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ISIJ International Vol. 30 (1990), No. 10

ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559
Publisher: The Iron and Steel Institute of Japan

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ISIJ International Vol. 30 (1990), No. 10

Mechanistic Interpretations of Some Empirical Correlations in Creep and Creep Fracture

Geoffrey W. Greenwood

pp. 795-801

DOI:

10.2355/isijinternational.30.795

Abstract

Creep failure occurs through excessive strain or premature fracture. From many empirical relationships proposed to predict the strain incurred in a given time at a specified stress and temperature, some are selected in terms of their practical usefulness and their scientific basis is discussed. It is shown that distinctive mechanisms can operate, dependent upon the material and on the creep conditions and it is important that these are individually considered. It is also noted that these mechanisms may be coupled in ways which may smooth the transitions between them. This may account for some empirical approaches being applicable for approximate predictions over relatively wide ranges. Different mechanisms leading to creep fracture can also be distinguished. Limits of extreme behavior can be clearly identified and explained but, under intermediate conditions, the form of coupling of different mechanisms leads to continuity in transitional regions. Concepts of creep damage can be developed in terms of cavity nucleation and growth, with the value of microstructural examination clearly recognised. Whilst much remains to be done, a scientific framework for the understanding of the mechanisms of creep and creep fracture is already in place and there is opportunity for detailed physical modelling to assist the prediction of behaviour in practical situations.

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Mechanistic Interpretations of Some Empirical Correlations in Creep and Creep Fracture

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Creep Deformation of Engineering Alloys: Developments from Physical Modelling

B. F. Dyson, M. McLean

pp. 802-811

DOI:

10.2355/isijinternational.30.802

Abstract

Differences in the creep behaviour of particle-strengthened engineering alloys relative to simple solid solutions are reviewed and their implications for the mechanisms of high temperature deformation in these materials are considered. Constitutive equations describing the shapes of creep curves, based on physical models of the important types of damage are considered. These are incorporated in a computer software package, designated CRISPEN, that allows the analysis of creep data, the development of a database of model parameters and the simulation of the strain/time trajectory for arbitrary loading conditions. Examples of the application of CRISPEN to a range of alloys and loading conditions are described.

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Creep Deformation of Engineering Alloys: Developments from Physical Modelling

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Prediction of Long-term Creep and Rupture Life

A. Plumtree, G. Shen

pp. 812-816

DOI:

10.2355/isijinternational.30.812

Abstract

Continuum damage mechanics has been applied to develop a constitutive equation which expresses the strain-time relation for the weakening effects occurring during long-term creep, caused by processes such as cavity formation and precipitate coarsening. Stress-life relations have been developed in a similar manner. This approach was found to accurately describe and predict creep curves as well as the rupture lives of low alloy steels. It is shown that long-life creep rupture and the time to achieve the minimum creep rate may be predicted satisfactorily from short-term tests.

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Prediction of Long-term Creep and Rupture Life

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Prediction of Long Term Creep Curve and Rupture Life of 2.25Cr-1Mo Steel

Kouichi Maruyama, Hideaki Kushima, Takashi Watanabe

pp. 817-822

DOI:

10.2355/isijinternational.30.817

Abstract

This paper aims at predicting long term creep curves and rupture lives of 2.25Cr-1Mo steel on the basis of the following creep equation:
ε=ε0+A{1–exp(–αt)}+B{exp(αt)–1},
where, ε0, A, B, α: parameters to be determined by curve fitting to a measured creep curve.
The parameters are expressed as simple functions of stress and temperature, and can easily be extrapolated to a lower stress or a lower temperature, i.e., to a long term creep condition. Inserting the extrapolated values into the creep equation, one can draw a long term creep curve up to the tertiary creep stage.
Rupture life is linearly related to the following rupture parameter P derived from the creep equation:
P=(1/α)ln{(εr–ε0–A)/B}, where, εr: the strain to rupture.
By the aid of this parameter long term rupture lives can be predicted.
The predicted creep curves and rupture lives were confirmed by long term creep tests up to about 400 Ms (13 years).

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Prediction of Long Term Creep Curve and Rupture Life of 2.25Cr-1Mo Steel

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Evaluation of Long-term Creep Strength of 2.25Cr-1Mo Heat Transfer Tube in Actual Service Stress Level Range

Masashi Nakashiro, Shigemitsu Kihara, Fumihiro Kishimoto, Tohru Fujimori

pp. 823-828

DOI:

10.2355/isijinternational.30.823

Abstract

The creep data sheet of 2.25Cr-1Mo steel tube by National Research Institute for Metals (NRIM) is widely used for the residual life assessment of boiler tube. The NRIM creep data sheet contains very useful creep test results such as lower stress level than 30 MPa and longer rupture time of over 100 000 h. Therefore, the master curve calculated from this data sheet has been noticed as a highly reliable curve. The influence of oxidation during long term creep test at low stress is very severe. As the tube's specimens of NRIM were tested on 6 mm diameter specimen, these data were influenced with oxidation scale more than 10 mm diameter specimen's creep data, and creep rupture time was become shorter.
This study was done to calculate the oxidation scale thickness and true stress variation during creep test from scale producing rate equation and to correct the rupture time with linear damage rule. As the difference of master curves between 6 and 10 mm diameter specimens was become very small after correcting scale effect, this correcting method was confirmed to be right.
The other's creep data sheet, ASTM and ISO, had been also investigated, and these master curves were not so much affected by oxide scale, and they had almost same creep strength level to the NRIM corrected master curve at the lower stress level.

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Evaluation of Long-term Creep Strength of 2.25Cr-1Mo Heat Transfer Tube in Actual Service Stress Level Range

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Creep Deformation and Creep-rupture Behavior of Cr-Mo-V Steel Forgings

Yoshikuni Kadoya, Toru Goto

pp. 829-837

DOI:

10.2355/isijinternational.30.829

Abstract

In order to investigate creep deformation and the fracture mechanism of Cr-Mo-V steel, creep tests up to 104 h were carried out in a temperature range of 550-675°C. Metallographic observations were also conducted on the fractured specimens.
The results obtained are summarized as follows:
(1) Stress dependence of steady-state strain rate changes at a certain stress level, at all the temperatures studied. The stress levels of the transition in creep rate are close to those in fracture mode from transgranular to intergranular.
(2) The stress level at which instantaneous plastic deformation starts (regarded as the Orowan stress), is estimated to be σ/E=1.5-2.0×10-3. This Orowan stress corresponds with the transition stress described in (1).
(3) Above the Orowan stress, creep occurs mainly by the motion of dislocations within grains, which is controlled by the Orowan-bowing mechanism. In this stress range, creep rupture ductility is high.
(4) Below the Orowan stress, dislocations slowly climb over particles. Because of the slow deformation within grains, the contribution of intergranular sliding increases. This results in a greater chance of intergranular cavitation, and consequently, results in a low creep rupture ductility.

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Creep Deformation and Creep-rupture Behavior of Cr-Mo-V Steel Forgings

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Surface Crack Propagation in Plate Specimens of 1Cr-1Mo-1/4V Turbine Rotor Steel under Creep-fatigue Condition

Ryuichi Ohtani, Takayuki Kitamura, Yoshinori Sakanoue, Masatoshi Wakabayashi

pp. 838-846

DOI:

10.2355/isijinternational.30.838

Abstract

It is important to know the crack propagation behavior under creep-fatigue condition in order to insure the security of high temperature components. In this study, crack propagation tests were conducted using two sizes of plate specimens of a 1Cr-1Mo-1/4V forged steel with a through or surface crack at 550°C. Regardless of the specimen size and crack shape, the crack propagation rate was correlated well with creep J-integral range, whereas it was not with elastic stress intensity factor nor net section stress. A numerical simulation method was successfully developed for the surface crack propagation on the basis of the relationship between crack propagation rate and creep J-integral range.

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Surface Crack Propagation in Plate Specimens of 1Cr-1Mo-1/4V Turbine Rotor Steel under Creep-fatigue Condition

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Characterization of Creep Crack Growth Behaviour of 316 Stainless Steel in Terms of Microscopical Fracture Mechanism

Masaaki Tabuchi, Koichi Yagi, Toshio Ohba

pp. 847-853

DOI:

10.2355/isijinternational.30.847

Abstract

In order to understand the correlation between creep crack growth rate and microscopical fracture mechanism, creep crack growth tests were carried out using CT specimens on 316 stainless steel at various temperatures and stresses. Three kinds of fracture mechanisms, i.e., 1) wedge-type intergranular fracture (W-type), 2) transgranular fracture (T-type), 3) cavity-type intergranular fracture (C-type), were observed depending on testing conditions. Creep crack growth rate was evaluated by C* parameter. For a given C* value the creep crack growth rate in W- and C-type fracture modes was higher than that in T-type fracture mode. The relationship between creep crack growth rate and C* value for these fracture modes was discussed concerning creep properties, creep ductility and microscopical feature ahead of crack tip.

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Characterization of Creep Crack Growth Behaviour of 316 Stainless Steel in Terms of Microscopical Fracture Mechanism

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Assessment of Remaining Life of Fossil Power Plant Parts by Means of a Miniature Creep Rupture Test

Yoshikuni Kadoya, Toru Goto, Shingo Date, Takayoshi Yamauchi, Tomikane Saida, Tetsuro Sada

pp. 854-861

DOI:

10.2355/isijinternational.30.854

Abstract

In order to validate a procedure for creep rupture tests using miniature specimens, a sampling technique and a method of making the miniature specimens were investigated. Finally, the use of the iso-stress creep rupture test on the miniature specimens to predict the remaining creep life of Cr-Mo-V, 21/4Cr-Mo and 11/4-1/2Mo steel was examined.
The creep curve, rupture life and ductility of the miniature specimens tested in argon were similar to those of conventional sized specimens in air. The fracture mode of the miniature specimens was also consistent with that of conventional sized specimens.
On a graph showing temperature T vs. log time to rupture tr, the creep rupture data were represented with an iso-stress line, respectively. Each iso-stress line shifted parallel with the stress level.
Moreover, the sampling device that was developed was applied to the operating plant components. Samples have been successfully removed from it and were obtained without affecting the material properties around the core hole.

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Assessment of Remaining Life of Fossil Power Plant Parts by Means of a Miniature Creep Rupture Test

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Creep Damage Evaluation for Boiler Tubes by Internally Pressurized Creep Tests

Atsuro Iseda, Yoshiatsu Sawaragi, Kunihiko Yoshikawa

pp. 862-868

DOI:

10.2355/isijinternational.30.862

Abstract

Creep damage behavior under multiaxial creep conditions has been investigated for the typical boiler tubes of T22(2.25Cr-1Mo) and TP321H(18Cr-10Ni-Ti) steels by means of long-term internally pressurized creep tests. Creep strain under pressurized creep for both steels was smaller than that under uniaxial creep. The strain ratios to the uniaxial creep for both steels were almost constant up to about 0.7 in the life fraction and the constant values of the strain ratios were about 0.13 for the T22 steel tube and 0.28 for the TP321H steel tube. For the ductile T22 steel tube, the extensive grain deformation was observed and voids were formed with difficulty even in the final stage of creep. For the less ductile TP321H steel tube, microcracks were observed on grain boundaries in the early stage of creep. However, they existed only in the outer surface side within 1 mm depth, even in the final stage of 0.78 in the life fraction. The replication method was applicable for the evaluation of the grain deformation for the T22 steel and of the microcracks for the TP321H steel.

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Creep Damage Evaluation for Boiler Tubes by Internally Pressurized Creep Tests

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Evaluation of Creep Properties of Degraded Cr-Mo-V Turbine Cast Steels through Hardness Measurement and Its Application to Life Assessment

Kazunari Fujiyama, Kazushige Kimura, Masamitsu Muramatsu, Masayuki Yamada

pp. 869-874

DOI:

10.2355/isijinternational.30.869

Abstract

Several 1.25Cr-1Mo-0.25V steam turbine cast components retired from fossil power plants after long time service at high temperatures were cut into specimens and degraded creep properties were investigated. Materials used were laboratory aged castings, used casings, valve chests, and a nozzle body.
The purpose of this paper is to characterize material degradation and to establish creep life estimation procedure for those old components.
It was observed that hardness was lower at high temperature portions of retired components than original production and this thermal softening was attributed to carbide coarsening and recovery of dislocation microstructure. Creep rate was increased and creep rupture time was reduced according to the thermal softening.
Quantitative relationships between hardness and creep rate, and between hardness and creep rupture time were developed. Using these relationships, creep life estimation procedure was established for cast components, which reasonably took into account material degradation during prolonged exposure to elevated temperatures through hardness measurement.

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Evaluation of Creep Properties of Degraded Cr-Mo-V Turbine Cast Steels through Hardness Measurement and Its Application to Life Assessment

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Interaction of Creep Damage and Low Cycle Fatigue Damage in a 1Cr0.5Mo Steel

Jan Storesund, Rolf Sandström

pp. 875-884

DOI:

10.2355/isijinternational.30.875

Abstract

Non-destructive replica studies of creep cavitation in power plant components of low alloy steels is a frequently used tool in order to estimate the reminant life. In addition to creep such components are exposed to thermal stresses and strains during start-ups and shut-downs, which can lead to fatigue damage. In order to study the role of creep-fatigue interactions, the influence of creep damage on low cycle fatigue (LCF) in a 1Cr0.5Mo steel was investigated. LCF-specimens were creep tested to 5% elongation at 560 and 600°C resulting in carbide precipitation, bainite coarsening and a significant amount of creep cavitation. The damage development during LCF was studied using replicas for test series of pre-crept and virgin material. In addition, the influence of a hold time was investigated. The development of creep cavities and micro-cracks, as well as the amount of intergranular crack propagation were evaluated. Creep exposure resulted in a reduction of the number of cycles to failure. The relationship between this reduction and the amount of creep damage as well as the consumed creep life fraction was analysed. The influence of softening and increased inelastic strain range due to creep exposure was also studied.
Continuous cycling LCF on creep exposed material resulted in a small increase of the cavity density. Compared to virgin material the number of cycles to failure Nf was smaller at low strain ranges. LCF with 5 min hold time resulted in a reduction of Nf by approximately a factor of 1.8. Creep exposure before hold time LCF resulted in an enhanced development of creep damage during LCF and smaller amounts of intergranular crack propagation whereas no significant further reduction of Nf could be observed.

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Interaction of Creep Damage and Low Cycle Fatigue Damage in a 1Cr0.5Mo Steel

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Creep Damage Simulation Test and Creep Life Assessment System for Elevated Temperature Plant Components

Fujimitsu Masuyama, Nobuhiko Nishimura, Toshihide Igari

pp. 885-894

DOI:

10.2355/isijinternational.30.885

Abstract

The creep damage experienced under the actual operating conditions, in many cases, occurs and progresses without observable strain. In order to reproduce such creep damage experienced in actual components of power plants a creep damage simulation test was developed. In this creep damage simulation test, large diameter and thick walled 21/4Cr-1Mo steel pipe was tightly fitted and constrained by the austenitic 18Cr-8Ni steel pipes with greater thermal expansion at the outside and inside diameter of tested 21/4Cr-1Mo steel pipe and subjected to thermal cycles of heating at 600°C and cooling down to 20°C.
Resultantly, progress of creep damage such as generation of creep voids, their linking and occurrence of cracks were observed with the increase of cycles in the tempered martensite structure of heat affected zone but not in the ferrite and peartite structure of base metal. These observation results clarified the relationship between the progresses of creep damage and the creep life consumption rate and suggested that it was needed to distinguish between the heat affected zone of creep-brittle material and base metal of creep-ductile material in the creep damage analysis and life assessment of CrMo steel such as 21/4Cr-1Mo steel. Finally a creep life assessment system based on those findings was proposed and operating procedures of the system were indicated with its practical applicability.

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Creep Damage Simulation Test and Creep Life Assessment System for Elevated Temperature Plant Components

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Evaluation of Creep Damage Progress by Metallurgical Examination in Aged Power Boiler Pressure Parts

Yuji Sugita, Yuhei Kato, Tomomitsu Yokoyama, Tetsuo Sada, Fujimitsu Sasuyama, Nobuhiko Nishimura

pp. 895-904

DOI:

10.2355/isijinternational.30.895

Abstract

In order to confirm the reliability of non-destructive plant life assessment method by metallurgical examination, creep damage was evaluated in aged power boiler pressure parts operated more than 100 000 h. Then the evaluated creep damage was compared with destructive tests, namely creep rupture test and creep fatigue test, of samples removed from the same positions of components inspected by the metallurgical method.
In the metallurgical method, metallurgical degradation was quantified by variation of type and shape of carbide, variation of distribution of alloying elements and nucleation of creep voids. All quantified values but shape of carbide varied systematically during the further operation for 1 or 2 years at all of inspected components. Comparing the quantified metallurgical degradations, with consumed life fraction assessed by destructive tests, relative fraction of M6C carbide, which was quantified type of carbide, and characteristics of Cr and Mo composition spectra, which was quantified distribution of alloying elements, were found to suit the life assessment during first half of life. The creep void density, which was quantified nucleation of creep voids, was found to suit during the latter half.
Consequently residual life of aged power boiler pressure parts can be assessed by the metallurgical method.

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Evaluation of Creep Damage Progress by Metallurgical Examination in Aged Power Boiler Pressure Parts

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