Search Results for solution heat treatment

To maximize the mechanical properties of Ni-base superalloys, solution heat treatment is essential to sufficiently homogenize the dendritic segregations formed during solidification. To investigate the homogenization behavior during solution heat treatment, a Ni-base single crystal superalloy, TMS-238, was heat treated under various conditions; temperatures ranging from 1573 to 1613 K for times ranging from 2 to 100 h. After solution heat treatment, the average concentrations of Re, an element that exhibits the highest degree of segregation, in dendrite core and inter-dendritic regions were analyzed. From these results, apparent diffusion constants, D app , were determined based on a proposed homogenization model. Obtained D app values were significantly smaller than the diffusion constant of Re in Ni, strongly suggesting that the apparent diffusion coefficients should be obtained experimentally when using the target alloy.

The broken elbow of the final superheater tube (ASME SA213 TP304H) from a coal-fired power plant was evaluated. The root causes were identified by metallographic observation, sensitization evaluation, hardness measurement, and EBSD analysis. The analysis results reached the following conclusions. (1) The tube bending was not performed in accordance with ASME Code requirements—a solid-solution heat treatment was not performed after cold working. (2) The hardness at the elbow is greater than 260 HV, exceeding the ASME code limit. (3) The sensitization was 19%, showing a performance degradation. (4) There are no obvious corrosion elements in the oxide layers of the cracks. (5) Metallographic microstructure analysis shows that there are many intergranular cracks and carbides such as Cr-rich phase and Fe-Cr are precipitated at the grain boundaries, ultimately resulting in strain-induced precipitation hardening damage.

The creep strength and ductility of Grade P22 steel (2¼ Cr) was measured at 600°C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitization temperatures ranging from 900°C - 1200°C followed by normalization at 900°C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitization temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600°C increased as function of PAG size up to approximately 70µm, but significantly decreased when the average prior austenite grain size measured approximately 108 µm. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.

The effect of grain size after solution treatment on the mechanical properties of FENIX-700, including its cooling rate, was investigated. In addition, the dependance of precipitation observed at grain boundaries on the heat treatment conditions was also discussed on the basis of the results of microstructure observations. It was confirmed that the tensile ductility, the creep rupture ductility, and the absorbed energy decreased as the grain size increased. The creep rupture strength, in contrast, increased remarkably as the grain size increased. The tensile strength increased as the cooling rate increased. Experimental results showed that satisfactory mechanical properties would be obtained for a grain size of ASTM G.S.No. 1.0-3.0.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

The behavior of strain-induced abnormal grain growth (AGG) in superalloy 718 has been investigated using compression testing and subsequent heat treatment below the d-phase solvus temperature of 980 °C. The nuclei of AGG grains were slightly newly recrystallized grains by a nucleation because small grains without dislocation was observed in the as- deformed microstructure. AGG was caused by the difference in intragranular misorientation (related to the stored strain energy in a grain) between dynamic recrystallized grains and deformed matrix. The initiation of AGG was retarded with decreasing plastic strain and produced microstructures consisted of larger grains having more complex morphology. It was observed that grain boundary migrated locally in the direction perpendicular to, or mainly in the direction parallel to the S3 {111} twin boundaries along with the formation of high-order twins. As a result of multiple twinning, AGG grains seemed to evolve with the growing directions changed.

A Ni-based superalloy named "TOS1X-2" has been developed as a material for A-USC turbine rotors. TOS1X-2 is based on Inconel Alloy 617 and has a modified chemical composition to achieve the higher strength needed for over 700°C-class A-USCs. Aging heat treatment conditions were determined from the mechanical properties and microstructure. We manufactured an actual-scale rotor model made of TOS1X-2. A 31 ton ingot was manufactured, followed by forging of the model rotor with a diameter of 1100 mm and length of 2400 mm without any defects. Metallurgical and mechanical analyses of the model rotor were carried out. All metallurgical and mechanical features of the TOS1X-2 rotor model satisfied the requirements for not only 700°C-class but also over 700°C-class A-USC turbine rotor.

Delivered condition of Inconel740H specified in ASME Code Case 2702 is solution heat treated and aged condition, fabricating performances are also based on the condition, and a re-annealing and aging shall be performed if cold forming to strains is over 5%. These almost harsh requirements bring great inconvenience for its engineering practice and utilization. The comparative bending tests on 740H tubes in solution heat treated + aged condition and solution heat treated condition were performed, and the rules’ reasonability of the specification on delivered condition was discussed and analyzed from point view of deformability and weldability in the paper. The bending test results showed that tube bent was difficult because of its high strength and limited deforming capacity in solution heat treated + aged condition. Therefore, the material supplied in the solution condition may be better from fabricating points. Whether re-solution for the bent tube is performed after bending depends on its bending radius, followed by welding and post weld heat treatment of component (this treatment can also be the aging treatment for annealed sector at the same time), this treating manner can meet regulatory requirements. For solution tubes, however, there are some inconveniences to its engineering application because fewer research studies were carried out on its properties up to now, and no regulations on them were given for the material in the specification. Suggestions are: 1) deeply investigating the properties of tubes in solution condition, including mechanical and fabricating performances, 2) adding the mechanical properties, maximum allowable cold forming to stain without performing re-solution and weld strength reduction factor of solution material to the code case.

For the safe operation of high temperature equipment, it is necessary to ensure creep rupture ductility of the components from the viewpoint of notch weakening. In this study, the effect of heat treatment conditions on creep rupture ductility was evaluated and its underlying metallurgical mechanism was investigated with using a forged Ni-based superalloy Udimet520. In order to improve the creep rupture ductility without lowering the creep rupture strength, it is important to increase both intragranular strength and intergranular strength in a balanced manner. For this purpose, it was clarified that 1) secondary γ' phase within grains should be kept fine and dense, 2) grain boundaries should be sufficiently covered by M 23 C 6 carbide by increasing its phase fraction, and 3) tertiary γ' phase within grains should be redissolved before the start of creep. To obtain such a precipitate state, it is essential to appropriately select the cooling rate after solution treatment, stabilizing treatment and aging treatment conditions.

In order to reduce CO 2 emissions and improve power generation efficiency, a development project involving an advanced USC (A-USC) plant has been carried out in Japan since 2008. Nibased alloys are candidate materials for boiler components with high temperature steam conditions, which are much stronger than conventional heat resistant steel. However, Ni-based alloys have never been applied with respect to the high pressure parts and thick walled components of USC coal-fired power plants. In this study, therefore, fabrication and characteristic properties, such as weldability, the weld joint and bent part properties, and weld cracking susceptibilities of Ni-based alloys such as HR6W, HR35 and two types of Alloy617 (High B and Low B) pipes were evaluated. Additionally, two types of HR6W header mock-ups and a HR6W tube element mock-up were fabricated. With the exception of Alloy617 (High B), the fabrication trials of Ni-based alloy pipes were conducted successfully, and the long-term creep strength of weldments and bends of Ni-based alloy pipes were found to be nearly equivalent to those of base metal. In the case of Alloy617 (High B), hot cracking was observed.

18Cr-9Ni-3Cu-Nb-N steel is widely used for heat exchanger tubes such as super-heaters and reheaters of ultra-super critical power generation boilers. In this study, long-term creep rupture tests were carried out on 18Cr-9Ni-3Cu-Nb-N seamless steel tubes of 7 heat materials, and the specimens of 2 heat materials with different creep rupture strengths were observed by ultra-low voltage scanning electron microscope after creep rupture tests. The results of the investigation of the creep rupture specimens and the coverage ratios of M 23 C 6 on grain boundary were different. The cause of this was estimated to be the difference in B content between the 2 heat materials. Creep rupture tests with different final ST temperatures were also carried out using the same heat material, and it was revealed that the higher final ST temperature, the higher the creep rupture strength. As the final ST temperature is higher, the amount of Nb(C, N) solid solution in the matrix increases, and the amount of precipitation of NbCrN and M 23 C 6 increases during creep, therefore it is assumed that the creep rupture strength increases.

Gas turbine blades are operated in a high temperature and a high pressure. In order to cope with that harsh condition, the blades are made of Nickel based superalloys which show excellent performance in such environment. Manufacturers of the blades usually provide the standards for the blade inspection and replacement. According to their guide, the blades are replaced after 3 times of operations and 2 times of refurbishments. Howsoever, purchase the new blades is always costly and burdensome to the power plant owners hence, the assessment of the blade lifespan and the rejuvenation of the degraded blades are indeed crucial to them. In this study, the optimal rejuvenation conditions for gas turbine blades were derived and verified. In addition to that, the creep durability was evaluated based on the actual blade inspection interval. LCF tests have been carried out on the rejuvenated blade and the result was compared with the fatigue life of the new blades. In order to secure the safety of the rejuvenated blade during operation, a heat flow analysis was performed to simulate the operating conditions of the gas turbine during operation, and the main stress and strain areas were investigated through the analysis results. And then LCF and creep considering the actual operating conditions were evaluated. The calculated life of fatigue and creep life is compared to the hot gas path inspection interval. For the rejuvenated blades, the creep life and the LCF interval were reviewed based on the temperature, stress, and strain acquired by computational analysis. The creep life was calculated as 59,363 hours by LMP curve, and the LCF was calculated as 2,560 cycles by the Manson Coffin graph.

Austenitic and super-austenitic stainless steels are a critical component of the spectrum of high temperature materials. With respect to power generation, alloys such as Super 304H and NF709 span a gap of capability between ferritic and martensitic high chromium steels and nickel-based alloys in boiler tube applications for both conventionally fired boilers and heat-recovery steam generators (HRSG). This research explores a wrought version of a cast austenitic stainless steel, CF8C-Plus or HG10MNN, which offers promise in creep strength at relatively low cost. Various manufacturing techniques have been employed to explore the impact of wrought processing on nano-scale microstructure and ultimately performance, especially in high temperature creep. Transmission electron microscopy has been used to quantify and characterize the creep-strengthening particles examining the relationship between traditional melting and extrusion as compared to powder metallurgy.

Ni-38Cr-3.8Al has high hardness and high corrosion resistance with good hot workability, and therefore, it has been applied on various applications. However, in order to expand further application, it is important to understand the high temperature properties. Then, this study focused on the high temperature properties such as thermal phase stability, hardness, tensile property, creep property and hot corrosion resistance. As the result of studies, we found that the thermal phase stability of (γ/α-Cr) lamellar structure and the high temperature properties were strongly influenced by the temperature. Although the high temperature properties, except for creep property, of Ni-38Cr-3.8Al were superior to those of conventional Ni-based superalloys, the properties were dramatically degraded beyond 973 K. This is because the lamellar structure begins to collapse around 973 K due to the thermal stability of the lamellar structure. The hot corrosion resistance of Ni-38Cr-3.8Al was superior to that of conventional Ni-based superalloys, however, the advantage disappeared around 1073 K. These results indicate that Ni-38Cr-3.8Al is capable as a heat resistant material which is required the hot corrosion resistance rather than a heat resistant material with high strength at high temperature.

Inconel alloy 740/740H (ASME Code Case 2702) is an age-hardenable nickel-based alloy designed for advanced ultrasupercritical (A-USC) steam boiler components (superheaters, reheaters, piping, etc.). In this work, creep testing, beyond 40,000 hours was conducted a series of alloy 740 heats of varying product form, chemistry, and grain size. Long-term creep-rupture strength was found to be weakly dependent on grain size. Analysis of the time-to-rupture data was conducted to ensure long-term strength projections and development of ASME stress allowables. Testing was also conducted on welded joints in alloy 740 with different filler metal and heat-treatment combinations. This analysis shows the current weld strength reduction factor of 30% (Weld Strength Factor of 0.70) mandated by ASME Code Case 2702 is appropriate for 740 filler metal but other options exist to improve strength. Based on these results, it was found that alloy 740 has the highest strength and temperature capability of all the potential A-USC alloys available today.

A combination of creep tests, ex-service blade samples, thermodynamic equilibrium calculations, combined thermodynamic and kinetic calculations, image analysis, chemical composition mapping and heat treatments have been conducted on PWA1483 to determine if microstructural rejuvenation can be achieved when taking the presence of oxidation coatings into account as part of a blade refurbishment strategy. The work has shown that the γ′ morphology changes during creep testing, and that through subsequent heat treatments the γ′ microstructure can be altered to achieve a similar γ′ size and distribution to the original creep test starting condition. Thermodynamic equilibrium calculations have been shown to be helpful in determining the optimum temperatures to be used for the refurbishment heat treatments. The interaction of oxidation resistant coatings with the alloy substrate and refurbishment process have been explored with both experimental measurements and coupled thermodynamic and kinetic calculations. The predictive nature of the coupled thermodynamic and kinetic calculations was evaluated against an ex-service blade sample which had undergone refurbishment and further ageing. In general there was good agreement between the experimental observations and model predictions, and the modelling indicated that there were limited differences expected as a result of two different refurbishment methodologies. However, on closer inspection, there were some discrepancies occurring near the interface location between the coating and the base alloy. This comparison with experimental data provided an opportunity to refine the compositional predictions as a result of both processing methodologies and longer term exposure. The improved model has also been used to consider multiple processing cycles on a sample, and to evaluate the coating degradation between component service intervals and the consequences of rejuvenation of the blade with repeated engine exposure. The results from the experimental work and modelling studies potentially offer an assessment tool when considering a component for refurbishment.

The growth behavior of oxide scale in a laboratory steam environment has been conducted for the shot-peened 18Cr-8Ni stainless steels differing in grain size. Both steels (fine grained and coarse grained) have demonstrated almost the same steam oxidation behavior reacted at 700°C for up to 2000h, which had excellent oxidation resistance due to formation of a protective Cr 2 O 3 scale. After the exposure of 4000h, however, nodule-like oxide occurred on the coarse grained steel, while the fine grained steel still remained the uniform Cr 2 O 3 scale. These behaviors well explained in terms of changes of the outward Cr flux due to recovery and recrystallization of the deformed structure. This result has proven that the shot-peened tube composed of fine grain structure is capable of combat against the steam oxidation at high temperatures.

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime size and distribution and the degree of rafting has been examined in detail using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM) after high temperature degradation and rejuvenation heat treatments. The relationship between the microstructure, mechanical properties and the applied heat treatment procedures has been investigated. It is shown that there are significant differences in the rafting behaviour, the size of the ‘channels’ between the gamma prime particles, the degree of rafting and the size of the tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical segregation investigations were carried out to establish the cause of reduced mechanical properties of the rejuvenated sample after high temperature degradation compared to an as-received sample after the same degradation procedure. The results indicate that although the microstructure of as-received and rejuvenated samples were similar, the chemical segregation was more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements during rejuvenation was not completely eliminated. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications.

Haynes 282 is a great candidate to meet advanced ultra-super-critical (A-USC) steam conditions in modern coal-fired power plants. The standard 2-step aging treatment has been designed for optimizing microstructure therefore providing excellent mechanical properties. We studied an alternative, more economical, 1-step aging treatment and compared microstructure, tensile properties at 750˚C and deformation behavior. Moreover, three cooling rates from the solution temperature were studied to simulate large-scale components conditions. We found that as much as about 20% of fine spherical intragranular γ' particles were successfully precipitated in all cases. Their average size increased as the cooling rate decreased. All four heat-treated alloys exhibited good mechanical properties at 750˚C with a yield strength well over 620MPa. As expected, the yield strength increased and the ductility decreased as the average γ' size decreased. The alloys exhibited a mixed mode of deformation, though the dominant deformation mechanism depended on the different γ' characteristics. The major operative deformation mechanism could be well predicted by strength increment calculations based on the precipitation strengthening model. Our results suggest that wrought Haynes 282 produced by a more economical 1-step aging treatment may be a reliable candidate for high temperature applications under A-USC conditions.

Titanium alloys are expected to be used as heat-resisting structural materials in the airplane and automotive industries. In this study, the creep properties of near-α Ti alloys consisting of a lamellar microstructure were studied. Ti–8.5wt%Al–8.0wt%Zr–2wt%Mo–1wt%Nb–0.15wt%Si alloy (alloy code, TKT34) and an alloy with 0.1 wt% of added boron (alloy code, TKT35) were used in this study. An ingot was hot forged at a temperature of 1,403 K and hot rolled (caliberrolling) at a temperature of 1,273 K to a reduction rate of approximately 90%. It then underwent solution treatment in a β single-phase region followed by air cooling. Finally, it was subjected to aging treatment for 28.3 ks at a temperature of 863 K and then air-cooled. Two solution treatment conditions were applied: a time of 1.8 ks at a temperature of 1,323 K (high temperature/short time (HS)) and a time of 3.6 ks at a temperature of 1,223 K (low temperature/long time (LL)). The average grain size of the prior β grains showed a tendency of the solution treatment temperature being low and the boron-added alloys tending to be small. The length and thickness of the lamellar of these alloys shortened or thinned owing to the addition of boron and at a low solution treatment temperature. The creep tests were carried out at an applied stress of 137 MPa and a temperature of 923 K in air. The creep rupture life of these alloys was excellent, in order of TKT35 (LL) < TKT34 (LL) < TKT35 (HS) ≦ TKT34 (HS). Therefore, the creep rupture life of these alloys was shown to be superior under the HS solution treatment condition as compared to the LL solution treatment condition. However, the minimum or steady-state strain rate of these alloys became slower in order of TKT 35 (LL)> TKT34 (LL)> TKT34 (HS) ≧ TKT35 (HS). The creep properties depended on the microstructure of the alloys.