Strain-amplitude dependent fatigue resistance of low-alloy pressure vessel steels in high-temperature water

Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in 561 K air and water over a wide strain amplitude range. It was found that fatigue resistance of the steels was enhanced in high-temperature water relative to high-temperature air under the low strain amplitude conditions (<0.3%) or in the high cycle regime (>2 × 10⁴ cycles), while it was remarkably degraded in high-temperature water under the higher strain amplitude conditions. Fatigue cracking and fractographic features suggested that effects of hydrogen be involved in the present corrosion fatigue process in high-temperature water. Possible environmentally assisted cracking mechanisms are discussed.     

Microstructural evaluation of fatigue damage in SA533-B1 and type 316L stainless steels

Transmission electron microscopy (TEM) examinations were made on fatigued SA533-B1 low alloy steel and Type 316L stainless steel specimens with the intention to investigate the mis-orientation changes among dislocation cells and the evolution of dislocation structures. Contrary to what might be expected for the cell structures, no clear relationship between fatigue damage and the mis-orientation changes of cell walls (or subgrain boundaries) was found in the fatigued samples of SA533-B1 steel (a bcc structure); however, significant changes of dislocation structures were observed in the fatigued samples of Type 316L stainless steel (an fcc structure). This could be accounted for by their different structures as well as complicated defect structures such as subgrain boundaries, small carbides, and dislocations inhomogeneously distributed in the SA533-B1 steel. It is interesting to note that at room temperature dislocations of fatigued SS316L specimens were observed to arrange themselves on {111} slip planes, in contrast, at 300°C the dislocations tend to move from their slip planes into subgrain boundaries in the surface layers rather than in the cross sectional layers.     

Effect of dynamic strain aging on fatigue crack growth behaviour of reactor pressure vessel steels

Abstract

Fatigue tests under constant amplitude load were conducted on compact tension specimens of SA533B3 steels with four levels of sulphur content at different temperatures. A modified capacitance type crack opening displacement (COD) gauge was shown to be suitable for fatigue crack length measurement at high temperatures. Test results obtained with different measurement techniques show good consistency. The observation that the Young's moduli measured at a strain rate of 4 × 10^?3 s^?1 for the SA533B3 steels at 150 and 300°C do not decrease with increasing temperature seems to be related to the presence of dynamic strain aging. The fatigue crack growth rates at 150 and 300°C are about two and half times slower than those tested at 400°C because dynamic strain aging prevails at 150 and 300°C. Fractographic examination results suggest that inclusions embedded in secondary cracks enhanced the fatigue crack initiation rather than the fatigue crack growth.     

Fracture toughness and crack growth resistance of pressure vessel plate and weld metal steels

Compact tension specimens were used to measure the initiation fracture toughness and crack growth resistance of pressure vessel steel plates and submerged are weld metal. Plate test specimens were manufactured from four different casts of steel comprising: aluminium killed C-Mn-Mo-Cu and C-Mn steel and two silicon killed C-Mn steels. Weld metal test specimens were extracted from five weld joints of Unionmelt No. 2 weld metal. The welds were of double V butt geometry having either the C-Mn-Mo-Cu steel (three weld joints) or one particular silicon killed C-Mn steel (two weld joints) as parent plate. On the upper shelf, a multiple specimen test technique was used to obtain crack growth data which were analysed by simple linear regression to determine the crack growth resistance lines and to derive the initiation fracture toughness values for each test temperature. These regression lines were highly scattered with respect to temperature and it was very difficult to determine precisely the temperature dependence of the initiation fracture toughness and crack growth resistance. The data were re-analysed, using a multiple linear regression method, to obtain a relationship between the materials' crack growth resistance and toughness, and the principal independent variables (temperature, crack growth, weld joint code and strain ageing).     

Low cycle fatigue behavior of constraint connections

The general influence of constraint effects on reversed low-cycle fatigue performance was investigated. Constraint effects are being considered to have contributed to the failure of welded Steel Moment Resisting Frames (SMRFs), were cracks initiated in constraint regions during recent earthquakes. However, this study revealed that high constraint enhanced the resistance to crack initiation during cyclic loading by reducing the local strains at the notch tip. Furthermore, changes in toughness due to various constraint severities had almost no influence on the low cyclic performance of steel under constraint. This applies to low and high stress levels as well as low and high constraint, covering the whole range of possible scenarios of low-cycle and even towards high-cycle fatigue. While high toughness is still important for enhancing the fracture strength to accommodate the maximum imposed stresses/strains in earthquake loading, it most likely does not play a key role in cyclic performance.     

Low cycle fatigue behaviour of HAYNES 230 alloy

Abstract

HAYNES® 230® alloy is a commercial solid–solution strengthened Ni-base superalloy that has found wide spread use in the hot sections of gas turbines. Recently, the alloy has been considered for other applications, e.g. in concentrated solar power plants. One common key performance criterion for all these applications is the ability of the material to withstand large numbers of thermal cycles due to start-ups, operatings, and shut-downs. Such thermal cycles may trigger low-cycle fatigue (LCF) due to differential thermal expansion and contraction. This paper reports isothermal LCF performance data of 230 alloy standard mill-annealed plate product for a wide range of temperatures 427–982°C and strain ranges (up to 1·5%), relevant to many of the aforementioned applications. Low-cycle fatigue lives are analysed in terms of the contributions of cyclic elastic and plastic strain components to fatigue damage. The fit parameters are discussed in the context of the alloy’s tensile properties.     

Low cycle fatigue of a titanium 829 alloy

The fatigue behaviour of titanium 829 in its oil quenched (‘basketweave’) and air cooled (‘aligned’) microstructural forms has been examined at 600°C and room temperature under fully reversed, total strain controlled conditions. Identical endurances are observed for each microstructure together with a low transition life. Similarly, almost perfect cyclic stability is exhibited irrespective of microstructure, temperature, strain range and rate. This is tentatively attributed to the ability of the aligned colonies present in both microstructures to accomodate plasticity. It is argued that a reduction in strain rate shortens life due to environmental effects. Multiple crack initiation is generally associated with facet-like features, with later growth surfaces bearing striations only after fatigue at 600°C.     

Low cycle fatigue behaviour of a plasma-sprayed coating material

Single crystal nickel-base superalloys employed in turbine blade applications are often used with a plasma-sprayed coating for oxidation and hot corrosion resistance. These coatings may also affect fatigue life of the superalloy substrate. As part of a larger programme to understand the fatigue behaviour coated single crystals, fully reversed, total-strain controlled fatigue tests were run on a ‘free standing’ NiCoCrAiY coating alloy, PWA 276, at 0.1 Hz. Fatigue tests were conducted at 650°C, where the NiCoCrAiY alloy has modest ductility, and at 1050°C, where it is extremely ductile, showing tensile elongation in excess of 100%. At the lower test temperature, deformation-induced disordering softened the NiCoCrAlY alloy, while at the higher test temperature cyclic hardening was observed which was linked to gradual coarsening of the two-phase microstructure. Fatigue life of the NiCoCrAlY alloy was significantly longer at the higher temperature. Further, the life of the NiCoCrAlY alloy exceeds that of coated, [001]-oriented PWA 1480 single crystals at 1050°C, but at 650°C the life of the coated crystal is greater than that of the NiCoCrAlY alloy on a total strain basis.     

Corrosion fatigue resistance of powder metallurgy and forged duplex stainless steels

The high cycle corrosion fatigue resistance of powder metallurgically (PM) fabricated and hot isostatically pressed (hipped) duplex stainless steels (DSSs) was investigated and compared with that of a conventional forged DSS. Tests were performed in rotating bending fatigue in a chloride and sulphate containing aqueous solution at room temperature. The hipped PM DSSs studied had small grain size and homogeneous microstructure and isotropic properties and thus, their high cycle corrosion fatigue resistance was generally markedly higher than that of the corresponding forged DSS tested in the longitudinal direction. Fatigue cracks were observed to grow preferentially in the ferrite phase and a retarding effect of the austenite phase on corrosion fatigue crack growth was observed both in the hipped PM and wrought DSSs. Slow cooling after solution annealing improved the corrosion fatigue resistance, even though it induced slight second phase precipitation. Localised corrosion attack was observed at inclusion/matrix interfaces, when oxide inclusions were cerium modified (forged DSS). When oxide inclusions were either unmodified or calcium modified (hipped PM DSSs), no localised corrosion attack was observed.

MST/3429     

Brittle fracture local criterion and radiation embrittlement of reactor pressure vessel steels

The application of local criteria for predicting brittle fracture of reactor pressure vessel steels is discussed with an emphasis on radiation embrittlement. An association of the radiation-induced damages and the processes of initiation and propagation of cleavage microcracks is analyzed from the standpoint of the local criterion for fracture. Physical-mechanical models are put forward to describe the influence of radiation damages on the cleavage microcrack initiation. The influence of the material hardening caused by neutron irradiation and plastic deformation on the fracture toughness is studied.     

Assessment of the master curve approach on three reactor pressure vessel steels

This study aims at assessing the applicability of the Master Curve procedure to the measurement of the reference temperature for three well-characterised reactor pressure vessel steels (22NiMoCr37, JSPS, JRQ). The following aspects of the methodology were investigated, using statistical tools such as the Generalised Maximum Likelihood (GML) and Monte Carlo methods: independence of T _o from test temperature and specimen type (configuration/dimensions); formula given in ASTM E1921 for evaluating the standard deviation of the reference temperature, and possibile expressions for estimating the standard deviation of the other Weibull parameters (m and K _min); proposed relationships for estimating the median toughness and standard deviation for T _o(_To) in the case of multi-temperature Master Curve analysis. In reference to the three Reactor Pressure Vessel Steels (RPVS) investigated, the independence of T _o was assessed from the test temperature (within the range prescribed by the following revision of the ASTM standard, T _o±50 °C) and, as far as C(T) specimens are concerned, from the sample dimensions; the well-known 10÷15 °C difference was however found between PCCv and C(T) geometries. Furthermore, using the Monte Carlo method, we assessed the relationship proposed by the ASTM standard for estimating the standard deviation of the reference temperature, which results fairly conservative; an alternative analytical function has been proposed. Clear trends have also been identified for the standard deviation of m (with respect to the number of valid data r) and K _min (with respect to the median toughness). Finally, the proposed relationships for estimating K _Jc,med and _To for the multi-temperature analysis have been validated by comparison with the results of the Monte Carlo method: an excellent agreement was found in terms of _To (better than 0.2 °C).     

Analysis of impact properties of A533 steel for nuclear reactor pressure vessel by instrumented Charpy test

Instrumented Charpy impact test can become more useful by connecting the digital memory and microcomputer, where yield load (P_y), maximum load (P_m), premaximum load energy (E_i) and post-maximum load one (E_p) are automatically and rapidly analyzed. In such analysis, cyclic oscillations in a load signal are corrected and smoothened by using a method of moving averages (the nonrecursive low pass digital filter). Dynamic J_1c value (J_1d) of A533 steel can be measured in a fatigue precracked type Charpy specimen, provided that a true deflection of specimen and a true crack initiation point in the load-deflection curve are known. For this purpose, elastic compliance values of the testing machine and the specimen are measured dynamically by the elastic low blow test to correct the apparent deflection. Crack initiation point, on the other hand, is detected by the plastic low blow test. It is shown that the crack initiation in the fatigue pre-cracked specimen of this material occurs prior to the maximum load, and that the relation between crack initiation energy (E_Δα) and pre-maximum load energy (E_i) is $\text{E}{}_{\mathrm{\Delta \alpha }}\text{E}{}_{\mathrm{i}}\text{? 0.8}$.     

Low cycle fatigue behaviour of particulate reinforced metal matrix composites

The low cycle fatigue (LCF) resistance of two different 6061 Al/20 vol% alumina particulate metal matrix composites (MMCs) in a peaked-aged condition has been evaluated under fully reversed strain control testing. Test results were combined with scanning electron and optical microscopy investigations to determine the effects of reinforcement particles and strain amplitude on the LCF behaviour of these MMCs. Both materials show three stages of response to LCF: initial fast hardening or softening in the first few cycles; gradual softening for most of the fatigue life; and a rapid drop in the stress carrying capability prior to failure. Both MMCs exhibit short LCF life which follows a Coffin-Manson relationship. All tested specimens demonstrate ductile fracture morphology at final failure. The experimental results are discussed in respect of strain amplitude, matrix composition and reinforcement shape and crack initiation.     

Validation of procedures for fatigue life assessment of a steel pressure vessel

A full‐size pressure vessel, made of steel plate P355NL1 (EN 10028‐3), was tested under repeated internal pressure until its failure was observed. Also, four representative structural details of the tested pressure vessel were fatigue tested under load control with a stress ratio of R= 0. These structural details are basically two seam‐welded joints, namely a butt‐welded joint and a joggle‐welded joint, one plate attachment using fillet‐welded joints and a nozzle‐to‐plate connection. S–N curves were generated for these details based on both nominal and structural stresses. These curves are critically compared with those proposed in pressure vessel design codes like the ASME VIII – Division 2, the PD 5500 and the recently approved EN procedures, the EN 13445 standard. Finally, predictions of the fatigue life of the pressure vessel, obtained using the previously referred procedures and the experimentally derived design curves are critically compared with the observed life of the vessel.     

Analysis of warm prestressing effect on fracture toughness of reactor pressure vessel steels

We have performed finite element calculation of precracked Charpy-like three-point bending specimen, in order to analyze the effect of warm prestressing on fracture toughness value of reactor pressure vessel steels. Two different hardening laws were applied in the calculation. J-integral was determined in both cases and comparative analysis was made.     

Statistical analysis of phosphorus segregation on intergranular fracture facets in pressure vessel steels

Abstract

A533B and C–Mn steels, widely used as nuclear pressure vessel steels, have been aged at 520°C after tempering at 650°C for various periods of time to produce different levels of embrittlement resulting from the segregation of P to grain boundaries. Metallographic observation and tensile test results showed that the embrittlement heat treatment did not have significant influence on the microstructures or tensile properties of the steels. P segregation at grain boundaries and on intergranular facets was investigated using field emission gun transmission electron microscopy and Auger electron spectroscopy. After such treatment, enhanced segregation was found to be a linear function of the square root of embrittling time. Statistical analysis of the AES measurements indicated that there is a minimum segregation level for intergranular fracture to occur.     

Probabilistic prediction of the crack resistance of nuclear pressure vessel steels on the basis of a local approach. Part 1

We analyze the stochastic nature of various critical parameters responsible for the brittle fracture of nuclear pressure vessel steels. It is shown that the critical stress of brittle fracture S_c governing the process of propagation of shear microcracks can be regarded (with sufficient accuracy) as a deterministic parameter. At the same time, the critical parameter controlling the process of initiation of microcracks should be regarded as stochastic. On the basis of the performed analysis, we modify our local criterion of brittle fracture proposed earlier to the case of probabilistic statement of the problem. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 1, pp. 5–20, January–February, 1999.