Abstract
In this work, the effects of various conditions of short-term rejuvenation heat treatment on room-temperature mechanical properties of long-term aged P92 boiler steel were investigated. Normalized and tempered P92 steel pipe was thermally exposed at 600 °C for time durations up to 5000 h in order to simulate high-temperature material degradation, as also occurring in service conditions. Thus, thermally embrittled material states of P92 steel were prepared, showing tempered martensitic microstructures with coarsened secondary phase precipitates of Cr23C6-based carbides and Fe2W-based Laves phase. Compared with the initial normalized and tempered material condition, thermally aged materials exhibited a slight decrease in strength properties (i.e., yield stress and ultimate tensile strength) and deformation properties (i.e., total elongation and reduction of area). The hardness values were almost unaffected, whereas the impact toughness values showed a steep decrease after long-term ageing. An idea for designing the rejuvenation heat treatments for restoration of impact toughness was based on tuning the material properties by short-term annealing effects at various selected temperatures somewhat above the long-term ageing temperature of P92 material. Specifically, the proposed heat treatments were performed within the temperature range between 680 °C and 740 °C, employing variable heating up and cooling down conditions. It was revealed that short-term annealing at 740 °C for 1 h with subsequent rapid cooling into water represents the most efficient rejuvenation heat treatment procedure of thermally aged P92 steel for full restoration of impact toughness up to original values of normalized and tempered material state. Microstructural observations clearly indicated partial dissolution of the Laves phase precipitates to be the crucial phenomenon that played a key role in restoring the impact toughness.
Highlights
P/T92 grade (EN X10CrWMoVNb9-2) creep-resistant steel belongs to a group of modern, high-alloyed structural materials, typically used in high-efficiency power engineering [1,2,3]
This conclusion can be supported by the findings of Komazaki et al [23], who found that the area fraction of the Laves phase precipitates of a certain critical size has a major effect on the impact toughness of P92 steel
The main reason for the impact toughness deterioration was ascribed to thermal embrittlement caused by coarsening of secondary phase precipitates, especially the Fe2 W-based
Summary
P/T92 grade (EN X10CrWMoVNb9-2) creep-resistant steel belongs to a group of modern, high-alloyed structural materials, typically used in high-efficiency power engineering [1,2,3]. In earlier experimental works [21,22], the research was focused on the effect of Laves phase on impact brittleness These works reported a notable decrease in the impact toughness values of thermally exposed materials after the. Another study [23] focused on the possibilities of improvement or even restoration of original values of impact toughness by employing short-term and long-term heat treatments of thermally exposed material states at temperatures above the. It has been reported that, by short-term heat treatment (700 ◦ C/1 h) of the thermally exposed state (600 ◦ C/2035 h) of P92 steel with the presence of the Laves phase in microstructure, it was possible to suppress thermal embrittlement of the material and restore its original impact toughness. Our current research is aimed at the investigation of the effects of various conditions of short-term rejuvenation heat treatment on the room-temperature mechanical properties of thermally aged P92 boiler steel and seeking feasible ways for its impact toughness restoration while maintaining the strength and hardness
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