Term Creep Research Articles

Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M23C6 carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M23C6 and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 105 h extrapolated creep rupture strength at 650°C is ∼100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.

Effect of Test Piece Size on Rheological Behavior of Wood Composites

The effect of size and the combined effect of the size and moisture sorption of test pieces on the long term creep behavior of wood composites were studied. Small-, wide-, and semisize test pieces from each of three commercial wood composites, particleboard (PB), oriented strand board (OSB), and medium density fiberboard (MDF) were tested. Three exposure regimes, constant 20°C/65%, a single change from 20°C/65% to 20°C/85%, and cyclic changes between 20°C/30% and 20°C/90% relative humidity (RH), were used. It was found that the width of test pieces had no effect while the length had a significant effect on long term behavior of wood composites, but the effect is in contrast to that of short term modulus of rupture (MOR) which ranged from 0.06 to 0.13 for the shape parameter and from 0.09 to 0.26 for the length parameter depending on the types of wood composites. The average ratio of the relative creep ( kc ) of small-:wide-:semisize was 1.14:1.13:1.00 for PB, 1.26:1.21:1.00 for MDF, and 1.24:1.24:1.00 for...

Application of the “Theory of Mixtures” to Temperature – Stress Equivalency in Nonlinear Creep of Thermoplastic/Agro-fibre Composites

The viscoelastic characterization of agro-filler based plastic composites is of paramount importance for these materials' long-term commercial success. To predict creep, it is imperative to derive a relationship between deformation, time, temperature, and stress. This work is the harbinger in modelling of the nonlinear creep behaviour of two-phase materials, where an extended “theory of mixtures” has been used to describe all the creep related parameters. The stress- and temperature-related shift factors were estimated in terms of the activation energy of the constituents. The combined effect of temperature and stress on creep strain was accommodated in a single analytical function where the interaction was shown to be additive. The model was validated under rigorous conditions and is unique because it describes creep not through curve fittings, but in terms of the creep constants of the constituents. This constitutive model is not only a vanguard in the prediction of long term creep of many biocomposites but also in the modelling of creep under step loading of temperature.

Estimate of Landfill Settlements Due to Mechanical and Decompositional Processes

Approximate closed form solutions are developed for estimating the mechanical and decomposition settlement of landfills. The mechanical settlements are the immediate (primary) settlement and long term creep. The decomposition settlements are generated by mass loss and gas generation. Based on the approximate solution, the decomposition settlement can constitute a significant portion of the total settlement and can be in the same order as the mechanical settlements. Settlement remaining after landfill closure depends on the decomposition environment during filling. For an average decomposition environment, postoperation settlement can still be significant 20years after completion of filling.

Behaviour of Z phase in 9–12%Cr steels

AbstractThe literature on the behaviour of modified Z phase Cr(V,Nb)N in creep resistant martensitic 9–12%Cr steels is briefly reviewed. Ten different 9–12%Cr steels were investigated after prolonged exposure at 600–660°C; the modified Z phase was found in all of them. In steels with high Cr content (11–12%), Z phase precipitates much faster than in 9%Cr steels. Precipitation of Z phase is associated with dissolution of MX carbonitrides, and causes a breakdown in long term creep strength in 9–12%Cr steels. High Cr steels show creep instabilities accompanied with Z phase precipitation, whereas low Cr steels show good long term creep stability. A niobium free CrVN variant of the modified Z phase was observed for the first time during the course of this work. The solution temperature of the Cr(V,Nb)N and CrVN modified Z phases was found to be close to 800°C for 11–12%Cr steels, much lower than the 1200–1250°C solution temperature of the unmodified CrNbN Z phase. Above the solution temperature the modified Z ...

Creep performance of OFP copper for the overpack of repository canisters

ABSTRACTTo experimentally assess the long term creep performance of oxygen-free phosphorus- doped (OFP) copper for the overpack of repository canisters, the combination of modestly elevated temperature and multi-axial stress state has been applied for accelerated testing. Multi-axiality was induced by using notched compact tension (CT) specimens, with interrupted testing to periodically inspect for creep damage. Uniaxial creep testing was also conducted to support creep analysis of the CT specimens. After about 10000 h of testing at 150°C/46 MPa (reference stress), the inspected CT specimens showed only marginal creep cavity indications near the notch tip. However, a distinct grain boundary zone with elevated Pcontent was observed to appear and widen during testing, mainly near the notch tip. The significance of the grain boundary zone is not well understood, but indicates stress-enhanced microstructural changes at relatively low temperatures. The predicted isothermal uniaxial creep life at 150°C/46 MPa agreed satisfactorily within a factor of two in time, when obtained independently from converted multi-axial testing results and directly from a creep model based on the available uniaxial data. Although the uncertainties in extended extrapolation remain large, the prediction would suggest safe long term service at leastagainst pure creep failure of intact parent material.

Effect of additions of Ti, B and Ce on microstructural stability, creep strength and creep damage in austenitic stainless steel

Optical and transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis of bulk extracted precipitate residues were carried out on long term (more than 80 000 h) creep tested (at 1023 K) type 304 austenitic stainless steels with different levels of Ti content to assess the microstructural stability and creep strength. B and Ce were added to the steels to suppress the creep cavitation. Finer Ti(C,N) particles with higher density and narrower size distribution were observed in steels with a higher Ti content, resulting in an increase in the creep rupture strength. However, higher Ti content increased the intergranular precipitation of the σ phase on longer creep exposure, resulting in the increase in creep cavitation and in the decrease in creep rupture strength. The study indicated an optimum level of Ti and {C + (6/7)N} content with the Ti/{C + (6/7)N} ratio close to the stoichiometric value of the Ti(C,N) precipitate particles that should also be close to their solubility limit at the solution heat treatment temperature.

Microstructural instability of a welded joint in P91 steel during creep at 600°C

The martensitic 10CrMoVNb 91 (P91) steel has been developed for applications under high ultra supercritical pressure for steam power plants. The creep rupture strength of welded joints of this steel is limited by the fine grained region of its heat affected zone (HAZ). The microstructural changes occurring in this region during long term creep tests at 600°C and 70 MPa were examined using optical microscopy and scanning and transmission electron microscopy for comparison with the microstructure of this region after post-weld heat treatment. The factors which result in the reduction of creep rupture strength of the fine grained region of HAZ are discussed. It was found that the most effective factors reducing the creep rupture strength of the fine grained region of the HAZ of a welded joint in P91 steel, in comparison with the coarse grained region of the HAZ and the base metal of the welded joint, are the finer prior austenite grain size of the fine grained region, which accelerates the rate of growth of martensite lath subgrains and enhances creep cavitation, and also the lower peak temperature during welding, which results in a softer martensite matrix.

Creep failure model of a tempered martensitic stainless steel integrating multiple deformation and damage mechanisms

A new model considering both deformation and damage evolution under multiple viscoplastic mechanisms is used to represent high temperature creep deformation and damage of a martensitic stainless steel in a wide range of load levels. First, an experimental database is built to characterise both creep flow and damage behaviour using tests on various kinds of specimens. The parameters of the model are fitted to the results and to literature data for long term creep exposure. An attempt is made to use the model to predict creep time to failure up to 105 h.

Time-dependent strength of large diameter PE100 low sag pipe

The qualification of large diameter polyethylene pipes in PE100-LS, a specially developed 'low sag' pipe grade of polyethylene, is described, taking into account long term creep. The extrapolated long term strain under hydrostatic pressure was 2·9% at 50 years which is lower than the 3·5% observed for more conventional PE100 polyethylenes. For gravity pipes of PE100-LS, a strain value of 5% at 50 years is recommended, which is the same as for common PE pipe materials subjected to stress relaxation. The present study also resulted in quality control recommendations for large diameter PE pipe, based on a 1000 h hydrostatic pressure test in which there is assumed to be a linear strain versus log pressure relationship. When the main load is due to external soil loading, a 1000 h ring test is proposed, at a deflection of 15%, focusing on stress relaxation measurements rather than pressure testing.

Long term creep of TiAl+W+Si with polycrystalline and columnar grain structures

Abstract The long term creep properties at 207 MPa and 760 °C of TiAl + W + Si with a fully lamellar microstructure and either polycrystalline grains or a columnar grain structure are compared. The results indicate that the columnar grain structure reduces the primary creep strain and delays the onset of tertiary creep, with only 1.2% creep strain after 10,000 h.

Prediction of creep life of X10CrMoVNbN-91 (P-91) steel through short term stress relaxation test methodology

The methodology proposed here adopts short-term stress relaxation testing (SRT) to predict creep strength/life of X10CrMoVNbN 91 (P-91) steel. SRT has been performed at three temperatures (853, 873, 898 K), and at each temperature two prestrains (0.7 and 1.2%) were applied with a hold time of 7 h. The initial data, stress versus time, were converted to stress versus inelastic strain rate by computation and compared with conventional steady state creep rate. Creep life was predicted from the SRT results using the Monkman-Grant relationship, and compared with creep data. The comparison showed the predicted life to agree extremely well with the creep-rupture life (≈30 000 h). The calculated activation energy for SRT (719 kJ mol-1) was found to be higher than the creep (415 kJ mol-1). Detailed SEM and TEM studies revealed that the degradation of microstructure in terms of coarsening of M23 C6, and martensitic lath grains, had taken place and was commensurable to long term creep exposure.

Transient dissolution patterns on stressed crystal surfaces

We present an experimental investigation on the dissolution of uniaxially stressed crystals of NaClO 3 in contact with brine. The crystals are immersed in a saturated fluid, stressed vertically by a piston and monitored constantly in situ with a CCD camera. The experiments are temperature-controlled and uniaxial shortening of the sample is measured with a high-resolution capacitance analyzer. Once the crystal is stressed it develops dissolution grooves on its free surface. The grooves are oriented with their long axis perpendicular to the direction of compressive stress and the initial distance between the parallel grooves is in accordance with the Asaro-Tiller-Grinfeld instability. We observe a novel, transient evolution of this roughness: The grooves on the crystal surface migrate upwards (against gravity), grow in size and the inter-groove distance increases linearly with time. During the coarsening of the pattern this switches from a one-dimensional geometry of parallel grooves to a two-dimensional geometry with horizontal and vertical grooves. At the end of the experiment one large groove travels across the crystal and the surface becomes smooth again. Uniaxial shortening of the crystal by pressure solution creep decays exponentially with time and shows no long term creep within the range of the resolution of the capacitance analyzer (accuracy of 100nm over a period of 14 days). This indicates that, while active, the fast transient processes on the free surface increase the solution concentration and thereby significantly slow down or stop pressure solution at the top of the crystal. This novel feedback mechanism can explain earlier results of cyclic pressure solution creep and demands development of a more complex theory of pressure-solution creep including processes that act on free surfaces.

Effects of Axial Shortening of Columns on Design and Construction of Tall Reinforced Concrete Buildings

Axial shortening of columns due to long term creep and shrinkage is inevitable in tall reinforced concrete buildings. However, calculation of exact values of axial shortening is not a straight forward task since it depends on a number of parameters such as the type of concrete, reinforcement ratio, and the rate and sequence of construction. All these parameters may or may not be available to the design engineer at the preliminary design stage of construction. Furthermore, long term shortening of columns could affect the horizontal structural members such as beams and floors and hence could affect the finishes and partitions. Therefore, a reasonable idea about the probable axial shortening could be important for construction engineers and project managers as well. In this study, the effects of construction sequence, rate of construction, and grade of concrete on axial shortening are determined based on a number of case studies covering 10–40 story range. These are presented as a set of guidelines so that the effect could be taken into account approximately, especially at the preliminary design stage and also during the construction phase.

Viscoplastic–plastic modelling of IN792

At high temperatures metallic materials behave in a viscous manner exemplified by strain rate dependence, stress relaxation and creep deformation. At low temperatures however, these effects are extremely small, and the behaviour is strain rate independent and shows no or very small relaxation effects. Finally there exists an intermediate region, in which the material behaviour is close to strain rate independent for high strain rates but at the same time shows time dependent inelastic effects, such as stress relaxation and creep. For IN792 this occurs at temperatures around 650 °C. The article describes the extension of a power-law viscoplastic model describing the behaviour of IN792 at 850 °C, also to describe the behaviour at 650 °C, by bounding the elastic–viscoplastic stress-space by a plastic yield surface. The model parameters have been estimated using data from creep test and tailored step relaxation tests, and the model fits well to both the step relaxation data aimed at resembling relevant component conditions and long term creep data.

The effect of long term creep exposure on the microstructure and properties of an underaged Al–Cu–Mg–Ag alloy

An underaged Al–Cu–Mg–Ag alloy shows zero secondary creep after 20,000 h at 130 °C and a stress of 200 MPa. Specimens exposed with and without an applied load reveal that dynamic precipitation of θ ′ and S ′( S) occurs on dislocations during primary creep, whereas σ phase forms in the matrix of the unloaded specimen.

Creep Characterization Of Ceramic BGA

Long term creep of a ceramic ball grid array (CBGA) solder ball under compressive loading was investigated. An experiment was conducted with two levels of loading and four of temperature. Analysis of the data assumed the composite ball structure could be simulated by an interconnection having the same shape, but of a single equivalent material. Curve fitting determined the stress exponent and activation energy of the equivalent material in the Norton creep model; the values were consistent with the range of values of the individual components available in the literature. Nonlinearity of the change in ball height with time was hypothesized to be due to geometric stiffening, a hypothesis which was confirmed by a simplified model. The model may be used to estimate creep behavior of other ball geometries having the same material set. The final result of this work - a closed form equation describing height decrease as a function of compressive force, temperature and time - can be used to simplify complex modeling of an entire package, and as an aid in designing accelerated thermal cycles which appropriately synchronize solder creep and fatigue.

Creep rupture ductility related to creep fracture mechanisms in 2.25Cr-1Mo steel

The long term creep rupture properties of 2.25Cr-1Mo ferritic steel have been studied at eight temperatures between 723 and 923 K and the rupture lives varied in the range of 100 to 113 000 h. Three fracture mechanisms have been identified on the basis of microstructural investigations, namely transcrystalline, cavitation, and recrystallisation. A transcrystalline fracture mode was observed at most of the test temperatures and often up to medium term rupture lives. A transition from transcrystalline fracture mode to cavitation mode was observed at very long rupture lives, whereas a transition to recrystallisation fracture mode was observed at the highest test temperatures. The variation of reduction in cross-sectional area of the sample at fracture, with the Larson-Miller parameter, has been found to reflect the transitions in fracture behaviour. Changes in the density of the material due to creep deformation were measured and found to exhibit a close correlation with reduction in cross-sectional area.

Creep resistant aluminium alloys and their applications

The CEC funded thematic network CREEPAL (BRRT -CT98 -5101) was established in 1998 to collect and disseminate information on the long term creep and thermal mechanical cycling behaviour of aluminium alloys. This paper presents some of the information gathered by consortium members and some additional industrial information related mainly to forging alloys. A short history of the use of aluminium alloys at elevated temperature is presented. The principal considerations required when evaluating aluminium alloys for use at temperatures in the 100-350°C range are detailed. The important microstructural mechanisms of creep in aluminium alloys are reviewed and the physical metallurgy of Hiduminium-RR.58 (2618A) is examined. The alloys that have found commercial application at elevated temperature are listed. Recent developments are described and opportunities for further development considered.

Creep deformation in a modified 9Cr-1 Mo steel

The short term creep properties and microstructural changes associated with creep were studied for constant stress specimens of a modified 9Cr-1Mo steel tested at 923, 873, and 823 K. The variation in the minimum creep rate with stress could be well described by the Norton equation (ε = Aσnexp(-Q/RT)) and many of the variations of this equation described in the literature. The values for the activation energy and stress exponent were comparable to values reported previously. The Monkman-Grant equation was found to describe well the relationship between the minimum creep rate and the time to rupture. The microstructural changes have been considered in terms of precipitates previously identified in these steels. A coarse structure (size ≤200 nm), comprising M23C6, M2X and a Nb rich MX type precipitate, was observed using SEM. A finer scale structure (size < 200 nm) was observed, using TEM, to be predominantly comprised of M23C6, M2X and a V rich MX type precipitate, with smaller amounts of M6X, V4C3, etc. After creep the coarse M23C6 and M2X precipitates were found to coarsen and the fine V rich MX type precipitates were found to increase in number. It is considered that precipitation of the V rich MX precipitates compensates for the coarsening of the M23C6 and M2X, thus offsetting the reduction in creep strength that would have been produced by the coarsening.