In order to determine the suitable manufacturing condition of 12%Cr alloy steel casing castings for advanced steam turbines, which improve efficiency for fossil fuel power plant by increasing steam condition, the research into chemistry, heat treatment and welding was carried out, and a partial model casting manufactured by this steel was evaluated. The laboratory test results indicated that the elevated temperature tensile strength and the room-temperature toughness were directly related to the room-temperature tensile strength. The stress rupture strength was found to be proportional to the tensile strength as well as the tendency toward increasing number of precipitates. The weldability of 12%Cr cast steel was likely to be the same as that of ordinary 1%CrMoV cast steels. On the base of laboratory studies and the evaluation of a partial model casing, 12%Cr alloy steel casing casting were successfully manufactured offering the integrity and the excellent creep rupture strength.
Use of the ultra-high temperature and ultra-high pressure steam to improve the efficiency of the fossil power plants, have been actively studied and the verification tests have also been carried out. As a material for the above steam conditions, 12%Cr heat resisting cast steel, has been newly developed, which is able to raise the steam temperature of the fossil power plants to 593°C. This material has better creep strength and creep rupture strength than the conventional Cr-Mo steel, in addition, it has high ductility and toughness, and the weldability is also considered. This newly developed material has been in the process of the manufacture to be used for the ultra-super critical fossil plant.
Creep rupture strength of 12 wt%Cr steel for boiler drastically changes due to chemical compositions and heat treatments. The relationship between microstructure, affected by alloying elements V, Nb and N and tempering conditions, and creep rupture strength was investigated by means of transmission electron microscopy. The results are summarized as follows. (1) Finely dispersed precipitate of vanadium nitride (VN) increased linearly according with nitrogen addition for 0.25 wt%V steels. The VN which did not change in size during creep, strongly improved long-term creep rupture strength. (2) The low niobium steels with 0.05 and 0.1 wt%Nb had high and stable creep rupture strength due to precipitation strengthening of VN and Nb (C, N) effectively. However, in case of higher niobium content, the amount of insoluble Nb (C, N) significantly increased and coarse Nb (C, N) coexisted with VN, which resulted in the deterioration of creep rupture strength. (3) Long-term creep rupture strength at 600 and 650°C significantly dropped for the steel tempered at 750°C because of softening and recovery due to high dislocation density in tempered martensite. In case of 800°C tempering, the long-term creep rupture strength was improved.
Effects of W addition, Ni and C contents on tensile properties, toughness, creep rupture strength and microstructure in 12Cr-Mo-V-Nb-N steel rotor forging were investigated, using specimens with various amounts of W(0 to 1.24wt%), Ni(0.40 to 1.35wt%) and C(0.12 to 0.19wt%). The results are summarized as follows. (1) Creep rupture strength was remarkably improved without the decreasing of toughness, by the addition of about 1%W with lower C and higher Ni contents. (2)The improvement of creep rupture strength in advanced 12Cr steel is mainly caused by the depression of dislocation recovery and the formation of sub-grain structure during creep deformation brought from solid-solution strengthening by W addition. (3)Trial rotor forging was manufactured based on the above basic research work, and it was proved that this forging had improved creep rupture strength and was satisfied enough with the mechanical properties and quality as a turbine rotor forging. (4) Also in the turbine blade and casing materials, it was proved that the creep rupture strength was remarkably improved by modifying the chemical composition similar to rotor material. It is expected that the advanced 12Cr steels obtained from the above R&D are applied for rotor, blade and casing in the future 593°C ultra-super critical turbine unit.
Effect of chemical compositions on creep rupture strength and fracture toughness in 12Cr type steel for 593°C/316 atg Ultra-Super-Critical (USC) steam turbine were investigated. In this study, as a rotor material for 593°C USC steam turbine, 11Cr-1.2Mo-W-V-Nb-N steel were newly developed. On the basis of preliminary studies, a trial large rotor forging 70 t in weight was manufactured by the developed material. This rotor forging had high creep rupture strength compared with conventional 12Cr steel rotor forging and was superior to quality and the other mechanical properties.
To improve the heat rate in the thermal power station, the development of an ultra super critical steam turbine has projected. A 12%Cr steel rotor forging which has higher creep rupture strength than conventional rotor forging has been required as the turbine rotor for a steam temperature of 593°C. Creep rupture strength at 593°C for 105 h has been required to be over 10 kgf/mm2. To achieve this requirement, the effect of carbon and nitrogen contents on creep rupture properties were studied using small laboratory heats based on low Si-10Cr-1.5Mo-0.05Nb-0.17V series steels. The excellent creep rupture properties were observed at the range of C: 0.130.15%, N : 0.040.05%. Based on the results of laboratory heats, the rotor forging for the demonstration tests was manufactured from 39 t ESR ingot. Creep rupture strength at the center core of the rotor forging was 12.4 kgf/mm2 at 593°C for 105 h. No drop of creep rupture ductility and no notch embrittlement were observed even for long term tests.
An extensive study has been made on the effect of rolling condition on recrystallization of austenite, dissolution behavior of alloying elements and mechanical properties of 9Cr-1Mo-V-Nb steel. Thereafter, metallurgical factors for improving mechanical properties have been investigated and a suitable rolling condition for thermo-mechanical control process was determined. The results obtained are as follows. 1) In the higher slab reheating temperature, especially above 1200°C, dissolution of Nb is accelerated and creep rupture strength increases. 2) The recrystallization in hot working occurs above 1 000°C, and makes fine grain structure, thereby improves toughness of the steel. 3) Recovery is mostly completed in the steel rolled at the non-recrystallizing region above 900°C. The steel shows higher resistance to the softening by tempering. 4) However, in the steel rolled at non-recrystallizing region below 900°C, there is a fear of decrease in tensile and creep rupture strength. 5) Suitable mechanical properties according to the required material specification can be obtained by thermo-mechanical control process through controlling precipitation behavior.
A trial has been made on the clarification of mechanism of embrittlement of 9Cr-2Mo dual phase steels during aging in the temperature range from 500 to 600°C. The focus is placed mainly on the relationship between toughness and precipitates of Laves phase and/or carbides. The precipitation of Laves phase causes a considerable decrease in toughness in rather short-term aging (600°C×100h for example). In the case of long-term aging (600°C×1000 h), however, the toughness depends mainly on the total amounts of the precipitates including both Laves phase and carbides. The preciptation of the Laves phase is markedly retarded by the decrease of Si content, resulting in excelent toughness. A harmful effect of P-addition on toughness is also observed. It is found, however, that no intergranular fracture is observed at below transition temperature, which probably suggests that the embrittlement is caused by some different mechanism from the so-called temper embrittlement.
At present, high strength ferritic steel is required as material for the superheater and reheater tubes of super-critical and advanced power plant, instead of austenitic stainless steels. Based on the finding that the replacement of a part of Mo of 912%Cr-Mo steels by W is very effective for increase in creep rupture strength, a 9Cr-0.5Mo-1.8W-Nb-V steel whose 105 h creep rupture strength at 600°C reached 157 MPa was developed for boiler tube. This steel also possesses excellent weldability and notch toughness, and its allowable tensile stress at 600°C is 85 MPa, which is significantly greater than that of 9Cr-Mo steels and 18Cr-8Ni austenitic steels. The creep rupture strength of TIG weld joint is nearly the same as that of the mother tube. With MITI's approval, several tertiary superheater tubes of SUS321HTB of a power plant were replaced by the boiler tubes made of the new steel. The boiler has subsequently operated without any trouble for more than 13000 h at a tube temperature of 600°C and a design stress of 85 MPa. These investigations demonstrate that 9Cr-0.5Mo-1.8W-Nb-V steel can replace the 18-8 series austenitic steels and that it is suitable for headers and piping as well as tubes.
The two oil crises of 1973 and 1975 spurred the development of ultra-supercritical power plants for greater energy savings by higher steam temperature and pressure. The development project has been mainly led by the Electric Power Development Co. Ltd. in Japan and Electric Power Research Institute in the United States. Since the existing steels are not strong and corrosion resistant enough to meet the intended application, it is a pressing need to develop high-strength steel for boiler tubes that can withstand the intended ultra-supercritical steam temperature and pressure. After study of solid solution strengthening of the matrix, precipitation strengthening, prevention of sigma phase formation and other factors involved in austenitic heat-resistant 20Cr-25Ni-1.5Mo-Nb-Ti-N steel has been developed. The steel has creep rupture strength of over 88 MPa at 700°C and 100000 h and excellent corrosion resistance, and can be produced by continuous casting.
A286 iron-base superalloy has been planned to be applied to rotor material of ultra-super critical steam turbine in which main steam temperature is 649°C, because of its high strength at high temperatures. In this case, a large ESR ingot weighing about 40 t is necessary. However, in the large ESR ingot of this material, "A" segregation is easily formed which may deteriorate the properties of the forgings. Therefore, characteristics of "A" segregation and mechanical properties of the segregated region, comparing with the normal region, in large A286 alloy forgings made from the 1 0001350 mm diameter ESR ingots with different Ti contents have been investigated. The following results are obtained; 1)Lowering Ti content is favorable for reducing "A" segregation. 2) Tensile properties, creep rupture properties, high and low cycle fatigue strength except tensile ductilities below 200°C and low cycle fatigue strength in higher strain than 1.0% are not deteriorated by "A" segregation in A286 alloy with 1.9% Ti. 3) Lowering Ti content improves the tensile ductility and low cycle fatigue strength in higher strain range. 4) Influence of "A"segregation on mechanical properties can be well explained by the difference in hardness between normal and segregated regions after work hardening by each test.
Optimum aging condition suitable to large parts made of low carbone low titanium 15Cr-26Ni-1.25Mo iron-base alloys was examined. The alloys are beneficial to making rotors for ultra-super critical steam turbine for generator. After solution treatment, the alloys were aged at 740°C for 1 through 64h, or 700 to 780°C for 16h. The experimental results are summarized as follows. 1) Aging at a range of 720 to 780°C for 16h yielded spheroidal γ' particles. But, after aging at 780°C for 16h coarsened γ' particles precipitated on the grain boundary. 2)Superior tensile properties were resulted after aging at 740°C for 16-32h or 740-760°C for 16h. 3) Creep properties at 650°C were affected by aging temperature. The long-term creep strength was hardly affected by the aging temperature in the range of 710 to 760°C for 16h. Creep ductility was significantly improved with elevating aging temperature up to 760°C. 4) Low cycle fatigue strength at 650°C was not affected by the aging conditions when tested under constant total strain amplitude in this work. 5) A drum with a weight of 4.5t was produced and solutionized and aged at 740°C for 32h. The test pieces were cut from typical four locations of the drum, and creep tested at 650°C. It was confirmed that dispersion of creep strength with respect to the location was negligible.
The effect of Nb, Ti and C contents on the creep rupture strength of a 20Cr-25Ni austenitic steel developed for ultra super critical pressure boiler tubes, has been investigated from metallographic viewpoints. In the case where only Nb was added, the creep rupture strength of the steel reached its maximum when the contents of Nb and C coincided with the NbC solubility curve and the stoichimetric ratio of NbC. When Ti was added together with Nb, the creep rupture strength of the steel was higher than if only Nb was added, and the strength was maximized by adding to the steel, maximum amounts of Nb and Ti which could be dissoluable at the solution treatment temperature. These results were explained by the fact that fine Nb (C, N) and Ti (C, N), precipitated in the early stage of creep, subsequently induced a uniform and fine precipitation of M23C6 which was delayed in coalescence and growth.
To develop a new low alloy steel for high pressure-low pressure mono-block steam turbine rotor, considerable studies have been performed. The targets of the new low alloy rotor steel development are to have same high temperature strength as those of conventional 1CrMoV steel for high pressure side of rotor, and to have excellent both room temperature strength (Yield strength is higher than 70 kgf/mm2) and fracture toughness (FATT is lower than + 80°C) for low pressure side of rotor. The effects of alloy composition on mechanical properties and the heat treatment characteristis of low alloy were investigated. As a result, the 21/4 CrMoV steel containing low silicon manufactured by vacuum carbon deoxidization process was found to show the best performance for mono-block steam turbine rotor. The 21/4 CrMoV steel rotor for mono-block steam turbine was produced by commercial base. Tensile, impact, stress rupture and metallurgical test were conducted on this rotor, and it is confirmed that this rotor has excellent yield strength and impact propertis at low pressure side and good stress rupture strength at high pressure side.
In order to investigate creep behavior and the fracture mechanism of a Cr-Mo-V steel, creep tests were carried out over long-term periods in a temperature range of 500675°C. Metallographic observations were also conducted on the fractured specimens. The results obtained are summarized as follows: (1)A transition in the plot of steady-state strain rate versus applied stress occurs at a certain stress level, yielding two values of stress exponents at any temperature within a range of 500650°C. These transition levels are much the same as those in transition behavior from transgranular to intergranular fracture. (2)In the transgranular fracture region, dislocations are nonuniformly distributed and clusterings of dislocations are seen. In the intergranular fracture region, dislocations are uniformly distributed and the formation of subgrain structures are seen. (3)The stress level at which instantaneous plastic deformation starts (regarded as the Orowan stress), was estimated to be σ/E=1.5 2.0×10-3. This Orowan stress level corresponds to the transition level which was determined by the change in stress exponent and fracture mechanism. From the above results, it is considered that creep occurs in a form of crystal deformation above the Orowan stress level. This is due probably to the dislocations by-passing particles. Below the Orowan stress level, dislocations move by climbing around particles; eventually, subgrain formation occurs.
A new corrosion resistant Fe-50%Ni-30%Cr-2%Mo austenitic alloy developed for super heaters has a high creep rupture strength at temperatures as high as 1073 K. The creep rupture strength of this alloy is equivalent to that of 17-14CuMo, one of the strongest commercial alloys. Unlike ordinary commercial austenitic heat resisting steels strengthened by carbide dispersion, which tend to decrease in the creep rupture strength at higher testing temperatures and under lower creep stresses, this alloy retains its strength in a temperature range between 823 and 1073 K for more than 10000 h. This excellent long term creep rupture strength was found to be due to a high density and a uniform dispersion of intragranular precipitates of bcc chromium phase. The dispersion strengthening due to the bcc chromium was unambiguously demonstrated by a creep test of a preaged specimen with a controlled dispersion of the precipitates. Coarsening of the intragranular bcc chromium precipitates was slower than that of the carbides, which lead to the smaller decrease in the creep rupture strength at higher testing temperatures and under lower creep stresses.
The high temperature creep resistance of as solution treated specimen of a Ni-30Cr alloy containing 0.04wt% carbon has been investigated by comparing with that of the specimen aged at 900°C for 1000 h, focusing on the difference in carbide morphology. Minimum creep rate of as solution treated specimen was two orders of magnitude less than that of the aged one under higher stress levels. In as solution treated specimen crept under higher stress levels, most of grain boundaries were covered with intergranular carbides and the region consisting of fine carbides precipitated on dislocations were formed along grain boundaries. Corresponding to the formation of this region, the marked reduction in minimum creep rate occurred. Carbide precipitation on dislocation during creep was attributed to the formation of the region with high dislocation density along the grain boundaries due to the high area fraction in grain boundaries covered by carbides.
Quantitative measurements of the eutectic carbide morphology have been conducted for HK40 and HP steels aged at 1100°C up to 3000h. The average area (S), perimeter (L) and volume fraction of eutectic carbides increased with the increase in aging time, and the precipitation density decreased. These morphological parameters of the HK40 steel changed monotonously with the increase in aging time. While, in the HP steel, there were no changes in these parameters up to 1000h, but after 1000h or more, all of these parameters changed abruptly. Using a parameter of L2/S to assess quantitatively the shape of eutectic carbides, it was found that the shape of eutectic carbides in the HP steel turned to complicated ones after long term exposure, whereas that in a HK40 steel turned to rounded ones. In a short time, the number of secondary carbides precipitated on dislocations in the HK40 steel was larger than that in the HP steel. After long term exposure, many secondary carbides were observed in the HP steel, irrespective of no carbides in the HK40 steel. Secondary carbides remaining after long term exposure in the HP steel coalesced with eutectic carbides and the shape of eutectic carbides turned to complicated ones. It is concluded that the changes in the morphology of eutectic carbides with aging time depends on the precipitation morphology of secondary carbides.