A novel approach to envisage effects of boron in P91 steels through Gleeble weld-HAZ simulation and impression-creep

This paper induced a novel methodology for the characterization of creep behavior of weld heat-affected zone (HAZ) for boron-free P91 (PM) and boron modified P91B (B-PM) steels. Gleeble-3800 thermo-mechanical simulator replicated specimens, representing coarse-grain HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and inter-critical HAZ (ICHAZ). Short-term impression creep tests were conducted at 625°C/270-410MPa on PM/B-PM and their simulated HAZs after being subjected to post-weld heat treatment (PWHT) of 760°C/3 h. Microstructural characterization and local strain analyses were accomplished by electron back-scattered diffraction. Simulated microstructures of P91B-FG/ICHAZ after PWHT exhibited lath martensitic structure and large prior-austenite grain size as regards P91-FG/ICHAZ, correspondingly. Average values of local microstructural strain from local average misorientation were relatively high in B-PM and P91B-ICHAZ than PM and P91-ICHAZ, respectively. Similar observations were found for P91-CG/FGHAZ with their counterparts. Stress dependent steady-state creep-rate (SSCR) followed power-law for all specimens except PM. The minimum and maximum ranges of SSCR for P91B specimens were observed to be in a narrower range than P91 specimens. The value of stress exponent for all specimens was evaluated, and corresponding mechanisms were discussed. The analyses of microstructures and corresponding impression creep behavior of P91/P91B samples suggested that modification of 100 ppm boron to P91 steel improved creep-rupture ductility that delayed type IV failure at outer HAZ of P91 steel weldments.

Creep Deformation Property and Creep Life Evaluation of Super304H

Creep deformation behavior, creep strength property and microstructural evolution during creep exposure were investigated on Super 304H steel for boiler tube. In the high stress and lower temperature regime, creep rupture strength of Super 304H steel is higher than that of SUS304H steel. The slope of stress vs. time to rupture curve of Super 304H steel, however, becomes steeper with increase in creep exposure time and temperature, and the creep rupture strength of Super 304H steel becomes closer to that of SUS304H steel after the tens of thousands of hours at 700°C and above. In the short-term, at 600°C, creep rupture ductility increases with increase in creep rupture life. However, it tends to decrease after showing the maximum value and the creep rupture ductility decreases with increase in temperature. The complex shape of creep rate vs. time curves, with two minima in creep rate, was observed at 600°C. Several type precipitates of niobium carbonitride (Nb(C,N)), Z phase (NbCrN), and copper were observed in Super 304H steel, as well as M23C6 carbide and sigma phase observed in SUS304H steel. The change in slope of stress vs. time to rupture curve is caused by disappearance of precipitation strengthening effect during creep exposure. Accuracy of creep rupture life evaluation was improved by stress range splitting method which takes into accounts of the change in slope of stress vs. time to rupture curves was demonstrated.

Creep Deformation Property and Creep Life Evaluation of Super304H

Creep deformation behavior, creep strength property and microstructural evolution during creep exposure were investigated on Super 304H steel for boiler tube. In the high stress and lower temperature regime, creep rupture strength of Super 304H steel is higher than that of SUS304H steel. The slope of stress vs. time to rupture curve of Super 304H steel, however, becomes steeper with increases in creep exposure time and temperature, and the creep rupture strength of Super 304H steel becomes closer to that of SUS304H steel after the tens of thousands of hours at 700°C (1292°F) and above. In the short-term, at 600°C (1112°F), creep rupture ductility increases with increase in creep rupture life. However, it tends to decrease after showing this maximum value and the creep rupture ductility decreases with increase in temperature. The complex shape of creep rate vs. time curves, with two minima in creep rate, was observed at 600°C (1112°F). Several type precipitates of niobium carbonitride (Nb(C,N)), Z phase (NbCrN), and copper were observed in Super 304H steel, as well as M23C6 carbide and sigma phase observed in SUS304H steel. The change in slope of stress vs. time to rupture curve is caused by disappearance of precipitation strengthening effect during creep exposure. Accuracy of creep rupture life evaluation was improved by stress range splitting method which takes into account the change in slope of stress vs. time to rupture curves was demonstrated.