Alloy 800: new stress rupture and creep data for pressurized components in high temperature reactors

In an HTR plant high temperature components of Alloy 800 materials are subjected to temperatures from about 550 to 850°C in the long term; higher temperatures may occur in the short term. Thus the creep and stress-rupture parameters govern the design of these components. Since in recent years the scope of experimental data available for Alloy 800 materials has considerably increased, a new evaluation of the creep and creep-rupture properties was performed using a data bank computer. The relationships between the characteristics of the creep and creep-rupture behaviour and the metallurgical parameters were investigated by multilinear regression analyses. On the basis of the results of these analyses and after discussion in material expert committees new material specifications were determined for different types of Alloy 800. They were included into the draft standards DIN 17459 and DIN 17460 under the material standard nos. 1.4958, 1.4958 Rk and 1.4959. Besides the new values for the 1% total plastic strain limit and the creep-rupture strength for the types of Alloy 800 under consideration, isochronous stress-strain relations were derived on the basis of creep curves of reference heats.     

Mechanical design methods for high temperature reactor components

The work concerned with design codes for high temperature reactor (HTR) components operating at temperatures above 800°C is summarized. Using the experimental results from the German HTR materials development programmes, in particular the time dependent properties, the structural design analysis for an intermediate heat exchanger is discussed, with reference to creep, fatigue, creep buckling and creep ratchetting. The analysis provides the basis for a critical consideration of ASME Code, Case N 47, and the applicability of the code case rules for service temperatures above 800°C.     

On the anomalous stress-dependence of creep rate in precipitation strengthened alloys

The stress and temperature dependence of secondary creep rate have been analysed in the temperature range of 773–823 K for a 22% Cr-34% Ni austenitic steel (alloy 800) strengthened by a small volume fraction of γ′(Ni₃Ti, Al). In this respect two regimes have been distinguished at low and high stresses with the activation energies corresponding to those of the grain boundary and bulk diffusions, respectively. The very high stress-dependence (20) observed at elevated stresses, in comparison with those at low stresses (3–5), is shown to be enhanced by the metastability of the matrix and the resultant marked deformation inequalities during secondary creep. The effect of prior cold working and ageing are discussed. A correlation between low stress and high stress data through internal friction stress estimations is sought, and the possibility of the Coble creep mechanism becoming operative at low stresses is foreseen.     

Effect of Microstructure on Failure Behavior of Light Water Reactor Coolant Piping under Severe Accident Conditions

In a severe accident of light water reactors, the reactor coolant system (RCS) piping might be subjected to thermal loads caused by the decay heat of the deposited fission products and the heat transfer from the hot gases, with an internal pressure in some accident sequences. Tests on the RCS piping failure were performed along with high temperature tensile and creep rupture tests including metallography to investigate the failure behavior. The prediction of the 0.2% proof stress by Arrhenius equation is in good agreement with the measured stress above 800°C for served RCS piping materials. The modified Norton's Law for the short term creep rupture model agrees with the experimental values between 800 and 1,150°C for type 316 stainless steel. The microstructural change was discussed with the effect of the very rapid formation and resolution of the precipitation on the strength at high temperature. The result of the piping failure tests which simulated the severe accident conditions, i.e., in short-term at high-temperature, could support the plastic limit load prediction of the flow stress model using the 0.2% proof stress.     

Material Properties of a Natural UF6 Transport Cylinder Vessel at High Temperature

Abstract

The IAEA Regulations for the Safe Transport of Radioactive Material are to be revised in 1996 and the fire test (800°C for 30 min) could become a requirement for the natural UF₆ transport cylinder. ASME SA 516 carbon steel is used as the structural material for this type of cylinder. It is very important to obtain high temperature data for SA 516 steel to be able to evaluate the integrity of the UF₆ transport cylinder vessel in the fire test. CRIEPI has therefore conducted material tests on SA 516 at high temperatures. The AC₁ and AC₃ transformation points of actual SA 516 steels have been measured. Tensile tests up to 900°C were conducted using USA, French and Japanese manufactured materials and the influence of phase transformation assessed. Preliminary creep tests show that assessment by creep strength can give a more conservative estimation than using the tensile strength. Creep deformation equations have been obtained using uniaxial creep tests and internal pressure creep tests. In addition, by the use of internal pressure creep rupture tests, the relation between the circumferential stress, the test temperature and the rupture time has been obtained.     

Characterization and modeling of creep mechanisms in Zircaloy-4

The deformation microstructure and creep mechanisms of Zircaloy-4 have been investigated. Four Zircaloy-4 specimens were tested at different temperatures and stress levels and the deformation microstructures of these specimens were analyzed using transmission electron microscopy. On the basis of microstructural observation of a-type screw dislocations in prismatic slip systems, the modified jogged-screw model has been applied as a rate controlling mechanism for creep of Zircaloy-4. In addition, the stress dependency of dislocation density, jog spacing, and jog height has been evaluated via modeling and experimental observations. The purpose of this study is to provide a detailed understanding of the creep deformation of Zircaloy-4 and prediction of creep rates in this alloy based on the microstructural information obtained from TEM analysis.     

Creep in phosphorus alloyed copper during power-law breakdown

During the first phase of storage, creep will take place in the copper canisters in the KBS-3 package for nuclear waste. The temperatures are below 100 °C, and the creep is well inside the power-law breakdown regime. Creep models for this situation have been developed. The analysed material is pure copper with about 50 ppm phosphorus. Constitutive equations for creep and other plastic deformation have been set up based on a generalised Norton expression and Kocks-Mecking’s model for the back stress. A model for the minimum creep rate based on fundamental principles for climb and glide has been derived. This model gives the correct order of magnitude for the creep rate in the temperature range from 400 to 20 °C without the use of fitted parameters. The creep exponent varies from 5 to 105 in this interval. The constitutive equations have also been formulated for multiaxial stress states.     

Liquid metal embrittlement of nuclear materials

The phenomenological features of liquid metal embrittlement (LME) are reviewed and the influence of metallurgical factors and testing conditions is described. The process is shown to be similar in many respects to the elastic fracture observed at lower temperatures in some bcc and hcp. metals. An important difference in the case of LME arises from the need for the embrittler to be present at the crack-tip during fracture. This condition imposes a low temperature limit which occurs when the embrittler is no longer mobile. The relationship between susceptibility to LME and alloying characteristics is discussed. The various theories have been considered and it is concluded that a reduction in surface energy leading to lower crack-tip cohesion and hence lower plastic deformation is consistent with most of the experimental evidence.The second section of the review uses this general understanding as a basis for speculation about possible synergy between LME and radiation effects. Those which alter the plastic deformation behaviour, i.e., radiation hardening or radiation annealing, are considered likely to have the same influences on embrittlement as on elastic fracture. Radiation creep will reduce any tendency to slow crack growth under restricted embrittler availability. Radiation-induced transmutation is seen as a possible source of supply of embrittling species, particularly as fission products, but interaction with high temperature creep-cavitation fracture by helium segregation is less likely because LME tends to be more severe at lower temperatures.The review concludes with a brief catalogue of LME-susceptible couples, concentrating as far as possible on nuclear materials (i.e. ferrous and zirconium alloys) to provide an initial source of data for materials selection by designers and operators and for failure analysis.     

稀土元素Gd对2308双相不锈钢热变形行为的影响

为解决含Gd双相不锈钢热加工不足问题,本文以含2%Gd的双相不锈钢为研究对象,在不同温度下开展热模拟压缩实验,研究含Gd双相不锈钢热变形行为及组织演变。利用Gleeble-1500D热模拟试验机对含Gd双相不锈钢进行变形量为50%的单道次热变形试验。根据真应力-真应变曲线计算了该合金的热变形激活能Q_d,建立本构方程。同时对热变形后的组织进行了分析,探究稀土元素Gd对含Gd双相不锈钢热变形行为的影响,结果表明,在热变形过程中,合金的动态软化机制主要为动态再结晶。合金包含两种含Gd析出相,即条带状的脆性析出相M₃Gd相和M₁₇Gd₂相（M=Fe、Cr、Ni）,均为六方结构。当变形温度为1 050 ℃时,脆性M3Gd相破坏了基体的连续性,无法与基体协同变形,降低了合金的热塑性,导致合金在热变形过程中出现沿晶开裂。含Gd双相不锈钢适宜的热加工工艺区间的应变速率为0.01~0.1 s^-1,变形温度为950~1 000 ℃。     

Effect of thermo-mechanical processing on microstructure and creep properties of the foils of alloy 617

The effect of rolling and annealing on the microstructure and high temperature creep properties of alloy 617 were investigated. Two types of foil specimens with different thickness reductions were prepared by thermo-mechanical processing. Recrystallization and grain growth were readily observed at specimens annealed at 950 and 1100 °C. The uniform coarse grains increase resistance against creep deformation. The grain size effect in creep deformation was dominant up to 900 °C, while dynamic recrystallization effect became dominant at 1000 °C. Dynamic recrystallization was observed in all the creep deformed foils, even though some specimens had already been (statically) recrystallized during annealing. Steady state creep rates decreased with increasing annealing temperature in the less rolled foils. The apparent activation energy Q_app for the creep deformation increased from 271 to 361 kJ/mol as the annealing temperature increased from 950 to 1100 °C.     

Creep Properties with Short Period Excessive Loadings on a Nickel-Base Heat-Resistant Alloy Hastelloy XR

A series of constant load & temperature creep tests and constant temperature creep tests with short period excessive loadings was carried out on a nickel-base heat-resistant alloy Hastelloy XR, which was developed for applications in the High-Temperature Engineering Test Reactor (HTTR), at temperatures ranging from 900 to 1,000°C. The excessive loading levels were set at the design stress intensity values Sm for Hastelloy XR indicated in the HTTR high-temperature structural design code.

Five to six time excessive loadings did not cause significant changes of the minimum creep rate or the time to onset of tertiary creep. Excessive loadings repeated around ten times did not cause significant changes of the time to rupture or the rupture elongation. The results suggest that the design stress intensity values Sm for Hastelloy XR have been determined reasonably.     

Simple Method for Evaluating Mesh Size Effects in Control Rod Worth Analysis

It has been pointed out that the reactor coolant system piping could fail prior to the meltthrough of the reactor pressure vessel in a high pressure sequence of pressurized water reactor severe accidents. In order to apply to the evaluation of the piping failure which influences the subsequent accident progression, models for the strength of piping materials at high temperatures were examined. It was found that 0.2% proof stress and ultimate tensile strength above 1,073 K obtained from tensile tests was reproduced by a quadratic equation of the reciprocal absolute temperature. Short-term creep rupture time and minimum creep rate at high temperatures were well correlated by the modified Norton's Law as a function of stress and temperature, which implicitly expressed the effect of the precipitation and the resolution of precipitates on the creep strength. The modified Norton's Law gave better results than the conventional Larson-Miller method. Relating applied stress vs. minimum creep rate and tensile properties vs. applied strain rate obtained from the creep and tensile tests, a temperature range where the dynamic recrystallization significantly occurred was evaluated.     

Analysis of the deformation and damage development of a dissimilar weld operating in the creep regime

The analysis of the deformation and damage behaviour of stress rupture tests with specimens out of the dissimilar metal weld seam 12% Cr-steel welded with a nickel base electrode for alloy 800 exhibits two competing processes:

• - Crack initiation occurs along the melting line due to high thermal stresses;

• - Creep deformation and damage concentrates in a heat affected zone of the ferritic 12% Cr steel due to long term stresses. The velocity of stress relaxation determines the resulting damage mechanism. At high temperatures with predominant creep deformation the cracks initiated in the melting line arrest and the creep deformation is concentrated in the heat affected zone (HAZ). At lower temperature the fracture area along the melting line increases. Long term tests at 535°C lead to lower stress rupture values compared to the scatterband of X 20 CrMoV 12 1 due to the reduced cross section after crack initiation in the melting line.

The analysis of stress rupture tests leads to the conclusion that grinding of melting line cracks is a reasonable measure because of sufficient stress relaxation.     

Materials for coal gasification plants utilizing nuclear process heat generated in a high temperature reactor

In coal gasification plants based on nuclear process heat, materials are subjected to high temperature corrosion in process gas atmosphere at 750 to 900° C. The process gas consists of steam, CO, CH₄, CO₂ and, depending on the gasified coal, low or high H₂S-concentrations. Materials for heat exchangers must be resistant to high temperature corrosion. They should also have adequate creep rupture strength. Therefore the commercial alloy Incoloy 800 and various model alloys were exposed to a process gas atmosphere to determine the corrosion behaviour and also stressed mechanically to investigate the interaction of high temperature creep behaviour and corrosion.

Compared with Incoloy 800, one of the new model alloys (30–32% Ni, 25–27% Cr, and Ce, Fe-balance) exhibits a very good corrosion resistance even when sulphur rich coal is gasified. The creep rupture strength at 900° C is in the range of the creep strength for Incoloy 800.     

In-reactor investigations of the creep of structural materials

Conclusions 1. A series of in-reactor tests was performed on a sample used to study radiation creep in 00X16H15M3B steel, XHM1 chrome-nickel alloy, the zirconium based alloys é110 and é635, and the vanadium-based alloy BTX8. The radiation creep modulus (in units of Pa⁻¹·(displacements/atom)⁻¹ equals 1.7·10⁻¹¹ for 00X16H15M3B steel, 4.6·10⁻¹¹ for XHM alloy with fluence up to 2.3·10²⁰ cm⁻² and 1.6·10⁻¹¹ for a fluence above 1·10²¹ cm⁻², (4.6–4.9)·10⁻¹¹ for é110 alloy, and 1.8·10⁻¹¹ for é635 alloy. For the alloy BTX8, at stresses below half the yield point and t=450°C, the modulus equals 3.3·10⁻¹² Pa⁻¹·(displacements/atom)⁻¹. At a higher stress, the deformation rate of the alloy increases progressively. 2. In the investigation of the temperature dependence of in-reactor creep of the alloy é110, it was found that at 350–370°C and higher, the thermal creep makes the predominant contribution to deformation. In the experimental range 370–455°C, the thermal activation energy of in-reactor creep was determined to be 36 ± 8 kcal/(g·atom). At temperatures below 350°C the creep of the alloy é110 is a temperature-independent radiation-stimulated process. 3. In the case of tests of zirconium alloys, a previously unobserved phenomenon of periodic rapid deformation of the material against the background of creep at stresses even well below the yield point of the irradiated material was discovered. The effect was manifested at a temperature of about 230°C. As the temperature increases up to 290°C and higher, no plastic movements are observed. Translated from Atomnaya énergiya, Vol. 80, No. 5, pp. 386–391, May, 1996.     

The viscoplasticity theory based on overstress applied to the analysis of beams under monotonic and cyclic creep loading

The viscoplasticity theory based on total strain and overstress can reproduce rate-dependent inelastic deformation without distinction between plastic and creep strain using two material functions. A viscosity function and an equilibrium stress-strain curve characterize rate-dependency and work hardening, respectively. The theory is used to analyze the creep and cyclic creep behavior of a beam subjected to a linearly increasing moment which is subsequently held constant.The analysis shows the existence of two possible states of equilibrium for creep deformation: termination of primary creep or secondary creep. They occur when the equilibrium stress-strain curve has positive or zero slope in the plastic range.The numerical experiments illustrate that the stress distribution at the end of the moment increase depends on the moment rate. The rate effects disappear with time when stress is redistributed. For practical purposes the equilibrium solution is obtained before 10⁷ s, when the material functions representing AISI Type 304 Stainless Steel at room temperature are used. The other equilibrium solution (secondary creep) is reached after primary creep when the constant moment is above the limiting equilibrium moment which corresponds to the plastic hinge moment of plasticity theory. The stress distribution during stationary creep is shown to be the solution corresponding to the Norton law of creep theory. The numerical experiments also illustrate the influence of various viscosity functions and equilibrium stress levels. A growth law for the equilibrium stress-strain curve is postulated and reversed loading as well as repeated loading are investigated.     

Overall mechanical properties of fiber-reinforced metal matrix composites for fusion applications

The high-temperature strength and creep properties are among the crucial criteria for the structural materials of plasma facing components (PFC) of fusion reactors, as they will be subjected to severe thermal stresses. The fiber-reinforced metal matrix composites are a potential heat sink material for the PFC application, since the combination of different material properties can lead to versatile performances. In this article, the overall mechanical properties of two model composites based on theoretical predictions are presented. The matrix materials considered were a precipitation hardened CuCrZr alloy and reduced activation martensitic steel `Eurofer'. Continuous SiC fibers were used for the reinforcement. The results demonstrate that yield stress, ultimate tensile strength, work hardening rate and creep resistance could be extensively improved by the fiber reinforcement up to fiber content of 40 vol.%. The influence of the residual stresses on the plastic behavior of the composites is also discussed.     

应力循环下T225NG合金塑性累积行为研究

对应力循环下T225NG合金的塑性累积行为进行了试验研究,提出了预测棘轮饱和应变的本构关系及描述棘轮应变演化规律的指数型演化方程。讨论了蠕变效应对T225NG合金棘轮行为的影响．结果表明．应力幅越低,循环蠕变分量在塑性累积中的贡献越大。     

Some questions of thermal strength in reactor construction

This paper considers some problems of the thermal strength of reactor materials, dealing with the nature of the stress and deformation states, creep, thermal fatigue, thermal shock, combination of thermal and mechanical stresses, and strength in temperature fields, varying in zones or with time. The possibility of micromechanical experiments to determine the mechanical properties of materials at various temperatures is also discussed.