Advanced High Cr Ferritic Steels Research Articles

Mechanism of microstructural deterioration preceding type IV failure in weldment of Mod.9Cr-1Mo steel

The objective of the present study was to elucidate the cavity formation mechanism of Type IV failure in weldment of advanced high-Cr ferritic steels. A welded joint of Mod.9Cr-1Mo steel was creep tested at 650 °C under 83 MPa. The creep fracture mode was Type IV failure in the heat affect zone (HAZ). Microstructural characterization of the HAZ and the fracture location, were performed before and after the creep test. The Type IV cracking started in the inter-critical HAZ at a location having fine grain size and coarse M23C6 precipitates. Moreover, the grain structure of the inter-critical HAZ, which is a mixture of soft α and hard α’ grains, plays an important role in the stage of cavity evolution into a crack along the grain boundary. This is due to the heterogeneity of local strain between the two kinds of grains. By a synergistic effect of the strain concentration, the coarse precipitates and heterogeneous strain distribution among grains in the inter critical HAZ, facilitates the nucleation and growth of creep cavities, resulting in premature failure of welded joints.

J0310301 A-USCボイラ用具材溶接継手のクリープ強度

The commercialization of a 700℃ class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project in Japan. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 650-700℃, and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 650℃. In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type-IV failure in the heat-affected zone is a concern. The high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in both base metal and weldment, is a candidate material for the Japanese A-USC boiler. In the present study, long-term creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. The creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5-10 times longer than that of the conventional 9Cr steel welded joints at 650℃. The effect of welding procedure was also investigated.

Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler

The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).

Creep Damage Evaluation by Hardness in Advanced High Cr Ferritic Steels

The apparent activation energy for rupture life sometimes changes from a high value of short term creep to a low value of long term creep. This change results in overestimation of rupture life recognized recently in advanced high Cr ferritic steels. The present study examined how to detect the decrease of activation energy in 9-12 %Cr steels with tempered martensitic lath microstructure. During aging without stress hardness of the tempered martensite microstructures remains almost constant in short term, whereas it decreases with increasing time after long term exposure. The onset of hardness drop can be a good measure of the decrease of activation energy. Causes of the hardness drop and the decrease of activation energy are discussed.

Prediction of Breakdown Transition of Creep Strength in Advanced High Cr Ferritic Steels by Hardness Measurement of Aged Structures at High Temperature

Recent researches have shown the premature breakdown of creep rupture strength in long term creep region of advanced high Cr ferritic steels. As safe operation of power plants becomes a serious problem we should be able to detect and predict the breakdown transition of creep rupture strength. Some methods for detecting the breakdown transition have been presented till now like the measurement of reduction of area after creep rupture and particle size of laves phase. However it will be more economic if we make use of non-destructive tests, for example, hardness testing. In this paper 3 types of ferritic steels with different Cr concentration have been studied. The results suggest that the hardness of aged structures is constant independently of exposure time in short term region, whereas the hardness breaks down in long term region. The boundary of breakdown in hardness coincides with that of breakdown in creep rupture strength.

Causes of breakdown of creep strength in 9Cr–1.8W–0.5Mo–VNb steel

Premature breakdown of creep strength is a serious problem to be solved in long-term creep of advanced high Cr ferritic steels. The material studied was ASTM grade 92 steel crept at 550–650 °C for up to 63 151 h. Stress exponent for rupture life decreases from 17 in short-term creep to 8 in long-term creep, confirming the breakdown in the steel. The steel shows ductile to brittle transition with increasing rupture life, and the breakdown accords with the onset of brittle intergranular fracture. Creep cavities are nucleated at coarse precipitates of Laves phase along grain boundaries. These findings suggest the following story of the breakdown of creep strength. Laves phase precipitates and grows during creep exposure. Coarsening of Laves phase particles over a critical size triggers the cavity formation and the consequent brittle intergranular fracture. The brittle fracture causes the breakdown. The coarsening of Laves phase can be detected non-destructively by means of hardness testing of the steel exposed to elevated temperature without stress.

Creep Fracture Analysis of W Strengthened High Cr Steel Weldment

Type IV cracking in heat affected zone (HAZ) of weldment is a problem for advanced high Cr ferritic steels. The present paper investigates the creep properties and microstructures of W strengthened P122 steel weldment at 923K. From the investigation of creep properties of simulated HAZ, it is clarified that heating up to around Ac3 during welding minimized the grain size and creep strength. Most of the welded joint specimens were type IV fractured in fine-grained HAZ and resulted in shorter creep lives than those of the base metals. Electron beam welded joint with very narrow HAZ also showed the brittle type IV fracture due to the formation of creep voids and cracks. The growth of intergranular precipitates was faster for pine-grained HAZ. On the basis of experimental results, the FEM code that simulates type IV crack growth behavior has been developed. The vacancy diffusion under multi-axial stress condition in HAZ of weldment is analyzed.

Effect of Co Addition on Microstructure in High Cr Ferritic Steels

Co is one of the interesting alloying elements in advanced high Cr ferritic steels used to improve their creep properties at elevated temperatures. However, it has been reported that Co bearing tends toward rapid deterioration in creep property at long-term testing. In this study, microstructures were studied in detail by comparing two kinds of 9% Cr ferritic steels with 3% Co and Co free for better understanding difference in those creep behaviors.Creep property of the steel with 3% Co was superior to that of the steel without Co within this study, as many researchers pointed out. Adding Co certainly suppressed δ-ferrite formation and there was large difference in prior-austenite grain size between two steels after tempering. In addition to this macroscopic difference, there was also remarkable change in precipitation behavior between them. It was found that heterogeneous precipitation of inter-metallic compounds and MX type carbonitrides was observed in Co free steel, while any extreme localization of such precipitation were not seen in 3% Co steel.It was deduced that such microstructural difference in connection with δ-ferrite formation between two steels was an important factor to understand better high temperature creep properties in 3% Co steel rather than those in Co free steel.

高W含有10Crフェライト系耐熱鋼における微細整合析出Laves相

In recent advanced high Cr ferritic steels, it is known that the Laves phase precipitates during creep test when the steels contain a relatively high level of Mo+W content. The Laves phase is believed to be granular in shape and to exist in the boundaries of lath, block, packet and prior austenite grains, but a number of fine Laves-phase precipitates are found to exist even inside the martensite lath in the tempered steels containing 10 mass% Cr and 4.6 mass% W. The shape of this Laves phase is plate-like, and the size is below 300 nm. The crystallographic relationship between the fine Laves phase and the matrix phase is as follows; (111)ferrite//(001)Laves and [011]ferrite//[110]Laves. In the microstructure aged at 923 K for 714 Ms, the fine precipitates of the Laves phase disappear inside the martensite lath, and coagulated Laves phase is observed at the lath-, block-, or packet- boundaries. Only the low C steel containing 0.08% C and 3% W shows a few amounts of the fine precipitates of the Laves phase after aging at 923 K for 7.268 Ms. This compositional dependence of the precipitation behaviour can be understood qualitatively with the aid of the System-Free-Energy Concept.

Current Progress in Advanced High Cr Ferritic Steels for High-temperature Applications.

The article presents recent developments and future trends in high Cr ferritic heat resistant steels. Research programs are underway worldwide to improve the performance of 8 to 13% Cr ferritic steels for high temperature applications of up to 650°C. We developed the super 12% Cr heat resistant steel called TAF steel in 1956. The creep rupture strength of TAF steel is two or three times higher than those of H46 and AISI422 at 600 and 650°C. Our research in this area was well ahead of work being conducted in other countries.Recently the author succeeded in developing TR1100, TR1200, TB9 and TB12 with excellent high temperature properties and room temperature toughness through the improvement of TAF steel. In the near future, these new steels will be applied to turbine rotors, blades, and boiler tubes for advanced supercritical power plants, to fuel cladding, wrapper and steam generator tubing for fast breeder reactors, and to first wall materials for conceptual fusion reactors. These steels are a key to making these plants and reactors practical.