Koichi Yagi

Results of a Japanese round robin on creep crack growth evaluation methods for Ni-base superalloys

Abstract Creep crack growth (CCG) tests on Ni-base superalloys were carried out in the Japanese VAMAS group as part of a round-robin program in order to assist the standardization of the CCG test method for creep-brittle alloys. The effect of temperature, load, specimen thickness and material microstructure on CCG behavior was investigated. The applicable range of fracture mechanical parameters to evaluate the CCG rate was evaluated. The CCG rate was characterized by the C∗ parameter independent of testing conditions in the range where the CCG rate accelerated. However, the acceleration stage occupied only a small portion of life time for the creep-brittle superalloys. In the range where the CCG rate was constant, the CCG rate and the fracture life could be predicted approximately by the Q∗ method based on the thermally activated process.

Inherent creep strength and long term creep strength properties of ferritic steels

Abstract Inherent creep strength is defined as a constant creep strength independent of time and microstructure. Sigmoidal inflection of a relation between stress and time to rupture is explainable in terms of a loss of creep strength followed by the advent of inherent creep strength. Although a large variation of short term creep rupture strength is observed, inherent creep strength is evaluated to be almost the same for several low alloy Cr-Mo steels. Creep rupture strength of the 12Cr-1Mo-1W-0.3V steel is higher than the other steels investigated in the short term region, however, it decreases with decrease in stress abruptly and it is almost the same as common inherent creep strength for low alloy Cr-Mo steels. On the other hand, creep rupture strength of the modified 9Cr-1Mo steel is higher than the other steels even at low stresses.

Thermal conductivity of SiC fine particles reinforced Al alloy matrix composite with dispersed particle size

The thermal conductivity of 10, 20, 30, and 40 vol % silicon-carbide-particulate-reinforced aluminum matrix composite was determined as a function of the volume fraction of SiC. The microstructures of the composites were examined with a scanning electron microscope. The size distributions of SiC particles were obtained by analyzing the micrographs. The interfacial thermal conductance was calculated with consideration of the particle size distribution. Both the thermal conductivity and interfacial thermal conductance decreased as the volume fraction of SiC increased.

The effect of carbide distributions on long-term creep rupture strength of SUS321H and SUS347H stainless steels

Abstract The creep fracture mode and long-term creep rupture strength have been investigated for SUS321H and SUS347H austenitic stainless steels. During creep, fine TiC and NbC precipitated in the matrix of the SUS321H and SUS347H steels, respectively, accompanying the re-dissolution of previously precipitated M23C 6 along grain boundaries, which accelerated grain boundary sliding and surface cracking. The creep rupture strength correlated with the stability and number density of grain boundary carbides.

Tetragonal to monoclinic transformation and microstructural evolution in ZrO2-9.7 mol% MgO during cyclic heating and cooling

The tetragonal (t) to monoclinic (m) transformation behaviour and its relationship to microstructural evolution were investigated by means of dilatometry and transmission electron microscopy for ZrO2–9.7 mol% MgO during cyclic heating and cooling between room temperature and 1490 K. In the as-sintered specimens, fine oblate ellipsoidal t-phase precipitates, 20–50 nm in diameter and 100–200 nm long, were distributed in the cubic (c)-phase matrix. They were below a critical size for transformation and exhibited no transformation in the first three cycles. In the fourth and further cycles, transformation occurred in two distinct stages. A low-temperature stage appeared at 850–1000 K on heating and at 400–700 K on cooling, while a high-temperature stage appeared at 1350–1400 K on heating and at 1000–1200 K on cooling. With the increasing number of cycles, at first the size of low-temperature stages increased and then decreased above ten cycles accompanying the development of the high-temperature stage. During cyclic heating and cooling, coarsening of ellipsoidal precipitates and decomposition of c- and t-phases occurred. As a result of the decomposition, MgO particles and a new m-phase containing a very low concentration of MgO were produced. The coarsened ellipsoidal t-phase precipitates were responsible for the low-temperature stage. The new m- or t-phase containing very low MgO produced by the decomposition was responsible for the high-temperature stage.

The influence of fracture mechanisms on the creep crack growth behavior of 316 stainless steel

The effect of creep fracture mechanisms on creep crack growth behavior was examined on 316 stainless steel. Three kinds of creep fracture mechanisms, i.e. wedge-type intergranular fracture, transgranular fracture and cavity-type intergranular fracture, were observed depending on testing conditions. The relationship between creep crack growth rate and the fracture mechanics C* parameter was dependent on the creep fracture mechanism. Creep crack growth rate was interpreted by the creep rupture ductility corresponding to each creep fracture mechanism, except for cavity-type fracture mechanism. The crack growth due to the cavity-type fracture mechanism must be investigated by taking into account the many cracks and cavities which are formed ahead of the crack tip.

Life prediction by the iso-stress method of boiler tubes after prolonged service

Abstract Internal pressure creep rupture strength was investigated on 1·25Cr-0·5Mo, 2·25Cr-1Mo and 9Cr-1Mo steel tubes which had been used for 153 500 h as the high temperature reheater tubes in a power station boiler. The applicability of an Iso-stress method to predict the remaining life was discussed. A linear relationship between the testing temperature and the logarithm of the time to rupture for 1·25Cr-0·5Mo and 2.25Cr-1Mo steels after prolonged service was found. The remaining life for these two steel tubes can be predicted using the Iso-stress method. However, 9Cr-1Mo steel after prolonged service was strengthened at higher temperatures in Iso-stress tests, and the relationship between testing temperature and logarithm of time to rupture was not linear. The strengthening during the Iso-stress tests at higher temperatures was caused by microstructural change of (Cr, Fe)2N nitrides precipitated during prolonged service.

Long-term creep strength and strength reduction factor for welded joints of ASME Grades 91, 92 and 122 type steels

Creep rupture data of base metal and welded joints of several creep strength enhanced ferritic (CSEF) steels was collected and long-term creep strength of those steels was evaluated by the SHC committee in Japan. The previous allowable tensile stress of those steels regulated in METI Thermal Power Standard Code was reviewed and weld strength reduction factor was investigated. The master creep life equations for not only base metal, but also welded joints were developed. According to the evaluated creep rupture strength, allowable tensile stress of ASME Grade 122 type steels was revised in December 2005 and those of ASME Grade 91 and 92 type steels was revised in August 2007. The creep strength reduction factor obtained from 100,000 hours creep rupture strength of welded joints and base metal was given as a function of temperature.

Degradation in tensile and creep properties of 2.25Cr-1Mo steel by long term service in plant

Abstract Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 105 h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.