The Effects of Various Conditions of Short-Term Rejuvenation Heat Treatment on Room-Temperature Mechanical Properties of Thermally Aged P92 Boiler Steel

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.

Comparative Study on Hot Metal Flow Behaviour of Virgin and Rejuvenated Heat Treatment Creep Exhausted P91 Steel

“…This is a typical microstructure reported in various studies of P91 steel [2,30,31]. The microstructure of the steel B was modified as a result of service conditions, with the grain boundaries being more apparent and an equiaxed grain structure [9,32]. understanding the metal flow pattern during hot deformation.…”

“…The rejuvenation of degraded metals and alloys can be achieved through various thermal procedures such as heat treatment and hot isostatic pressing [9]. These processes can modify the material's microstructure, thus improving the mechanical properties of damaged or exhaust components [10].…”

“…Rejuvenation heat treatment, as mentioned earlier, provides an economic benefit in terms of the expensive process of replacing parts and time and energy savings [9]. In addition, an appropriately designed refurbishment process can significantly improve the lifetime service of components or a production plant.…”

Comparative Study on Hot Metal Flow Behaviour of Virgin and Rejuvenated Heat Treatment Creep Exhausted P91 Steel

“…Restoration of the microstructure of degraded components has been achieved through various thermal procedures such as heat treatment and hot isostatic pressing [10]. Thermal rejuvenation procedures can modify the material's microstructure, thus improving mechanical properties [11].…”

“…Rejuvenation heat treatment, as mentioned earlier, can be used for complete recovery of material properties and reuse of exhaust components or partial recovery for alternative applications [10]. Should the alternative application require the manufacturing of a new structural part from either processing or machining, it is imperative to understand the response of the effect of rejuvenation heat treatment on creep-exhausted steel to thermomechanical processing parameters.…”