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 .

Remaining-life assessment of high temperature components using the small punch (SP) creep testing technique necessitates the evaluation of SP load (F)/uniaxial stress (σ) conversion factor, F/σ, obtained by comparing the SP and uniaxial creep test results. In the present study, the SP creep tests were carried out at 850°C on various Ni-base alloys having different reduction of area in the range of 0.05-0.67 to investigate the influence of creep ductility on the value of F/σ. The F/σ value was determined for each alloy by correlating SP creep rupture data with corresponding uniaxial creep ones. The experimental results revealed that the F/σ value was not well correlated with Vickers hardness, but it increased almost linearly with increasing reduction of area up to around 0.4. This result indicated that the SP creep rupture data could be converted to the uniaxial data if the creep ductility on a given material was available.

The change in hydrogen desorption characteristic due to creep was investigated to examine the possibility of hydrogen as tracer for detecting and evaluating the creep damage accumulated in high Cr ferritic boiler steel, Gr.91. Hydrogen charging into the creep specimen was conducted by means of cathodic electrolysis. Next, the thermal desorption analyses (TDA) were carried out at temperature range from room temperature to 270°C for measuring the hydrogen evolution curve. The experimental results revealed that the amount of hydrogen desorbed during analysis, C H , increased with increasing creep life fraction, although the trend of increase in C H was strongly dependent on the stress level. Moreover, there was an almost linear correlation between the logarithm of C H measured on the creep ruptured specimen and the Larson-Miller parameter (LMP), which was approximated by “log C H = 0.39 LMP – 13.4”. This can be a criterion for creep rupture and means that as far as the C H does not reach the line, the rupture never occurs.

A new 9%Cr steel with high boron levels (boron steel) has been developed by optimization studies on steels and alloys that are applicable to advanced ultra-super critical power plants operated at steam conditions of 700°C and 30 MPa and above. The composition and heat treatment condition of boron steel was optimized by the initial hardness, tensile strength, yield strength, and Charpy impact values on the basis of the fundamental investigation with the stability of the long-term creep strength. Creep testing of boron steel was conducted at temperatures between 600 and 700°C. The creep rupture strength at 625°C and 105 h is estimated to be 122 MPa for the present 9% Cr steel with high boron by Larson-Miller parameter method. Furthermore, physical properties as a function of temperature, metallurgical properties, tensile properties, and toughness were examined to evaluate the applicability of the steel for a 625°C USC power plant boiler. It was also confirmed that the steel has good workability for such an application by the flaring and flattening tests with tube specimens having an outer diameter of approximately 55 mm.

Recent advances in materials technology for boilers materials in the advanced USC (A-USC) power plants have been reviewed based on the experiences from the strengthening and degradation of long term creep properties and the relevant microstructural evolution in the advanced high Cr ferritic steels. P122 and P92 type steels are considered to exhibit the long term creep strength degradation over 600°C, which is mainly due to the instability of the martensitic microstructure strengthened too much by MX carbonitrides. This can be modified by reducing the precipitation of VN nitride and by optimizing the Cr content of the steels. An Fe-Ni based alloy, HR6W strengthened by the Fe2W type Laves phase is found to be a marginal strength level material with good ductility at high temperatures over 700°C and to be used for a large diameter heavy wall thick piping such as main steam pipe and hot reheat pipe in A-USC plants, while Ni-Co based alloys such as Alloys 617 and 263 strengthened by a large amount of the y’ phase are found to be the high strength candidate materials for superheater and reheater tubes, although they are prone to relaxation cracking after welding and to grain boundary embrittlement during long term creep exposure. A new Ni based alloy, HR35 strengthened by a-Cr phase and other intermetallic phases has been proposed for piping application, which is specially designed for a good resistance to relaxation cracking as well as high strength and a good resistance to steam oxidation and fire-side corrosion at high temperatures over 700°C.