Thick-walled Components Research Articles

Effect of prior cooling rate on the grain size of fully-lamellar TiAl-base alloy developed by tempering/quenching

The near-gamma titanium aluminides are attractive for aerospace and automobile applications, because of their low density, good high-temperature strength and acceptable oxidation resistance. Investment casting appears to be the lowest cost alternative for the manufacture of near-net-shape TiAl components; but, the unavoidable remnant of coarse ingot grains in the final product deteriorates the advance of cast alloy. Recently, the authors found that the fine lamellar structure can be developed in cast TiAl-base alloy by a quenching/tempering process. By recrystallization of the quenched massive {gamma} structure at temperature above the {alpha} transus, a fine fully-lamellar structure with a grain size of 150 {micro}m can be obtained. For the engineering application of this process, it must be considered that, in some TiAl-base alloys, microcracks might be introduced during water quenching, and for thick-wall TiAl components, the cooling-rate in center region is lower than that in the surface. In this context, this paper aims to study the effect of prior cooling rate on the tempered lamellar grain size in a cast Ti48Al2W0.5Si alloy.

Fatigue Crack Growth Properties of a Cryogenic Structural Steel at Liquid Helium Temperature

The structural materials of the coils of superconducting magnets utilized in thermonuclear fusion reactors are used at liquid helium (4.2 K) temperatures and are subjected to repeated thermal stresses and electromagnetic forces. A high strength, high toughness austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N) has recently been developed for large, thick-walled components used in such environments. This material is non-magnetic even when subjected to processing and, because it is a forging material, it is advantageous as a structural material for large components. In the current research, a large forging of 12Cr-12Ni-10Mn-5Mo-0.2N austenitic stainless steel, was fabricated to a thickness of 250 mm, which is typical of section thicknesses encountered in actual equipment. The tensile fatigue crack growth properties of the forging were examined at liquid helium temperature as function of specimen location across the thickness of the forging. There was virtually no evidence of variation in tensile strength or fatigue crack growth properties attributable to different sampling locations in the thickness direction and no effect of thickness due to the forging or solution treatment associated with large forgings was observed. It has been clarified that there are cases in which small scale yielding (SSY) conditions are not fulfilled when stress ratios are large. ΔJ was introduced in order to achieve unified expression inclusive of these regions and, by expressing crack growth rate accordingly, the following formula was obtained at the second stage (middle range). da/dN = CJ ΔJmJ, CJ = AJ/(ΔJ0)mJ, where, AJ = 1.47 × 10−5 mm/cycle, ΔJ0 = 2.42 × 103N/m.

On-Line Thermal Stress Monitoring Using Mathematical Models

Thermal stresses of thick-walled components in power plants are the limiting factors for rapid start-up, shut-down or load changes. In current industrial practice, thermal stresses are calculated from differential temperature measurements. In this paper a method is described to calculate thennal stresses from readily available plant measurements. Mathematical models of the critical thickwalled components were used for unit 8 at the GKM power plant in Germany in order to reduce the instrumentation costs and improve the quality of the thermal stress calulation. Operational experience has shown that the estimated inner and mean temperatures are considerably more accurate than the prevoiusly used measured values.

Failure behaviour of thick-walled, austenitic pipe bends under cyclic bending loading at constant internal pressure

The deformation behaviour and the stress-strain characteristics of pipe bends depend on geometrical parameters and the type of loading. The basic parameter is the ratio of the outer to inner diameter. With thin-walled bends, the most highly stressed location in the bend shell is at the inside surface in the region of the bend flanks, whereas the highest loading occurs at the outside surface for thick-walled components. An additional parameter which affects the pipe bend behaviour is the arc angle. This paper describes the deformation and failure behaviour of thick-walled pipe bends made of austenitic material. Four pipe bends with angles of 90° and 60° were investigated experimentally, and these failed under loading through constant internal pressure and simultaneously applied cyclic in-plane and out-of-plane loading, respectively, by way of leakage. The location and orientation of the cracks corresponded well to predictions made on the basis of numerical results.

The prediction of breakaway oxidation for alumina forming ODS alloys using oxidation diagrams

AbstractThe oxidation induced loss in wall thickness of alumina forming high temperature alloys is in most cases not a limiting factor for component life because of the slow growth rates of the alumina surface scales. However, scale growth and repealing after spalling lead to depletion of the scale forming element, aluminium, in the bulk alloy. If, during long time service at high temperatures, the concentration of aluminium has decreased beneath a critical level, the protective alumina scale can no longer be formed and oxidation of the base elements Fe, Ni or Co occurs, leading to a catastrophic breakaway oxidation of the component. A model is presented which allows the calculation of the time at which this breakaway occurs using FeCrAl based ODS alloys as an example. The parameters which are necessary for the calculations, such as scale growth rates, scale spalling und bulk aluminium diffusion can be determined from relatively short time laboratory experiments. The calculated und measured results for the commercial ODS alloys ODM 751, PM 2000 und MA 956 at 11.00 und 1200°C are presented in form of Oxidation Diagrams in which the time to breakaway oxidation is plotted as a function of component wall thickness. On the basis of the theoretical considerations possible measures for increasing the oxidation limited life of the mentioned alumina forming alloys are discussed.

Oxidation induced lifetime limits of thin walled, iron based, alumina forming, oxide dispersion strengthened alloy components

Abstract The long term oxidation resistance of most high temperature alloys depends on the ability to form protective alumina or chromia surface scales during service. Scale growth and scale rehealing after spalling leads to depletion of the scale forming element, aluminium or chromium, in the bulk alloy. If the concentration of this element decreases below a critical level, oxidation of the base elements, e.g. iron, nickel, or cobalt, occurs leading to catastrophic oxidation and destruction of the metallic component. Using iron based, alumina forming, oxide dispersion strengthened alloys as an example, a model is presented which allows the calculation of the time at which the catastrophic breakaway oxidation for a given component occurs. The calculated data are presented as oxidation diagrams in which the time to breakaway is plotted as a function of component wall thickness. Experimental data for the oxidation of the oxide dispersion strengthened alloys MA 956, PM 2000 and ODM 751 in air in the temperat...

Different code requirements for workability and non-destructive testing of thick-walled austenitic components / Forderungen verschiedener Regelwerke an Verarbeitbarkeit und zerstörungsfreie Prüfung dickwandiger austenitischer Komponenten.

The requirements for workability and non-destructive testing of thick-walled austenitic components as laid down in the German KTA-standards, the French RCC-M and RSEM Codes and the American ASME Code are compared. Austenite-specific problems arising during material processing are mastered by strict delimitation of the choice of material, and by taking into account of procedure related peculiarities. Of the methods of non-destructive testing, the ultrasonic technique must still be considered as problematical. The Codes mentioned provide for the problems by different technical specifications and by restricted scopes

Problems of the mechanics of winding thick-walled composite structures

Dividing the winding process into individual stages, examination of the interply compliance of the composites, and the introduction of the circular metals made it possible to link the parameters of this process and, in particular, the tensioning force, with the properties of the completed component. It was thus possible to propose engineering methods of calculating thick-walled wound components with an allowance made for technological prior history. Structures for specific applications were constructed and a new technological method proposed and applied. These methods include the methods of producing spatial reinforced driveshafts whose load-carrying frame is produced on the basis of the system of the single filament. The studies examined in this work were used as a starting point for further investigations and development of the mechanics of winding in a number of scientific centers of the country.

Sensors for narrow-gap welding

Narrow-gap welding is an important technique for increasing productivity in the manufacture of thick-walled components. The nature of the process demands an automated approach and requires precise control to ensure consistently high weld quality. The work described in the paper gives the design, construction and development of appropriate sensors and sensing techniques for the control of the narrow-gap TIG welding (GTAW) process. Sensing systems based on arc-voltage sensing and direct vision sensing were developed and successfully tested on an experimental welding system. The arc-voltage sensing included the design of a digital technique for calculating the size of the voltage drop as the welding torch approaches a sidewall. The vision-based work involved the development of a very fast access framestore and highly optimised techniques for extracting the position of contrast features within a live image. The relative merits of arc-based and vision-based techniques are discussed.

Prediction of transient heat transfer coefficients in thick-walled pressure components of cylindrical geometry

This paper presents a procedure for determining the transient heat transfer coefficient in cylindrical, thick-walled pressure parts. From theoretical considerations, the temperatures can be predicted at discrete locations throughout the wall, when input data such as thermocouple responses are known at one or several interior locations.

Non-destructive detection and analysis of residual and loading stresses in thick-walled components

Components and systems are designated to withstand loads. These loads are primarily mechanical ones, additionally thermal, chemical and other ones. The constructed part has to support these loads with safety margins. Additionally, peak values for stresses have to be avoided. During the last ten years the theoretical background as well as the numerical evaluation were developed and introduced in practice. Especially finite-element methods at least for loading stresses are the common aid of the designing engineer. The experimental verification of calculated stresses and stress distributions fails up to now. There is no experimental measurement of stress at all. For strain measurements - restricted to elastic strains and to the surface region, not exceeding some ten micrometres - only the X-ray technique allows static and dynamic measurements. Strain variations by changes of loads or stresses can be measured with strain gages, optical holographical interferometry and other methods - also restricted to the surface only. Nothing is known about the volume-distribution of strains and stresses, and for scientists and engineers it is a surprising phenomenon how practicians here rely on theories and calculations. During the last five years several approaches were developed as non-destructive methods for strain/stress-measurements not only on the surface but in the interior, too. The state of the art is described in this contribution covering ultrasonic as well as micromagnetic methods. Examples are given from research results and applications in practice. The most important literature differentiated referring to physics, review contributions and applications is listed in an appendix.

Modelling and Simulation of Thermal Stress Dynamics in a Steam Generator

Enormous price increases and limited reserves of fossil fuels have resulted in increased use of nuclear power plants for the base load of electricity supply. Consequently, conventional steam power plants have been used in variable and peak load ranges in the dailly load curves of electricity neneration. Economical control of non-stationary operations (start-up, shut-down, fast and large load changes) is possible only by approachinn and maintaining the maximum allowable material stresses at the critical points of power plants. These points occur during the steam generator operation, on the inside surface of the insulated thick wall components containing high temperature and hinh pressure steam. The rate of load change of a steam generator is mainly limited by thermal stresses occuring in the thick wall elements in which the working fluid flows. The appropriate design and effective application of a new control system for the nonstationary operating conditions require knowledqe of the dynamic behaviour of the thermal stresses at the critical points in the system. Thermal stress dynamics in steam generator components depend on the dynamics of the working fluid as well as on the design data of the component concerned. Extension of the working fluid dynamics to the thermal stress dynamics in steam generators is investigated in this study. The study comprises modeling and simulation of working fluid dynamics and their extension to thermal stress dynamics of an actual once-through steam generator. The mathematical model developed for the thermal stress dynamics can be used for developing and optimization of new control and measurement systems used for the control of power plants under nonstationary operating conditions.

Interpretation of acoustic emission signals

Acoustic emission (AE) testing of large components, such as pressure vessels, must consider signal distortion by sound propagation. The transfer functions for energy, peak amplitude and signal duration of crack propagation signals in steel plates were determined experimentally, as were group velocity and change in the frequency spectrum. The results are presented and discussed in the context of AE testing of thick-walled components.

Hydrogen embrittlement and segregation of impurities in type 10MnNi2Mo pressure vessel steel

This paper summarizes the findings of a study in which the brittle fracture resistance and hydrogen embrittlement resistance of a 10MnNi2Mo steel were investigated both in the quenched and tempered state, and after a subsequent heat treatment that simulated the thermal cycling in the heat-affected zone around a weld joining thick-walled components. This simulating treatment included two alternative stress relieving procedures, at 650 and 580°C respectively. The simulating treatment caused marked coarsening of the prior austenitic grains; had no substantial effect on the static and dynamic fracture toughness, nor on the transition temperatures; but reduced the resistance against hydrogen embrittlement, and during the development of hydrogen-induced cracks produced some intercrystalline facettes. A supplementary study of surface segregation, by Auger electron spectroscopy on the free surfaces of the specimens, indicated that this incidence of intercrystalline failure may be at least partly ascribed to the pronounced segregation of phosphorus during stress relieving at both of the temperatures employed. This made an interesting comparison with 20Mn steel, where the simulating treatment profoundly reduced both the fracture and notch toughness levels but led to favourable resistance to hydrogen embrittlement. The fact that no intercrystalline failures were detected in hydrogen-charged 20Mn steel is ascribed in part to the lower observed segregation activity at the stress relieving temperatures, in part to the resultant microstructure of this non-alloyed steel.

Recording length criteria as applied in ultrasonic testing

An appreciable method used to assess the quality and integrity of safety-related components in light water reactors is the ultrasonic examination, in which case great importance is attributed to the criteria pertaining to recording length and permissible defect size. The development of the recording length criteria as applied when employing this method of examination is portrayed, the latter being based on the criteria which have proven themselves throughout long years of practice in the examination of conventional components. When taking these criteria into account the application of conventional ultrasonic techniques often leads to problems in the case of thick-walled components the reason being that indications are overrated. Taking the design of reactor components as the basic point of consideration, modified criteria are derived particularly when the size of discontinuities calculated by fracture mechanics analyses is taken into account. The introduction of new ultrasonic examination techniques such as, for example, focussed probes revealed that a considerably more realistic assessment is possible and consequently results in a reduction of unnecessary repairs. A comparison of the size of indications determined using conventional and analytical techniques renders possible the anchoring of an intermediate stage in the evaluation of indications which is encompassed in the consideration of the bundle divergence. Thus a new concept is realized for the evaluation of ultrasonic indications detected in reactor components, which in the meantime has found its way into the associated regulatory guides.

Thermal shock in nuclear reactors

Most types of liquid cooled reactors undergo rapid changes in coolant temperature during abnormal operating conditions, which cause components adjacent to the coolant to undergo rapid surface temperature changes. The differential expansion between surface and bulk material caused by repeated thermal shocks can induce fatigue or cyclic creep damage. In thick walled components individual thermal shocks may cause rapid fracture from pre-existing defects. This paper reviews the reported experimental and analytical work defining the temperature, stress and resultant damage in reactor structural components subject to thermal shock.

An axisymmetric model of acetabular components in total hip arthroplasty

An axisymmetric finite element model with nonaxisymmetric loading of an acetabular arthroplasty and the surrounding pelvic bone is presented. Model variations include ultra high density polyethylene acetabular components of varying wall thicknesses and metal backed ultra high density polyethylene components. Each of the two component types is modeled as implanted within an acetabulum with intact subchondral bone and within an acetabulum without subchondral bone. Thin wall polyethylene acetabular prostheses are predicted to increase, relative to thick wall components, maximum stresses in the cement-bone composite. Trabecular bone stresses are predicted to increase with the removal of subchondral bone. Stiffer metal-backed acetabular components are predicted to reduce maximum cement and bone stresses and to abridge the effects of altered component wall thickness and of subchondral bone removal.

The relationship between notched bar impact bending test, robertson test and drop-weight test

The temperature of the Robertson-Test referred to as crack-arrest-temperature determines the change of the fracture mode from brittle to tough. It was found out that for 22 NiMoCr 37 (similar to A 508 Cl 2) and St 37 steels this temperature was corresponding with the upper shelf temperature of the impact test. This is also the temperature at which there cannot be initiated any cleavage fracture (fracture transition characteristic temperature) in the modified drop-weigh-test, and it is corresponding with the FTE-point for large wall thickness in the FAD by Pellini which is also 65–70 K above NDT. Thick walled components as well can be considered at this temperature to be absolutely safe in terms of brittle fracture, if possibly generated brittle areas of base or HAZ material will remain localized in small isolated volumina. The latter requirement has to be verified by adequate manufacturing.