Two materials with different purity of 2.25Cr-1Mo steel thick weld joint were prepared and creep rupture behavior was investigated by large sized specimens. For high purity material, two types of challenging heat treatment was tried to modify the original microstructural conditions. Weld joints were made and large sized creep test specimens were machined. Creep tests were performed at 903K, 40MPa. Specimen made from low purity material fractured at fine grained heat affected zone (FGHAZ) and showed so-called Type IV cracking. On the other hand, specimen made from high purity material showed maximum creep damage at weld metal. In the case of specimens applied challenging heat treatment, remarkably high ductility were observed at fracture. Regarding 2.25Cr-1Mo steel, it was confirmed that the suppression of Type IV cracking had been basically achieved by past improvement on purity level. At the same time, improvement of heat treatment condition was found to have further effect. Because of improved creep properties of high purity material, properties of weld metal had rose up to be the next issue to be examined. At least, taking care on layout design of weld beads to avoid creating wide spread fine grained portion is desired.

The effect of a reduced-temperature austenization treatment on the microstructure and strength of two ferritic-martensitic steels was studied. Prototypic 9% and 12% Cr steels, modified 9Cr-1Mo (ASME T/P91) and Type 422 stainless (12Cr-1Mo-W-V), respectively, were austenized at the standard 1050°C and an off-normal 925°C, both followed by tempering at 760°C. The reduced austenization temperature was intended to simulate potential inadequate austenization during field construction of large structures. The microstructure, tensile behavior, and creep strength were characterized for both steels treated at each condition. While little change in microstructure was observed for the modified 9Cr-1Mo steel, the creep strength was reduced at higher temperatures and in long duration tests. The microstructure of the Type 422 stainless in the off-normal condition consisted of polygonized ferrite instead of tempered martensite. In this case the creep strength was reduced for short duration tests (less than ~1000 hr), but not for long duration tests. Slight reductions in tensile strength were observed at room temperature and elevated temperatures of 450,550, and 650°C.

The addition of boron without the formation of any boron nitrides during normalizing heat treatment at high temperature minimizes the degradation in creep strength of both base metal and welded joints of 9Cr steel at 650 °C and long times. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the reduction of coarsening rate of M 23 C 6 carbides in the vicinity of PAGBs, enhancing boundary and subboundary hardening, and also for the production of same microstructure between the base metal and heat-affected-zone (HAZ) in welded joints, indicating no Type IV fracture in HAZ. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalizing, which reduces the soluble boron concentration and accelerates the degradation in creep rupture ductility at long times. 9Cr- 3W-3Co-VNb steel with 120 - 150 ppm boron and 60 - 90 ppm nitrogen (MARBN) exhibits not only much higher creep strength of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture at 650 °C. The pre-oxidation treatment in Ar gas promotes the formation of protective Cr 2 O 3 scale on the surface of 9Cr steel, which significantly improves the oxidation resistance in steam at 650 °C.

This paper explores the low cycle fatigue (LCF) and creep-fatigue properties of a hot-forged, normalized, and tempered 9Cr-1Mo ferritic steel. This steel offers good performance in high-temperature applications (up to 873K) in power plants and reactors. The steel was forged into 70 mm diameter rods and then heat-treated with normalizing (1313K for 1 hour, air cooling) and tempering (1033K for 1 hour, air cooling). LCF tests were conducted at 300-873K with varying strain amplitudes and strain rates to understand the influence of both factors. Additionally, some specimens were aged at different temperatures for 10,000 hours before testing. Finally, creep-fatigue interaction tests were performed at 823K and 873K using tensile hold times ranging from 1 to 30 minutes.

Modified 9Cr-1Mo steel was manufactured via laser powder bed fusion (LPBF) using gas atomized powders under various building conditions. Dense samples were obtained at an energy density of 111-125 J/mm 3 . As-built samples were subjected to a normalization and tempering heat treatments. The microstructure of the as-built sample exhibits a duplex structure, comprising coarse columnar δ-ferrite grains and fine martensite grains. In addition, a small amount of retained austenite phase was observed at the interface between δ-ferrite and martensite. The formation of δ-ferrite is attributed to the extremely rapid solidification that occurs during the LPBF process, while martensite is obtained through the phase transformation because of the thermal cycles experienced during the process. The area fraction of δ-ferrite and martensite can be controlled by adjusting the LPBF parameters. Typical as-built microstructure morphology characterized by the columnar δ- ferrite was eliminated after the heat treatments, resulting in a tempered martensitic microstructure that is identical with that obtained through the conventional process. However, an increase in prior austenite grain size was observed when the area fraction of δ-ferrite in the as-built condition was high, due to faster phase transformation kinetics of martensite than that of δ-ferrite during the normalization. This suggests that the prior austenite grain size can be controlled by optimizing the area fraction of δ-ferrite and martensite in the as-built microstructure.

The effects of chemical composition and heat treatment on the creep properties of ASME Grade 91 type steel were experimentally investigated using materials whose chemical compositions and heat treatment conditions in the steel making process were completely controlled. Regarding chemical composition, only the Al, Cr, and Ni contents were systematically varied while keeping the contents of the other elements and heat treatment conditions constant. Regarding heat treatment, the normalizing and tempering temperatures were varied while keeping the contents of chemical components constant. The creep tests of materials were performed for approximately up to 50,000 h at 650°C. The creep strength of Grade 91 type steel decreased with increasing Al content under the test conditions of short-term to long-term range. On the other hand, the effect of Cr content on the creep life of Grade 91 type steel depended on the stress or time range, and the creep strength of the steel decreased at high Cr contents under test conditions of only the longterm range. No effect of Ni content on the creep life of the materials was observed in the test data obtained in this study. As creep tests are currently being conducted at 625°C and 60 MPa, which are conditions closer to the actual service conditions of main steam piping at ultra-super critical power plants, the creep deformation data at present indicate that the above trends hold true in the long-term range. Regarding the effect of heat treatment, the creep life of the materials tended to increase with increasing normalizing temperature or decreasing tempering temperature. The results obtained in this work indicate that within the scope of the material standards for Grade 91 type steel, the effect of chemical composition on creep life is greater than that of heat treatment.

MarBN steels, originally developed by Professor Fujio Abe at NIMS Japan, have undergone significant advancement in the UK through a series of government-funded collaborative projects (IMPACT, IMPEL, INMAP, IMPULSE, and IMPLANT). These initiatives have achieved several major milestones, including operational power plant trials, full-scale extruded pipe production, matching welding consumable development, and most notably, the creation of IBN-1—a new steel demonstrating 30-45% higher creep strength than Grade 92. However, like other creep strength-enhanced ferritic steels, IBN-1 shows reduced creep ductility under the lower stress conditions typical of operational use. Since adequate creep ductility is essential for component damage tolerance and effective in-service monitoring, this study investigates the effects of an alternative normalizing and tempering heat treatment on cast IBN-1. The research presents creep rupture test results showing improved ductility and analyzes the microstructural mechanisms responsible for this enhancement.

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.

The effect of boron nitrides (BN) and aluminum nitrides (AlN) on long-term creep life and rupture ductility has been investigated for martensitic 9 to 12Cr steels at 550 to 700 °C. The BN particles form in 9 to 12Cr steels during normalizing heat treatment at 1050 to 1150 °C, suggesting no change in the amount of BN particles during creep. On the other hand, the AlN particles gradually form during creep at 550 to 700 °C, decreasing the concentration of nitrogen free from the AlN particles. The degradation in creep life takes place more significantly with test duration in the steels containing high Al but not in those containing the BN particles. The rupture ductility is evaluated by using a semi-logarithmic diagram of the RA and total elongation, showing the necking dominant and void swelling dominant regions. The BN and AlN particles are responsible for the degradation in RA at low stresses and long times by accelerating the formation of creep voids at interfaces between the BN and AlN particles and alloy matrix.

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.

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

The microstructures of an advanced Ta-added 9Cr-3Co-2W-Mo steel with increased boron content that has been homogenized at different temperatures were investigated. The chains of coarse W-rich particles were observed in the steel after homogenization at 1150°C for 24 h. These particles remained in the microstructure after normalization and tempering. Such additional dispersion hardening in the initial state of the studied steel decreased the creep rate in transient region. However, the duration of steady state creep and overall creep time was increased in the samples homogenized at 1200°C. Despite of the presence of coarse W-rich particles, the impact toughness of the low-temperature- homogenized steel in the tempered condition was significantly higher than that of the steel homogenized at 1200°C

Modified 9Cr-1Mo alloy steel has been developed over the last few decades and has since gained wide acceptance in the boiler industry for the production of a variety of pressure-critical components, including tubing, piping and headers. The properties of creep-strength enhanced ferritic steels such as grade 91 are critically dependent on manufacturing parameters such as steelmaking, hot deformation, heat treatment and welding. Since the applications for which this material is used impose strict requirements in terms of resistance, corrosion, and creep behavior, poor process control can severely compromise the service behavior. This work discusses the impact of total deformation during the rolling process, and heat treatment parameters on time-independent and time-dependent properties for grade 91. For this study, two heats with similar chemical composition were produced with different reduction ratios: to which, several normalizing and tempering combinations were applied. For each combination, the microstructure was characterized, including evaluation of segregation by metallographic examination, and analysis of secondary phase precipitates by means of X-ray powder diffraction. Mechanical testing and creep testing were performed. A comparison of results is presented, and recommendations on the optimal process parameters are provided to ensure reliable performance of grade 91 material.

Microstructure change during creep at 650°C has been examined for a high-B 9%Cr steel by FIB-SEM serial sectioning 3D observation, Nano-SIMS, SEM, EBSD and TEM. The precipitates formed in the steel were M 23 C 6 , Laves phase, and a quite small amount of MX. For as-tempered steel, precipitation of M 23 C 6 on the prior austenite grain boundaries was clearly found, while precipitation of the Laves phase was not confirmed during tempering. The volume fraction of the Laves phase gradually increased with elapsed time, while M 23 C 6 appeared to increase once and decrease afterward, based on the comparison between the 2,754 h ruptured sample and the 15,426 h ruptured sample. Nano-SIMS measurements have revealed that B segregates on the prior austenite grain boundaries during normalizing, and it dissolves into M 23 C 6 .

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.

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.

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in grade 91 steel. Failed hot tensile specimens have been sectioned at various positions along the specimen axis, and therefore at different levels of hot tensile strain, to obtain material for creep strength evaluation. Because of the limited amount of material available for creep testing obtained in this way, creep testing has been carried out using the specialised small-scale impression creep testing technique. The grade 91 material has been tested in both the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% Ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant with grade 91 pipework systems.

The long-term performance of superheater super 304h tube during the normal service of an ultra-supercritical 1000mw thermal power unit was tracked and analyzed, and the metallographic structure and performance of the original tube sample and tubes after 23,400h, 56,000h, 64,000 h, 70,000 h and 80,000 h service were tested. The results show that the tensile strength, yield strength and post-break elongation meet the requirements of ASME SA213 S30432 after long-term service, but the impact toughness decreases significantly. The metallographic organization is composed of the original complete austenite structure and gradually changes to the austenite + twin + second phase precipitates. With the extension of time, the number of second phases of coarseness in the crystal and the crystal boundary increases, and the degree of chain distribution increases. The precipitation phase on the grain boundary is dominated by M 23 C 6 , and there are several mx phases dominated by NbC and densely distributed copper phases in the crystal. The service environment produces a high magnetic equivalent and magnetic induction of the material, the reason is that there are strips of martensite on both sides of the grain boundary, and the number of martensite increases with the length of service.

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.