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Strain controlled fatigue and creep-fatigue tests at 550°C were carried out on two kinds of SUS304 welded joints. The results obtained are summarized as follows;
(1) Fatigue strength reduction factor of welded joints, in the case without strain holding, shows a maximum value at the strain range of 0.3%, and decreases rapidly with a decrease in strain range.
(2) Creep-fatigue strength of welded joints, in the case of strain range of more than 0.5%, can be reasonably evaluated by strain concentration in a weld metal region, which is strongly affected by thermal softening and/or dynamic strain aging behaviors of weld metal and base metal.
(3) Creep-fatigue strength of welded joints in a low strain range region is considerably affected by creep damage, which increases due to elastic follow-up during strain holding period.

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Creep-Rupture Behavior of Butt Welded Joint of 304 Stainless Steel Thick Plate Using Small and Large Specimens

Masayoshi YAMAZAKI, Yoshio MONMA, Hiromichi HONGO, Takashi WATANABE, J ...

1990 Volume 39 Issue 440 Pages 509-515
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.509

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Creep-rupture properties of 50mm thick 304 stainless steel joints by narrow-gap submerged arc welding were investigated at 550°C using small (round bar) and large (full-thickness plate) specimens. For the small round-bar specimens rupture occurred at the base metal, while fracture was at the weld metal for large plate specimens. This difference in rupture location was attributed to the stress distribution in the welded joint specimens during the creep process. The stress distribution can be caused by the non-uniformity of strength in the weld metal and the suppression of deformation due to hardening of the heat affected zone in the base metal.
The deformation behaviors of base metal and weld metal in the joint can be estimated by a FEM computation, and it allow us to predict the location of fracture in the weldment by comparison of relafive strength with base and weld metals.

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Creep-Fatigue Phenomena and Creep-Fatigue Damage in Cr-Mo-V Steel Forgings

Yoshikuni KADOYA, Toru GOTO, Kazuo KAWAMOTO, Katsumi SUGAI

1990 Volume 39 Issue 440 Pages 516-521
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.516

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In order to investigate the characteristics of creep-fatigue interaction and its mechanism for Cr-Mo-V steel forgings, the creep-fatigue tests were carried out under a strain-controlled condition for long-term and the metallographic observations were conducted on the interrupted creep-fatigue specimens.
The results obtained are summarized as follows:
(1) The data obtained were evaluated using the linear life fraction damage rule. The value of creep damage (D_c) was lower than the value of fatigue damage (D_f) by two orders of magnitude for the tests conducted. Consequently, D_c+D_f≅D_f. The fracture mode was transgranular as is generally seen in the fatigue dominant fracture. However, fracture occurred at a small value of D_f when the hold time was added.
(2) Metallographic observation, however, showed that creep cavities nucleated and grew inside the specimen with increasing number of cycles in the case of long hold time and low strain-range. Moreover, it was found that fatigue micro-cracks of the case where creep cavities and creep cracks were observed in the fractured specimen had a tendency to propagate and link up along grain boundary.
(3) It is suggested from the above mentioned results that, even though D_c is very small, creep cavities and creep cracks nucleate, and that they reduce the fatigue life by the effect of their linkage with fatigue crack.

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Damage Evaluation of Inconel 738LC Nickel-Base Superalloy in Creep-Fatigue

Hiroaki IBUCHI, Hirofumi KIMURA, Masao SAKANE, Masateru OHNAMI, Masami ...

1990 Volume 39 Issue 440 Pages 522-528
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.522

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This paper studies creep-fatigue damage evaluation of a nickel-base superalloy Inconel 738LC by X-ray diffraction, hardness and ultrasonic wave methods as well as replication method. Since its deformation behavior is almost elastic at 1123K, the nondestructive measures, which detect the plastic deformation, such as hardness and ultrasonic wave are not effective to evaluate the creep-fatigue damages. X-ray diffraction is occasionally effective for the creep-fatigue damage evaluation. On the other hand, the replication method is available for creep-fatigue damage evaluation. Cracks were mostly initiated and propagated along the grain boundaries of the [001] oriented crystal. Simulation of the 3-dimensional FEM analysis, taking account of the elastic anisotropy of the grains, revealed that the crack nucleation along such crystals is mainly attributed to the large stressing due to the elasticity anisotropy. Therefore, as a damage evaluation method, it is effective to detect the cracks by replication method on the [001] grains and to measure the misorientation of the most [001] oriented crystals.

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Surface Small Crack Initiation and Growth of Heat Resisting Alloys in Creep-Fatigue

Ryuichi OHTANI, Takayuki KITAMURA, Muneaki ABE, Yoshihide KURIYAMA, Hi ...

1990 Volume 39 Issue 440 Pages 529-535
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.529

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In order to investigate the behavior of small crack initiation and growth in creep-fatigue at high temperature, c-p type fatigue (slow tension and fast compression) tests were carried out using smooth specimens of Type 304 stainless steel, 21/4Cr-1Mo steel and Ti-17 titanium alloy. Their average grain sizes were 50, 20 and 5μm, respectively. The results obtained are summarized as follows:
(1) Multiple small cracks initiated and grew along the grain boundaries on the specimen surface.
(2) The increasing rate of crack density showed stronger dependence on the strain range than the crack growth rate.
(3) Remarkable fluctuation of the crack growth rate, which is characteristic of microstructurally small cracks, was found for the cracks of the half length being smaller than 4 or 6 times of the average grain diameter.
(4) The average growth rate of the small cracks coincided with the rate extrapolated from the large crack growth law.
(5) The normalized relation between crack growth rate and crack length, being available for representing the region of microstructurally small cracks, was successfully derived on the basis of large crack growth law and average grain diameter of materials. This relation was independent of material and strain range.

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Acceleration of Crack Growth under Intermittent Overloading at Elevated Temperature

Ryoichi KOTERAZAWA, Takayoshi NOSHO, Toshikazu MURAKAMI, Nobuhiro YAMA ...

1990 Volume 39 Issue 440 Pages 536-542
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.536

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Acceleration of crack growth under intermittent overloading was studied at 650°C by using two types of center notched plate specimens of different thickness made of a SUS 304 stainless steel. When the hold time of overload was very short (20 seconds), the crack growth rate was significantly accelerated to be about 20-50 times as large as that of static creep cracks, and the fracture surface morphology was different from that under static loading, indicating extremely ductile transgranular fracture by glide plane decohesion or microvoid coalescence. The acceleration in growth rate could be ascribed to recovery of the material during the low stress period. In the thinner plate specimens, the crack growth rate under intermittent loading was correlated well with modified J-integral J and agreed with the growth rate of static creep cracks in J-da/dt diagram. In the thicker plate specimens, however, the crack growth rate under intermittent loading did not agree with that of the thinner plate specimens in J-da/dt diagram, being about 1/5 times. Transgranular fatigue type crack growth appeared in the low growth rate region, and the growth rate was different from that of creep cracks.

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High Temperature Fatigue Properties and Life Prediction of Mod.9Cr-1Mo Steel under PP Type Variable Straining

Mitsuo MIYAHARA, Katsuyuki TOKIMASA

1990 Volume 39 Issue 440 Pages 543-548
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.543

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Twc-step variable pp type straining tests were conducted for Mod.9Cr-1Mo Steel at 600°C in air, where pp type strain range level varied from high to low (HL tests) or from low to high (LH tests). It was found that the experimental results of the two-step tests could be predicted by the linear damage rule when both the lower and the higher total strain range levels were greater than 0.8%. When the lower strain range level was smaller than 0.8%, discrepancy was found between the experimental results and the linear damage rule prediction. In HL tests the sum of life ratios was smaller than unity and in LH tests the sum of life ratios was larger than unity. These tests results were well explained by the proposed fatigue life prediction model, in which it was assumed that the crack initiation life is negligible in the straining level larger than 0.8% while it is not negligible in the straining level smaller than 0.8% and that the crack growth curve is described by the linear relation between ln (a/a₀) and n/N_pp, a being the half crack length, a₀ the initial half crack length, n the applied cycles and N_pp the total fatigue life. The ratio of the crack initiation life to the total fatigue life, α, was determined quantitatively. The smaller the straining level, the larger the value of α.

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Observation of First Stage High-Cycle Fatigue Crack Growth Behavior Using Directionally-Solidified Al-Li Alloy at High Temperature

Masakazu OKAZAKI, Shigeki YAMAHIRA, You KOJIMA

1990 Volume 39 Issue 440 Pages 549-555
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.549

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High-cycle fatigue tests were carried out using directionally-solidified Al-Li alloy at 453K, and the first stage fatigue crack growth behavior was investigated by employing an elaborate replication technique. It was shown that the directionally-solidified alloy predominantly revealed the typical stage I fatigue fracture; that is, the fracture occurred on the crystallographic {111} slip planes and preferentially in <101> directions, which are the most important slip system in f.c.c. materials. It was also found that the rate of the stage I crack growth, whose crack tip was far apart enough from neighbouring grain boundaries, proportionally increased with crack length, and that it rapidly decreased as the crack tip approached to neighbouring grain boundaries. Such a stage I crack growth behavior was compared with Tanaka model, which was proposed based on the phenomenon that the crack tip plastic deformation at small crack tip was blocked by neighbouring grain boundaries. The stage I crack growth behavior not only far apart from but also near grain boundaries obtained in this work could be successfully evaluated by Tanaka model. The above results suggested that the crack tip sliding displacement played an important role in the stage I crack growth.

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Deformation and Failure of 21/4Cr-1Mo Steel Under Push-Pull and Torsion Fatigue Condition

Shoji KITADE, Masafumi YAMAUCHI, Toshihide IGARI, Keiichi NAKASHIMA

1990 Volume 39 Issue 440 Pages 556-561
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.556

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Biaxial strain-controlled fatigue tests for 21/4Cr-1Mo steel at 550°C were performed under both proportional and nonproportional loading of combined tension and torsion, in order to examine the deformation property and the failure mode under biaxial stresses. Further, by using the results obtained, the accuracy of plastic constitutive equations and life prediction methods under biaxial stresses was investigated.
The results obtained are summarized as follows.
(1) The degree of cyclic softening under both proportional and nonproportional loading was uniformly arranged by using a parameter of Δσ_ASME, that is defined as the length of diagonal line in stress history.
(2) The stress-strain behavior at half fatigue life under biaxial stresses could be predicted by the combination of conventional constitutive equation and cyclic stress-strain curve under uniaxial stress.
(3) The life prediction under biaxial stresses by the equivalent strain range defined in ASME Code, was conservative.
(4) The life prediction method based on energy approach was valid for the improvement of the accuracy of life prediction under biaxial stresses.

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A Study on Life Evaluation Method of 304 Stainless Steel Under Biaxial Creep-Fatigue Condition

Takashi OGATA, Akito NITTA, Kazuo KUWABARA

1990 Volume 39 Issue 440 Pages 562-568
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.562

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In this study, biaxial creep-fatigue tests were carried out under in-phase and out-of-phase strain conditions at 550°C, and the fracture surface appearance and biaxial creep-fatigue life characteristics were assessed. The fracture modes in the in-phase creep-fatigue tests were characterized as Mode I and Mode II similar to those under fatigue conditions at 550°C. The mixed intergranular and transgranular fracture was observed in both the in-phase and out-of-phase tests.
In order to extend the biaxial LCF total-strain-based criterion to creep-fatigue life evaluation, the inelastic-strain-based parameter, Δγ_in was defined based on the iso-failure lines in the modified inelastic Γ-plane. By the new criterion combined with the damage rate rule, the creep-fatigue life was estimated well within a factor of two scatter band.

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SEM With High Temperature Fatigue Testing Capability and Its Application to Crack Behavior of Si₃N₄

Masaki KITAGAWA, Yuji OHSHIMA

1990 Volume 39 Issue 440 Pages 569-574
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.569

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A common difficulty associated with high temperature (above 800°C) in-situ observation of matter in a SEM is the emissions of thermal electrons and light from a heating device and the heated specimen itself, obscuring the image and degrading the resolution. This difficulty was overcome by placing a thermal electron barrier that works as a light shield also.
By optimizing the design of testing jigs and these heat shields as well as the bias voltage, in-situ observations of propagation and self-healing of cracks in Si₃N₄ were successfully conducted at temperatures as high as 1200°C. Also, difference of the effects of moisture on static and cyclic fatigue crack propagations at room temperature was presented.

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The Application of Fracture Mechanics to Advanced Problems

5. New Developments on Fracture Analysis and Monitoring in Experimental Fracture Mechanics

K. Ogura, Y. Miyoshi, S. Kubo, K. Minoshima

1990 Volume 39 Issue 440 Pages 575-581
Published: May 15, 1990
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.39.575

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