## 1 Introduction  
P91 steel belongs to a family of 9-12% Cr ferritic creep-resisting steels [1]. This steel was initially developed in the 1970s for application in steam generators of fast breeder reactors by Oak Ridge National Laboratory [2, 3]. At present, P91 steel has wide-ranging applications in high-temperature and high-pressure components such as headers, heat exchangers, water-wall tubing, and piping systems of fossil-fuel power plants [4, 5]. The suitability of this steel for these applications is due to its superior thermo-mechanical properties that include high corrosion resistance, good weldability, and low thermal expansion. It also exhibits excellent fatigue strength, toughness and good creep performance [6]. P91 steel is commonly used in the temperature range of 540 \({}^{\circ}\)C to 650 \({}^{\circ}\)C. Some of its mechanical properties conforming to ASTM A335 are a minimum tensile strength of 585 MPa, a minimum yield strength of 415 MPa, and elongation of 19.5%. At room temperature, it has a hardness of HB 250 and a minimum creep rupture strength of 69 MPa at 650 \({}^{\circ}\)C [7]. After prolonged service, steel experiences internal damage due to microstructural changes from long-term exposure to high temperatures under stress. These changes are detrimental to mechanical properties, especially toughness, and cause degradation of the creep properties [8]. This leads to the replacement of degraded components at the end of the design lifetime. The high cost of replacement has increased research interest in alternative methods of extending the components' service life. The implementation of an appropriate rejuvenation processes has the potential to significantly extend the operational service time of degraded components. 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]. Proper heat treatment schedules cause grain refinement and can re-establish the original microstructure and thus the material properties of service-exhausted components [11]. Information in the literature about the effect of rejuvenation heat treatment on metal flow behaviour, especially P91 steel, is scarce, and therefore there is a need for further research. In 9-12% Cr ferrite steels, heat treatment is generally performed by means of normalising, cooling and tempering processes [12]. Normalising is conducted above the upper critical temperature, Ac\({}_{3}\), and tempering is conducted below the lower critical temperature, Ac\({}_{1}\)[13]. Various authors [2,13,14,15] have applied different normalisation and tempering temperatures for P91 steel within the Ac\({}_{3}\) and Ac\({}_{1}\) requirements. [PERSON] et al. [2] reported the phase transformation temperatures of P91 as 810 \({}^{\circ}\)C to 825 \({}^{\circ}\)C (Ac\({}_{1}\)) and 912 \({}^{\circ}\)C to 930 \({}^{\circ}\)C (Ac\({}_{3}\)). According to [PERSON] [16], the Ac\({}_{1}\) was 800 to 830 \({}^{\circ}\)C and Ac\({}_{3}\) 890 to 940 \({}^{\circ}\)C in P91. The variations in heat treatment temperatures of P91 are due to minor differences in chemical composition (especially Ni + Mn), rate of heating and prior austenite grain boundaries (PAGBs) [14]. The American Society of Mechanical Engineers (ASME) [16] recommended an austenitisation temperature of 1040 to 1080 \({}^{\circ}\)C and tempering at 730 to 780 \({}^{\circ}\)C in P91 pipes. In 9-12% Cr ferrite steels, creep deformation mechanisms are mainly associated with changes in the microstructure. These microstructure changes cause a reduction in dislocation density, the migration of sub-grain boundaries, the formation of new secondary phases (Laves and Z-phase) and the growth of sub-grain structure [17,18]. During creep, the sub-grains grow, causing a decrease in the creep strength of the 9-12% Cr steels.