Low Creep Ductility Research Articles

Effect of solid-solution treatment on high-temperature properties and creep fracture mechanism of laser powder bed fused Inconel 625 alloy

Laser powder bed fusion (LPBF) has been widely used for fabricating Inconel 625 (IN625) alloy, and it is widely regarded as a promising manufacturing technology. However, during high-temperature service, the γ″ strengthening phase of LPBFed IN625 gradually transforms into the detrimental δ phase. Controlling the morphology and distribution of precipitates is necessary for optimizing high-temperature performance. This study compared the as-built and 1090 °C solution-treated LPBFed IN625 alloy and investigated the microstructure evolution, tensile properties, and creep fracture behavior at a high temperature of 815 °C. The results show that the solid-solution treatment improves the creep ductility of LPBFed IN625 alloys at 815 °C by forming a high percentage of twinned boundaries and eliminating dislocations. The formation of coarsened needle-like δ phases and carbides during creep reduces the strength of the grain boundaries and is responsible for creep fracture failure. During creep of the as-built samples, the irregular Laves phase between the columnar dendrites gradually transforms into plate-like δ phases and M6C carbides uniformly and densely distributed within the columnar grains. The precipitation-strengthening effect of these fine precipitates, combined with the initial high density of dislocations and strong texturing effect, brings favorable high-temperature strength and creep fracture life but consequently leads to low creep ductility.

Effect of Grain Size on Mechanical and Creep Rupture Properties of 253 MA Austenitic Stainless Steel

The effect of grain size on the mechanical properties and creep rupture of 253 microalloyed (MA) austenitic stainless steel (ASS) was investigated. The cold rolling process with a 53% reduction in thickness was applied to the steel followed by annealing at 1100 °C over 0, 900, 1800, and 3600 s to obtain grain sizes of 32.4, 34.88, 40.35, and 43.77 µm, respectively. Uniaxial tensile and micro-Vickers hardness tests were carried out to study the effect of grain size on mechanical properties at room temperature. The creep rupture test was performed at 700 °C under a load of 150 MPa. The results showed that there was a correlation between grain size, mechanical properties, and creep rupture time. The fine initial grain size showed relatively good mechanical properties with a short creep rupture time, while the coarse initial grain size produced low mechanical properties with a long creep rupture time. The initial grain size of 40.35 µm was the optimum grain size for a high value of creep rupture time due to the low hardness and elongation values at room temperature and low creep ductility value. The intergranular fracture was found on the initial grain size below 40.35 µm, and a mixed mode of intergranular and transgranular fracture was found on the initial grain size above 40.35 µm after the creep rupture test.

Microstructural characterisation of creep tested advanced austenitic stainless steel super 304H

ABSTRACT Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples.

Effect of applied stress on creep properties of Sanicro 25 welded joint made with a composition-matched weld filler metal

Sanicro 25 is a candidate material for reheaters and superheaters used in 650–700 °C advanced ultra-supercritical power plants. A composition-matched weld filler metal has been developed to join Sanicro 25 tubes through tungsten inert gas welding. Creep tests of the Sanicro 25 welded joint were conducted at 973 K under a stress of 180–240 MPa. The creep lifetime decreased with an increase in applied stress. The welded joint crept at 180 MPa fracture at the base metal (BM) in a mixed fracture type, and voids were the main damage form in the BM, which had low ductility in the welded joint. Creep fractures occurred in the weld metal (WM) in an intergranular manner at 200–240 MPa, with wedge cracks representing the main damage form and a lower creep ductility in WM. The distribution and strengthening mechanism of the nano-MX and nano-Cu phases in the WM of joint were close to those of pure BM after creep. The growth of carbides in the WM was restrained by element B during the 180 MPa creep test, which caused increasingly serious carbide coarsening in the BM. The amount of low misorientation angle boundaries in the WM following the creep substantially increased. Besides, the creep deformation of the welded joint at 180 MPa is dominated by BM, and the change in the weld grain shape is small. For tests under 200–240 MPa, the creep deformation of the welded joint was dominated by the weld, and the shape of the weld grain changed significantly.

Mechanical properties of T23 steel welded joints without post-weld heat treatment for fossil fired boilers

Abstract

Anisotropy in creep properties of DS200 + Hf alloy

The directionally solidified Ni-based superalloy DS200 + Hf is used in aero-engines and industrial gas turbines for the manufacturing of intermediate and low-pressure turbine blades. This material consists of ~<0 0 1> columnar grains oriented along the solidification direction and with random secondary orientations. In this study, the creep behaviour of the DS200 + Hf alloy has been investigated along longitudinal and transverse directions. The main objective was to investigate the durability of this alloy and the damage mechanisms for four different crystalline microstructures: DS200 + Hf composed of fine grains loaded longitudinally, the same alloy composed of coarse grains or fine grains, loaded transversally to the solidification direction and finally, the single crystal version, Mar-M200 + Hf, loaded along a <0 0 1> crystallographic orientation. Creep tests were performed from 750°C up to 1100°C. Under all conditions, a pronounced decrease in the strain to failure is observed for specimens loaded along the transverse direction compared to specimens tested along longitudinal direction or single crystals. Fracture surface observations revealed that the lower creep ductility along such a loading direction results from an intergranular failure mode. Furthermore, the creep life anisotropy significantly decreases when temperature increases. Additional creep experiments under vacuum at 900°C and experiments using single crystalline specimens far away from the <0 0 1> crystallographic orientation show a prominent damaging role of oxidation at very high temperature and a major contribution of the local crystallography at low temperatures. In addition, the grain size effect observed when testing in the transverse direction only results from the grain boundary oxidation.

Effects of creep ductility and notch constraint on creep fracture behavior in notched bar specimens

The creep rupture life of U-type notched specimens and smooth specimens has been calculated based on the ductility exhaustion damage model using stress-dependent creep ductility. Effects of creep ductility and notch constraint on creep fracture behaviour in notched bar specimens have been investigated. The results show that the U-type notch exhibits notch strengthening effect under a wide range of stress level and notch constraint condition (notch acuity) for creep ductile materials. The lower equivalent stress in notched specimens plays main role for reducing creep damage and increasing rupture life. The rupture life of notched specimens of creep brittle materials (with lower creep ductility) decreases with the increase in stress level and notch constraint. With increasing creep ductility and decreasing notch constraint, the degree of the notch strengthening effect increases. In creep life designs and assessments of high-temperature components containing notches, the material creep ductility, notch constraint and stress levels need to be fully considered.

Creep, low cycle fatigue and creep-fatigue properties of a modified HR3C

Creep, low cycle fatigue (LCF) and creep fatigue tests have been conducted for modified HR3C (25Cr20NiNbN) at high temperatures ranging of 650-750 °C. Both LCF and creep fatigue test results could be described with the Coffin-Manson relationship. The number of cycles to failure in the creep fatigue tests was more than one order of magnitude lower compared with LCF. The effect of the total hold time in tension (the total creep time) was compared to creep rupture data. The creep fatigue results were in reasonable agreement with the creep tests. The short creep fatigue lives may be due to the low creep ductility which was found in the creep tests. Fractography showed that the rupture mode was intergranular. Cavities were observed at grain boundaries due to the fracture of the primary Z phase particles in both LCF and creep fatigue tests. In comparison to Sanicro 25, the modified HR3C showed better LCF properties.

Influence of phosphorus on the creep ductility of copper

Around 1990 it was discovered that pure copper could have extra low creep ductility in the temperature interval 180–250°C. The material was intended for use in canisters for nuclear waste disposal. Although extra low creep ductility was not observed much below 180°C and the temperature in the canister will never exceed 100°C, it was feared that the creep ductility could reach low values at lower temperatures after long term exposure. If 50ppm phosphorus was added to the copper the low creep ductility disappeared. A creep cavitation model is presented that can quantitatively describe the cavitation behaviour in uniaxial and multiaxial creep tests as well as the observed creep ductility for copper with and without phosphorus. A so-called double ledge model has been introduced that demonstrates why the nucleation rate of creep cavities is often proportional to the creep rate. The phosphorus agglomerates at the grain boundaries and limits their local deformation and thereby reduces the formation and growth of cavities. This explains why extra low creep ductility does not occur in phosphorus alloyed copper.

A Microstructural Sensitivity Study of 316H Austenitic Stainless Steel to Inter-Granular Creep Fracture

A preliminary sensitivity examination of the ductility exhaustion based creep damage prediction model, currently used in the R5 high temperature assessment procedure, showed that material property inputs had significant effects on damage prediction. In the present work, the link between the microstructural factors and the susceptibility to inter-granular high temperature creep failure is considered. The latter was judged to be associated with the low creep ductility. Here, the longitudinal section of a creep specimen and the fracture surface were examined. Auger electron spectroscopy was used to investigate the grain boundary composition in this specimen, which failed after a creep test of 1038h at 550°C under a triaxial stress state. The present results demonstrate that there is a possibility to correlate the susceptibility to high temperature inter-granular fracture from the low temperature fracture investigations. Finally, the susceptibility of the pre-treated 316H stainless steel to inter-granular high temperature failure and the contribution to the creep damage model are briefly discussed.

Estimation of Creep Crack Growth Properties Using Circumferential Notched Round Bar Specimen for 12CrWCoB Rotor Steel

Most heat resisting materials in structural components are used under multi-axial stress conditions and under such conditions ductile materials often exhibit brittle manner and low creep ductility at elevated temperature. Creep crack initiation and growth properties are also affected by multi-axial stress and it is important to evaluate these effects when laboratory data are applied to structural components. Creep crack growth tests using circumferential notched round bar specimens are a simple method to investigate multi-axial stress effects without using complicated test facilities. Creep crack growth tests have been performed using a 12CrWCoB turbine rotor steel. In order to investigate the effects of multi-axial stress on creep crack growth properties, the tests were conducted for various notch depths at 650°C. The circumferential notched round bar specimen showed brittle crack growth behaviour under multi-axial stress conditions. Creep crack growth rate was characterized in terms of the C* parameter. A 12CrWCoB turbine rotor steel has been tested using circumferential notched round bar specimens with different multi-axiality. Circumferential notched round bar specimens show increased brittle creep crack growth behaviour due to the multi-axial stress condition. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multi-axial conditions.

Influence of Extra Coarse Grains on the Creep Properties of 9 Percent CrMoV (P91) Steel Weldment

With modern welding methods, satisfactory microstructures in 9%CrMoV (P91) steel can be obtained with a modest variation in hardness and prior austenite grain size. However, there is always a risk that significant deviations in the properties can be obtained, if the welding parameters are not optimized. In the present paper the role of extra coarse grains in the heat affected zone (HAZ) has been studied. Creep tests were carried out at 600°C for parent metal, weld metal, cross weld, simulated extra coarse grained HAZ, and simulated intercritical HAZ of a 9%CrMoV (P91) steel. The parent metal, the cross welds, the weld metal, and the simulated intercritical HAZ had about the same rupture strength except at long rupture times, where the values for the cross welds were considerably lower. In the cross welds, rupture took place in the intercritical HAZ at longer times (Type IV cracking). The simulated extra coarse grains gave considerably longer rupture times, lower strain rate and lower creep ductility than the parent metal and the weld metal. The creep strain behavior was successfully analyzed using the Omega model where the log creep strain rate is linear in the creep strain.

The θ projection method and low creep ductility materials

The θ projection method of creep analysis produces the poorest projections of creep properties at low strains. This paper describes a modification to the basic curve equation which eliminates this problem. The detailed equations are derived together with the parameter estimation procedures and the new method is shown to give very considerably better descriptions of creep curves. The implications with respect to the projection of properties are discussed.

High-strain-rate deformation of pure aluminum reinforced with 25% alumina submicron particles near the solidus temperature

Recently, the mechanical properties at room and elevated temperatures of dispersion-strengthened-cast aluminum (DSC-Al) with high volume fractions (>25 vol.%) of submicron Al{sub 2}O{sub 3} oxide dispersoids have been investigated. The material has characteristics of both Al-MMCs (e.g., high modulus, strength, and low coefficient of thermal expansion) and conventional PM oxide-dispersion-strengthened aluminum with lower reinforcement content (e.g., high creep resistance). While the room-temperature ductility of DSC-Al is quite high (9.4% for extruded DSC-Al with 25 vol.% Al{sub 2}O{sub 3} oxide particles), its formability at elevated temperatures is limited by its high creep strength and low creep ductility. Because of the similarity of microstructure between DSC-Al and other Al-MMCs, the question arises whether DSC-Al containing 25% Al{sub 2}O{sub 3} particles can exhibit high-strain-rate superplasticity near the solidus temperature. The purpose of the present note is to investigate this issue.

Low temperature creep ductility of OFHC copper

Abstract Creep tests have been carried out on oxygen-free high conductivity (OFHC) copper between 75 and 245°C. Two of the batches exhibited poor ductility at temperatures between 180 and 245°C and ruptured at creep strains of

Superplastic Foaming of Titanium and Ti-6Al-4V

ABSTRACTSolid-state foaming of metals can be achieved by hot-isostatic pressing of powders in presence of argon followed by expansion of the resulting high-pressure argon bubbles at ambient pressure and elevated temperature. This foaming technique was first demonstrated by Kearns et al. [1] for Ti-6Al-4V, but is limited by its low creep rate and ductility, which lead to early cell wall fracture. We address these issues by performing the foaming step under superplastic conditions. Rather than using microstructural superplasticity (requiring fine grains which are difficult to achieve in porous powder-metallurgy materials), we used transformation superplasticity, which occurs at all grain sizes by biasing with a deviatoric stress (from the pore pressure) of internal stresses (from the allotropic mismatch during thermal cycling about the allotropic temperature range). As compared to control experiments performed under isothermal creep conditions, superplastic foaming under temperature cycling of unalloyed titanium and alloyed Ti-6Al-4V led to a significantly higher pore volume fraction and higher foaming rate.

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.

Approximate inelastic analysis of defective components

In the inelastic analysis of defective components at high temperature, the crack tip characterising parameters are of interest for a range of loading conditions. In this paper, four situations are examined: (1) steady state creep under constant load; (2) the transitional behaviour under constant load prior to attainment of steady state conditions; (3) behaviour under displacement control; and (4) behaviour under displacement-controlled cycling with hold periods. For steady state creep, it is shown by comparison with finite-element solutions and experimental data that a reference stress approximation may be used to predict the parameter C ∗ with reasonable accuracy. The transitional period prior to steady-state conditions is examined by finite-element calculations, and the time-dependent parameter C(t) [C(t) → C ∗ as t → ∝] is evaluated for a number of geometries under constant load. The results are predicted by extending the steady state reference stress approximation. Comparisons are made between C( t) and the C t parameter of Saxena. Finite-element results are also reported for geometries creeping under a constant applied displacement. Under this loading, the parameter C( t) falls rapidly with time. The reference stress method is used to predict the rate at which the applied loading falls, and then to estimate the value of C( t). It is shown that the amount of crack growth associated with displacement-controlled loading is much less than under load control at the initial load. A simplified estimate is made of the total crack growth during a period of constant displacement, and compared with the experimental data for steels of both high and low creep ductility. Under displacement-controlled cycling, finite-element calculations involving hold periods would be extremely time consuming. It is shown that the simplified results developed for constant-displacement loading may also be applied to predict the experimental behaviour under cyclic loading provided the material creep data used in the estimates are obtained under relevant cyclically hardened or softened conditions.

Multiaxial creep of fine grained 0·5Cr–0·5Mo–0·25V and coarse grained 1Cr–0·5Mo steels

To explore the multiaxial creep response of materials used from electrical power generating plant, two steels, a fine grained 0·5Cr–0·5Mo–0·25V steel in a normalised and tempered condition with high creep ductility and a coarse grained 1Cr–0·5Mo steel in a quenched and tempered condition with low uniaxial creep ductility, have been selected. A range of multi axial stress testing techniques which span the stress states that would allow identification of any technique dependent variables has been used. The deformation andfailure of the normalised and tempered 0·5Cr–0·5Mo–0·25V steel for a range of multi axial test techniques and, therefore, stress states may be described by an equivalent stress criterion. The results from the multiaxial tests carried out on the fully bainitic 1Cr–0·5 Mo steel show that the multiaxial stress rupture criterion (MSRC) varies with stress state, at high triaxialit y (notch) , it is controlled by the maximum prinicipal stress, whereas, at low triaxiality (shear), it is dependent on both maximum principal stress and equivalent stress. Furthermore, a simple description of stress state based on maximum principal and equivalent stress does not define this uniquely, since the MSRC derived from uniaxial and torsion testing does not describe the failure of notch, tube, or double shear tests.MST/1390

Influence of Weld Factors on Creep-Rupture Cracking at Elevated Temperature

The purpose of this paper is to identify the weld effects which are of primary importance in elevated temperature design. A full-scale Fast Flux Test Facility (FFTF) Intermediate Heat Exchanger (IHX) was tested at Westinghouse to investigate weld effects at elevated temperature. The IHX was subjected to two and a half times the design pressure. In addition, one of the four welded nozzles of the IHX was subjected to 26 severe thermal downshock transients, which were interspersed with 156 hr of creep hold time at 1100°F (593°C). At the end of testing, creep rupture cracks were observed in the weldments at the nozzle to cylinder intersections, whether or not they experienced downshock transients. Detailed three-dimensional inelastic analyses were performed to investigate the effects of welding on the creep-rupture strength of weldments. The analyses suggest that the weldment material property variation contributed to creep-rupture cracking at high primary pressure loading. The weld metal and heat-affected zone had higher yield strength, but lower creep ductility compared to the nozzle base material. The analytical predictions and metallurgical observations suggest that the role of residual stresses on creep-rupture cracking is of secondary importance, and need not be numerically simulated in the elevated temperature design of weldments.