Steady-state Creep Rate Research Articles

Creep failure analysis of tie rod used in lifting steel vessels

Tie rod is a part of lifting and lowering mechanism of skirt of steel making vessel. The design stress on tie rod was1000 MPa and working temperature is 650–700 °C. Tie rods were approximately 1.9 m (1900 mm) long and 65 mm in diameter. The presently used EN24 material has average life of 6 months. The tie rod thinned as the diameter reduced to 45 mm and length increased upto 2055 mm. The unaffected microstructure showed martensite with bainite and the thinned region shows grain boundary chromium carbide precipitates which indicated microstructural degradation due to high temperature exposure over long period. The grain boundary precipitates is indication of creep which led to precipitate-induced void formation, and their growth and propagation. The steady state creep rate was calculated for 6 months with a change in length of 0.175 m and it is found to be 1.12 × 10−8 m/s which is 3 order magnitude lower than the creep resistant steel. The yield strength and UTS deteriorated due to creep and found to be much lower than the designed stress. At 650 °C which is working temperature of tie rod with a time of 105 h and the rupture strength was compared for different materials. Recommended martensitic steel AISI 431 rupture strength at the same condition is around 100 MPa. The comparison of two materials at 650 °C showed better service life for recommended material.

Effect of microstructure and microtexture evolution on creep behavior of hot-rolled and mushy-state rolled Al-4.5Cu-5TiB2 in-situ composite

Tensile creep behavior of the Al-4.5 wt.%Cu-5wt.%TiB2 in-situ composite, subjected to hot rolling, and mushy state rolling with or without prior cold rolling has been examined at two different temperatures (275°C and 300°C) under 25 MPa stress in order to study the effects of the microstructural and microtextural evolutions introduced by the thermo-mechanical treatments. The creep behavior of the investigated samples has been analyzed in terms of minimum or steady-state creep rate, time to rupture, and strain fractions in primary, secondary or tertiary stages of creep along with a thorough assessment of microstructural evolution and related damage examined using scanning electron microscopy, transmission electron microscopy, and electron back-scattered diffraction technique. The creep resistance of the pre-cold rolled mushy state rolled composite (PCMRC) at both the test temperatures has been found to be superior to that of the hot rolled composite or mushy state rolled composite. When tested at 275°C, the PCMRC composite has exhibited the lowest steady-state creep rate (∼1.36 × 10−10 s−1) and the highest time to rupture (∼903 h). The homogeneity of elemental Cu in the solid solution, a uniform distribution of CuAl2 and TiB2 particles in the matrix, and the highest low angle boundary density achieved due to mushy state rolling with prior cold rolling have imparted higher creep resistance through restricting dislocation glide. The formation of fine CuAl2 precipitates at the pre-existing dislocations and sub-grain boundaries within grains in course of the creep test has further improved the creep resistance through Orowan strengthening. The fracture surfaces have revealed the presence of dimples containing fine CuAl2 precipitates confirming partial ductile failure, along with intergranular cracking due to de-cohesion of CuAl2 and TiB2 particles at the α-Al grain boundaries.

Nanoindentation Investigation on the Size-Dependent Creep Behavior in a Zr-Cu-Ag-Al Bulk Metallic Glass

Nanoindentation technology has been widely adopted to study creep behavior in small regions. However, nanoindentation creep behavior of metallic glass is still not well understood. In the present work, we investigated nanoindentation size effects on creep deformation in a Zr-based bulk metallic glass at room temperature. The total creep strain and strain rate of steady-state creep were gradually decreased with increasing holding depth under a Berkovich indenter, indicating a length-scale-dependent creep resistance. For a spherical indenter, creep deformations were insignificant in elastic regions and then greatly enhanced by increasing holding strain in plastic regions. Strain rate sensitivities (SRS) decreased with increasing holding depth and holding strain at first, and then stabilized as holding depth was beyond about 500 nm for both indenters. SRS values were 0.4–0.5 in elastic regions, in which atomic diffusion and free volume migration could be the creep mechanism. On the other hand, evolution of the shear transformation zone was suggested as a creep mechanism in plastic regions, and the corresponding SRS values were in the range of 0.05 to 0.3.

Investigation on the microstructure and creep behavior of laser remelted thermal barrier coating

Laser surface modification of the thermal barrier coating was investigated with the aim of increasing creep resistance. Yttria-stabilized-zirconia (YSZ) with a CoNiCrAlY bond coat was deposited by air plasma spraying on equiaxed Ti–6Al–4V substrates. Analysis was carried out comparing uncoated samples with as-sprayed and laser remelted ones. A cross section and detailed characterization of coating surface was carried out by scanning electron microscopy and X-ray diffraction techniques. Mechanical properties in terms of microhardness and creep resistance were evaluated. Constant load creep tests were conducted at stress levels of 125 to 319 MPa at 500 °C and 600 °C. A dense ceramic layer of thickness about 40 μm was formed by laser remelted treatment and its microhardness surface was higher than the other layers. As-sprayed YSZ had higher creep resistance than other samples. Analysis of creep behavior showed that the steady-state creep rate of laser remelted samples had about 42% reduction in 600 °C condition, evidencing a higher creep resistance than uncoated material. SEM images revealed a ductile fracture with presence of equiaxed dimples.

Creep behaviors of MC carbide reinforced nickel based composite

IN718C alloy and MC carbide reinforced IN718C composites (MC/IN718C) were prepared by casting. Creep behaviors of the two materials were studied. The steady-state creep rate of the MC/IN718C composite at 704 °C under 620 MPa was reduced by an order of magnitude. The higher creep resistance for the MC/IN718C composite was not caused by the increase in the stress exponent or the creep activation energy. Based on the quantitative research of load partitioning in the MC/IN781C composite by using in situ synchrotron high energy X-ray diffraction under tensile loading, it was found the load bearing effect of the MC particles played an important role in the creep resistance improvement of MC/IN718C composite, which reduced the stress on the matrix, slowed down the creep of the matrix, and delayed the failure of the MC/IN718C composite.

Novel in situ synthesized carbide reinforced Ni base composite for structural castings with high creep resistance

Ni base superalloys for structural castings of aero engines demand higher service temperatures and lower costs while retaining their processing characteristics. To achieve these properties, alloying with the rare precious metals inevitably leads to a decrease in the castability and a marked increase in the raw material cost. In this work, we develop a novel Ni base composite (MC/IN718C) with a high creep resistance, excellent microstructure stability and excellent castability by optimizing the chemistry and microstructure of the commercial IN718C superalloy. In this work, both tensile strengths at room temperature and at 704 °C were obviously improved, while the steady-state creep rate of the MC/IN718C composite at 704 °C was reduced by one order of magnitude. The rupture life of MC/IN718C at 704 °C/620 MPa was enhanced by one order of magnitude compared to the IN718C alloy due to the load bearing effect of the MC carbide, the elimination of the metastable strengthening precipitation phase γ″ and a decrease in the harmful Laves and δ phases. MC/IN718C Ni base composite presents a new strategy for the design and preparation of high-temperature materials potentially for critical structural castings.

Free volume and internal structural evolution during creep in model amorphous polyethylene by Molecular Dynamics simulations

All-atom Molecular Dynamics (MD) simulations were employed to investigate the structural and free volume evolution (correlated with damage) during creep of model amorphous polyethylene (PE) at various applied stress states (tension, shear, compression), stress levels (10–200 MPa), and temperatures (175–325 K). The Modified Embedded-Atom Method for saturated hydrocarbons is applied to show that the phenomenological macroscale creep response of PE can be captured through MD simulations. The model adequately predicts the three classical stages of creep (primary, secondary, and tertiary) and provides detailed insight into the underlying molecular mechanisms. The calculated glass transition temperature (Tg) was found to be very close to the experimental Tg. Simulations were performed at temperatures below Tg (175 K) to above Tg (325 K) and demonstrate that the transition from glassy to rubbery state is reflected in the chain dynamics and damage evolution. Under all the stress states and temperatures simulated, the evolution of void volume, nucleation, growth, and coalescence are shown to directly correlate with specific stages of the creep response and the underlying chain dynamics within each stage. A correlation between the steady-state creep rate and steady-state void nucleation rate is found, suggesting that secondary creep is heavily driven by void nucleation, while tertiary creep is driven by void growth and coalescence.

Impacts of stress and roughness on steady-state creep of synthetic rock joints under shear

This study presents a systematic experimental investigation on the effects of joint roughness on the steady stage creep of rock joints. Multi-stage creep tests were conducted on synthetic rock joints with different roughness characterised by joint roughness coefficient (JRC, 0–2, 6–8 and 18–20) under different normal stress levels (10–30% of the uniaxial compressive strength, σc). Each creep test includes five loading steps at shear stress of 50–90% of the peak shear strength (τp). This study focuses on the steady-state creep rate of rock joints, which is characteristic of the time-dependent deformation behaviour, and stability of rock mass. The experimental results show that steady-state creep rate is highly affected by the magnitude of JRC and normal stress level. The creep rate decreases as the JRC increases and the relationship exhibits an exponential trend. The normal stress level affects heavily the creep rate at low values. The influence diminishes when normalised normal stress exceeds 0·2σc because the influence of joint roughness becomes less important and the creep rate is close to that of intact rock. The experimental results reported herein provide valuable data regarding the time-dependent deformation behaviour of rock joints, which have been rarely reported. This will be of significance to the improvements in theoretical models and engineering design.

Cyclic loading–unloading creep behavior of composite layered specimens

Soft and hard interbedded rocks show obvious time-dependent deformation after deep tunnel excavations, and it is therefore necessary to research the mechanical behavior of the layered rock. However, it is hard to obtain ideal transversely isotropic rocks in fields, so rock-like specimens were poured by using artificial materials. Cyclic loading–unloading creep experiments were performed on the artificial layered cemented specimens with various layer angles (0°, 30°, 60° and 90°) at a 20 MPa confinement. Time-independent deformations and time-dependent deformations of the rock-like specimens were distinguished to investigate the visco-elasto-plastic deformation characteristics. Instantaneous elastic strain and instantaneous plastic strain had linear correlations with stress ratio, whereas creep strain, including visco-elastic strain and visco-plastic strain, increased nonlinearly with an increasing stress ratio. The specimens with a small layer angle had more noticeable time-independent and time-dependent deformations and larger steady-state creep rates than those of the specimens with a large layer angle. Attenuation creep and secondary creep could be observed at relative low stress levels, whereas accelerating creep until failure occurred at the creep failure stress level. The time for creep failure can be predicated according to the axial steady-state creep rate or volumetric creep curve. Damage in the rock-like specimens showed linear correlation with the stress ratio. Dip angle has a significant effect on the creep failure mode under cyclic loading–unloading conditions.

Creep-permeability behavior of sandstone considering thermal-damage

This investigation presented conventional triaxial and creep-permeability tests on sandstones considering thermally-induced damage (TID). The TID had no visible effects on rock surface color, effective porosity and permeability below 300oC TID level. The permeability enlarged approximately two orders of magnitude as TID increased to 1000oC level. TID of 700oC level was a threshold where the influence of TID on the normalized mass and volume of the specimen can be divided into two linear phases. Moreover, no prominent variations in the deformation moduli and peak strength and strain appeared as TID<500oC level. It is interesting that the peak strength increased by 24.3% at 700oC level but decreased by 11.5% at 1000oC level. The time-related deformation and steady-state creep rate had positive correlations with creep loading and the TID level, whereas the instantaneous modulus showed the opposite. The strain rates under creep failure stresses raised 1-4 orders of magnitude than those at low-stress levels. The permeability was not only dependent on the TID level but also dependent on creep deformation. The TID resulted in large deformation and complexity of failure pattern for the sandstone.

Creep tests of red-bed sandstone after high temperature

Temperature is an important factor affecting rock deformation and stability. The influence of temperature on the creep characteristics of red-bed sandstone is revealed through the creep test of red-bed sandstone after different high temperatures, and a creep damage model considering the temperature effect based on the Burgers model is established to analyze the influence rule of temperature on model parameters. The research results indicate that 510~550 °C is the nodal temperature range for the change in mechanical properties of red-bed sandstone. Below this temperature range, with the temperature increases, the peak strength of specimen increase, ductility increases, and the long-term strength decreases. Above this temperature range, with the temperature increases, the peak strength and peak strain of specimen decrease, and the long-term strength increases. The instantaneous strain increases linearly with the increasing temperature. Under the same stress conditions, with the temperature increases, the instantaneous strain of specimen, the steady-state creep rate and the time to enter the stable creep stage increase, the time to enter the accelerated creep stage decreases, and the acceleration creep curve becomes steeper.

Assessment of creep deformation and rupture behaviour of 304HCu austenitic stainless steel

Reduction in CO2 gas emission and decrease in fuel consumption can result in increase in efficiency of the thermal power plants. Increase in efficiency is directly linked to the steam temperature and pressure which requires materials having high creep strength. This resulted in development of Advanced Ultra Super Critical (AUSC) power plant which aims to increase efficiency to more than 45 % and significant decrease in CO2 emission. The 304HCu stainless steel is one of the candidate materials to be used in AUSC power plant. It contains around 3 wt. % of copper, certain amounts of niobium and nitrogen and increased carbon content for enhancing creep strength. Creep tests are conducted for 304HCu stainless steel at 923K, 973K and 1023K over a stress range of 100-240MPa. The creep curve exhibited shorter primary regimes followed by marginal secondary regimes and extended tertiary regime. The variation of steady state creep rate with applied stress exhibited Norton’s power law relationship (εs = Aσn). The value of n (stress exponent) decreased with increase in temperature but decrease was more pronounced at 923K. The product of steady state creep rate and rupture life obeyed Monkman-Grant relation. The contribution of tertiary creep was found to increase with temperature. Microstructural degradation in the form of coarsened precipitates, dislocation cell formation and deformation bands was the primary reason for increase in tertiary regime of creep curve and increase in the value of damage tolerance factor (λ). A mathematical model based on finite element analysis coupled with continuum damage mechanics has been used to predict the creep deformation and rupture life of 304HCu SS. The prediction of creep curve based on this model was found to be in good agreement with experimental results.

Experimental Study on the Creep Behavior of Red Sandstone under Low Temperatures

In cold regions, the deformation characteristics and long‐term mechanical properties of rocks under low‐temperature conditions are considerably different from those in other regions. To study the deformation characteristics and long‐term mechanical properties of rocks in a low‐temperature environment and the effect of different temperatures, we perform a multilevel loading‐unloading uniaxial creep test on red sandstone samples and obtain the creep curves at different temperatures (20°C, −10°C, and −20°C). The results demonstrate that the total strain at each temperature can be divided into instantaneous and creep strains; the instantaneous strain includes instantaneous elastic and plastic strains, and the creep strain includes viscoelastic and viscoplastic strains. Temperature has a significant effect on the deformation properties of red sandstone. A decrease in temperature reduces the instantaneous and creep deformations of the rocks at all levels of stress. In addition, a decrease in temperature exponentially attenuates the total creep and viscoplastic strains of the rocks. 0°C is a critical point for the reduction of the total creep and viscoplastic strains of the rocks. When the temperature is greater than 0°C, the total creep and viscoplastic strains of the rocks decrease rapidly and linearly with decrease in temperature; however, when the temperature is less than 0°C, the decrease in the total creep and viscoplastic strains of the rocks is slow. The steady‐state creep rate of the rock samples decreases with decrease in temperature, whereas the creep duration increases with decrease in temperature, especially in the case of the accelerated creep stage. The accelerated creep durations of the rock samples S4 (20°C) and S7 (–10°C) are 0.07 h and 0.23 h, respectively.

Creep Features of Ti-600 Alloy at the Temperature of 650°C

Creep tests were carried out on one kind of near alpha titanium alloy named after Ti-600 alloy at the temperature of 650°C, and with the stresses of 150MPa, 200MPa, 250 MPa, 300 MPa and 350 MPa, respectively. The alloy ingot was conventionally forged and rolled to diameter 18mm bars. The creep samples were cut from the rolling bars and were solutioned at 1020°C for 1 h, air cooling, then aged at 650°C for 8 h, air cooling (STA). Steady state creep rate and the stress exponent n at different stresses were calculated for the alloy. Threshold stress σ0 was introduced to get the true stress exponent p. Creep deformation mechanism was also investigated. The results indicated that the steady state creep rate will increase with the rise of stress, and the creep time will also be shortened at the same time. At 650°C, the threshold stress is 83.8MPa. The value of n and p is 7.7 and 3.3 respectively for the alloy crept at lower stress region (150-200MPa); and which is 2.1 and 4.7 respectively for the alloy crept at relatively higher stress region (200-350MPa). Constitutive equations of steady state creep rate were also established for the alloy crept at 650°C. The creep deformation for the alloy is controlled by dislocation slipping at lower stress region, and which is mainly controlled by dislocation climbing and subordinately controlled by dislocation slipping at higher stress region.

Creep behavior and deformation mechanisms of a novel directionally solidified Ni-base superalloy at 900 °C

A novel directionally solidified Ni-base superalloy with excellent creep-rupture strength has been developed. By means of measurement of the creep properties and microstructural observations, the variations of the creep deformation behavior and corresponding deformation mechanisms of the new superalloy with the applied stress are investigated at 900 °C. As the applied stress increases from 300 to 375 MPa, the steady-state creep rate increases gradually, whereas the creep-rupture life decreases. Transmission electron microscope observations on the crept specimens reveal that dislocation climb plays an important role in the steady-state creep deformation, and the process which two matrix dislocations with different Burgers vectors combine into one a〈001〉 dislocation and subsequently shear through γ′ precipitates operates more actively with increasing the applied stress. Based on the experimental observations, the reasons that account for the variation of the creep properties with the applied stress are discussed.

Effect of annealing treatment on microstructure evolution and creep behavior of a multiphase Ni3Al-based superalloy

The microstructure evolution and creep behavior of a novelly designed as-cast Ni3Al-based superalloy with multiphase configuration are investigated by high-temperature annealing treatments. The as-cast microstructure comprises dominant γ′+γ dendritic and less (~19.37 vol%) interdendritic β areas, annealing treatments at 1160–1280 °C promoted the growth of γ'Ⅰ phase in the γ'+γ dendrite, during which the interdendritic β phase aggregated and coarsened rapidly in width with keeping its relatively constant volume fraction. Meanwhile, the annealing treatments significantly promoted the precipitation of quasi-spherical α-Cr phase particles within the β phase. The original as-cast microstructure exhibited inferior creep resistance at 800 °C/200 MPa with the shortest creep rupture life of 194 h, however, the annealing treatments at 1160–1280 °C prolonged the creep rupture life to beyond 611 h. Although the steady-state creep rate was gradually reduced with increasing annealing temperatures, the creep ductility was degraded by higher-temperature annealing at 1240 and 1280 °C compared with original as-cast microstructure. The 1160 °C annealed microstructure exhibited the longest creep rupture life of 665 h and the maximum creep strain to fracture of 3.21%, while the minimum steady-state creep rate was obtained on account of the largest γ'Ⅰ phase sized 0.91 µm after annealing at 1280 °C. Despite its positive role in the castability, thermoplasticity and weldability of the explored multiphase Ni3Al-based superalloy, interdendritic β phase has negative effects on the creep properties due to its incoherent existence with the dominant γ′+γ dendrite.

Microstructure and mechanical properties of the (Fe,Ni)Al-based alloy produced by SLM and HIP of spherical composite powder

The microstructure and properties of spherical composite powders produced from the β-(Fe, Ni)Al-based alloy by plasma rotating electrode process (PREP) using a steel-shelled electrode were studied. Parameters of the selective laser melting (SLM) mode (laser power density, laser intensity, scan speed and regime) were optimized to ensure the minimal porosity of < 0.08% as well as the absence of cracks or structural defects. Pore distribution and pore volume fraction in the fabricated samples were determined by 3D microCT visualization. The evolution of structural transformations in the matrix of the as-SLM (Fe, Ni)Al alloy was observed as the alloy was heated to the aging temperatures of 850 and 1250 °C. At 1250 °C, the secondary α-Fe precipitates are transformed to coherent cuboids with a cube-on-cube orientation relationship limited by the {100} plane. The in situ transmission electron microscopy (TEM) studies of structural phase transformations occurring upon heating in the temperature range of 20–850 °C demonstrated that coherent α-precipitates with a cubic morphology are homogenously nucleated via the mechanism of spinodal decomposition of an oversaturated solid solution of the B2 phase. Segregation of the pre-precipitation zones < 3 nm in size was observed at 450–650 °C. At the initial stage of ageing, α-Fe precipitates have a near-spherical shape. The positive effect of ageing at 850 and 1250 °C on properties of the alloy produced by SLM of spherical composite powders was demonstrated. Ageing at 1250 °C enhances the ultimate compressive strength (σb) and ductility (ε) from σb = 1490 ± 34 MPa at ε = 1.2% to σb = 1650 ± 41 MPa at ε = 8.0%. The maximum mechanical properties were observed in the samples produced by HIP: conditional ultimate compressive strength: 2120 ± 60 MPa (which corresponds to the true ultimate compressive stress σb = 1765 ± 50 MPa) and plastic yield ε = 16.25% (e = 0.1774) at an offset yield point (true yield stress) σ0.2 = 872 ± 21 MPa. Thermomechanical properties of the (Fe,Ni)Al- based alloy at 873–1373 K was studied. The temperature-force dependence of the steady-state creep rate has been analyzed. The true yield stress equal to 566 MPa was measured at 873 K, logarithmic strain rate 0.001 s−1, and degree of induced plastic strain 0.2%. It was shown that high-temperature mechanical properties of (Fe,Ni)Al- based alloy has intermediate values in between intermetallic alloy Ni41Al41Cr12Co6 and Fe-Ni-Gr-Al alloy (EP747).

The High-Temperature Creep Behavior of In-Situ TiB2 Particulate Reinforced Al12Si4Cu2NiMg Composite

In the present work, the in-situ TiB2/Al12Si4Cu2NiMg (denoted as ‘Al-12Si’) composites were successful synthesized through the salt-metal reaction route. The influences of weight fraction (0, 4, and 9 wt.%) and heat treatment (T5 and T7) on the tensile creep deformation were studied at ≥623 K under constant load in air. At the investigated temperature and stress condition, TiB2 particles increased creep deformation resistance, as compared to the unreinforced alloy, while the composites presented similar strength when the weight fraction of reinforcement increased from 4% to 9%. It was found that the steady-state creep rate was lower in the 4 wt.% TiB2/Al-12Si composite (T5), as compared with that in the 4 wt.% TiB2/Al-12Si composite (T7). The result has been rationalized by using the load-partitioning model and relative to the evolution of the rigid phase. The creep deformation of the 4 wt.% TiB2/Al-12Si composite was controlled by the climb of dislocations in the aluminum alloy matrix.

Effect of Cr and Zr on the Tensile Creep Behavior of Al-5Fe Based Semisolid Alloys

Determining the dominant creep mechanism is the key feature for design and development of Al-Fe alloys. The tensile creep behavior of Al5Fe4Cu2ZnMnMg alloy (AF1) and Al5Fe4Cu2CrZnZrMnMg alloy (AF15) by prepared semisolid forming has been investigated. The results reveal that the steady-state creep rates of the AF1 and AF15 alloys are increased with increasing temperature and applied stress. The creep property of the AF15 alloy is slightly higher than that of the AF1 alloy. There is threshold stress during creep. The creep true stress exponent is close to 5. The creep mechanism of the alloys is controlled by the lattice diffusion of aluminum. The creep resistance of the AF15 alloy is significantly improved by adding a small amount of Cr and Zr elements The reasons for the improved creep life of the AF15 alloy are discussed.

The influence of sensitization and corrosion on creep of 5083-H116

Elevated temperature exposure of 5XXX alloys causes sensitization, which leads to changes in room temperature mechanical properties and may lead to intergranular corrosion (IGC) susceptibility. The influence of sensitization and corrosion damage on the creep response of 5083-H116 at various temperatures (representative of fire damage) from 200 °C to 400 °C has been examined in this experimental study. Sensitization had a negligible effect on creep properties, while corrosion damage led to a reduction in creep rupture time and steady state creep rates. After accounting for sensitization and corrosion damage, creep rupture can be predicted based upon secondary creep/steady state creep rates.