In this work, the effects of phosphorus addition on the creep properties and microstructural changes of wrought γ’-strengthened Ni-based superalloys (Haynes 282) were investigated, focusing on the effects of carbides precipitation. In an alloy with a phosphorus content of 8 ppm, precipitation of M 23 C 6 carbides was observed in both grain boundaries and the grain interior prior to the creep tests. Grain boundary coverage by carbide increased with phosphorus content up to approximately 30 ppm. On the other hand, the amount of M 23 C 6 in the grain interior decreased with phosphorus content. The results of the creep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbides. The microstructure of the crept specimens showed the existence of misorientation at the vicinity of grain boundaries without carbides, as demonstrated via electron backscattered diffraction (EBSD) analysis. These results suggest that the observed improvement in the time to rupture is due to a grain-boundary precipitation strengthening mechanism caused by grain boundary carbides and that phosphorus content affects the precipitation behavior of M 23 C 6 carbides in the grain interior and grain boundaries. These behaviors were different between alloys with the single addition of phosphorus and alloys with the multiple addition of phosphorus and niobium.

Dissimilar metal welds between T91 ferritic steels and TP347H austenitic alloys are commonly used in fossil power plants in China. Premature failure of such dissimilar welds can occur, resulting in unplanned plant outages that can cause huge economic losses. In this article, microstructural evolution of T91/TP347H dissimilar welds after different service conditions were studied, mechanical properties before and after service were also analyzed, a full investigation into the failure cause was carried out. The results show, the dissimilar metal welds in the as-welded condition consists of a sharp chemical concentration gradient across the fusion line, failure is attributed to the steep microstructural and mechanical properties gradients, formation of interfacial carbides that promote creep cavity formation.

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M 23 C 6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23 C 6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M 23 C 6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M 23 C 6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

Three Ni-base wrought alloys with different hardening mechanisms (INCONEL 706, Waspaloy and INCONEL 617) were investigated as candidates for steam turbine rotor applications at temperatures up to 700 °C in respect to their microstructure and microstructural stability. The Nb containing alloy Inconel 706 had a complex microstructure with γ', γ" and η phases which are stable in long term service up to 620 °C. At higher temperatures significant particle coarsening and phase transformation were observed. Waspaloy is hardened by γ' particles and after ageing at 700 °C and higher, it tended to a stable microstructure. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M 23 C 6 carbides. After long term ageing at temperatures of 650 °C to 750 °C the carbides tended to form carbide films along the grain boundaries and at 700 °C to 750 °C γ' precipitated as homogeneously distributed particles with low coarsening during long term service. In order to optimize the candidate alloys Inconel 706 and Waspaloy were modified to the new alloys DT 706 and DT 750. The aspects of modification and first experimental results are reported.

The effects of Cr and W on the creep rupture life of 8.5-11.5Cr steels at 650°C were evaluated. Throughout this paper the specimen composition is expressed in mass percent. The creep rupture life of 8.5Cr steel is the longest in 8.5-11.5Cr steels at 650°C under the stress of 78MPa. The creep rupture life of 9Cr steel at 650°C was extended with increasing W content. The creep strength of the modified steel, 9Cr-4W-3Co-0.2V-NbBN steel, at 650°C did not decrease sharply up to 32000h. The 105h creep rupture temperature of this steel under the stress of 100MPa was estimated to be approximately 635°C using Larson-Miller parameter. M 23 C 6 type carbides and VX type carbonitrides were observed on the lath boundary of the modified steel. The stability of these precipitates in the modified steel is likely to suppress the degradation of the long term creep strength at 650°C.

Microstructure in the gage sections of ruptured GX12CrMoWVNbN10-1-1 cast steel specimens was examined after creep tests under applied stresses ranging from 120 to 160 MPa at T=893 K. The microstructure after tempering consisted of laths with an average thickness of 332 nm. The tempered martensite lath structure was characterized by M 23 C 6 -type carbide particles with an average size of about 105 nm, and MX carbonitrides with an average size of about 45 nm. Precipitation of Laves phase occurred during creep test. The structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. MX carbonitrides and M 23 C 6 -type carbide particles increase in size slightly under long-term creep. Microstructural degradation mechanisms during creep in GX12CrMoWVNbN10-1-1 cast steel are discussed.

In this study, a possibility of application of advanced 9%Cr steel containing 130 ppm boron for boiler components utilized at around 650 °C to higher temperature steam turbine rotor materials has been investigated by means of reduction in silicon promoting macro-segregation in the case of large size ingots, using laboratory heats. Tempered martensitic microstructure without proeutectoid ferrite in all steels studied is obtained even at the center position of a turbine rotor having a barrel diameter of 1.2 m despite lower amounts of nitrogen and silicon. The strength at room temperature is almost the same level of practical high Cr steels such as X13CrMoCoVNbNB 9-2-1 for ultrasuper critical steam turbine rotors. The toughness is sufficient for high temperature rotors in comparison with CrMoV steels utilized as sub-critical high pressure steam turbine components. The creep rupture strength of the steels is higher than that of the conventional 9-12Cr steels used at about 630 °C. The creep rupture strength of 9%Cr steel containing 130 ppm B, 95 ppm N, 0.07 % Si and 0.05 % Mn is the highest in the steels examined, and it is therefore a candidate steel for high temperature turbine rotors utilized at more than 630 °C. Co-precipitation of M 23 C 6 carbides and Laves phase is observed around the prior austenite grain boundaries after the heat treatments and the restraint of the carbide growth is also observed during creep exposure. An improvement in creep strength of the steels is presumed to have the relevance to the stabilization of the martensitic lath microstructure in the vicinity of those boundaries by such precipitates.

Nickel-base alloys were exposed to flowing supercritical CO 2 (P = 20MPa) at temperatures of 700 to 1000°C for up to 1000 h. For comparison, 316L stainless steel was similarly exposed at 650°C. To simulate likely service conditions, tubular samples of each alloy were internally pressurised by flowing CO 2 , inducing hoop stresses up to 35 MPa in the tube walls. Materials tested were Haynes alloys 188, 230 and 282, plus HR120 and HR160. These alloys developed chromia scales and, to different extents, an internal oxidation zone. In addition, chromium-rich carbides precipitated within the alloys. Air aging experiments enabled a distinction between carburisation reactions and carbide precipitation as a result of alloy equilibration. The stainless steel was much less resistant to CO 2 attack, rapidly entering breakaway corrosion, developing an external iron-rich oxide scale and internal carburisation. Results are discussed with reference to alloy chromium diffusion and carbon permeation of oxide scales.

9-10%Cr-3%Co martensitic steels are the prospective materials for elements of boilers, tubes and pipes for fossil power plants which are able to work at ultra-supercritical parameters of steam (T=620-650°C, P=25-30 MPa). The effect of creep on the microstructure of the 10 wt.%Cr-3Co- 3W-0.2Re martensitic steel was investigated in the condition of 650°C and an applied stress of 140 MPa, time to rupture was more than 8500 h. Previously, this steel was subjected to the normalizing at 1050°C and tempering at 770°C. This heat treatment provided the hierarchical tempered martensite lath structure with the mean size of prior austenite grains of 59 μm and with high dislocation density (2×10 14 m -2 ) within martensitic laths. Boundary M 23 C 6 and M 6 C carbides and randomly distributed within matrix Nb-rich MX carbonitrides were detected after final heat treatment. The addition of Re in the steel studied positively affected creep at 650°C/140 MPa and stabilized the tempered martensite lath structure formed during 770°C-tempering. The formation of the subgrains in the gage section was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase with fine sizes during creep. No depletion of Re and Co from the solid solution during creep was revealed whereas W content decreased from 3 to 1 wt.% for first 500 h of creep. Reasons of improved creep as well as mechanisms of grain boundary pinning by precipitates are discussed.

The size and distribution of the Laves phase particles in a 9.85Cr-3Co-3W-0.13Mo-0.17Re- 0.03Ni-0.23V-0.07Nb-0.1C-0.002N-0.008B steel subjected to creep rupture test at 650°C under an applied stresses of 160-200 MPa with a step of 20 MPa were studied. After heat treatment consisting of normalizing of 1050°C and tempering of 770°C, M 23 C 6 and Fe 3 W 3 C carbides with the mean sizes of 67±7 and 40±5 nm, respectively, were revealed along the boundaries of prior austenite grains and martensitic laths whereas round NbX carbonitrides were found within martensitic laths. During creep metastable Fe 3 W 3 C carbides dissolved and the stable Laves phase particles precipitated; volume fraction of Laves phase increases with time. The Laves phase particles nucleated on the interfacial boundaries Fe 3 W 3 C/ferrite during first 100 h of creep and provided effective stabilization of tempered martensitic lath structure until their mean size less than 150 nm.

Detailed knowledge of the creep and creep crack behavior is essential for a safe operation of thick-walled components in thermal power plants. High mechanical loads and temperatures of more than 700 °C often require the application of nickel-based alloys, e.g. alloy C-263. Unfortunately, manufacturing and non-destructive evaluation (NDE) of thick-walled components (> 50 mm) made of nickel-based alloys are quite challenging. Tolerable critical flaw sizes, experimentally validated for long service durations, play an important role in the quality assurance of such components. It is commonly accepted that manufacturing parameters, e.g. heat treatment procedures, have a significant influence on creep ductility and time-dependent crack behavior. By means of adjusting the process parameters, the ductility and the creep life of notched specimen can be significantly improved in the case of alloy C-263. Essential root cause is the decoration of grain boundaries with carbides which drastically influences creep crack initiation and growth. This results in significant differences for allowable critical flaw sizes and thus, the potential use of the candidate material. On a first generation of alloy C-263 “G1”, a dense population of carbides on the grain boundaries was found, which resulted in an inadmissible creep crack behavior. The resulting critical flaw sizes were only a few tenths of a millimeter. On a second generation “G2”, the grain boundary occupation was positively influenced, so that a satisfactory creep crack behavior could be found. The critical flaw sizes are in the order of one millimeter or more. A critical or impermissible material behavior under creep conditions can be demonstrated by testing smooth and notched round specimens. For example, the first generation “G1” notched round specimens fails earlier than the smooth round specimens, indicating notch sensitivity. On the second generation “G2”, however, a notch insensitivity was found. The critical defect sizes can be determined by a method that takes into account a simultaneous examination of the crack tip situation and the ligament situation.

9-12%Cr martensitic steels can be applied to the next highest temperature components such as boiler tracts, steam pipelines and turbines of advanced ultra-supercritical power plants with steam temperatures of 650°C. New 10%Cr martensitic steels with high B and low N contents can be a worthy candidate for use in production because it has superior creep resistance. At the same time, resistance to cyclic and dynamic loads is very important. In this work, we studied the low cycle fatigue (LCF) properties at room and elevated (500-650°C) temperatures and Charpy impact toughness at temperatures ranging from -196…100°C of advanced 10% Cr martensitic steel with high B and low N contents. The effect of new alloying scheme and corresponding peculiarities of M 23 C 6 carbides on the low cycle fatigue resistance and impact toughness of the 10%Cr martensitic steel is analyzed. It is revealed that fine and densely distributed carbides has no effect on the fatigue resistance except for the slight improvement of fatigue life at small strain amplitudes and shift the ductile-brittle transition temperature (DBTT) to higher but satisfactory value of +10°C as compared to other high-chromium martensitic steels.

In order to clarify the effect of stress state on microstructural changes during creep, the microstructure was observed in the central part of the cross section of the fine-grained heat-affected zone (FGHAZ) and in the surface region of the FGHAZ in Gr.92 steel welded joint. Creep tests were performed under constant load in air at 650°C, using cross-weld specimens. The creep strength of welded joint was lower than that of base metal. Type IV fracture occurred in the long-term. Creep voids were detected in the FGHAZ after the fracture. Number of creep voids was higher in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. It was checked the multiaxiality of stress during creep was higher in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. The recovery of dislocation structure occurred after creep in the base metal and the FGHAZ. Mean subgrain size increased with increasing time to rupture. However, there was no difference of change of subgrain size during creep in the central part of the cross section of the FGHAZ and in the surface region of the FGHAZ. The growth of M 23 C 6 carbide and MX carbonitrides was observed during creep in the base metal and the FGHAZ. Laves phase precipitation occurred during creep. There was no difference of the change of mean diameter of MX carbonitrides in the central part of the cross section of the FGHAZ and in the surface region of the FGHAZ after creep. However, the growth rate of M 23 C 6 carbide in the FGHAZ was much higher in the central part of the cross section than in the surface region.

Research on low-alloyed, heat-resistant 12Cr2MoWVTiB steel, implemented in China to power plants in 50’s last century, was performed to investigate a possibility of its application for pressure elements of boilers, in particular for membrane walls. The microstructure of the as-received 12Cr2MoWVTiB tube, investigated by light microscopy, scanning- and transmission electron microscopy, consists of ferritic grains with some bainite areas between them as well as primary carbides (MC) and secondary carbides (VC, M 23 C 6 , M 6 C) formed during tempering of the steel. Results of mechanical tests of 12Cr2MoWVTiB welded joints (butt- and fillet welded joints) as well as microstructure analyses of are satisfactory.

Tempered martensitic 9-12%Cr steels bearing tungsten, such as P92 and P122 showing higher creep rupture strength than the conventional steel P91, have been developed for thick section components in ultra-supercritical (USC) boilers. However, their creep strength is not sufficient for applying at the steam condition of 650°C/35MPa or above, which is a recent target condition in order to increase plant efficiency. The research and development project in NIMS on advanced high-Cr steels which can be applied at the steam condition of 650°C/35MPa as boiler components with large diameter and thick section has been carried out since 1997. In this project, it has been revealed that the addition of boron more than 0.01 mass% to the 0.08C-9Cr- 3W-3Co-V,Nb-<0.00ЗN steel remarkably improves creep strength. The boron enriched in M 23 C 6 carbides near prior-austenite grain boundaries suppresses coarsening of these carbides during creep deformation, leading to excellent microstructural stability and creep strength. Further improvement of creep strength is achieved by the addition of appropriate amount of nitrogen which enhances precipitation of fine MX. Excess addition of nitrogen to the high-B containing steel reduces creep rupture lives and ductility. The highest creep strength is obtained in the 0.08C-9Cr-3W-3Co-0.2V-0.05Nb-0.0139B-0.0079N (mass%) steel, resulting in excellent creep strength in comparison with that of P92 and P122. This steel shows good creep ductility even in the long term. It is, therefore, concluded that this high-B bearing 9Cr-3W-3Co-V,Nb steel with the addition of nitrogen in the order of 0.008 mass% is the promising candidate which shows superior creep strength without impairing creep ductility for thick section components in the 650°C-USC plant.

This study examines the influence of carbon and austenite stabilizing elements (Ni, Mn, Co, Cu) on Laves phase precipitation, Fe 2 W formation, and creep rupture strength (CRS) in 9-12% Cr steels at 600-700°C. Nickel and manganese had minimal impact on Laves phase and coarse carbide formation up to 1% content. While cobalt increased Laves phase fraction at 650°C, it did not improve long-term CRS and even caused a rapid decrease in short-term CRS. Copper, on the other hand, promoted the precipitation of fine Cu-rich particles that acted as nucleation sites for Laves phase and M 23 C 6 carbide. This resulted in a different needle-like Laves phase morphology compared to the globular type observed in nickel and cobalt alloys, leading to improved CRS in the copper alloy. Increasing carbon content from 0.1% to 0.2% effectively suppressed Laves phase formation, as confirmed by Thermo-Calc calculations. Notably, for cobalt alloys with higher tungsten content, higher carbon content (0.09% to 0.19%) improved CRS at 650°C, whereas the opposite effect was observed in nickel and nickel-manganese alloys. Copper alloys maintained improving CRS trends even with increased carbon, leading to the overall best CRS performance among the tested alloys with 0.2% carbon.

In order to clarify the effect of stress and strain on microstructural changes during creep for T91 steel, creep interrupted tests were performed at 600°C for 10000h, 20000h, 30000h, 50000h and 70000h. The steel studied was T91 steel with high Ni content (0.28mass%) in the range of specification. Changes of dislocation structure and precipitates distributions were observed for the grip and gauge portions of creep interrupted samples. The subgrain size gradually increased with increasing creep time up to 50000h in both the grip and gauge portions. However, the subgrain size abruptly increased after 50000h in the gauge portion as compared with the grip portion. Decrease in dislocation density inside subgrain was promoted in the gauge portion as compared with the grip portion. The size of M 23 C 6 gradually increased with increasing creep time up to 50000h in both the grip and gauge portions. The increase in M 23 C 6 size was accelerated after 50000h in the gauge portion as compared with the grip portion. The Z phase formation was promoted in the gauge portion as compared with the grip portion. The number density of all kinds of particles gradually decreased with increasing creep time in the gauge and grip potions. After 50000h, the number density rapidly decreased in the gauge portion as compared with the grip portion.

The delivery state of austenitic heat resistant steel boiler tubes is paramagnetic, such as TP304H, TP347H and S30432, the material state, however, appears obviously magnetic after long-time high-temperature service. Vibrating Sample Magnetometer (VSM) has been employed to test the magnetism difference after high-temperature service, and XRD, SEM, TEM, SAED and EDS has been adopted to observe and analyze their microstructure, phase structure and composition. The research results show that compared with the delivery state, the lath α´-Martensite and sometimes the lamellar ε-Martensite will occur in areas adjacent to grain boundaries due to martensite transformation in the microstructure of austenitic heat resistant steel boiler tube after high temperature service. There are high density dislocations tangled together in the substructure of α´-Martensite, and lamellar stacking faults arrayed orderly by a large number of dislocations in the substructure of ε-Martensite. The magnetism of α´-Martensite, its internal stress and carbides is the reason why the austenitic heat resistant steel boiler tubes appear obviously magnetic after high temperature service, and the α´-Martensite plays a major role.

The present study presents a detailed investigation on the evolution of the microstructure during welding on virgin and long-term service exposed (creep aged 1 = 535°C; 16.1 MPa; 156 kh and creep aged 2 = 555°C; 17.0 MPa; 130 kh) 12% Cr (X20CrMoV11-1) martensitic steel. This study was carried out in order to understand the impact of welding on prior creep exposed Tempered martensite ferritic (TMF) steel and to explain the preferential failure of weldments in the fine grained heat affected zone (FGHAZ) of the creep aged material side instead of the new material side. Gleeble simulation (Tp = 980°C; heating rate = 200 °C/s; holding time = 4 seconds) of the FGHAZ was performed on the materials to create homogeneous microstructures for the investigation. Quantitative microstructural investigations were conducted on the parent plate and simulated FGHAZ materials using advanced electron microscopy to quantify: a) voids, b) dislocation density, c) sub-grains, and d) precipitates (M 23 C 6 , MX, Laves, Z-phase) in the materials. Semi-automated image analysis was performed using the image analysis software MIPARTM. The pre-existing creep voids in the creep aged parent material and the large M 23 C 6 carbides (Ø > 300 nm) in the FGHAZ after welding are proposed as the main microstructural contributions that could accelerate Type IV failure on the creep aged side of TMF steel weldments.

Additive manufacturing (AM) is a process where, as the name suggests, material is added during production, in contrast to techniques such as machining, where material is removed. With metals, AM processes involve localised melting of a powder or wire in specific locations to produce a part, layer by layer. AM techniques have recently been applied to the repair of gas turbine blades. These components are often produced from nickel-based superalloys, a group of materials which possess excellent mechanical properties at high temperatures. However, although the microstructural and mechanical property evolution during the high temperature exposure of conventionally produced superalloy materials is reasonably well understood, the effects of prolonged high temperature exposure on AM material are less well known. This research is concerned with the microstructures of components produced using AM techniques and an examination of the effect of subsequent high temperature exposures. In particular, the paper will focus on the differences between cast and SLM IN939 as a function of heat treatment and subsequent ageing, including differences in grain structure and precipitate size, distribution and morphology, quantified using advanced electron microscopy techniques.