Fujimitsu Masuyama

10-Year Experience With T23(2.25Cr-1.6W) and T122(12Cr-0.4Mo-2W) in a Power Boiler

Tungsten strengthened ferritic steels, 2.25Cr-1.6W-V-Nb and 12Cr-0.4Mo-2W-V-Nb-Cu have been developed and approved by the ASME Boiler and Pressure Vessels Code Committee for use in Section I construction, designated as T23 and T122, respectively. A field exposure test installing both steel tubes in service along with comparative materials in the tertiary superheater and secondary reheater of a 156 MW utility power boiler has been conducted since April 1993. The tubes were removed to confirm their material properties and corrosion/steam oxidation behaviors after 1-year, 3-year, 6-year, and 10-year periods of service. The tensile and creep rupture strengths of both steels showed no remarkable change during service. Examination of steam oxidation scale on the inner surface of the tubes indicated that the scale growth rate of T122 was extremely small following 1-year service. The growth rate and morphology of steam oxidation scale is discussed, as compared with conventional materials.

Japanese experience with steam oxidation of advanced heat-resistant steel tubes in power boilers

Abstract Japanese experience of steam oxidation behavior of advanced heat-resistant steel tubes in power boilers is discussed. Severe scale separation, cracking, and exfoliation were observed in T91 pendant reheater tubing in a Japanese utility boiler after around 40,000 hours of operation. Separation occurred at the interface between the inner and outer layers of scale. A high-pressure steam oxidation test rig in which the steam conditions could be controlled in a similar manner to that of an actual boiler was developed and T91 steel samples were tested up to 10,269 hours. The scale separation behavior of T91 was reproduced in the steam oxidation test. The growth rate of T91 was lower than that for conventional 9Cr-1Mo steel reported by EPRI. The scale separation was related to void formation at the interface between the inner and outer layers of scale, as well as the transformation of magnetite into hematite in the outer layer. Field exposure testing was carried out for T23 and T122 for 80,000 hours, and the properties of steam oxidation scale were obtained.

Long-Term Service Experience With Advanced Austenitic Alloys in Eddystone Power Station

A service exposure test of advanced austenitic alloys was performed at Eddystone Power Station Unit 1. The alloys tested were TP347HFG, SUPER304H and HR3C as well as 17-14CuMo as a reference material. They were installed in the final super-heater section in 1991 and were removed in 2004. The service exposure duration is 75,075 h with the steam temperature and pressure estimated 615 °C and 35 MPa, respectively. Post exposure examination has proved no marked hot-corrosion and steam oxidation have occurred for the TP347HFG, SUPER304H and HR3C specimens, while severe hot-corrosion and exfoliation of steam-oxide films was pronounced in 17-14CuMo specimens. It is also found that HR3C exhibits the highest tensile strength with a lowest ductility and TP347HFG does vice versa, while the strength of SUPER304H is equivalent to that of 17-14CuMo with much better ductility. Short-term creep rupture strengths of these alloys have been reduced to some extent after the long-term service exposure and a longer-term creep rupture testing has been in progress. Microstructural evolution of the TP347HFG, SUPER304H and HR3C as well as 17-14CuMo during the long-term service exposure have been analyzed using a detailed TEM examination and the chemical analysis of extraction residue of the exposed specimens.Copyright

Phase Transformation and Properties of Gr.91 at Around Critical Temperature

During the fabrication processes of boiler components, heating, forming and welding processes are commonly applied to the materials used. The effect of heating and strain induced by the these fabrication processes are very important in maintaining the integrity of creep strength enhanced steels, because it is well known that the mechanical properties of normalized and tempered high strength steels are relatively sensitive to thermal and work history. In particular, while Gr.91 steel has been used extensively for boiler and pressure components over the last two decade, the heat treatment and forming procedures for this steel are not fully established based on fundamental transformation and strain behavior. This paper deals with the phase transformation behavior of Gr.91 during tempering at around the Ac1 temperature, and the hardness/creep properties of Gr.91 steel heated at temperatures from just below Ac1 to above Ac3 combined with strain.Copyright

Experimental observation of creep damage evolution in seam-welded elbows of mod. 9Cr-1Mo steel

Seam-welded elbows and straight pipe of the same size as actual high temperature reheat piping for boiler applications were manufactured, and internal pressure creep testing was conducted. Uniaxial creep testing was also performed in order to compare creep damage behaviours. Comparing the observed creep damage evolution and stress analysis results, a creep damage estimation method was discussed. Creep damage distribution varied depending on ovality in the cross sections of the seam-welded elbows. It is important that FEM models for creep analysis incorporate measurement results for the cross-sectional shape of seam-welded elbows. Using the nominal stress of the specimens and the average creep rupture data from small size uniaxial welded joint tests, a simplified prediction method was discussed, which is applicable to creep rupture time prediction for both large creep specimens and seam-welded elbows.

Advanced technology in creep life prediction and damage evaluation for creep strength enhanced ferritic steels

Abstract Creep damage mechanisms of recently developed boiler and steam-turbine steels are not well clarified, however, creep life prediction technology for such creep strength enhanced ferritic steels, grades 91, 92 122, etc. with tempered martensitic structure including welds has been strongly demanded by power plant operators and equipment manufacturers. In this paper, the technologies related to the creep life prediction and damage evaluation for creep strength enhanced ferritic steels being studied from the various aspects in Japan will be surveyed and presented. The creep life prediction of this type of steel must be grounded on the findings on microstructural degradation and creep softening in martensitic structure composed of very fine martensite lath, block, packet and prior austenite grains, and precipitation and dislocation structures. Physical properties response to the creep degradation and measurement and detection of localized creep damage/strength would be useful tools to develop diagnostic techniques for life prediction of creep strength enhanced ferritic steels. Other important techniques to support the creep life prediction based on creep-strain/rupture data are creep modelling and data analysis which have been successfully investigated to date.

Phase transformation behaviour of creep-strength enhanced 9%Cr steels

Abstract The phase transformation behaviour of Grade 92 steel has been investigated using differential thermal analysis (DTA) and hardness measurements. In our DTA experiments, disk-shaped samples were normalized at 1070°C and then tempered at temperatures between +10 and −40°C from the ferrite-to- austenite transformation temperature of 878°C (Ac1) at a rate of 30°C/min. From the DTA heating curve during normalization, the magnetic transition and Ac1 transformation were found to occur at 744 and 778°C, respectively. Two overlapping exothermic peaks corresponding to the magnetic transition and the formation of ferrite from austenite were observed in the temperature range 700 – 800°C in the DTA cooling curves after tempering at temperatures between −10 and +10°C from Ac1. The hardness values measured after the DTA experiments decreased with increasing tempering temperature up to around Ac1−15°C and then increased slightly with increasing temperature.

Cold Work Effect on Creep Rupture Strength of Austenitic Boiler Steels

In order to clarify the effect of cold work, warm work at working temperatures of up to 400°C and chemical compositions on the creep rupture strength of austenitic steels used for boiler tubing and high temperature support structures, long-term creep rupture tests were carried out on typical 18Cr-8Ni system steels consisting of TP304H, TP316H, TP321H and TP347H grade tubes and of TP321 plates. The long-term (100,000 hours) creep rupture strength of these steels was evaluated in terms of working ratio and Ni-equivalent. It was consequently clarified that creep rupture strength was substantially reduced in the cold-worked TP321 and TP321H materials, although warm-work resulted in less work-induced deterioration. It was also found creep rupture strength was enhanced by the higher Ni-eq in 18Cr-8Ni austenitic steels, and that the combined conditions of working ratio and Ni-eq govern the creep rupture strength criteria of weaker or stronger than as-received strength. Additionally the effect of cold work on the creep rupture strength and ductility of recently developed creep-strength enhanced 23Cr austenitic stainless steel (a candidate material for the hot end of superheaters in ultra-high temperature fossil-fired power plants) was considered. The strength of cold worked 23Cr austenitic steel was observed to fall below the as-received strength at stresses within about 120MPa, while re-solution annealing recovered the creep strength level to the as-received strength across the entire stress region.Copyright

Effect of Service Exposure on Material Properties of Austenitic Boiler Steels

A long-term field exposure test of austenitic stainless steels developed through the early 1980s was conducted at Eddystone Power Station Unit No.1, which has highest steam parameters worldwide to date. The experimental lengths include TP347 chromized, Tempaloy A-1, TP347HFG and 17-14CuMo/310S coextruded, and were service-exposed in the final superheater operated at steam conditions of 1170°F (632°C) and 4550 psig (31MPa) for about ten years. The test lengths were removed from the final superheater at operating duration points of about 20,000 and 50,000 hours. In this paper the post-service creep rupture properties and changes in mechanical properties are presented and discussed through comparative study with unused materials. Steam oxidation scale growth behavior of the test steels during high-temperature superheater service is also reported.Copyright

New Ferritic Steel Beyond Grade 92 and its Creep Degradation Assessment by Hardness Method for Grade 91

The alloy development R&D activities for weld construction components, such as boiler piping and headers of high-efficient fossil-fired power plants have been conducted to introduce creep strength enhanced ferritic (CSEF) steels, grades 91, 92, 122, 911, 23 and 24 for last three decades. The grade 92 among these CSEF steels has the highest creep rupture strength to increase the steam temperature up to 620°C however the weld heat affected zone is much weaker in creep than the base metal due to the Type IV failure. Alloy design trials or proposal of the candidate steels in laboratory level has been conducted to improve the creep rupture strength than that of grade 92. Presently it is becoming possible in a very near future to introduce the new high strength ferritic steels beyond grade 92 on the commercial basis, and also the new steels could mitigate the Type IV failure at the welds to be applicable at the maximum use temperature of 625°C and above. But the creep degradation behavior in such new advanced steels is not yet well investigated and it is not verified that the creep degradation/life assessment techniques studied for the existing CSEF steels are applicable. Therefore the creep degradation and softening behavior of new advanced CSEF steels were studied and the hardness creep life assessment technique developed using grade 91 was applied to investigate the similarity of degradation process with the conventional CSEF steels. The present paper introduces the development status of new advanced steels beyond grade 92 in Japan and deals with the creep degradation and softening behavior of the new steels in comparison with grades 91 and 92. INTRODUCTION To improve thermal efficiency and survive the frequent shutdowns and startups and load swing operations of fossil-fired power plant with higher steam parameters, it is essential to select materials having high creep strength, high thermal conductivity and low thermal expansion for the high-temperature components. In consideration of these requirements, high strength martensitic steels have been developed and introduced for practical application from the 1980s through to the present. Grades 91, 92, 122, etc. are the creep strength enhanced ferritic (CSEF) steels most well known and used in the modern high efficiency power plants. Grade 92 among the CSEF steels developed to date has maximum creep rupture strengths of approximately 130 MPa at 600°C and 65 MPa at 650°C for 100,000 hours. However there is strong demand for the development of CSEF steels having strength of 100 MPa level at 650°C for future advanced plants, including a 700°C class. The development of this type of high-strength martensitic steel beyond grade 92 has been conducted, and further enhancements in the creep resistance of 9-12%Cr steels used for boiler header/piping and steam turbine rotor applications are vital in order to achieve steam temperatures in excess of 625°C. This will lead to a further increase in the thermal efficiency of power plants whenever innovative steels have been commercially implemented. The feature of the conventional CSEF steels, grades 91, 92 and 122 has resulted in extensive utilization for high temperature components in high steam parameter fossil-fired power plants for last three decades. However a number of creep failure experiences with these steels used in the superheater/reheater tubes, main-steam/hot-reheat pipes and headers due to the uncertainty and instability of long-term creep properties have been reported. Therefore it is necessary to clarify the creep degradation and softening behavior in the newly developed advanced CSEF steels beyond grade 92 to confirm that the advanced CSEF steels exhibit similar creep degradation and softening behavior with the conventional CSEF steels, and that the life assessment methods for conventional CSEF steels are also applicable. In the research presented here, the development status of new advanced CSEF steels beyond grade 92 in Japan is introduced and evaluated, and the creep degradation and softening behavior of the new steels are investigated in comparison with grades 91 and 92 in terms of hardness based creep life assessment results. DEVELOPMENT OF ADVANCED CSEF STEELS Figure 1 shows elevation of the creep rupture strength of heat resistant steels (ferritic, meta-stable austenitic and stable austenitic) for boilers, viewed in terms of change in the 100,000 ETAM2014-1007 1 Published with permission. D ow naded rom htt://asm edigitallection.asm e.org/PT/edings-pdf/ETAM 2014/4074/4448174/etam 201007.pdf by gest on 10 N ovem er 2019 Figure 1 Historical Improvement of Creep Rupture Strength of Boiler Steels Figure 2 Development Progress of Ferritic Boiler Steels h creep rupture strength at 600°C for materials developed during the 20th century and to date. Regarding ferritic steels, low alloy steels or 9-12%Cr steels at about 40MPa of 100,000 h creep rupture strength had been used over a long period of years, and the problem of cost increases existed because, especially in the case of the superheater and the reheater, there was an alloy gap between the low alloy steels and 18Cr-8Ni austenitic steels, arising as a result of steam temperature elevation. Accordingly, development of high strength 9-12%Cr steels was initiated in order to fill this gap, and the materials for 60MPa class (first generation) were developed over the period from 1960 to 1970. Further developments were advanced, and creep rupture strength reached to the 100MPa class (second generation) in the 1980s, with the 130MPa (65MPa at 650°C for grade 92) class (third generation) achieved in the 1990s. The steels developed in the second generation and the third generation should be called as the CSEF steels which is defined as “a family of ferritic steels whose creep temperature strength is enhanced by the creation of a precise condition of microstructure, specifically martensite or bainite, which is stabilized during tempering by controlled precipitation of temper-resistant carbides, carbo-nitrides, or other stable and/or meta-stable phases” in ASME Code, Section IX, QW/QB-492 Definitions. Ferritic steels for 100MPa at 650°C (150MPa at 600°C) class, as the next generation, had been expected to emerge, and alloy design trials or proposal of the candidate steels in laboratory level had been conducted to improve the creep rupture strength than that of grade 92 Presently it is becoming possible to introduce the new advanced CSEF steels beyond grade 92 on the commercial basis, and also the new steels could mitigate the Type IV failure at the welds to be applicable at the maximum use temperature of 625°C and above. Figure 2 shows the latest development progress diagrams for conventional CSEF steels and advanced CSEF steels in the family of ferritic steels used for power boiler components. Three kinds of advanced CSEF steels developed in Japan (Low C-9Cr, B-9Cr and 9Cr-3W-3Co-Nd-B (SAVE12AD)) can be introduced as indicated in the figure. Low C-9Cr steel has been proposed based on the fundamental study on the effect of chemical composition of Cr, C, Ni and Al on the long-term creep rupture strength and precipitation behavior [1, 2]. B-9Cr has been studied to propose a new concept of alloy design to mitigate the creep degradation in weld heat affected zone (Type IV) and significantly improve the long-term and high temperature creep rupture strength based on the fundamental findings of the effect of soluble boron and nitrogen on the mechanism of the microstructural evolution and creep strengthening [3-7]. 9Cr-3W-3Co-Nd-B (SAVE12AD) has been developed through the extensive creep and characterization tests to optimize the chemical composition and heat treatment conditions for the determination of the allowable stresses to meet the use for temperatures exceeding that for the conventional CSEF steels. The development of this steel have been conducted mainly from the industrial stand point rather than the laboratory aspect with limited publication, but the original alloy design concept and particularly the role of unique compositional element of Neodymium (Nd) in the steel have been studied [8, 9] and the chemical composition and mechanical properties have been specified for industrial applications. Table 1 shows nominal chemical composition for three kinds of advanced CSEF steels in comparison with those for grades 91 and 92 conventional CSEF steels. Figure 3 compares the extrapolated 100,000 h creep rupture strengths of the steels [10], and those of grades 91 and 92 which are calculated from the METI allowable stresses. Since the strength values for B-9Cr are not yet officially given due to the limited data points when evaluated, the estimated band for the 2 Published with permission. D ow naded rom htt://asm edigitallection.asm e.org/PT/edings-pdf/ETAM 2014/4074/4448174/etam 201007.pdf by gest on 10 N ovem er 2019 100,000 h strength from the published data at the temperatures is shown in the figure. B-9Cr looks strongest among three Table 1 Nominal chemical composition of advanced CSEF steels, and grades 91 and 92 Figure 3 Comparison of 100,000 h creep rupture strengths of advanced CSEF steels Figure 4 Performance improvement of boiler steels steels, but the long-term creep tests to extrapolate the 100,000 h rupture strengths are presently ongoing. Low C-9Cr is weakest and shows remarkable drop in the strength at 650°C. In case of 9Cr-3W-3Co-Nd-B (SAVE12AD) it is found that the creep rupture strength at 650°C and 100,000 h is approximately 20% stronger than that of grade 92. Figure 4 shows the performance improvement of advanced CSEF steels comparing with conventional steels demonstrating the relationship between Cr content in steels and maximum applicable temperature at the design stress of 50MPa which is relatively low stress conditions, but the relative comparison between the steels can be done from the figure. The strength level of advanced CSEF is closing to the high