The Effect of Normalizing Temperature on the Short-Term Creep Rupture of the Simulated HAZ in Gr.91 Steel Welds

Abstract

As-received Gr.91 steel tube was normalized at either 940 or 1060 • C for 1 h, followed by Ar-assisted cooling to room temperature, then tempered at 760 • C for 2 h. Those samples were designated as 940NT or 1060NT samples. An infrared heating system was used to simulate HAZ microstructures in the weld, which included over-tempering (OT) and partial transformation (PT) zones. The results of short-term creep tests showed that normalizing at higher temperature improved the creep resistance of the Gr.91 steel. By contrast, welding thermal cycles would shorten the creep life of the Gr.91 steel. Among the tested samples in each group, the PT samples had the shortest life to rupture, especially the 940NT-PT sample. The microstructures of the PT samples comprised of fine lath martensite and ferrite subgrains with carbides decorating the grain and subgrain boundaries. Excessive dislocation recovery, rapid coalescence of refined martensite laths, and growth of ferrite subgrains were responsible for the poorer creep resistance of the PT samples relative to those of the other samples.

Microstructural Evolution and Short-Term Creep Rupture of the Simulated HAZ in T92 Steel Normalized at Different Temperatures

“…T92 steel tubes have been widely used in advanced supercritical boilers to replace Gr.91 tubes. As reported in prior works, the simulated HAZ microstructures of Gr.91 steel are sensitive to thermal cycles [24,29]. Moreover, normalizing Gr.91 steel at higher temperature is beneficial to improving its creep resistance [29].…”

“…As reported in prior works, the simulated HAZ microstructures of Gr.91 steel are sensitive to thermal cycles [24,29]. Moreover, normalizing Gr.91 steel at higher temperature is beneficial to improving its creep resistance [29]. To date, the microstructural evolutions and short-term creep tests of simulated HAZs present in a T92 steel weld have not been studied extensively.…”

“…To investigate the effects of the welding thermal cycles on the creep resistance of the simulated specimens at elevated temperature, the simulated tempered specimens were loaded with dead weight under one of two conditions-the 903 K/120 MPa or 933 K/80 MPa. The samples for creep tests [24,29] were cut from the steel tube by an electro-discharged wire cutter. The microstructures of various specimens were inspected with a BX51 optical microscope (OM, Olympus, Tokyo, Japan) and a JSM-7100F scanning electron microscope (SEM, JEOL, Tokyo, Japan).…”

“…The tensile strength [27] and creep-rupture life [6] of P91 steel increase with an increase in the normalizing temperature. Increasing the normalizing temperature also improves the strength of the ICHAZ [14] and prolongs the creep-rupture life of Gr.91 steel [28,29]. It is reported that ausforming T/P91 steel can obviously increase the amount of MX precipitates and improve the creep strength of the steel [30].…”

Creep and High-Temperature Deformation of Metals and Alloys

“…The paper by Wu et al [11] deals with the effect of heat treatment process parameters, and specifically the normalizing temperature, in grade 91, a 9-12% Cr steel, which is discussed on the basis of a careful microstructural analysis of crept samples. Other critical features for the industrial applications of these steels have been considered in the Issue, such as weld joint behaviour (Hu et al [5] investigated the effect of residual stresses left by these processes on creep damage) or creep-fatigue and creep relaxation phenomena (Jürgens et al [10]).…”