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Ni-based model alloys with a base composition of Ni-20 mass pct Cr-3 mass pct Mo that were precipitation strengthened by the γ′ phase were studied in regards to their failure mechanisms as part of the fundamental research for achieving a creep rupture strength of 100 MPa at 1023 K (750 °C) and 105 hours. The microstructure, which was interrupted by transient creep, as well as the minimum creep rate and accelerated creep at 1123 K (850 °C) and 80 MPa was observed. The microstructure around the grain boundaries was altered remarkably with strain-induced grain boundary migration, while the γ′ particle size increased linearly inside the grains with increasing temperature and time. Furthermore, the volume fraction of the γ′ phase and the amount of precipitation on the grain boundary were associated with the size of the precipitate-free zone (PFZ), which is a major factor in creep damage. The appropriate precipitations inside the grains and at the grain boundaries were very effective for suppressing PFZ. Consequently, the creep properties can be improved by controlling PFZ in the proximity of grain boundaries for a superior balance of creep strength and ductility.

Creep rupture life and microstructural degradation have been studied in two heats of Gr.91 steels. The coarsening of subgrains and precipitates, mainly M23C6 and MX, has been evaluated during static aging and creep. Hardness of head (static aging) and gauge (creep) portions of crept samples were measured to know their relation with microstructural degradation during long-term exposure. The correlation between subgrain size and spacing of precipitates and hardness has been equated. As an example, there is a close correlation between hardness value and inverse subgrains size in Gr.91 steels, regardless of aging or creep conditions. The appearance of three recovery mechanisms was found during long-term creep, namely: strain-induced recovery, pure static recovery and strain-assisted static recovery. By equated correlations between subgrain size, precipitates and hardness, the contribution of three recovery mechanisms to subgrain coarsening and hardness drop were calculated for two creep conditions at 700 °C in long-term creep region, where breakdown of creep strength has happen. The calculated data accord well with experimental data obtained from aged and crept samples. The contribution of static recovery to the subgrain coarsening and consequent hardness drop during long-term creep increases with increasing creep time. The significant contribution of both static recovery mechanisms can result in the breakdown of creep strength in long-term creep region.

Strain-induced coarsening of precipitates, mainly M23C6, has been studied in several types of Gr. 122, Gr. 92 and Gr. 91 steels in a wide range of creep conditions. Strain-induced coarsening of precipitates should be separately evaluated in high stress and short-term creep region (or called “H”), where precipitates and subgrains are thermally stable, and low stress in long-term creep region (or called “L”), where thermal coarsening of precipitates and subgrains appear. Creep plastic deformation does not accelerate the coarsening of precipitates during the primary and secondary creep regions in short-term creep region “H”. However, the coarsening of M23C6 precipitates occurs during the acceleration creep region and increases with decreasing of applied stress. The onset of long-term region “L” is accompanied by the thermal coarsening of M23C6 precipitates as well as strain-induced coarsening of M23C6 precipitates during secondary and acceleration creep regions and maybe in primary creep region under very low creep stress. In long-term region “L”, recovery of the dislocation substructure is controlled simultaneously by creep plastic deformation and pinning force from precipitates. On the other hand, the coarsening of precipitates is due to both thermal coarsening and strain-induced coarsening of precipitates. In fact, the stability of precipitates is the most important factor for the suppression of subgrain recovery in long-term region “L” rather than short-term creep region “H”. In long-term region “L”, greater thermal recovery of subgrains and M23C6 precipitates due to higher creep temperature or/and longer creep rupture life results in greater strain-induced coarsening of M23C6 precipitates.

There are two creep regions with different creep characteristics: short-term creep region “H”, where precipitates and subgrains are thermally stable, and long-term creep region “L”, where thermal coarsening of precipitates and subgrains appear. In region “H”, the normalized subgrain size ( λ - λ 0 ) / ( λ ∗ - λ 0 ) has a linear relation with creep strain and its slope is 10 e −1 . But, region L is the time range in which the static recovery and the strain-induced recovery progress simultaneously. In this region, the static recovery accelerates the strain-induced recovery, and subgrain size is larger than that line which neglects the contribution of the static recovery. In region “L”, the Δ λ / Δ λ ∗ -strain present a linear relation with a slope 35 e −1 . There is a linear relation between hardness and subgrain size. Hardness drop, H 0 − H , as a function of Larson–Miller parameter can be a good measure method for assessment of hardness drop and consequently degradation of microstructure. Hardness drop shows an identical slope in creep region “H”, whereas hardness drop due to thermal aging and creep in region “L” show together a similar slope. In region “H”, degradation of microstructure is mainly due to recovery of subgrains controlled by creep plastic deformation, and precipitates do not have a major role. However, in creep region “L”, there are three degradation mechanisms that accelerate creep failure; (1) strain-induced recovery of subgrains due to creep plastic deformation, (2) static-recovery of subgrains and precipitates and (3) strain-induced coarsening of precipitates due to the appearance of static-recovery.

Creep behavior and degradation of subgrain structures and precipitates of Gr. 122 type xCr-2W-0.4Mo-1Cu-VNb (x = 5, 7, 9, 10.5, and 12 pct) steels were evaluated during short-term and long-term static aging and creep with regard to the Cr content of steel. Creep rupture life increased from 5 to 12 pct Cr in the short-term creep region, whereas in the long-term creep region, it increased up to 9 pct Cr and then decreased with the addition of Cr from 9 to 12 pct. Behavior of creep rupture life was attributed to the size of elongated subgrains. In the short-term creep region, subgrain size decreased from 5 to 12 pct Cr, corresponding to the longer creep strength. However, in the long-term creep region after 104 hours, subgrain size increased up to 9 pct Cr and then decreased from 9 to 12 pct, corresponding to the behavior of creep rupture life. M23C6 and MX precipitates had the highest number fraction among all of the precipitates present in the studied steels. Cr concentration dependence of spacing of M23C6 and MX precipitates exhibited a V-like shape during short-term as well as long-term aging at 923 K (650 °C), and the minimum spacing of precipitates belonged to 9 pct Cr steel, corresponding to the lowest recovery speed of subgrain structures. In the short-term creep region, subgrain coarsening during creep was controlled by strain and proceeded slower with the addition of Cr, whereas in long-term creep region, subgrain coarsening was controlled by the stability of precipitates rather than due to the creep plastic deformation and took place faster from 9 to 12 pct and 9 to 5 pct Cr. However, M23C6 precipitates played a more important role than MX precipitates in the control of subgrain coarsening, and there was a closer correlation between spacing of M23C6 precipitates and subgrain size during static aging and long-term creep region.

The cause of the breakdown of creep strength has been studied in Gr.91 steel. The results show that the contribution of the static recovery of subgrains to creep deformation causes the breakdown of creep strength. The subgrain boundaries are mainly stabilized by M23C6 and MX precipitates. MX precipitates are thermally stable even in the time range when coarsening of subgrains takes place. Whereas M23C6 precipitates are not thermally stable and the aggregation of M23C6 precipitates takes place in the time range when coarsening of subgrains happens. Therefore, loss of pinning force from M23C6 precipitates is responsible for the static recovery of subgrains. MX has nothing to do with the static recovery. The microstructural stability of Gr.91 steel in the time range longer than 105 h at 600 °C has also been assessed based on the creep data of Gr.91 steel tested up to 90,408 h at 600 °C and 22,900 h at 650 °C. The aggregation of precipitates and the coarsening of subgrains start to take place at round 105 h at 600 °C. Therefore, the breakdown of creep strength is expected to happen in the time range a little longer than 105 h at 600 °C.

Creep rupture data and microstructural degradation during aging of high Cr ferritic boiler steels with enhanced creep strength have been studied with special attention to prediction of long term creep rupture life. Tempered lath martensite structure in the high Cr ferritic steels remains unchanged during short term aging, whereas static recovery of the lath martensite structure proceeds when diffusion distance during aging becomes sufficiently long as is the case in long term creep. The static recovery brings about premature failure in long term creep and decreases in apparent activation energy for creep life. The decrease in activation energy is responsible for overestimation of rupture life reported in strength enhanced high Cr ferritic steels. The boundary from a short term region with high activation energy QH to a long term region with low activation energy QL moves towards longer time with decreasing Cr concentration. The difference in activation energy (QH − QL) primarily determines the extent of overestimation of rupture life predicted from short term data. In general, the extent of overestimation is less serious at 9%Cr as compared to 12%Cr.

Creep rupture life in the time range of 105 h and longer has been evaluated on the basis of creep rupture data available on modified 9Cr–1Mo (Gr.91) steel. The steel is strengthened by a tempered martensite lath structure, which degrades by means of strain induced recovery in short-term creep (Region H). The degradation leads to creep rupture. In long-term creep (Region L), static recovery of the martensite lath structure takes place in addition to the strain induced recovery, resulting in premature creep rupture and the consequent decrease in stress exponent n for rupture life. The premature creep rupture always occurs in Gr.91 steel, but the activation energy QL for rupture life in region L is usually close to the value QH in Region H in the steel. In such a case, long-term rupture life can be predicted correctly by simple extrapolation of short-term data. In some heats of Gr.91 steel, however, another region with low values of n and Q, namely Region L2, appears in the time range longer than 104 h. Simple extrapolation of short-term data results in overestimation of rupture life in Region L2. The static recovery and the consequent premature creep rupture are controlled by migration of sub-boundaries stabilized by precipitates. M23C6 as well as MX precipitates in Gr.91 steel have higher thermal stability than those in other high Cr ferritic steels, resulting in the later appearance or absence of Regions L and L2 in the steel.

Long-term creep-rupture strength of low C–2.25Cr–1.6W–V–Nb steel (HCM2S; KA-STBA/PA24J1, T23/P23, ASME CC2199) has been studied and reassessed using the oxidation correction method after Nakashiro et al. Their long-term creep strengths are found to be very stable at temperatures between 500 and 600 °C. A severe oxidation of the creep-rupture specimens has been identified at high temperatures above 600 °C and the longer term rupture time tends to decline below the extrapolated curve. An oxidation correction has been found to be effective to evaluate the long-term creep-strength stability of the steel and worked well in the case of 2.25Cr–1Mo steel (ASME T22/P22). It is thus concluded that in low C–2.25Cr–1.6W–V–Nb steel no creep-strength degradation apart from that due to oxidation at high temperatures is expected, in contrast to results on advanced high Cr steels.

Long-term creep rupture strengths and the microstructural stability of ASME P92 and P122 pipes have been studied using creep testing at the temperatures from 550 to 700 °C and detailed scanning transmission electron microscopy. Creep rupture strength of P92 is found to be more stable than that of P122 at temperatures over 600 °C, which is mainly due to the difference in their Cr content. P122 type model steel with reduced Cr content, 9%Cr, has been prepared to explore the effect of Cr on the stability of MX and formation of Z-phase during creep deformation. MX in 9%Cr steel is found to be stable even after prolonged exposure at 650 °C, while Cr and Fe concentration to MX without marked coarsening has been observed in 10.5%Cr steel after aging for 10,000 h at 650 °C. This seems to lead to the transition of MX carbonitride into the Z-phase after aging for 23,000 h, which requires ordering in a M 2 N lattice to achieve a tetragonal Z-phase to be stable. Creep deformation behavior in the transient creep region of the steels is almost same up to about 7000 h, while in the acceleration creep region the creep rate of 10.5%Cr steel becomes much faster than that of 9%Cr steel, resulting in shorter rupture life. It is obvious that the creep rupture strength degradation starts prior to the formation of Z-phase in 10.5%Cr steel. It is thus concluded that Z-phase is not a necessary factor for degradation of creep rupture strength but the instability of the fine precipitates such as Cr 2 (C, N) caused by the compositions change like Cr supply to MX carbonitride is more essential.

Long-term creep rupture life is often evaluated from short-term data by a time–temperature parameter (TTP) method. However the conventional TTP methods sometimes fail in understanding creep rupture behavior of strength enhanced high Cr ferritic steels and overestimate creep rupture life in long-term creep. In the present paper, creep rupture data of seven kinds of heat resistant steels with different W and Cr concentrations have been analyzed. The conventional TTP method like Orr–Sherby–Dorn analysis evaluates long-term creep rupture life assuming a unique value of activation energy for all the creep rupture data. This analysis is called single region analysis in this paper. The single region analysis can represent well the creep rupture data of steels containing less than 8% Cr. The creep rupture analyses of steels containing more than 8% Cr exhibit that apparent activation energy changes from a high value in short-term creep region to a low value in long-term creep region. In each case a creep data was divided into several data sets, and then the conventional single region analysis was applied to each divided data set. This analysis is referred to as multiregion analysis. The multiregion analysis describes very well all the data points, whereas regression curves of the single region analysis deviate from the data points, resulting in overestimation of long-term rupture life. The difference between the two activation energies of short-term and long-term creep increases with increasing Cr concentration. Therefore, the overestimation due to singles region analysis is expected to be more serious at higher Cr concentration.

Creep deformation mechanism of the steels with a different matrix, α (ferrite), α’ (martensite) and γ (austenite), and precipitates such as MX and M23C6 has been analyzed using positron annihilation lifetime measurement. The positron annihilation lifetime has been found to be a very useful tool for evaluating the characteristic creep damage of the steels with different microstructure and the corresponding microstructural evolution during creep deformation. The creep deformation process of the α steel is heterogeneous, while the α’+M23C6 steel exhibits gradual changes in the creep rate in both transient and acceleration creep regions with the largest off-set strain, implying the homogeneous creep deformation. The α’+M23C6+ MX steel is in between the α and α’+M23C6 steels. The homogeneous creep deformation takes place in the γ steel.

The apparent activation energy for rupture life sometimes changes from a high value of short term creep to a low value of long term creep. This change results in overestimation of rupture life recognized recently in advanced high Cr ferritic steels. The present study examined how to detect the decrease of activation energy in 9-12 %Cr steels with tempered martensitic lath microstructure. During aging without stress hardness of the tempered martensite microstructures remains almost constant in short term, whereas it decreases with increasing time after long term exposure. The onset of hardness drop can be a good measure of the decrease of activation energy. Causes of the hardness drop and the decrease of activation energy are discussed.

Recent researches have shown the premature breakdown of creep rupture strength in long term creep region of advanced high Cr ferritic steels. As safe operation of power plants becomes a serious problem we should be able to detect and predict the breakdown transition of creep rupture strength. Some methods for detecting the breakdown transition have been presented till now like the measurement of reduction of area after creep rupture and particle size of laves phase. However it will be more economic if we make use of non-destructive tests, for example, hardness testing. In this paper 3 types of ferritic steels with different Cr concentration have been studied. The results suggest that the hardness of aged structures is constant independently of exposure time in short term region, whereas the hardness breaks down in long term region. The boundary of breakdown in hardness coincides with that of breakdown in creep rupture strength.

Creep rupture and deformation behavior of T122-type steels with different matrix phases such as α′ (martensite) and α ′ + δ (martensite and delta-ferrite) at different stress levels has been studied using creep testing and a detailed FE-STEM microscopy. Long-term creep rupture strength of the dual phase steel is found to be lower than that expected from the short-term creep testing. Fine grain microstructure enhances the creep deformation of the dual phase steel at lower stress region, but is not the major factor on the degradation. Decrease in fine MX and unequal distribution of MX in the ferrite matrix are found to be the major causes of the strength degradation in the dual phase steel with the higher Cr content. It is thus concluded that the heterogeneous creep deformation is much more pronounced at lower stress level in the dual phase steel, which is due to inhomogeneous microstructure consisting of α ′ + δ phase matrix and the relevant heterogeneous distribution of fine precipitates such as MX and M23C6 in the δ-ferrite matrix and near the interface between δ / γ .

A new attempt has been demonstrated using carbon free Fe–Ni–Co martensitic alloys strengthened by Laves phase, such as Fe 2 W or Fe 2 Mo, and μ phase, such as Fe 7 W 6 or Fe 7 Mo 3 , to achieve homogeneous creep deformation at high temperatures under low stress levels. Creep behavior of the alloys is found to be completely different from that of the conventional high-Cr ferritic steels. The alloys exhibit gradual change in the creep rate with strain both in the transient and acceleration creep regions, and give a larger strain for the minimum creep rate. In these alloys the creep deformation takes place very homogeneously and no heterogeneous creep deformation is enhanced even at low stress levels. The fine precipitates are found to be stable even at 973 K and effectively decrease the minimum creep rate much lower than that of the conventional high-Cr ferritic steels. Moreover, superior creep resistance is achieved by addition of Cr to the alloy.

A phenomenological calculation of phase equilibria by combining the cluster variation method (CVM) with the Lennard-Jones-type (L-J-type) pairwise atomic interaction energies has been attempted for the Fe-Pd-Ni ternary system in order to examine the possibility of Ni as the substituting element for Pd. The calculated Ni content in an Fe-Pd-based L10 ordered phase at 863 K shows fairly good agreement with the experimental analysis performed for Fe-5Pd-5Ni alloy by energy-dispersive X-ray spectroscopy of a transmission electron microscope. The detailed calculation indicates that Ni is preferentially substituted in a Pd site in the L10-ordered phase, which can be interpreted by the Lennard-Jones-type pairwise potentials employed for the present study. It is concluded from the thermodynamic point of view that Ni is a promising candidate as the substituting element for Pd in an Fe-Pd-based L10 ordered phase.

The steam oxidation behavior of 9Cr ferritic steel containing Pd was studied in comparison with that of modified 9Cr-1Mo steel (P91). It was found that the oxidation of 9Cr steel containing 3 mass% Pd, the surface of which was finished with 320-grit silicon carbide (SiC) paper, was suppressed by the formation of a protective oxide layer when it was exposed to steam at 650°C. This experimental result is unusual in high-Cr ferritic steels, including mod.9Cr-1Mo. A similar suppression of oxidation was also observed in specimens of mod.9Cr-1Mo steel that were coated with Pd or Pt. When the steel containing Pd was exposed to steam at 650°C for 50 h, the thickness of the oxide layer became ∼0.3 μm. Analyses of the oxide layer by thin film x-ray diffraction, transmission electron microscopy (TEM), and electron diffraction suggest that the protective layer was made of (Fe, Cr)2O3 with a rhombohedral structure. A supply of Cr from the metal matrix because of the residual strain in the surface region of sp...

Tungsten strengthened ferritic steels, 2.25Cr-1.6W-V-Nb and 12Cr-0.4Mo-2W-V-Nb-Cu have been developed and approved by the ASME Boiler and Pressure Vessels Code Committee for use in Section I construction, designated as T23 and T122, respectively. A field exposure test installing both steel tubes in service along with comparative materials in the tertiary superheater and secondary reheater of a 156MW utility power boiler has been conducted since April 1993. The tubes were removed to confirm their material properties and corrosion/steam oxidation behaviors after 1-year, 3-year, 6-year, and 10-year periods of service. The tensile and creep rupture strengths of both steels showed no remarkable change during service. Examination of steam oxidation scale on the inner surface of the tubes indicated that the scale growth rate of T122 was extremely small following 1-year service. The growth rate and morphology of steam oxidation scale is discussed, as compared with conventional materials.

In order to seek inexpensive substituting elements for Pd in an FePd based L10 ordered phase, a phenomenological calculation based on the cluster variation method combined with a Lennard-Jones type pair-wise atomic interaction energies has been attempted for a FeNi binary system. The phase equilibria for FeNi3-disorder and FeNi-disorder are investigated. In particular, a tetragonal distortion is explicitly considered for the FeNi L10 ordered phase. The calculated FeNi3-disorder phase boundary shows good agreements with an experimental result. An FeNi L10 ordered phase which does not appear in a conventional FeNi binary phase diagram is predicted as a stable phase, implying that Ni is a potential substituting element for Pd in an FePd based L10 ordered phase.

Lennard-Jones type pair-wise atomic interaction energies for FeFe, FePd and PdPd are derived from extrapolated thermodynamic and structural data of pure elements and an ordered compound. In order to estimate the values at zero Kelvin, extrapolation was performed with the aid of thermodynamic database. Also, additional measurement of ordering energy was attempted by Differential Scanning Calorimetry to determine the L1 0 -disorder transition temperature. The resultant pair interactions are combined with the Cluster Variation Method and the concentration dependency of heats of formation at 1565K was calculated. In addition, the phase boundary for L1 0 -disorder was obtained. Fairly good agreements with experimental results are achieved.

For the construction of ultra super critical (USC) power plant, 9Cr-3W base ferritic heat-resistant steels with relatively high B and no N have been investigated. Authors have been revealed in the previous report that the addition of 139ppm B significantly improves creep strength of the steels, whereas most of added B forms unidentified borides, which are deemed almost ineffective to creep strength. The effect of improved heat treatment on creep strength and distribution of B in precipitates is investigated to effectively utilize and decrease added B. As a result of the analysis of the extracted residue and characterization of precipitates using field emission Auger electron spectroscopy (FE-AES), most of added B still forms borides in the 92ppm B added steel. These composites are almost dissolved and the B content in M 23 C 6 carbides is significantly increased by normalizing at 1423K. It is also found by FE-AES analysis that B content in M 23 C 6 carbides near prior-austenite grain boundaries is relatively higher than that inside grains. Creep strength at 923K for the 92ppm B added steel normalized at 1423K is not improved at short times, but it is remarkably improved to almost the same level as the 139ppm B added steel at long times. This excellent creep strength is achieved resulting in improving microstructural stability through the effective utilization of added B by high-temperature normalizing.

Microstructure of C-free martensitic alloys at elevated temperatures was studied for understanding the origin of their superior creep properties over 923 K. These alloys initially have martensitic structure and the heterogeneous creep deformation seems to be suppressed even under low stress region in distinction from the conventional heat resisting steels because there is no rapid growth of carbides during creep at high temperatures. Inter-metallic compounds such as Laves or μ phase were dominantly observed over 923 K and these precipitation had been followed by the low temperature reaction that greatly contributed to the typical aging behavior like in the usual maraging steels. It was found that the density of the precipitation was still kept high even at 973 K because its gathering and coalescence rate was small comparing with that of high Cr steels. Furthermore, the reversely transformed austenite phase certainly appeared by the shearing mechanism in these alloys when the materials were rapidly heated up to the temperature for creep testing, because their As points were measured to be at around 873 K. This austenite phase represented terrace like feature in SEM observations. Fortunately, this characteristic structure was resulted from a selective electropolishing in specimen preparation due to the compositional fluctuation. However, it was also confirmed that partitioning of alloying element was not completed in the short-term aged materials. It was concluded that the characteristic microstructure of these alloys over 923 K was continuously formed as follows. Firstly, uniform fine precipitation of inter-metallic compounds occurred at early stage and then the reversely transformed austenite was broken out without diffusion process. Finally, dissolution of precipitation in the retained martensitic region was assisted by long-term keeping at high temperatures. Consequently, it was expected that specific creep resistance of these alloys over 923 K be achieved by existence of large amount of austenitic phase strengthened by fine precipitation.

Precipitation strengthening is important to improve creep strength of heat resisting steels at elevated temperatures. Especially, in the high-Cr ferritic steels recently developed for Ultra Super Critical Power Plants, precipitation behavior is complicated and should be clarified because the correlation between various strengthening factors are still not well understood. In this study, we focus on the change in precipitation behavior of MX type carbonitrides in model steels with different heat treatment conditions. It is found that dominant precipitation behavior has been changed with cooling rate from normalization temperature before tempering and better creep properties have been obtained in the steel quenched rather than in the one air-cooled. It is confirmed that precipitation of M 23 C 6 is suppressed and total fraction of MX is relatively increased by the quenching procedure. It is thought that better creep resistance is mainly due to fine distribution of MX that is achieved by suppressing typical co-precipitation of MX such as Nb(C, N)-VN during cooling after normalization and tempering.

Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels has been studied using model steels with the initial microstructures consisting of the various combination of a, MX and M 23 C 6 to clarify the role of each precipitate on creep deformation and the resultant creep strength of the steels. A carbon free steel strengthened by the a phase has exhibited high creep resistance at the initial stage of transient creep region but a quick transition to the acceleration creep at a small strain, due to less precipitation along boundaries such as lath, block, packet and the prior austenite grain boundary. Precipitation of M 23 C 6 along these boundaries has been found to delay the transition to the acceleration creep, which gives a larger offset creep strain. Fine dispersion of MX has reduced much the creep rate in the transient creep region but enhances the heterogeneous creep deformation in the acceleration creep region. It is thus concluded that an optimum microstructure consists of fine dispersion of the a phase and MX carbonitride inside lath grain decorated with M 23 C 6 along lath, block, packet and the prior austenite grain boundary.

Static recovery of tempered lath martensite microstructures of strength enhanced ferritic steels has been investigated during very long-term aging up to 2 × 104 h at 650 °C for 3 steels containing 9 to 12% Cr. Static recovery of tempered lath martensite microstructure occurs after an incubation period for 1–2 × 103 h in the steels. The static recovery is controlled by the loss of strengthening due to M23C6 precipitates, and disappearance of MX carbonitrides cannot be the main cause of the static recovery.

The effect of trace elements such as Mn and B on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb has been investigated from the standpoints of a long-term microstructural stability. The chemical analysis of extracted residues and TEM observation show that the M23C6 carbide and/or M7C3 carbides, precipitated by tempering are replaced by M6C with a concentration of W; i.e., the amount of W in solution reduces during creep. On the other hand, MX type carbides such as VC and NbC are very stable during long-term aging and contribute to the creep strength by obstructing the dislocation annihilation. One of the most significant results is that a reduction in Mn-content lowers the minimum creep rate, resulting in an increase in the creep rupture time. The nucleation and/or growth of M6C are retarded with reducing Mn-content, thereby the increase in dissolved W seems to enhance the resistance to creep deformation. Another significant result is that an increase in B-content has delayed the transition from the primary creep to a tertiary creep stage. The principal roles of B are stabilizing M23C6 or M23(C, B)6 on former austenite grain boundaries and retarding the dynamic recrystallization during creep. In addition, in the specimens with higher amount of B, the bainite lath interface is covered by MX and some filmy precipitates with high density, thereby the softening resistance is enhanced.

Both of the newly developed tungsten strengthened ferritic steels considered here, i.e., 2.25Cr-1.6W V Nb (HCM2S) steel and 12Cr-0.4Mo-2W-1Cu-V-Nb (HCM12A) steel, have the practical capability to be used for steam generator tubing due to their excellent fabricability and very high creep strength, These steels have already been approved by the ASME Boiler and Pressure Vessel Code Committee for use in Section I construction. A field service test putting these steel tubes in service along with comparative materials in the tertiary superheater and secondary reheater of a 156 MW utility power boiler has been conducted since April 1993, and the tubes were removed to confirm their material properties and corrosion/steam oxidation behaviors after one-year and 3 year periods of service. The investigation results showed that the practical performances of HCM2S and HCM12A steel tubes were good, demonstrating superior corrosion resistance and less material degradation than the comparative materials. This paper deals with the practical properties and examination results of these newly developed steels compared with conventional boiler steels.

Summary This paper describes an investigation of the effects of tungsten on the precipitation behaviour of intermetallic compounds in the HAZ of duplex stainless steel and the mechanism of these effects to clarify the advantage of having some Mo replaced by W. Fine precipitates of sigma phase are observed in electron micrographs of the simulated HAZ of a 25% Cr-7% Ni-0.3% N sample steel containing 4% Mo, whereas fine precipitates of the chi phase are observed in that containing 3% Mo and 2% W. The addition of tungsten delays sigma phase precipitation in steel aged at 850–950 °C × 600 sec. The sigma phase and austenite phase are identified in steel containing less than 8% W aged at 800–900°C, whereas the sigma, chi, and austenite phases are detected in steel containing more than 12% W. Based on these results and the chemistry data of the respective phases determined with an electron probe microanalyser (EPMA), the phase diagram at 900°C is experimentally derived. The chi phase is precipitated with a smalle...

High temperature deformation in Ni-(11-l2.5)Al-l0Cr-0.005B(mass%) with β(B2-ordered)+γ'L12-ordered)+disordered γ triple phase alloys has been studied in order to clarify their potentiality for high temperature structural applications, comparing with γ'/γ, β/γ' dual phase and β single phase alloys. The β/γ'/γ alloy has been found to consist of coarse β precipitates surrounded by y' phase and a mixture of γ'/γ dual phase and primary γ' single phase matrix. The β/γ'/γ alloy with 12.2 % Al shows high compressive strength at high temperatures, which is equivalent to that of IC218(Ni3Al based commercial alloy with γ'/γ dual phase), and excellent ductility, about 9% in elongation at 1373K. Compressive creep resistance of the β/'γ'/γ alloys is much higher than that of the β/y' dual phase and the β single phase alloys. The stress exponents of the minimum creep rate according to Dwn equation for the β/γ'/γ (12.2Al) and β/'γ' (16Al) alloys are determined to be 5.1 and 3.1, respectively. It is, therefore, considered that the creep rates of the β/γ'/γ alloys are controlled by dislocation climb, while those of the β/'y' alloys are controlled by viscous motions of dislocations. The activation energy for creep of the β/'γ'/'γ alloy (12.2Al) is estimated as 513kJ/mol, which is much higher than that for Ni diffusion in Ni-Al system Detailed TEM observation has shown that the y'/γ interface contributes to creep strength and the β/'γ' interface enhances ductility. It is, thus, concluded that the high temperature strength with enough hot workability can be obtained for the β/γ'/'γ alloys.

In order to clarify the role of deformation in hydrogen cracking, effect of ordering (disordered fcc → NiCr-type ordered phase) on tensile properties in air and with hydrogen charging has been investigated by using a single crystalline Ni-Cr alloy (70 % Ni30 % Cr), grown by an optical floating-zone furnace and aged for from 0h to 1000h at 773K The hydrogen cracks have been found to initiate at the intersection of slip lines and to propagate along (001)-plane when deformed in tension along with hydrogen charging in the disordered condition. The mode of fracture, however, has been changed by ordering and the hydrogen cracks have initiated and propagated preferentially along {111}-plane. TEM observation has shown that, in the ordered specimens, there are two types of dislocations, superdislocation triplet and ordinary single dislocation which is enhanced by hydrogen charging. The interaction of these dislocations has led to anisotropic deformation identified with twinning deformation and pile-ups of dislocation on {111}-plane. It is concluded that the anisotropic deformation plays a major role in the fracture along {111}-plane in Ni2Cr ordered alloys.

Summary This paper describes an investigation of the effect of tungsten on the HAZ properties of duplex stainless steel in a programme to develop highly pitting corrosion resistant duplex stainless steel. The steels tested were modified SUS329J4L duplex stainless steel containing added tungsten. The welded joints produced by gas tungsten arc (GTA or TIG) welding and the simulated HAZ were evaluated in terms of the pitting corrosion resistance, impact toughness, and microstructure, including an analysis of precipitates. The results suggest that tungsten added up to 2 mass% is effective for improvement of the pitting corrosion resistance without heavy loss of impact toughness in the HAZ. Tungsten addition exceeding 3 mass%, however, decreases both the HAZ impact toughness and pitting corrosion resistance by accelerating the precipitation of intermetallic compounds during the welding thermal cycles. It is thus demonstrated that the pitting corrosion resistance in the HAZ of duplex stainless steel is improved...

The effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy (alloy C-276) has been investigated by means of tensile tests in air and with hydrogen-charging in 1N-H2SO4 solution. The annealed specimen has exhibited intergranular fracture by hydrogen-charging, resulting in a marked reduction in tensile elongation and ultimate tensile strength. The mode of fracture was changed by aging at 773 K, and the transgranular fracture has been found to be dominant in the aged specimens. The susceptibility to hydrogen embrittlement, as identified by the test method used in this study, seems to be reduced by short-term aging, though it turns out to be increased again by further aging. The fractured boundaries have been characterized using electron channeling pattern (ECP) analysis of adjacent grains. It is found that the misorientation of grain boundaries plays an important role in fracture, and ∑3 boundaries, twin boundaries in a face-centered cubic (fcc) lattice, are most likely to fracture in the aged specimens. Transmission electron microscopy (TEM) observation has shown that a short-range ordering reaction from a disordered fcc lattice into an ordered Ni2(Cr, Mo) (Pt2Mo type) super-lattice takes place by aging, and hence, superdislocation triplets with APB (antiphase boundary) become predominant when deformed. It is also seen that in the aged specimens, deformation twinning is another mode of deformation, and this leads to the transgranular fracture at twin boundaries by hydrogen-charging. These results suggest that a change in the mode of deformation after aging plays a major role in fracture due to hydrogen embrittlement as a consequence of the heterogeneous interaction between slip dislocations and twin boundaries.

Creep-fatigue properties of Fe-Ni base 0.08C-23Cr-45Ni-7W alloy (HR6W) are investigated at 700°C and compared with Ni base superalloy Alloy617 in order to examine applicability of these materials to piping in 700°C A-USC power plants. It is clarified that the fatigue lives of HR6W and Alloy617 are almost the same under PP (fast-fast) type strain waveform, but the life of HR6W is longer than that of Alloy617 under CP (slow-fast) type strain waveform. The crack initiation lives at the ligament parts of superheater outlet headers, which are made of HR6W and Alloy617, are predicted from the results of three-dimensional FE-analyses. The transient heat transfer analysis and the elastic-plastic-creep thermal stress analysis are conducted on several kinds of typical operating conditions to evaluate the inelastic strain caused by the thermal cycles during start-up and shut-down of power plants. The maximum steam temperature and pressure are selected as 700°C and 34.3MPa, respectively, and the maximum cycle time of operation is 1 year. On the basis of the analytical results, creep-fatigue and creep rupture properties of the materials, the crack initiation lives of the headers are predicted with the strain range partitioning method. Inelastic strain in the header of HR6W is larger than that of Alloy617, because HR6W has lower creep deformation resistance and a slightly higher thermal expansion coefficient. However the predicted life of the header of HR6W is longer than that of Alloy617, because HR6W is much superior to Alloy617 in creep ductility and creep-fatigue properties.Copyright © 2007 by ASME

Hydrogen environment embrittlement (HEE) susceptibility in high pressure gaseous hydrogen was investigated on 300 series austenitic stainless steels and A6061-T6 aluminum alloy. Tensile properties of these materials were evaluated by Slow Strain Rate Testing (SSRT) in gaseous hydrogen pressurized up to 90MPa (13053 psig) in the temperature range from −40 to 85 degrees C (−40 to 185 degrees F). HEE susceptibilities of austenitic stainless steels strongly depended upon the chemical compositions and testing temperatures. A6061-T6 aluminum alloy showed no degradation by hydrogen. Fatigue properties in high pressure gaseous hydrogen were evaluated by the external cyclic pressurization test using tubular specimens. The tubular specimen was filled with high pressure hydrogen gas, and the outside of the specimen was cyclically pressurized with water. Type 304 showed a decrease in the fatigue life in hydrogen gas, while as for type 316L and A6061-T6 the difference of the fatigue life between hydrogen and argon environments was small. HEE susceptibility of investigated materials was discussed based on the stability of an austenitic structure.Copyright © 2007 by ASME

A service exposure test of advanced austenitic alloys was performed at Eddystone Power Station Unit 1. The alloys tested were TP347HFG, SUPER304H and HR3C as well as 17-14CuMo as a reference material. They were installed in the final super-heater section in 1991 and were removed in 2004. The service exposure duration is 75,075 h with the steam temperature and pressure estimated 615 °C and 35 MPa, respectively. Post exposure examination has proved no marked hot-corrosion and steam oxidation have occurred for the TP347HFG, SUPER304H and HR3C specimens, while severe hot-corrosion and exfoliation of steam-oxide films was pronounced in 17-14CuMo specimens. It is also found that HR3C exhibits the highest tensile strength with a lowest ductility and TP347HFG does vice versa, while the strength of SUPER304H is equivalent to that of 17-14CuMo with much better ductility. Short-term creep rupture strengths of these alloys have been reduced to some extent after the long-term service exposure and a longer-term creep rupture testing has been in progress. Microstructural evolution of the TP347HFG, SUPER304H and HR3C as well as 17-14CuMo during the long-term service exposure have been analyzed using a detailed TEM examination and the chemical analysis of extraction residue of the exposed specimens.Copyright © 2007 by ASME

Six new grade austenitic stainless steel tubes including SUPER304H® (C.C. 2328), TP347HFG, NF709® , TEMPALOY A-3® , HR3C® (TP310HCbN) and TEMPALOY CR30A® were selected and installed in the superheater of the Eddystone No.1 boiler for the purpose of extending tube service life by reducing fire-side and steam-side corrosion. No significant corrosion was observed on the outer surfaces at 75,075h of exposure. Precipitates due to exposure were identified by TEM-EDS analysis. The thickness of steam oxidation scale was less in high-Cr materials, and high Cr content was thus demonstrated to have effectively improved steam oxidation resistance. HR3C showed comparatively poor impact properties, while the other materials exhibited acceptable impact values of over 30J/cm2 at 20°C. All materials showed reduced creep strength compared with unexposed materials after service exposure under high stress test conditions. However the differences became smaller at lower stresses.Copyright © 2007 by ASME

Tungsten strengthened ferritic steels, 2.25Cr-1.6W-V-Nb and 12Cr-0.4Mo-2W-V-Nb-Cu have been developed and approved by the ASME Boiler and Pressure Vessels Code Committee for use in Section I construction, designated as T23 and T122 respectively. A field exposure test installing both steel tubes in service along with comparative materials in the tertiary superheater and secondary reheater of a 156MW utility power boiler has been conducted since April 1993. The tubes were removed to confirm their material properties and corrosion/steam oxidation behaviors after one-year, 3-year, 6-year and 10-year periods of service. The tensile and creep rupture strengths of both steels showed no remarkable change during service. Examination of steam oxidation scale on the inner surface of the tubes indicated that the scale growth rate of T122 was extremely small following one-year service. The growth rate and morphology of steam oxidation scale is discussed, as compared with conventional materials.Copyright © 2004 by ASME

The electronic structures of Al2O3 thin film are calculated within the framework of the local density approximation using the full-potential-linearized augmented-plane-wave method, for comparison to those of the bulk. The band gap of Al2O3 thin film is about 2.6 eV, which is much smaller than that of the bulk (~6.6 eV). It is found that Al2O3 thin film is not a good insulator.