Minimum Creep Rate Research Articles

Creep behaviors of Fe-18Ni-12Cr based alumina-forming austenitic steels with ultralow carbon

Two kinds of Fe-18Ni-12Cr-based alumina-forming austenitic (AFA) steels with ultralow carbon content were prepared by vacuum induction melting. Creep rupture tests were carried out with the applied stress range between 80 and 180 MPa at the target application temperature of 700 °C. Microstructure evolution was characterized using glancing X-ray diffraction, scanning electron microscopy equipped with a detector of electron backscatter diffraction and transmission electron microscope. Compared with 18-12-Al, 18-12-AlNb exhibited superior creep properties including ten times longer creep rupture life ( $${t}_{\mathrm{rup}}$$ ) and one magnitude smaller minimum creep rate ( $$\dot{\varepsilon }_{{\min }}$$ ). For example, the $${\dot{\varepsilon }}_{\mathrm{min}}$$ at the applied stress of 140 MPa was 4.4 × 10–5 and 2.1 × 10–4 h–1 for 18-12-AlNb and 18-12-Al, respectively. The strengthening effect of B2-NiAl would weaken at 700 °C due to its ductile-to-brittle transition. The addition of Nb not only promoted the formation of Laves-Fe2(Mo,Nb), but also resulted in formation of nanoscale NbC. The GB Laves-Fe2(Mo,Nb) did not impair the creep ductility by strengthening GBs, and all the creep ruptured AFA steels exhibited typical ductile fracture mode. The GB Laves-Fe2(Mo,Nb) and NbC precipitates contributed to the superior creep resistance of 18-12-AlNb.

A Minimum Creep Rate for 2-1/4Cr-1Mo Steel Consistent With the ASME Section III, Division 5 Rules

Abstract This technical note describes a minimum creep rate model for 2-1/4Cr-1Mo steel that is consistent with the current creep strain equation embedded in the ASME Boiler & Pressure Vessel Code Section III, Division 5, Subsection HB, Subpart B isochronous stress–strain curves. Minimum creep rate models for all the Section III, Division 5 Class A materials are required for the development of improved high temperature design methods. Of all the Class A materials, only 2-1/4Cr-1Mo does not have a readily identifying minimum creep rate term in the current isochronous stress–strain curve model.

Creep behavior of 316 L stainless steel manufactured by laser powder bed fusion

Additive manufacturing as a new processing technique can produce unique microstructure that is difficult to achieve using conventional techniques. In this study, we have investigated the creep behavior of 316 L stainless steel produced by a laser powder bed fusion process at temperatures of 550, 600 and 650 °C and stresses between 175 and 300 MPa. We found that additively-manufactured 316 L stainless steel had a higher stress dependence of the minimum creep rate than conventionally-made Type 316 SS, which could be attributed to the dislocation cell structure resulting from the printing process. The dislocation cell structure was unstable under creep, evolving into a uniform dislocation structure under the test conditions. While internal porosity in AM 316 L SS may serve as nucleation sites of creep voids and may be responsible for a relatively lower creep life, additively-manufactured 316 SS did not show inferior creep ductility when compared with conventionally-made 316 SS. The creep life of AM 316 L SS could be improved by stabilizing dislocation cell structure and/or reducing internal porosity through an optimized additive manufacturing process.

Thermal stability of additively manufactured austenitic 304L ODS alloy

Thermal stability and high-temperature mechanical properties of a 304L austenitic oxide dispersion strengthened (ODS) alloy manufactured via laser powder bed fusion (LPBF) are examined in this work. Additively manufactured 304L ODS alloy samples were aged at temperatures of 1000, 1100, and 1200 °C for 100 h in an argon atmosphere. Microstructure characterization of LPBF 304L ODS alloy before and after the thermal stability experiments revealed that despite the annihilation of dislocations, induced cellular substructure by the LPBF process was partially retained in the ODS alloy even after aging at 1200 °C. The size of Y-Si-O nanoparticles after aging at 1200 °C increased from 25 to 50 nm. EBSD analysis revealed that nanoparticles retained the microstructure of LPBF 304L ODS and hindered recrystallization and further grain growth. At 600 °C and 800 °C, the yield stress of the 290 and 145 MPa were measured, respectively, which are substantially higher than 113 MPa, and 68 MPa for 304L at the same temperatures. Furthermore, the creep properties of LPBF 304L ODS alloy were evaluated at a temperature of 700 °C under three applied stresses of 70, 85, and 100 MPa yielding a stress exponent (n) of ∼7.7; the minimum creep rate at 100 MPa was found to be about two orders of magnitude lower than found in the literature for wrought 304L stainless steel.

Effect of Low-Temperature Annealing on Creep Properties of AlSi10Mg Alloy Produced by Additive Manufacturing: Experiments and Modeling

The effects of postprocessing annealing at 225 °C for 2 h on the creep properties of AlSi10Mg alloy were investigated through constant load experiments carried out at 150 °C, 175 °C and 225 °C. In the range of the experimental conditions here considered, the annealing treatment resulted in an increase in minimum creep rate for a given stress. The reduction in creep strength was higher at the lowest temperature, while the effect progressively vanished as temperature increased and/or applied stress decreased. The minimum creep rate dependence on applied stress was modeled using a physically-based model which took into account the ripening of Si particles at high temperature and which had been previously applied to the as-deposited alloy. The model was successfully validated, since it gave an excellent description of the experimental data.

High Temperature Creep Behaviour of Cast Nickel-Based Superalloys INC 713 LC, B1914 and MAR-M247

Cast nickel-based superalloys INC713 LC, B1914 and MAR-M247 are widely used for high temperature components in the aerospace, automotive and power industries due to their good castability, high level of strength properties at high temperature and hot corrosion resistance. The present study is focused on the mutual comparison of the creep properties of the above-mentioned superalloys, their creep and fracture behaviour and the identification of creep deformation mechanism(s). Standard constant load uniaxial creep tests were carried out up to the rupture at applied stress ranging from 150 to 700 MPa and temperatures of 800–1000 °C. The experimentally determined values of the stress exponent of the minimum creep rate, n, were rationalized by considering the existence of the threshold stress, σ0. The corrected values of the stress exponent correspond to the power-law creep regime and suggest dislocation climb and glide as dominating creep deformation mechanisms. Fractographic observations clearly indicate that the creep fracture is a brittle mostly mixed transgranular and intergranular mode, resulting in relatively low values of fracture strain. Determined main creep parameters show that the superalloy MAR-M247 exhibits the best creep properties, followed by B1914 and then the superalloy INC713 LC. However, that each of the investigated superalloys can be successfully used for high temperature components fulfils the required service loading conditions.

Enhancing High-Temperature Creep Resistance of <i>In Situ</i> TiAl-Based Matrix Composite by Low Volume Fraction of Ti<sub>2</sub>AlC Particles

Samples of TiAl-based matrix in-situ composite with the chemical composition Ti-46.4Al-5.1Nb-1C-0.2B (at.%) reinforced with a low volume fraction of primary Ti2AlC particles were prepared by vacuum induction melting in graphite crucibles and centrifugal casting into graphite moulds. The hot isostatic pressing (HIP) of the as-cast samples and subsequent heat treatments leads to the formation of equiaxed grains with fully lamellar α2(Ti3Al) + γ (TiAl) microstructure and uniformly distributed Ti2AlC and TiB particles. The minimum creep rates of the in-situ composite are significantly lower compared to those measured for the counterpart low carbon benchmark alloy with the chemical composition Ti-47Al-5.2Nb-0.2C-0.2B (at.%) at temperatures ranging from 800 to 900 °C and applied stress of 200 MPa. The studied in-situ composite shows also significantly improved creep resistance compared to that of some TiAl-based alloys with fully lamellar, convoluted and pseudo-duplex microstructures at a temperature of 800 °C and applied stress of 200 MPa.

High-Temperature Creep Behavior and Microstructural Evolution of a Cu-Nb Co-Alloyed Ferritic Heat-Resistant Stainless Steel

The creep behavior of Fe–17Cr–1.2Cu–0.5Nb–0.01C ferritic heat-resistant stainless steel was investigated at temperatures ranging from 973 to 1123 K and stresses from 15 to 90 MPa. The evolution of precipitates after creep deformation was analyzed by scanning electron microscopy, energy dispersion spectrum, and transmission electron microscopy. The minimum creep rate decreased with the decrease in the applied load and temperature, thereby extending the rupture life. Cu-rich phase and Nb-rich Laves particles were generated in dominant quantities during the creep process, and the co-growth relationship between them could be detected. Creep rupture was featured by ductile fracture with considerable necking. As increasing the temperature and decreasing the stress, the softening of the metal matrix was accelerated, showing more obvious plastic flow. The true stress exponent and activation energy were 4.9 and 375.5 kJ/mol, respectively, indicating that the creep deformation was dominated by the diffusion-controlled dislocation creep mechanism involving precipitate-dislocation interactions. Based on the creep rupture data obtained, the Monkman–Grant relation and Larson-Miller parameter were established, which described the creep rupture life for the studied steel well.

Creep behavior and modeling of high-density polyethylene (HDPE)

Creep behavior of high-density polyethylene (HDPE) was investigated. HDPE has many engineering applications in different industries such as piping systems, cable and wiring, automotive parts, storage containers, and biomedical implants. The temperatures chosen were 23, 53, and 82 °C based on the service temperature range for application in design of automotive fuel tanks. Creep strength decreased and creep strain as well as creep strain rate increased by increasing temperature. The Larson-Miller parameter widely used for metallic materials was able to correlate time to rupture, stress, and temperature data of HDPE. The Monkman-Grant relation was successfully used to correlate minimum creep rate and time to rupture. The Findley power law and time-stress superposition principle (TSS) were used to represent nonlinear viscoelastic creep curves. Some longer-term creep tests were also conducted at room temperature to evaluate the accuracy of extrapolation of the short-term creep test results to longer creep life predictions. The models used based on the data presented can be used in the design of parts and components made of HDPE where creep failure may be a concern.

Creep in high Al ferrite based low density steel

Low density steels have attracted the attention of researchers across the globe as substitute alloys in structural and locomotive applications with promising mechanical properties and reduced density. Most of the research in low density steels is concentrated on room temperature properties after the thermomechanical processing of the same. In the present study, a high Al Fe-11Al-5Mn-0.1C low density steel was melted by vacuum arc melting in argon atmosphere. The as cast alloy was homogenized at 1200 °C and then hot rolled from 20 mm to 2 mm at 1100 °C. Hot rolled strips were annealed at 1000 °C for 1 h followed by furnace cooling. After thermomechanical processing, creep testing of alloy samples was carried out at 600 °C for 50 MPa and 100 MPa stress. The processed alloy has shown a total creep life of 6.21 h and minimum creep rate of 8.5 × 10 −1 h −1 for 100 MPa stress and for a stress of 50 MPa, creep life of 1005.1 h with minimum creep rate of 1.4 × 10 −3 h −1 . A dominating tertiary creep with appreciable secondary and negligible primary creep regime was observed. Fracture occurred due to cavitation at ferrite-kappa carbide interface. Ductile mode of fracture with fine dimples was observed.

The Effect of Predeformation on Creep Strength of 9% Cr Steel.

Martensitic creep-resistant P92 steel was deformed by different methods of severe plastic deformation such as rotation swaging, high-pressure sliding, and high-pressure torsion at room temperature. These methods imposed significantly different equivalent plastic strains of about 1–30. It was found that rotation swaging led to formation of heterogeneous microstructures with elongated grains where low-angle grain boundaries predominated. Other methods led to formation of ultrafine-grained (UFG) microstructures with high frequency of high-angle grain boundaries. Constant load tensile creep tests at 873 K and initial stresses in the range of 50 to 300 MPa revealed that the specimens processed by rotation swaging exhibited one order of magnitude lower minimum creep rate compared to standard P92 steel. By contrast, UFG P92 steel is significantly softer than standard P92 steel, but differences in their strengths decrease with increasing stress. Microstructural results suggest that creep behavior of P92 steel processed by severe plastic deformation is influenced by the frequency of high-angle grain boundaries and grain coarsening during creep.

Pt–Al bond coat dependence on the creep stress distribution, deformation and fracture behaviour in a second generation Ni-based single crystal superalloy

An evaluation was conducted on the impact of a Pt–Al bond coat on the creep behaviours of a second-generation Ni-based single crystal superalloy at 750 °C/820 MPa, 850 °C/630 MPa, 1038 °C/137 MPa and 1100 °C/112 MPa, respectively. As revealed by the creep results, the creep behaviours of the coated superalloy were inferior to those of the bare superalloy to some extent under all testing conditions. The maximum deterioration in the strain-to-fracture and time-to-rupture was observed at 1100 °C/112 MPa (7.3%) and 750 °C/820 MPa (74.7 h), respectively. Following the creep test, deconvolution calculations were performed, which indicated that the creep stress for all core superalloys was as low as approximately 1.7–1.9% of the applied stress while the bond coat stress was 4.0–10.4% that at the minimum creep rate. Moreover, TEM analyses revealed that within the superalloy substrate adjacent to the bond coat, there were fewer stacking faults and an abundance of dislocation debris observed within γ′-Ni3Al at 750 °C/820 MPa. The misfit of the γ/γ′ phase was insignificant and the dislocation networks became irregular at 1100 °C/112 MPa. With an increase in temperature from 750 °C to 1100 °C, the fracture mechanism of both the substrate and bond-coat was found to shift from quasi-cleavage to micro-void coalescence. The quasi-cleavage cracks occurring within the bond coat increased the number of connections with creep cracks to cause spreading from the interior of the samples, thus resulting in premature failure at 750 °C/820 MPa and 850 °C/630 MPa. However, the degradation at 1038 °C/137 MPa and 1100 °C/112 MPa was ascribed to the increasing thickness of the inter-diffusion zone, as well as the volume of the blocky γ′-Ni3Al phase inside the bond coat and topologically close-packed phases in the superalloy near the bond coat.

Creep Deformation Behavior of Inconel 617 Alloy in the Temperature Range of 650 °C to 800 °C

Creep behavior of the Inconel 617 alloy has been investigated through tests carried out in the temperature range of 650 °C to 800 °C under 95 to 350 MPa. Creep curves obtained from tests on the alloy exhibit a non-classical nature, with the primary creep rate decreasing to a minimum value, followed by a typical increase, which is either continuous at temperatures ≤ 700 °C or has an intermediate steady-state regime at 750 °C or 800 °C. The variation of minimum creep rate (έmin) with stress (σ) obeys the power law relationship (έmin = Aσn), with the stress exponent, n, increasing with temperature to a peak value at ~ 700 °C. The value of n > 5 has been rationalized by considering the existence of threshold stress, which like n has been found to decrease sharply with temperature beyond 700 °C. Investigation of the post-creep microstructures by transmission electron microscope has revealed the formation of γ′ and M23C6 precipitates, which obstruct dislocation motion during the primary stage. Microstructures of the samples creep-tested at ≥ 750 °C have exhibited relatively smaller amounts of γ′. The existence of threshold stress and the steady-state regime in the tertiary creep stage has been ascribed to obstruction of dislocation motion by M23C6 and γ′ precipitates.

Creep deformation and failure properties of 316 L stainless steel manufactured by laser powder bed fusion under multiaxial loading conditions

316 L stainless steel has long been used in high temperature applications. As a well-established laser powder bed fusion (LPBF) alloy, there are opportunities to utilise additive manufacturing in such applications. However, the creep behaviour of LPBF 316 L under multiaxial stress conditions must first be quantified before such opportunities are realised. Uniaxial and double notched bar creep tests have been performed and characterised using power-law relations to evaluate the creep strain and rupture properties of LPBF 316 L. The creep response was found to be anisotropic with specimen build orientation, with samples loaded perpendicular to the build direction (Horizontal) exhibiting 8 times faster minimum creep rates than samples built parallel to the build direction (Vertical) and significantly shorter rupture lives. This was mainly attributed to the columnar grain structure, which was aligned with the build direction of the LPBF samples. The multiaxial creep rupture controlling stress was determined and found to be a combination of the equivalent and max. principal stress. X-Ray CT measurements in selected samples illustrated that the samples were approximately 99.6% dense post-build and the quantity of damage post testing was determined. Optical and EBSD microstructural characterisation revealed intergranular creep damage present in the specimens, however rupture was ultimately trans-granular in nature and influenced by the presence and orientation of pre-existing processing defects relative to the sample build and loading direction.

WITHDRAWN: Influence of processing routes on the creep deformation and damage evolution of Zr-2.5Nb alloy

Abstract Tensile creep behavior of the Zr-2.5Nb alloy tubes having outer diameters of 90 mm and 110 mm, processed by extrusion and forging, respectively has been studied in the temperature range of 350-450°C under the stress range of 82-319 MPa. The creep curves have shown that the minimum creep rate and time to rupture changes with the applied stress and temperature. The apparent activation energy of creep ( Q C ) values, have been determined by using the power law for 90 mm and 110 mm tube samples are found to be ∼ 231 kJ/mol and ∼ 264 kJ/mol respectively, which is significantly higher than that for lattice self-diffusion of Zr (113 kJ/mol). The microstructural examination of the creep-ruptured samples through transmission electron microscopy (TEM) has shown evidence for multiplication of dislocations, along with their interaction with the β-(Nb, Zr) precipitates in the α-Zr matrix, which is mainly contributed to origin of threshold stress during creep. By incorporating the threshold stress data in the modified power-law relation, the true activation energy of creep ( Q t ) values from the samples of 90 mm and 110 mm tubes are found as 118 kJ/mol and 132 kJ/mol, respectively, which are close to that for lattice self-diffusion in Zr. The obtained true stress exponent ( n t ) values of 3.6 and 4.9 and formation of dislocations sub-grains in the power-law regime of steady state creep, which suggests that the dislocation climb is the rate-controlling mechanism. Further, creep data of the alloy are validated by the Monkman-Grant and Modified Monkman-Grant relationship, and time to rupture effectively predicted using the Larson-Miller Parameter. The creep damage tolerance factors are in the range of 1.5-2.5 for the tubes suggest cavity growth mechanism. Additionally, the comparison of creep data with the Ashby-Dyson maps indicates early formation of cavities due to creep.

The effect of Y and Nd additions on the microstructure and creep behavior of AZ80

The effect of yttrium (0.2Y wt.%) and yttrium + neodyminum (0.1Y+0.1Nd wt.%) additions on the long-term (>1200 h) tensile creep and microstructural evolution of AZ80 was investigated at temperatures ranging between 100 °C and 150 °C and stresses ranging between 50 MPa and 100 MPa. The alloying resulted in both the formation of Y- and/or Nd containing particles, and lower tensile strengths and elongation-to-failure. The minimum creep rate (ε˙min) increased with increasing applied creep temperature and stress. Y addition resulted in greater creep-resistance compared with that for (Y + Nd) additions. The ε˙min of AZ80 + 0.2Y was approximately half that of AZ80 at 50 MPa at 150 °C. The apparent creep activation energy for each alloy ranged between 73 and 93 kJ/mol, and pipe-diffusion-assisted dislocation creep was suggested to be the dominant secondary stage creep deformation mechanism. Discontinuous precipitation (DP) occurred during the creep exposure but was impeded by precipitate-containing twins. The DP was less concentrated in the Y and (Y + Nd) containing alloys, which exhibited lower Vickers hardness values than AZ80. Grain boundary and twin boundary cavitation/cracking were prevalent during creep. Minor Y or (Y + Nd) addition, resulted in less oxidation and extended the intermediate-temperature creep life.

The effect of η phase precipitates on the creep behavior of alloy 263 and variants

Phase stability is an important design parameter for Ni-based superalloys to be used in future advanced ultra-supercritical (AUSC) power plants as exposure times in this type of environment are considerable. In this investigation, microstructures based on candidate alloy 263 were obtained with varying amounts of η precipitates using isothermal exposure at 800 °C for times ranging from 1000 h to 10,000 h. The effect of η phase stability on the creep properties was determined using creep specimens isothermally aged at 800 °C for 8 h, 3000 h, 5000 h and 10,000 h prior to creep screening. The creep life was found to exponentially decrease with increasing density of η phase while the elongation to failure was found to increase. Furthermore, the minimum creep rate was related to the density of η phase; a relationship that did not depend on the alloy formulation. Modification to the Ti and Al concentrations slowed down the γ′ to η transformation while doubling the γ′ fraction after standard heat treatment. By modifying the Ti and Al content, and thereby improving γ′ stability over η, the creep lives of specimens isothermally aged for up to 5000 h were greater than that of the nominal alloy in its standard aged condition.

Modelling the creep behavior of an AlSi10Mg alloy produced by additive manufacturing

AlSiMg alloys produced by additive manufacturing possess an extremely fine and complex microstructure that in many ways defies the most widely used phenomenological models, which, in fact, have turned out to be poorly suited for predicting their mechanical properties. The underlying rationale for the peculiar properties of these alloys has been qualitatively established, however the need for a constitutive model with better predictive capability is still strong. To this aim, the ultra-fine microstructure was described by using a model-material (MM) consisting of soft and hard zones deforming under a similar strain rate. A physically-based set of constitutive equations which took into account also the coarsening/ripening phenomena of the Si-particles was used to predict the creep behavior of the MM. In parallel, the creep response of an AlSi10Mg alloy produced by additive manufaturing and tested in the as-deposited condition was investigated at temperatures ranging from 150 to 225 °C. The minimum creep rate curves obtained for the MM by the constitutive model were then compared with the experimental data obtained by testing the real alloy under constant load in different initial states. The excellent correlation between model curves and experimental results was discussed, taking into account the evolution of the microstructure during creep.

Processing 4th generation titanium aluminides via electron beam based additive manufacturing – characterization of microstructure and mechanical properties

Lightweight intermetallic γ-titanium aluminides (TiAl) based alloys are used for high temperature applications in advanced aerospace and automotive engines. Normally, these alloys are processed via investment casting as well as wrought processing, e.g. hot-forging. The feasibility of processing γ-TiAl via electron beam powder bed fusion (PBF-EB/M) is already demonstrated for various titanium aluminides. Due to the high reactivity of titanium aluminides, electron beam powder bed fusion is a promising manufacturing technique which provides dense components with only small variations in the aluminum (Al) content when using accurately optimized process parameters. However, the processing and machining of γ-TiAl based alloys is challenging due to their high reactivity as well as low ductility and fracture toughness at room temperature (RT). The present paper is focusing on the characterization of a 4th generation γ-TiAl based alloy with the nominal composition Ti–47.5Al–5.5Nb–0.5 W (at.%), customized for additive manufacturing process. The high amount of aluminum ensures a peritectic solidification and improves the room temperature ductility. Niobium and Tungsten provide sufficient high temperature strength and creep resistance. However, the processability of a novel alloy via PBF-EB/M is challenging. Therefore, accurately optimized PBF-EB/M parameters are used for the processing of this 4th generation γ-TiAl powder in order to avoid defects and inhomogeneous microstructures. The subsequent material characterization is based on numerous cylindrical specimens additively manufactured in 0°, 45°, and 90° orientation. Micrographs reveal a non-directional NLγ+βo microstructure consisting of lamellar α2/γ colonies, globular γ, and βo-phase. It is shown, that the microstructure possesses no directional dependence for any specimen. Tensile tests are performed at RT, 300 °C and 850 °C with samples in PBF-EB/M state and after hot isostatic pressing. The tensile tests show a room temperature ductility of up to 2.1% and high temperature strength around 380 MPa. Additionally, a minimum creep rate of 1.0 × 10−8 s−1 can be derived from the creep tests at 750 °C with 150 MPa load.

Creep behaviour, creep damage and precipitation in the austenitic cast steel HK30 at 750 °C

The austenitic cast steel HK30, used in exhaust manifolds of heavy-duty truck engines, was studied with respect to creep, creep damage, oxidation and precipitation at 750 °C in air. Even though creep clearly affects the lifetime of the manifolds, there are very few studies available on the topic. The results of the present study provide a basis for materials selection as well as creep data input to component lifetime simulations.In the present study, five specimens were subjected to constant loads resulting in creep rupture times between 1 and 94 days. The minimum creep rate follows Norton's creep law with a Norton parameter (n) of 8.9 and the logarithmic creep rupture time increases linearly with reduced stress. At low stresses secondary creep is predominant observing oxide intrusions from the specimen surface, precipitation of sigma and G-phases, creep cavitation at grain boundaries and weak dislocation networks. At high stresses direct transitions from primary to tertiary creep are observed as well as stronger dislocation networks, but no oxide intrusions, no precipitation, or cavitation are observed. The material has grains of cm size and intergranular fracture is predominant at all stress levels.The segregation of elements during solidification was studied by Scheil/Gulliver simulations and precipitations of sigma and G-phases in the last solidified regions were predicted using Thermo-Calc.