Damage Susceptibility of Grain Boundaries in HT9 Steel Subjected to High-Temperature Creep

HT9 steel is an attractive ferritic/martensitic steel that is used in components of nuclear and fossil power plants because of its high strength and good swelling resistance. Specific phenomena (such as segregation, voiding, cracking, etc.) are prevalent along grain boundaries since these interfaces act as efficient sources for vacancies. The accumulation of vacancies in grain boundaries may result in intergranular fracture. In this study, HT9 steel was subjected to creep tests at elevated temperature (about 0.5 T _m) and two different creep conditions (where creep lifetimes were about 100 and about 1000?hours, respectively). The grain boundaries in HT9 steel after creep tests were studied by the use of scanning electron microscopy in order to establish the relationship between the grain boundary structure and creep damage. Images and data obtained using electron backscatter diffraction reveal a high susceptibility of high-angle boundaries to creep cavitation, as expected. In addition, the ??3 boundaries are also susceptible to damage under these conditions at a similar or even higher rate as compared with random high-angle boundaries.     

Creep Damage Assessment and Lifetime Predictions for Metallic Materials under Variable Loading Conditions in Elevated Temperature Applications

In this paper the problem of creep damage assessing as well as of creep life prediction of metallic materials under variable loading conditions is considered. A non‐linear model for creep damage accumulation which is based on a more general physical‐phenomenological model describing the mechanical behaviour of metallic materials under creep conditions, is presented. The model is suitable for the prediction of the creep damage accumulation under variable load and/or temperature. The proposed model takes into account the previous damage history and, definitely, the loading order. In any case the creep strain accumulation is taken into account as this is directly associated with the creep damage accumulation. Theoretical results agree well with the experiments.     

Thermomechanical fatigue,oxidation, and Creep: Part II. Life prediction

A life prediction model is developed for crack nucleation and early crack growth based on fatigue, environment (oxidation), and creep damage. The model handles different strain-temperature phasings(i.e., in-phase and out-of-phase thermomechanical fatigue, isothermal fatigue, and others, including nonproportional phasings). Fatigue life predictions compare favorably with experiments in 1070 steel for a wide range of test conditions and strain-temperature phasings. An oxide growth (oxide damage) model is based on the repeated microrupture process of oxide observed from microscopic measurements. A creep damage expression, which is stress-based, is coupled with a unified constitutive equation. A set of interrupted tests was performed to provide valuable damage progression information. Tests were performed in air and in helium atmospheres to isolate creep damage from oxidation damage.     

P92钢的蠕变损伤容许量系数及蠕变断裂机理

利用P92钢在595、610、640、670℃的高应力试验条件下的蠕变试验数据,得出其Norton应力指数,依据Norton应力指数的大小判定其蠕变机理为位错蠕变。同时结合1种新的蠕变变形及断裂模型,引入将蠕变损伤看作1个内在的阶段变量的蠕变损伤容许量系数,根据蠕变损伤容许量λ=2.94,判断其蠕变变形和断裂是位错运动控制的。微观组织的观察也表明,蠕变后的试样中位错密度大大降低,高密度位错是P92钢持久强度高的原因,伴随着位错密度的下降,P92钢持久强度降低直至断裂。     

309S耐热不锈钢蠕变行为及寿命预测

以工业化生产的309S奥氏体耐热不锈钢为研究对象,在不同温度和应力条件下进行单轴拉伸蠕变实验.结果表明:700℃/100 MPa蠕变断裂时间最长为480.13 h,蠕变曲线有明显的3个阶段;800℃/100 MPa蠕变断裂时间最短为4.61 h,蠕变曲线只有第3阶段,直至断裂.扫描电子显微镜(SEM)观察分析表明,蠕变...     

Microstructural parameters for creep damage quantification

This paper is concerned with the use of mechanistically based parameters that can be used to quantify cavitation creep damage for the purpose of life assessment of high temperature components. They are referred to as “line parameters” in that they relate the number of damaged grain boundaries intersected by or very close to a reference line to the total number of grain boundaries intersected by this reference line. The parameters considered in this work are (i) the “A-parameter” based on the concept of constrained cavity growth and the phenomenological damage equations by Kachanov and Rabotnov, (ii) the “ϱ-parameter” again based on the model of constrained cavity growth but relating to a model of a creeping solid containing microcracks and (iii) a new parameter A¹, which is based on the “A-theory” but takes the mechanical interaction of cavitating facets during creep into account. A¹ performs best in relating creep damage to consumed life fraction. It is more sensitive to the consumed life fraction than “A” and the metallographic data are in better agreement with the predictions of the corresponding damage evolution law than in the case of “ϱ”. The results of a Monte Carlo simulation performed in this study inicate that one of the reasons for scatter in creep rupture lives of engineering steels is the statistical variation in the time required for the formation of highly damaged regions.     

Creep cavity growth from tritium-induced helium bubbles in nickel

The growth of grain boundary cavities in nickel under creep conditions was investigated. Growth could be studied unambiguously and without the complications of radiation damage due to a unique sample preparation technique making use of the tritium-to-helium’ decay reaction. Resultant helium bubbles served effectively as creep cavity nuclei, the growth of which led to premature intergranular fracture. Constant stress tension creep tests under argon were performed on helium embrittled samples, revealing information on the stress and temperature dependence of the creep-fracture process. The cavity spacing developed during primary creep by bubble migration and coalescence persisted to fracture. Bulk plastic deformation was strongly suppressed by a matrix bubble population which stabilized a finer subgrain network than is characteristic of virgin nickel. This enhanced creep resistance permitted observation of a stress region in which void growth was controlled by classical Hull-Rimmer grain boundary vacancy diffusion. At higher stresses a transition to plasticity control appeared to take place. These results are interpreted in terms of a coupled grain boundary diffusive/ matrix plasticity model. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Creep deformation and damage evaluation of service exposed reformer tube

Abstract

This paper aims at studying the creep deformation behaviour and quantifying creep damage of ～11 years service exposed primary hydrogen reformer tube made of HP40 grade of steel in a petrochemical industry, in terms of Kachanov’s continuum damage mechanics model (K model) and Bogdanoff model (B model) based on Markov process. Hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography were extensively carried out. The as received tubes did not possess any sign of degradation including voids or creep cavitations and decarburisation. There was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to aging and overheating. Accumulation of damage due to creep has been quantified through microstructural studies in terms of two damage parameters A and A*. Experimental scatter observed in creep deformation and creep strain rate curves of the material at 870°C and at various stress conditions, is probably due to scatter in creep cavitations/voids and also due to variation in the mode of fracture in top as well as bottom portion of the tube. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area, shifts with increase in true strain towards the higher population of void. The estimation of average time to reach a specific damage state from K model and B model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well.     

Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel

The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress.     

CLAM钢的高温蠕变行为

 为了保证CLAM钢在服役条件下的安全性,避免蠕变对设备造成破坏,对CLAM钢的蠕变行为进行了研究。首先,在高温持久蠕变试验机上对CLAM钢进行蠕变试验,绘制了蠕变曲线并讨论了CLAM钢在试验条件下的蠕变变形机制,结果表明,CLAM钢具有很强的温度敏感性,第二相粒子强化机制为主要蠕变机制;其次,采用双对数等温线外推法和Larson-Miller参数法对CLAM钢的蠕变寿命进行了预测,确定了两种蠕变寿命预测模型;最后,比较了两种蠕变寿命预测模型的精度,结果显示,Larson-Miller参数法的外推精度更高,更能反映实际情况。     

Type IV Creep Damage Behavior in Gr.91 Steel Welded Joints

Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and 923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution of Type IV creep damage.     

Creep of fibre composite beams in bending

Large scale deformation of fibre reinforced composites (Mg alloy/Al₂O₃ fibres) has been observed after creep in bending at 310°C. The deformation is considered in terms of the continuum theory of fibre reinforced materials in which only elastic deformation occurs in the fibres but the matrix is allowed to deform in shear. It is shown that the continuum theory is capable of accounting for time-dependent microcracking or fracture when creep is allowed for. However, the continuum theory does not predict two stage creep or the shear cracking which was observed in the experiments. It is necessary to consider microstructural features in order to explain many aspects of the observations and some of these are considered. In particular, damage accumulates at the fibre-matrix interface and this is thought to lead to an increase in the creep rate after a critical deformation.     

循环加卸载下闪长玢岩蠕变特性及损伤本构模型

分级加载压缩蠕变试验未能充分考虑稳定蠕变中的黏塑性应变，故采用三轴循环加卸载压缩蠕变试验来实现岩石的黏弹、塑性应变分离，从而使岩石黏弹、塑性应变在岩石蠕变的各个阶段得以充分考虑。以某水电站闪长玢岩为例，探讨该类岩石蠕变特性。在破坏前，岩石的瞬时弹性应变以及瞬时塑性应变随着偏应力逐级增大呈线性增长；随着偏应力的增加，黏弹性应变和黏塑性应变呈非线性增长。引入一个分数阶Abel黏壶与Kelvin模型串联形成新型黏弹性模型；用分数阶Abel黏壶代替传统的黏塑性模型中的线性牛顿体并基于损伤建立黏塑性损伤模型。然后将新型黏弹性模型和黏塑性损伤模型与瞬时弹性模型和瞬时塑性模型串联组成一个新的岩石蠕变损伤模型。最后将该模型与岩石蠕变曲线进行拟合，从而证明该模型的适用性。      

Creep cavitation in a NiCr steel

Through creep tests at 823 K and intermittent cryogenic separation of grain boundaries of a NiCr steel with well-defined secondary stage creep, the continual cavity nucleation and diffusive growth models based on the linear relation between the cavity number and creep strain were examined. A one-dimensional (1-D) model^[1] can predict rupture times within a factor of 2 and their stress exponents accurately and was supported by the stress dependence of the Monkman-Grant product. The area fraction of fully coalesced area (FCA) increased linearly with the density change of specimens and rupture occurred at FCA ∼ 0.5. Then, by treating FCA as a damage parameter and applying the Avrami equation, it was shown that the extended area of FCA was proportional toσ ²ⁿt³, which led tot _f ∼ σ^−2n/3.     

Creep of Polymer Matrix Composites. II: Monkman-Grant Failure Relationship for Transverse Isotropy

This research concerns polymer matrix composite (PMC) materials having long or continuous reinforcement fibers embedded in a polymer matrix. The objective is to develop comparatively simple, designer friendly constitutive equations intended to serve as the basis of a structural design methodology for this class of PMC. Here (Part II), the focus is on extending the damage/failure model of an anisotropic deformation/damage theory presented earlier. A companion paper (Part I) by the writers deals with creep deformation of the same class of PMC. The extension of the damage model leads to a generalization of the well known Monkman/Grant relationship to transverse isotropy. The usefulness of this relationship is that it permits estimates of (long term) creep rupture life on (short term) estimates of creep deformation rate. The current extension also allows estimates of failure time for various fiber orientations. Supporting exploratory experiments are defined and conducted on thin-walled specimens fabricated from a model PMC. A primary assumption in the damage model is that the stress dependence of damage evolution is on the transverse tensile and longitudinal shear traction acting at the fiber/matrix interface. We conjecture that a supplemental mechanism of failure is the extensional strain in the fiber itself. The two postulated mechanisms used in conjunction suggest that an optimal fiber angle may exist in this class of PMC, maximizing the time to creep failure.     

Developing Dislocation Subgrain Structures and Cyclic Softening During High-Temperature Creep–Fatigue of a Nickel Alloy

The complex cyclic deformation response of Alloy 617 under creep–fatigue conditions is of practical interest both in terms of the observed detriment in failure life and the considerable cyclic softening that occurs. At the low strain ranges investigated, the inelastic strain is the sole predictor of the failure life without taking into consideration a potentially significant environmental influence. The tensile-hold creep–fatigue peak stress response can be directly correlated to the evolving dislocation substructure, which consists of a relatively homogenous distribution of subgrains. Progressive high-temperature cycling with a static hold allows for the rearrangement of loose tangles of dislocations into well-ordered hexagonal dislocation networks. The cyclic softening during tensile-hold creep–fatigue deformation is attributable to two factors: the rearrangement of dislocation substructures into lower-energy configurations, which includes a decreasing dislocation density in subgrain interiors through integration into the subgrain boundaries, and the formation of surface grain boundary cracks and cavity formation or separation at interior grain boundaries, which occurs perpendicular to the stress axis. Effects attributable to the tensile character of the hold cycle are further analyzed through variations in the creep–fatigue waveform and illuminate the effects of the hold-time character on the overall creep–fatigue behavior and evolution of the dislocation substructure.     

A Model Based on the Assessment of Intrinsic Creep Resistance Concept

In the present study a simple model is proposed to assess creep behavior. The model is applied to experimental results performed on austenitic steel X8 CrNiMoNb 16 16. The model is based on a modification of the Levy‐Mises equation for plasticity to consider creep time effects, introducing as a parameter the intrinsic creep resistance. The assessment of creep behavior applied for monotonic and two stages loading data is good. The model could assess negative creep strain rates as well as damage accumulation observed as an increase of the minimum creep rate after each reloading at the same stress level in two stages tests.     

Microstructural Parameters Controlling High-Temperature Creep Life of the Nickel-Base Single-Crystal Superalloy MC2

The influence of both topologically close-packed (TCP) phase precipitation and pores on the creep life of a single-crystal superalloy has been studied at 1323?K (1050?°C)/160?MPa. Despite very reproducible primary and secondary creep stages, the creep life is scattered for this specific condition where a very steep tertiary creep stage is observed, corresponding to a highly localized failure process. Image processing was performed after failure to determine the stereological parameters characterizing pores and TCP-phase particles. It was determined that pores are major determinants of creep life under these temperature and stress conditions. It was also observed that the average surface area or the density of pores is not sufficient to explain creep life variability. A homogenization method including modified ??/???? microstructure area surrounding pores and TCP-phase particles was developed and correlated to creep life. It is shown that the greater the extent of the modified microstructure, the lower the creep life. Moreover, a better understanding of the TCP-phase role in controlling the creep life was obtained: TCP-phase particles modified the local stress field and disturbed the local ??/???? microstructure. They enhance the generation of vacancies and subsequent nucleation and growth of pores.     

Creep damage evaluation of a power plant header using combined FEM analysis and quantitative metallography

The actual operating conditions of a component may vary from the original design conditions either constantly or variably as a consequence of deviation from design parameters. Hence, the knowledge of operating temperatures and stresses, both historically and for anticipated future operation is required for an accurate life assessment. A number of high temperature and high stress components of Indian thermal power utilities are being monitored round-the-clock for damage assessment by acquiring real-time process data using an on-line damage monitoring system BOSSES, developed by BARC, Mumbai. A number of surface replicas have been obtained from the critical location of a shell-nozzle junction of a super-heater header and analyzed by quantitative metallographic methods. The aim of these activities is to assess service damage of critical components to prevent any unforeseen failure based on a closed loop on-line damage monitoring system-cum-surveillance-cum-life management programme.