High temperature crack initiation and defect assessment of P22 steel weldments using time dependent failure assessment method

High temperature deformation and crack resistance of low alloy ferritic grade P22 steel weldments applied in power plants are reported. The creep crack initiation (CCI) and creep crack growth (CCG) data were determined using compact type (C(T)) and C-Shape (CS(T)) fracture mechanics specimens at 550 °C. The deformation and crack growth behaviour of similar weldment zones and significance of CCI and CCG in defect assessment of components were addressed. The weldments with industrially relevant properties were produced in butt welded pipe joint from which test specimens are sampled. The studied material covers a spectrum of microstructures and ductility over the weldment zones to give representative for a welded component. The emphasis is placed on the measurement and particularly analysis of crack initiation for failure assessment in P22 steel weldments. The particular importance of construction of isochronous curves for time dependent failure assessment diagram (TDFAD) method is reported. It is aimed to contribute to establishing guidelines for acceptable methodologies for testing, analysis and assessment of welded components using TDFAD for high temperature service.     

Creep crack growth rate predictions in 316H steel using stress dependent creep ductility

Abstract

Short and long term trends in creep crack growth (CCG) rate data over test times of 500–30?000 h are available for Austenitic Type 316H stainless steel at 550°C using compact tension, C(T), specimens. The relationship between CCG rate and its dependence on creep ductility, strain rate and plastic strain levels has been examined. Uniaxial creep data from a number of batches of 316H stainless steel, over the temperature range 550–750°C, have been collected and analysed. Power-law correlations have been determined between the creep ductility, creep rupture times and average creep strain rate data with stress σ normalised by flow stress σ_0·2 over the range 0·2<σ/σ_0·2<3 for uniaxial creep tests times between 100 and 100?000 h. Creep ductility exhibits upper shelf and lower shelf values which are joined by a stress dependent transition region. The creep strain rate and creep rupture exponents have been correlated with stress using a two-stage power-law fit over the stress range 0·2<σ/σ_0·2<3 for temperatures between 550 and 750°C, where it is known that power-law creep dominates. For temperature and stress ranges where no data are currently available, the data trend lines have been extrapolated to provide predictions over the full stress range. A stress dependent creep ductility and strain rate model has been implemented in a ductility exhaustion constraint based damage model using finite element (FE) analysis to predict CCG rates in 316H stainless steel at 550°C. The predicted CCG results are compared to analytical constant creep ductility CCG models (termed NSW models), assuming both plane stress and plane strain conditions, and validated against long and short term CCG test data at 550°C. Good agreement has been found between the FE predicted CCG trends and the available experimental data over a wide stress range although it has been shown that upper-bound NSW plane strain predictions for long term tests are overly conservative.     

Effect of tempering temperature on Z-phase formation and creep strength in 9Cr–1Mo–V–Nb–N steel

The effect of the tempering temperature on Z-phase formation and creep strength in 9Cr–1Mo–V–Nb–N steel was examined with particular attention to the precipitation sequence of MX, M₂X, and Z-phase during creep exposure. Tempering at a lower temperature provided a high dislocation density and a fine lath structure. Tempering at 953, 1003, and 1038 K provided [M₂₃C₆, M₂X, NbX], [M₂₃C₆, M₂X, NbX, VX], and [M₂₃C₆, NbX, VX] phases, respectively. The creep strength of steel tempered at 953 K was the highest among the steels studied, even in the long term. No large decrease in creep strength was observed in steel tempered at 953 K. The Z-phase was observed after long-term creep in steel tempered at 1003 or 1038 K. In steel tempered at 953 K, a VX rather than a Z-phase formed during creep, and this was accompanied by consumption of M₂X. Retardation of the Z-phase formation can retard the creep strength degradation in steel tempered at 953 K.     

Comparison of different approaches for estimation of creep crack initiation

In large components such as rotors defects due to manufacturing processes have to be taken into account and crack assessments based on findings of non-destructive evaluation are necessary. Approaches are used in remaining life estimations, for example:

• Time Dependent Failure Assessment Diagram (TDFAD),

• Two Criteria Diagram (2CD) and

• Nikbin–Smith–Webster-Model (NSW-Model).

The TDFAD approach is currently being developed within the R5 procedures as an alternative to conventional methods for predicting incubation and the early stages of Creep Crack growth. A key requirement of TDFAD approaches is the evaluation of a time dependent creep toughness, denoted K_c mat. The 2CD approach has been developed independently in Germany to assess Creep crack incubation in ferritic steels. This approach uses crack tip and ligament damage parameters, R_K and R_σ, respectively. Furthermore the NSW-Model is employed for the estimation of creep crack initiation by using the creep fracture mechanics parameter C^*. Calculations and used parameters were compared for a ferritic 1CrMoV-steel.     

Book Reviews

The results of astudy of the multi-axial creep rupture of thick-walled tubes at 600°C are presented here as asequel to aprevious report on the characteristic creep behaviour of the same type 316austenitic steel under purely tensile stress conditions.

It is demonstrated that simply devised and inexpensive direct double-shear creep rupture tests can be used to identify the correct creep rupture multi-axial stress criterion appropriate to pure shear-dominated conditions for the steel being studied. It is also shown that the introduction of this criterion into the reference stress method of creep analysis enables acceptable predictions of the creep lifetimes of thick-walled tubes under torsional stress to be made.

In addition, anumber of preliminary supporting test results on thick-walled tubes under internal pressure and under pure bending are briefly reported and the results discussed with respect to the central theme of the study.     

Application of the reference stress method for interpreting impression creep test data

Impression creep tests have been performed on a 316-stainless steel at 600°C, for which conventional uniaxial creep test data are available. It is shown that the technique, based on the reference stress approach, for converting impression creep test data to equivalent uniaxial creep data, is accurate. The results show that the impression creep technique can be used for obtaining creep properties for materials which have high creep resistance at high temperature and test pressure conditions. The difficulties and limitations associated with such situations are described and methods of dealing with them are outlined.     

Modeling and experimental study of long term creep damage in austenitic stainless steels

One of the main challenges for some reactors components in austenitic stainless steels at high temperature in-service conditions is the demonstration of their behavior up to 60 years. The creep lifetimes of these stainless steels require on the one hand to carry out very long term creep tests and on the other hand to understand and to model the damage mechanisms in order to propose physically-based predictions toward 60 years of service. Different batches of austenitic stainless steels like-type 316L with low carbon and closely specified nitrogen content, 316L(N), are subjected to numerous creep tests carried out at various stresses and temperatures between 525 °C and 700 °C up to nearly 50 • 10³ h.Interrupted creep tests show an acceleration of the creep deformation only during the last 15% of creep lifetime, which corresponds to macroscopic necking. The modeling of necking using the Norton viscoplastic power-law allows lifetime predictions in fair agreement with experimental data up to a transition time of about ten thousand hours which is temperature dependent. In fact, one experimental result together with literature ones, shows that the extrapolation of the ‘stress–lifetime’ curves obtained at high stress data leads to large overestimations of lifetimes at low stress. After FEG–SEM observations, these overestimates are mainly due to additional intergranular cavitation as often observed in many metallic materials in the long term creep regime. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel is carried out. For each specimen, ten FEG–SEM images (about 250 observed grains) are analyzed to determine the rate of cavity nucleation assumed to be constant during each creep test in agreement with many literature results. This measured constant rate is the only measured parameter which is used as input of the Riedel damage model. Lifetimes for long term creep are rather fairly well evaluated by the lowest lifetime predicted by the necking model and the Riedel model predictions. This holds for experimental lifetimes up to 200,000 h and for temperatures between 525 °C and 700 °C. A transition time as well as a transition stress is defined by the intersection of the lifetime curves predicted by the necking and Riedel modelings. This corresponds to the change in damage mechanism. The scatter in lifetimes predicted by the Riedel model induced by the uncertainty of some parameter values is less than a factor of three, similar to experimental scatter. This model is also validated for various other austenitic stainless steels such as 304H, 316H, 321H (creep rupture data provided by NIMS). A transition from power-law to viscous creep deformation regime is reported in the literature at 650 °C–700 °C for steel 316H. Taking into account the low stress creep rate law, it allows us to predict lifetimes up to 200,000 h at very high temperature in fair agreement with experimental data.     

The effect of prior fatigue on the creep behaviour of type 321 stainless steel

An investigation was made of the influence of prior elevated-temperature fatigue (at 400°C, 650°C and 750°C) on the creep behaviour of type 321 stainless steel stressed at 200 MPa at 650°C. Prior fatigue was found to accelerate the creep rate, increase the creep ductility, and decrease the time to failure. The acceleration in creep rate was ^∝e^1.8t^0.68, where e is the strain amplitude and t is the duration of the fatigue test. Transmission electron microscopy showed that the dislocation structures produced by prior fatigue tended to persist throughout the subsequent creep test. A stress-aided recovery mechanism is suggested to account for these effects.     

一级应变硬化F316奥氏体不锈钢的高温蠕变性能

研究了在200 MPa应力下一级应变硬化F316奥氏体不锈钢在650℃、680℃和700℃的蠕变性能和蠕变断裂行为。结果表明： 在200 MPa 恒定应力下蠕变温度越高其蠕变寿命越短,稳态蠕变速率越大,由应力加载引起的瞬时应变越大。蠕变断裂方式主要为韧性断裂。蠕变孔洞主要分布在三叉晶界等脆弱部位,距离断口越远试样中孔洞的平均尺寸和孔洞面积百分比越小。在与断口距离相同的位置上,随着蠕变温度的提高蠕变孔洞的平均尺寸和面积百分比均明显增大。与未预应变的F316不锈钢相比,具有高密度孪晶的一级应变硬化F316不锈钢具有更大的蠕变抗力。分别基于Larson-Miller 参数法和θ参数法外推计算了350℃/200 MPa下的蠕变寿命,θ参数法的拟合曲线与实际蠕变曲线吻合得较好。根据Larson-Miller参数法和θ参数法,探讨了350℃/200 MPa下一级应变硬化F316奥氏体不锈钢长期服役蠕变可靠性。     

Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr-1 Mo pressure vessel steel

Tests were performed on a 2¼  Cr–1  Mo steel to measure the fracture toughness at initiation, K _Ic and at arrest, K _Ia . The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure K _Ia : (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to −196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (∼500  m  s⁻¹ ) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic–plastic finite element method to determine K _Ia values. These tests reveal a greater temperature shift (∼100 °C) between K _Ic and K _Ia in 2¼  Cr–1  Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼  Cr–1  Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism     

Creep crack growth in low alloy steel weldments

Developments in the determination and analytical representation of creep crack growth property data during the past 30 years are reviewed. The testing and data analysis of weldments involve additional complexities, and these are appraised with respect to low alloy steel weldments. Creep crack initiation and growth properties are dependent on creep deformation and rupture ductility characteristics. Consideration is given to the relationship between these properties using data determined for a ½Cr½ Mo¼V/2½CrMo pipe joint.     

Through-thickness creep damage in a service-exposed header of 2.25Cr1 Mo steel

A creep damage investigation was made on a service-exposed header of 2.25Cr 1Mo steel with a service time of 200 000 h. Weldments and the flanged parts at both outer and inner surfaces were studied as well as the damage in the through-thickness direction. Extensive cavitation was observed not only at the outer surface, but also well inside the component in the weldments. A model for the prediction of rupture position is introduced to explain the appearance of the observed creep damage.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

Evaluating creep cracking in welded fracture mechanics specimens

At present there lacks a unified approach in understanding the mechanistic role of weldments cracking in fracture mechanics specimens at elevated temperatures. The effects of residual stress, the development and crack tip damage due to it and the subsequent creep relaxation at high temperatures can be evaluated using relevant test data, numerical modelling and residual stress measurements. These problem areas are considered in this paper and discussed in context under the newly formed collaborative international effort in the Versailles Agreement of Materials and Standards committee, VAMAS TWA31. The plans for this collaborative effort are to evaluate tests at elevated temperatures in a number of high strength steels and also model and measure weldments containing residual stresses. The aim of the four year programme will be to make recommendations and establish pre-standardisation methods for testing, measuring and analysing creep crack initiation (CCI), creep crack growth (CCG), and low frequency creep fatigue crack growth (CFCG) (where creep dominates) characteristics in weldments containing residual stress. The fracture mechanics geometries that will be considered have already been validated in the previous TWA25 collaboration [VAMAS TWA25, Draft Code of Practice, ‘Creep/fatigue Crack Growth in Components’, VAMAS document. Nikbin K, editor. May 2005.] for testing of parent material. Examples of testing and analysis techniques are presented in this paper to highlight the future objectives for this work.     

Rationalization and extrapolation of creep and creep fracture data for Grade 91 steel

Abstract

New relationships are shown to allow straightforward rationalization and extended extrapolation of multi-batch creep data sets for Grade 91 steel (9Cr –1MoVNb). Specifically, after normalizing the applied stress through the appropriate UTS value, creep property measurements at different temperatures are superimposed onto sigmoidal ‘master curves’ using the activation energy for lattice diffusion in the alloy steel matrix (300 kJ mol^–1). In contrast to currently-adopted analysis procedures, applying the new methodology to results from tests lasting less than 30,000 h allows the minimum creep rates, the times to various creep strains and the creep lives to be predicted accurately for stress-temperature conditions causing failure in times approaching or even exceeding 100,000 h.     

Correlation between microstructure and creep performance of martensitic/austenitic transition weldment in dependence of its post-weld heat treatment

This paper deals with the influence of post-weld heat treatment (PWHT) of T92/TP316H martensitic/austenitic transition weldment on the resulting microstructure and creep characteristics. Experimental weldments were fabricated by gas tungsten arc welding using a nickel-based weld metal (Ni WM). After the welding, two individual series of produced weldments were heat-treated according to two different PWHT procedures. The first “conventional PWHT” was carried out via subcritical tempering (i.e. bellow Ac₁ temperature of T92 steel), whereas the other one, the so-called “full PWHT” consisted of a complete reaustenitization of the weldments followed by water-quenching and final tempering. The use of “conventional PWHT” preserved microstructural gradient of T92 steel heat-affected zone (HAZ), consisting of its typical coarse-grained and fine-grained subregions with tempered martensitic and recrystallized ferritic–carbidic microstructures respectively. In contrast, the “full PWHT” led to the complete elimination of the original HAZ via transformation processes involved, i.e. the reaustenitization and back on-cooling martensite formation. The observed microstructural changes depending on the initial PWHT conditions were further manifested by corresponding differences in the weldments’ creep performance and their failure mode. The weldments in “conventional PWHT” state ruptured after long-term creep tests by premature “type IV failure” within their recrystallized intercritical HAZs. On the contrary, the long-term creep behavior of the weldments processed by “full PWHT” was characterized by their remarkable creep life extension but also by the occurrence of unfavorable “decohesion failure” along T92/Ni WM interface.     

CREEP-FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr-1Mo-0.25V STEELS. PART I: ESTIMATION OF CRACK TIP PARAMETERS

Abstract— Non-linear finite element (FEM) analyses, involving various creep deformation laws, as well as experiments with hold times of 100 s and 15 min were performed on compact type specimens with stationary cracks. The work was aimed at developing accurate expressions for estimating the small-scale creep parameters, ( C _t)_avg, for 1Cr-1Mo-0.25V steel at 538°C (1000°F). Here ( C _t)_avg is a representative value for the small-scale creep parameter C _t, averaged over a hold period under the conditions of creep-fatigue loading. These expressions were then applied to crack growth data obtained from specimens tested under the various hold times. When an elastic-plastic-primary creep-secondary creep constitutive model was used in the FEM analyses, the calculated values of C _t compared well with the measured values. The FEM results also showed that the accumulated creep deformation during the hold time was not significantly reversed during the unloading portion of the creep-fatigue cycle for this material. Therefore, a new method of estimating (C_t)_avg is proposed on the basis of these numerical and experimental results.     

The effect of prior cyclic loading variables on the creep behaviour of ex-service Type 316H stainless steel

Abstract

Initial tests have been conducted for a systematic survey of the effect of prior cyclic loading on subsequent creep properties. Samples of 316H stainless steel were subjected to prior cyclic loading at 550°C at different combinations of strain range and cycles experienced. These samples were then remachined into uniaxial creep specimens and tested under a constant load at 250 MPa at 550°C.

The initial results from specimens subject to prior cyclic loading show significant decreases in the minimum true creep strain rate of between 30 and 94%. A consistent decrease in the minimum creep strain rate was found with increases in both the strain range of the prior cyclic loading and the number of cycles experienced by the sample. In addition, the prior-cyclic loading has significantly changed the shape of the creep curve to varying degrees depending upon the applied cyclic loading.     

Interpretation of creep crack initiation and growth data for weldments

Creep crack growth testing of macroscopically homogeneous materials is well established and standardised test procedures are detailed in ASTM E1457. In ASTM E1457 the use of the compact tension C(T) specimen is specified and crack growth data are interpreted using the fracture mechanics parameter C^∗. The resulting benchmark crack growth data are used in assessment procedures, together with estimates of the value of C^∗ in the component, to predict creep crack growth behaviour. In this work, the results of a series of creep crack growth tests performed on a Type 316 stainless steel weldment at a temperature of 550 °C are presented. The initial crack is located within the heat affected zone (HAZ) of the weldment. Since there are currently no agreed methods for determining C^∗ in inhomogeneous laboratory specimens, this paper examines the application of procedures in ASTM E1457 for the characterisation of crack growth in weldments. In addition, the creep toughness parameter is evaluated for the material. It is shown that the creep crack growth rates in the weldment may be described by the C^∗ values obtained following ASTM E1457 in conjunction with parent material properties. Comparison of the results with similar data for Type 316H stainless steel parent material shows that the crack growth rates for the crack located within the HAZ are higher and the initiation times lower than the parent values, for the range of test conditions examined. Simple analytical models based on ductility exhaustion arguments have been shown to bound the crack initiation and growth data for the weldment.     

Applicability of improved Dyson–McLean approach to creep deformation behaviour of tempered martensitic P9 steel

Modelling creep deformation of tempered martensitic P9 steel in quenched and tempered, and simulated post weld heat treatment conditions has been performed in the framework of improved Dyson–McLean approach for wide range of stresses at 873 K. In this approach, kinetic creep law coupled with the set of first-order differential equations representing the evolution of microstructural internal-state-variables with strain/time has been employed to describe creep deformation behaviour of tempered martensitic steel. The optimised material constants associated with the model such as dislocation storage parameter (K_d) and rate constants associated with the precipitate coarsening (K_p) and solute depletion (K_s) reflect the influence of two different heat treatments on creep characteristics examined in the present investigation. At all test conditions, good agreement between the predicted and experimental creep strain/strain rate-time data at 873 K has been observed. Further, good correlations have been obtained between the experimental and predicted steady-state creep rates and time to reach the specified strain levels for both the heat treatment conditions.