Masaaki Tabuchi

Variation in the Type IV cracking behaviour of a high Cr steel weld with post weld heat treatment

In this study creep tests were conducted on P122 steel cross weld specimens at 70 MPa after post weld heat treatment (PWHT) for different durations. Results indicated that the duration of PWHT did not influence the rupture time and all the specimens failed in a typical Type IV manner. However, differences were observed with respect to creep damage and microstructural changes that occurred in the fine grained heat affected zone (FGHAZ), the location of the fracture. It appears that factors other than changes in microstructure also affect the rupture life. One such factor could be the heterogeneity introduced in the weld joint due to the existence of different zones like weld metal, coarse grained HAZ (CGHAZ), FGHAZ and base metal, which differ vastly in their properties.

Microstructure and creep strength of welds in advanced ferritic power plant steels

Abstract The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 104 h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.

Estimation of creep crack growth rate in IN-100 based on the Q* parameter concept

Since the high-strength Ni-based superalloy, cast IN-100, is considered to be brittle at high temperatures, the stable creep crack growth region is limited. Therefore, technically, it is very difficult to perform creep tests and there are few experimental results on the creep crack growth behaviour of this material. We performed creep crack growth tests using Ni-based superalloy, cast IN-100, and derived the Q* parameter for this material, which characterizes the creep crack growth rate. Using this Q* parameter, we derived a law for the creep rupture life of this material.

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.

Suppressing type IV failure via modification of heat affected zone microstructures using high boron content in 9Cr heat resistant steel welded joints

Abstract Creep rupture strength at 923 K and microstructural evolution of welded joints have been investigated for high boron–low nitrogen–9Cr heat resistant steels developed at the National Institute for Materials Science (Japan). Welded joints were prepared from plates containing 47–180 ppm boron using gas tungsten arc welding and Inconel type filler metal, and showed superior creep properties to those of welded joints of conventional high chromium steels such as P92 and P122. No type IV failure was observed in the boron steel welded joints. A large grained microstructure was observed in the heat affected zone heated to Ac 3 (Ac 3 HAZ) during welding, whereas the grains are refined at the same location in conventional steel welded joints. The simulated Ac 3 HAZ structures of the boron steels have a creep life almost equal to that of the base metal. Large grained HAZ microstructures and stabilisation of M23C6 precipitates are probable reasons for suppression of type IV failure and improved creep resistance of the boron steel welded joints.

Evaluation of Creep Strength Reduction Factors for Welded Joints of Modified 9Cr-1Mo Steel (P91)

In order to review the allowable creep strength of high Cr ferritic steels, creep rupture data of base metal and welded joints have been collected and long-term creep strength have been analyzed in the SHC committee in Japan since 2004. In the present paper, the creep rupture data of 370 points for welded joint specimens of modified 9Cr-1Mo steel (ASME Grade 91) offered from seven Japanese companies and institutes were analyzed. These data clearly indicated that the creep strength of welded joints was lower than that of base metal due to Type IV fracture in HAZ at or above 600°C. From the activities of this committee, the master curve for life evaluation of welded joints of Gr.91 steel could be represented as follows: LMP==34154+3494(log σ)−2574(log σ)2, C=31.4 The reduction factor of 100,000 hours creep rupture strength of welded joint to base metal was concluded to be 0.75 at 600°C and 0.70 at 650°C for the Gr.91 steel.Copyright

Effect of welding process and groove angle on type IV cracking behaviour of weld joints of a ferritic steel

Abstract Creep tests were carried out at 923 K and various stress levels on weld joints of 11CrMoWVNb (P122) steel prepared via gas tungsten arc, electron beam, and laser welding processes. All the weld joints exhibited typical type IV cracking. However, the rupture life of the weld joints was found to increase with decreases in the width of the heat affected zone (HAZ) and the groove angle. A comparison of the results with those for the simulated fine grained HAZ (FGHAZ) revealed that the creep damage and the type of fracture of the weld joint differ considerably from those of the simulated FGHAZ specimen. In the weld joints fracture was characterised by low ductility and formation of creep cavities, whereas the FGHAZ specimens exhibited good ductility and hardly any creep cavities were observed in them. These differences are attributed to the presence of a triaxial stress state in the weld joints, due to the existence of zones such as parent metal, weld metal, FGHAZ, and coarse grained HAZ, which differ considerably in their creep properties. Changes in the width of the HAZ or the groove angle of the joints alter the stress state in the weld joint, leading in turn to corresponding changes in the creep behaviour of the weld joints. Finite element analysis of the stress–strain distribution in the weld joint specimens during creep also confirmed these influences.

Mechanisms for boron effect on microstructure and creep strength of ferritic power plant steels

Abstract Boundary and sub-boundary hardening are shown to be the most important strengthening mechanism in creep of 9%Cr steel. Soluble boron reduces the coarsening rate of M23C6 carbides near prior austenite grain boundaries during creep, enhancing the boundary and sub-boundary hardening for long times at 650°C. The enhancement of boundary and sub-boundary hardening retards the onset of acceleration creep, which decreases the minimum creep rate and improves the creep life. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalising heat treatment, which significantly reduces soluble boron and soluble nitrogen concentrations. The boundary and sub-boundary hardening are significantly reduced in the fine grained region of the heat affected zone (HAZ) of Gr.92 welded joints, promoting type IV fracture. Soluble boron produces substantially the same microstructure between base metal and HAZ in 9Cr steel welded joints, resulting in no type IV fracture at 650°C.

Evaluation of creep crack growth properties using circular notched specimens

It is important to evaluate the effect of multiaxial stress conditions on initiation and growth of creep cracks, when the laboratory data are subsequently applied to structural components under the same or similar stress state. The round robin tests of creep crack growth using circular notched specimens of 1CrMoV steel at 538 and 594 °C and 12CrWCoB steel at 650 °C were conducted by the Japanese VAMAS TWA25 group. The effect of notch depth and specimen size, i.e. stress multiaxiality on crack growth properties was investigated. The test procedure including criteria for crack length measurement by electric potential drop was established. The circular notched specimens fractured intergranularly and showed different crack growth behaviour from that of a CT specimen due to the multiaxial stress field. The creep crack growth rate for the same C* value increased as the ratio of the notch depth to specimen diameter, i.e. stress multiaxiality increased. The Q* evaluation method based on the thermally activated process can also be applied to the circular notched specimen.

Evaluation of Microstructures and Creep Damages in the HAZ of P91 Steel Weldment

The creep strength of welded joints in high Cr steels decreases due to the formation of Type IV creep damage in heat-affected zones (HAZs) during long-term use at high temperatures. This paper aims to elucidate the processes and mechanisms of Type IV creep damage using Mod.9Cr-lMo (ASME Grade 91) steel weldments. Long-term creep tests for base metal, simulated fine-grained HAZ, and welded joints were conducted at 550 °C, 600°C, and 650°C. Furthermore, creep tests of thick welded joint specimens were interrupted at 0.1, 0.2, 0.5, 0.7, 0.8, and 0.9 of rupture life and damage distributions were measured quantitatively. It was found that creep voids were initiated at an early stage of life inside the specimen thickness and coalesced to form cracks at a later stage of life. Creep damage was observed mostly at 25% below the surface of the plate. Experimental creep damage distributions were compared with computed versions using finite element method and damage mechanics analysis. Both multi-axial stress state and strain concentration in fine-grained HAZ appear to influence the formation and distribution of creep voids.