Search Results for creep-rupture test

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.

Welding processes and fabrication techniques have been studied in the development of Advanced USC boilers. Advanced 9Cr steels, Fe-Ni alloy (HR6W) and Nickel base alloys (HR35, Alloy 617, Alloy 263, Alloy 740 and Alloy 740H) have been selected as candidate materials for the boiler. The weld joints of these alloys were prepared from plates, small diameter tubes and large pipes, and welding procedure tests were performed. In this study, TIG and SMAW were applied. Both welding process produced good weld joints, and they showed good results in bending tests, tensile tests and the Charpy impact test. To select the annealing conditions for stress relief, stress relaxation tests and hardness tests were conducted on the weld joints after various heat treatments. The microstructure was also evaluated by SEM and TEM. Creep rupture tests are being performed for the weld joints with and without heat treatment. The maximum creep rupture tests are expected to take over 100,000 hours. In the study of fabrication techniques, hot bending tests by high frequency induction heating for large pipes and cold/hot bending tests for small diameter tubes were established. After the bending tests, mechanical property tests such as tensile tests, impact tests and creep rupture tests were conducted. The effect of pre-strain on creep strength was studied to take the creep test results after bending into consideration. The creep rupture test will be continued for specimens from weld joints and bending pipes to show their long term reliability.

Inconel alloy 740H is designated for boiler sueprheater/reheater tubes and main steam/header pipes application of advanced ultra-supercritical (A-USC) power plant at operating temperatures above 750°C. Microstructure evolution and precipitates stability in the samples of alloy 740H after creep-rupture test at 750°C, 800°C and 850°C were characterized in this paper by scanning electron microscopy, transmission electron microscopy and chemical phase analysis in details. The phase compositions of alloy 740H were also calculated by thermodynamic calculation. The research results indicate that the microstructure of this alloy keeps good thermal stability during creep-rupture test at 750°C, 800°C and 850°C. The precipitates are MC, M 23 C 6 and γ′ during creep-rupture test. The temperature of creep test has an important effect on the growth rate of γ′ phase. No harmful and brittle σ phase was found and also no γ′ to η transformation happened during creep. Thermodynamic calculations reveal almost all the major phases and their stable temperatures, fractions and compositions in the alloy. The calculated results of phase compositions are consistent with the results of chemical phase analysis. In brief, except of coarsening of γ′, Inconel alloy 740H maintains the very good structure stability at temperatures between 750°C and 850°C.

The influence of holding time during tempering on the long-term creep rupture strength of mod.9Cr-1Mo steel was investigated in this study, so as to elucidate proper heat treatment for boiler applications. Tempering was conducted at 770°C for 0.5h, 1h, 3h, 10h and 100h for the test materials, after re-normalization at 1050°C for 1h in all cases. Creep rupture tests were conducted at 600°C, and ruptured specimens were investigated to better understand the microstructural changes, including changes in the number density of precipitates, in order to observe and discuss their creep strength. All creep rupture test results for materials tempered within 10h exceeded the average creep strength of T91. Shorter tempering times such as 0.5h and 1h were clearly correlated with longer time to rupture at 600°C under 80MPa to 100MPa stress conditions. Reduction of area in creep-ruptured specimens decreased principally with lowered creep stress. Materials tempered for 0.5h and 100h showed the lowest reduction of area at 90MPa and 100MPa respectively, and their reduction of area recovered at lower than those stress levels. These stresses, showing minimum reduction of area, met inflection stress in the creep rupture strength curve.

This paper presents results and analyses from long-term creep-rupture testing of alloy CCA617 (also known as alloy 617B) in wrought and welded forms at temperatures and stresses relevant to power generation under advanced steam conditions. The refined controlled chemical composition of CCA617 resulted in increased creep-rupture strength compared to the conventional alloy 617 chemistry at applied stress levels of ~150 MPa and above. Long-term creep rupture testing of weldments (in one case, over 100,000 h) showed that their creep-rupture lives were dependent on the welding process. Gas-tungsten-arc and shielded metal-arc weldments of CCA617 performed nearly equivalent to standard alloy 617 base metals in creep, but there was some debit in creep-rupture resistance when compared to CCA617 base metal. Submerged arc welding produced weldments that were notably weaker than both versions of alloy 617 base metal under creep conditions, possibly due to lack of optimization of filler wire composition and flux.

As flux cored wires for gas metal arc welding offer several technical and economic advantages they are becoming more and more popular. Matching flux cored wires for welding P92 have already been available for several years. A matching flux cored wire for welding the Co-alloyed cast steel CB2, which is used for turbine and valve casings operating at steam temperatures of up to 620°C, was developed recently. To connect casings with P92 pipes, dissimilar welding of CB2 to P92 is necessary. This can be done with filler metal that matches either CB2 or P92. Pre-tests have confirmed that flux cored arc welding (FCAW) can generally be used for dissimilar joint welding of CB2 to P92. To evaluate creep rupture strength dissimilar welds were performed with filler metal matching CB2 and P92, respectively. TIG welding was used for the root and the second pass and FCAW for the intermediate and final passes. Cross-weld tensile tests, side bend tests and impact tests of weld metals and heat-affected zones were carried out at ambient temperatures after two post-weld heat treatments (PWHT), each at 730°C for 12 hours. Creep rupture tests of cross-weld samples were performed at 625°C. This study compares the results of the mechanical tests at ambient temperature and the creep rupture tests, and discusses why P92 filler metals are preferred for such welds.

Creep strength degradation behavior of normalized and tempered 2.25Cr-1.6W-V-Nb(Gr.23) steel was investigated by conducting extra long-term creep rupture tests. Creep strength drop was observed in long-term creep range at 600°C and above, while signs of a creep strength drop were not identified at 550°C and 575°C. Creep strength drop of around 110 MPa at 600°C was caused not by the effect of oxidation but rather by a change of the deformation mechanism or the weakening of deformation resistance by the microstructural change during creep. The effect of oxidation was significant for causing a further creep strength drop in the range which exceeded 20,000 h in rupture time at 600°C. As a result, the creep strength at 60 MPa and 600°C was almost the same regardless of long tempered or aged steel.

A Japanese national project has been undertaken since Aug. 2008 with the objective of developing an advanced ultra-supercritical power plant (A-USC) with a steam temperature of 700°C. Fe-Ni and Ni-based alloys, namely HR6W, HR35, Alloy617, Alloy740, Alloy263 and Alloy141, were taken as candidate materials for piping and superheater/reheater tubes in an A-USC boiler. Weldments of these alloys were manufactured by GTAW, after which long term creep rupture tests were conducted at 700°C, 750°C and 800°C. Weldments of HR6W, HR35 and Alloy617 showed similar creep strength as compared with these base metals. Weldments of Alloy740 tended to fail in the HAZ, and it is considered that voids and cracks preferentially formed in the small precipitation zone along the grain boundary in the HAZ. The creep strength of Alloy263 in weldments exhibited the highest level among all the alloys, although HAZ failure occurred in the low stress test condition. A weld strength reduction factor will be needed to avoid HAZ failure in Alloy740 and Alloy263. Also, to prevent premature failure in weld metal, optimization of the chemical composition of weld filler materials will be required.

Development of the advanced USC (A-USC) boiler technology has been promoted in recent years, which targets 700°C steam condition. HR6W (Ni-23Cr-7W-Ti-Nb-25Fe) and HR35 (Ni-30Cr-6W-Ti-15Fe) have been developed for A-USC boiler tubes and pipes. The former alloy is mainly strengthened by Fe 2 W type Laves phase. The latter one employs precipitation strengthening of α-Cr phase in addition to Laves phase. Characteristic alloy design of both alloys, which does not use precipitation strengthening of γ′ phase (Ni 3 Al), leads to superior ductility and resistance to stress-relaxation cracking. Stability of creep strength and microstructure has been confirmed by long-term creep rupture tests. The 100,000h average creep rupture strength of HR6W is 85MPa at 700C. That of HR35 is 126MPa at 700°C which is comparable with conventional Alloy617. Tubes of both alloys have been evaluated by the component test in Japanese national A-USC project with γ′ hardened Alloy617 and Alloy263. Detailed creep strength, deformation behavior and microstructural evolution of these alloys are described from the viewpoint of the difference in strengthening mechanisms. Capability of these alloys for A-USC boiler materials has been demonstrated by the component test in the commercial coal fired boiler as the part of the A-USC project.

Long-term creep rupture tests up to 10 5 hours at 600℃ and 650℃ were carried out on mod.9Cr- 1Mo steel base metal and weldments from five different materials, consisting of various chemical compositions and heat treatments as well as welding conditions. As a result, positive correlations of creep rupture strength were clarified between the base metal and weldments from the same materials. Microstructural observations and thermokinetic calculations revealed that the strength correlations were attributed to the precipitation strengthening behavior of finely dispersed M 23 C 6 carbides and V-type MX carbonitrides, where their precipitation distribution characteristic in the fine-grained HAZ microstructures partially or almost entirely took over those in base metal. This finding implies that the long-term creep rupture strength of mod.9Cr-1Mo steel weldment might be able to be evaluated as long as the corresponding base metal strength is obtained.

ASME Grade 91 steel seam-welded elbow pipe, which has been used in a USC plant (A-Plant) for about 6 × 10 4 h, was investigated to clarify the microstructure and remaining creep life of the material at long-term region. SEM and TEM observations were conducted on specimens cut from the welded portions of the intrados and extrados of the elbow, and the number density of creep voids in fine-grained HAZ was measured in the wall-thickness direction. Then, creep rupture tests were performed to examine the remaining life of each portion of the base metal and welded joint. On the basis of the results, it was suggested that the microstructural changes were small and that the cumulative creep damage was also small for the elbow pipe during its use at the USC plant for about 6 × 10 4 h. The present result was compared with the result of an investigation on Grade 91 steel elbow used in another USC plant (B-Plant) for about 5 × 10 4 h. The A-Plant material had a creep life about ten times longer than that of the B-Plant material for not only the base metals but also the welded joint. Through the comparison of the investigation results, it was suggested that the difference in the creep deformation property between the base metals of the elbows was the main reason for the difference in their creep lives.

10CrMoWVNbN (X 12 CrMoWVNbN 10 1 1) steel trial forgings has been manufactured to clarify the effect of austenitizing temperature on the creep rupture strength and microstructure. From the results of creep rupture tests up to 30,000 hours, higher austenitizing temperature improves the rupture strength without large degradation of the rupture ductility. The microstructural investigations demonstrate that the prior austenite grain size and the precipitation behavior of fine M2X particles are presumed to contribute to the improvement of creep rupture strength.

SUPER304H (18Cr-9Ni-3Cu-Nb-N, ASME CC2328) and TP347HFG (18Cr-12Ni-Nb, ASME SA213) are advanced fine-grained microstructure steel tubes developed for high strength and superior steam oxidation resistance. Their exceptional performance is demonstrated by the longest creep rupture tests, with SUPER304H tested at 600°C for 85,426 hours and TP347HFG at 700°C for 55,858 hours, both maintaining stable strength and microstructure with minimal σ phase formation and absence of other brittle phases compared to conventional austenitic stainless steels. HR3C (25Cr-20Ni-Nb-N, ASME CC2115) was specifically developed for high-strength, high-corrosion-resistant steel tubes used in severe corrosion environments of ultra-supercritical (USC) boilers operating at steam temperatures around 600°C. The longest creep test for HR3C, conducted at 700°C and 69 MPa for 88,362 hours, confirmed its high and stable creep strengths and microstructural integrity across the 600-800°C temperature range. These innovative steel tubes have been successfully installed in the Eddystone No. 3 USC power plant as superheater and reheater tubes since 1991, with subsequent microstructural investigations after long-term service exposure revealing their remarkable performance. The paper provides an up-to-date analysis of the long-term creep rupture properties and microstructural changes of these steels following extended creep rupture and aging processes, highlighting their successful application as standard materials for superheater and reheater tubes in newly constructed ultra-supercritical boilers worldwide.

To achieve the necessary creep-rupture lifetimes at the temperatures and pressures associated with advanced ultrasupercritical (A-USC) steam conditions (100,000 h at 100 MPa and 760°C), precipitation-strengthened nickel-based alloys are required for the superheater and reheater tubing in A-USC boilers. Two alloys were considered to have potential for this application: Inconel 740 and Haynes 282 alloy. In support of this application, creep-rupture testing of several heats of Inconel 740 was conducted over a range of temperatures and stresses to develop confidence in qualitatively predicting creep lifetimes under conditions relevant to A-USC steam conditions, with the longest rupture times exceeding 30,000 h. For comparison, the creep-rupture behavior of Haynes 282 alloy was mapped as a function of temperature and stress, but with a significantly smaller dataset. Only a small difference in creep-rupture results between Inconel 740 and Inconel 740H was found although the latter alloy showed significantly greater resistance to η phase formation during testing. Little effect of prior aging treatments (for setting the γ′ precipitate structure) on creep-rupture behavior was observed. Results from a modified power law analysis showed that, while both Inconel 740 and Haynes 282 are projected to meet the A-USC lifetime requirements, the latter offered the potential for better long-term creep resistance.

Type IV damage was found at several ultra-supercritical (USC) plants that used creep-strength-enhanced ferritic (CSEF) steels in Japan, and the assessment of the remaining life of the CSEF steels is important for electric power companies. However, there has been little research on the remaining life of material that has actually served at a plant. In this study, the damage and remaining life of a Gr.91 welded elbow pipe that served for 54,000 h at a USC plant were investigated. First, microscopic observation and hardness testing were conducted on specimen cut from the welded joint; the results indicated that the damage to the elbow was more severe in the fine-grain heat-affected zone near the inner surface. Furthermore, creep rupture tests were performed using specimens cut from the welded joint of the elbow, and from these results, the remaining life was evaluated using the time fraction rule as almost 110,000 h. Finite-element analysis was also conducted to assess the damage and remaining life, and the results were compared with the experimental results.

Large heat-to-heat variation of creep rupture strength in weldments of mod.9Cr-1Mo steels was observed in the creep rupture tests conducted for two different heats at 600°C and 650°C. One heat showed consistently lower time-to-rupture than the other for 130-60MPa at 600°C. Detailed microstructural investigations revealed that the number density of precipitates in the weaker heat was remarkably lower than that associated with the stronger heat through most of the creep region. Accordingly, heat-to-heat variation of creep rupture strength was attributed to the difference in the precipitate strengthening effects throughout creep. Equilibrium calculation predicted that the smaller phase fraction of M 23 C 6 and VN precipitates due to the lower content of chromium and lower ratio of nitrogen/aluminum in the weaker heat. However, given that long-term creep rupture strength at 650°C converged for the two heats, the microstructure including precipitates may settle into a similar level for subsequent longer hours even at 600°C.

Continuous and active works have been going to develop 700°C A-USC (Advanced Ultra Super Critical) power plants in Europe, United States and also Japanese national project has launched in 2008. In this new Japanese project Fe-Ni based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti) is one of the candidate materials for boiler tube and pipe as well as Ni based alloys such as well-known Alloy617, Alloy263 and Alloy740. The most important issue in boiler fabrication is the welding process of these alloys and long-term reliability of their weldments. Authors investigated the weldability of HR6W thick-wall pipe. The integrity of the weldment was confirmed with metallurgical investigation, mechanical testing and long term creep rupture test. It is proved that the narrow gap HST welding procedure can meet the requirements for Ni based or Fe-Ni based alloys and provides excellent strength properties.

Recent years high strength 9Cr1MoNbV steel developed in USA has been major material in boiler high temperature components with the increase of steam parameters of coal fired thermal power plants. As the microstructure of this steel is tempered martensite, it is known that the softening occurs in HAZ of the weldment. In the creep rupture test of these welded joints the rupture strength is lower than that of the parent metal, and sometimes this reduction of strength is caused by TypelV cracking. To develop an effective method to improve the rupture strength of welded joint, advanced welding procedure and normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with thickness of 40-50mm were welded by 10mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1,050°C and tempering at 780°C, material properties of the welded joints were examined. Microstructure of HAZ was improved as before weld, and rupture strength of the welded joints was equal to that of the parent metal. The long term rupture strength of the welded joints was confirmed in the test exceeded 30,000hours. This welding procedure has been applied to seam weld of hot reheat piping and headers in USC boilers successfully.

Advanced Ultra-supercritical (A-USC) steam power-plant technology is being developed for better efficiency and lower emissions at 700°C and above, but is based mainly on Ni-based alloys. The ability to include lower-cost alloys with appropriate high-temperature performance should have substantial technological and economic benefits. CF8C-Plus is a cast austenitic stainless steel recently developed for other applications at 600-900°C, which has creep-strength comparable to many solid-solution Ni-based alloys. EPRI and Carpenter Technology produced a 400 lb heat of CF8C-Plus steel and hot-forged it at 5:1 and 12:1 reductions, to assess feasibility of the alloy as a wrought advanced stainless steel for potential use as steam headers and piping for A-USC power plant applications. The hot-forged alloy has a recrystallized grain structure 6-9 times finer than the as-cast dendritic structure, resulting in better strength and impact resistance at room-temperature, and about 20% higher yield-strength (YS) at 760°C, and similar or better ductility compared to the as-cast material. The initial creep-rupture testing at 700-800°C for up to 2000h also indicates similar or better rupture resistance and better creep-ductility for wrought compared to cast material. The next steps needed to test performance of the wrought austenitic stainless steel for extruded headers and piping are discussed.

This study investigated the creep rupture strength and microstructure evolution in welded joints of high-boron, low-nitrogen 9Cr steels developed by NIMS. The welds were fabricated using the GTAW process and Inconel-type filler metal on steel plates with varying boron content (47-180 ppm). Creep rupture tests were conducted at 923K for up to 10,000 hours. Despite their higher boron content, these steels exhibited good weldability. Welded joints of the boron steel displayed superior creep properties compared to conventional high-chromium ferritic steel welds like P92 and P122. Notably, no Type IV failures were observed during creep testing. Welding introduced a large-grained microstructure in the heat-affected zone (HAZ) heated to the austenite transformation temperature (Ac3 HAZ). This contrasts with the grain refinement observed in the same region of conventional heat-resistant steel welds. Interestingly, the grain size in this large microstructure was nearly identical to that of the base metal. Analysis of the simulated Ac3 HAZ revealed crystal orientation distributions almost identical to those of the original specimen. This suggests a regeneration of the original austenite structure during the alpha-to-gamma phase transformation. Simulated Ac3 HAZ structures of the boron steel achieved creep life nearly equivalent to the base metal. The suppression of Type IV failure and improved creep resistance in welded joints of the boron steels are likely attributed to the large-grained HAZ microstructures and stabilization of M 23 C 6 precipitates. The optimal boron content for achieving the best creep resistance in welded joints appears to lie between 90 and 130 ppm, combined with minimized nitrogen content.