Hiromichi Hongo

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.

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.

Theoretical Investigation of Stabilizing Mechanism by Boron in Body-Centered Cubic Iron Through (Fe,Cr)23(C,B)6 Precipitates

Small amounts of boron improve the mechanical properties in high-chromium ferritic heat-resistant steels. In this work, the stabilizing mechanism by boron in body-centered cubic iron (bcc Fe) through (Fe,Cr)23(C,B)6 precipitates was investigated by first-principles calculations. Formation energy analysis of (Fe,Cr)23(C,B)6 reveals that the compounds become more stable to elemental solids as the boron concentration increases. Furthermore, the interface energy of bcc Fe(110) || Fe23(C,B)6(111) also decreases with boron concentration in the compounds. The decreased interface energy caused by boron addition is explained by the balance between the change in the phase stability of the precipitates and the change in the misfit parameter for the bcc Fe matrix and the precipitates. These results show that boron stabilizes the microstructure of heat-resistant steels, which is important for understanding the origins of the creep strength in ferritic steels.

Evaluation of long-term creep damage in high Cr ferritic steel welds

Abstract The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.

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

The present paper aims to clarify the Type IV creep damage process of Mod.9Cr-1Mo (Gr.91) steel weldment. Long-term creep tests for base metal and simulated fine-grained HAZ and welded joints were conducted at 550, 600 and 650 °C. Furthermore, creep tests of thick welded joint specimens were interrupted at 0.2, 0.5, 0.7, 0.8, 0.9 of rupture life, and damage distributions were measured quantitatively. It was found that creep voids initiated at the early stage of life inside the specimen thickness, and grew into cracks at the later stage of life. Experimental creep damage distributions were compared with computed ones using FEM and damage mechanics analysis. The effect of multiaxial stress condition on creep damage evolution is discussed.Copyright

Strain-Rate Sensitivity Enhanced by Grain-Boundary Sliding in Creep Condition for AZ31 Magnesium Alloy at Room Temperature

This study investigated strain-rate sensitivity (SRS) in an as-extruded AZ31 magnesium (Mg) alloy with grain size of about 10 mm. Although the alloy shows negligible SRS at strain rates of >10-5 s-1 at room temperature, the exponent increased by one order from 0.008 to 0.06 with decrease of the strain rate down to 10-8 s-1. The activation volume (V) was evaluated as approximately 100b3 at high strain rates and as about 15b3 at low strain rates (where b is the Burgers vector). In addition, deformation twin was observed only at high strain rates. Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates. Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance. Therefore, GBS enhances and engenders SRS in AZ31 Mg alloy at room temperature.

Study on Type-IV Damage Prevention in High-Temperature Welded Structures of Next-Generation Reactor Plants: Part II — Effect of Boron on Creep Properties and Microstructures of HAZ for High Cr Steels

Creep strength of welded joints for high Cr ferritic steels is decreased for long-term services at high temperatures due to Type IV creep damages that occur in heat affected zone (HAZ). Aiming at improving the creep strength of HAZ, we have investigated the effect of boron and nitrogen content on the microstructures and creep strength of HAZ for the 9Cr and 12Cr steels produced based on the P91 and P122 steel. It was found that the effect of boron appeared remarkably for the 12Cr steel. By increasing boron and decreasing nitrogen, the formation of fine-grained HAZ structure during weld thermal cycle could be suppressed and the creep strength of HAZ was considerably improved. For the 9Cr steel, while the formation of fine-grained HAZ structure could be inevitable independent of boron and nitrogen contents, the creep strength of fine-grained HAZ was improved by grain boundary strengthening effect of boron.Copyright

Long-term creep strength and fracture of Gr.91 steel welds

Abstract Mod.9Cr–1Mo steel (ASME Gr.91 steel) is widely used as a main structural material for boiler components in ultra-supercritical (USC) thermal power plants at about 600 °C. Decrease of the creep strength of welded joints due to Type-IV failure is a critical issue, however creep rupture data longer than 10,000 h are scarcely obtained. The present paper conducted long-term creep tests of welded joints from 30,000 to about 70,000 h at 600 and 650 °C. For the specimens fractured after about 35,000 h at 600 and 650 °C, creep damage was mainly observed in the fine-grained heat-affected zone (HAZ) (Type-IV), however damage was also observed in the weld metal. The welded joints crept at 600 °C and 50 MPa for about 50,000 and 70,000 h fractured in the weld metal. It was found that the hardness of 9Cr weld metal decreased after about 30,000 h creep at 600 °C due to the recovery of microstructures. Therefore it is considered to be necessary to monitor creep damage not only in the HAZ but also in the weld metal.

Creep-induced nonlinear ultrasonic change in an austenitic stainless steel

We investigated the evolutions of two nonlinear acoustic characterizations: resonant frequency shift and three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in an austenitic stainless steel, JIS-SUS 304. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk- shear-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests with plate specimens were carried out at 973 K, and 100 MPa and interrupted at several time steps. Two nonlinear acoustic parameters showed peas at 40% of creep life. After that they deceased, they increased from 60% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring and the initiation and growth of creep void, with support from the SEM and TEM observation.

Change of nonlinear acoustics in ASME grade 122 steel welded joint during creep

In this paper, we described the changes of two nonlinear acoustic characterizations; resonant frequency shift and three-wave interaction, with electromagnetic acoustic resonance (EMAR) throughout the creep life in the welded joints of ASME Grade 122, one of high Cr ferritic heat resisting steels. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). These nonlinear acoustic parameters decreased from the start to 50% of creep life. After slightly increased, they rapidly increased from 80% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring related to the initiation and growth of creep void, with support from the SEM and TEM observation.