# Thermomechanically Induced Precipitation in High-Performance Ferritic (HiperFer) Stainless Steels
## 1 Introduction  
Fully ferritic, Laves-phase strengthened [1] stainless steels with high chromium content show potentially sufficient resistance to steam oxidation at operating temperatures as high as 650 \({}^{\circ}\)C [2; 3; 4; 5]. Based on this knowledge, advanced High-performance Ferritic (HiperFer) stainless steels were designed for application in the next generation of supercritical thermal power plants [5; 6; 7; 8; 9; 10]. The heightened operation parameters (650 \({}^{\circ}\)C/300 bar) of these plants necessitate increased resistance to creep of the applied structural materials up to 650 \({}^{\circ}\)C, in combination with sufficient steam-oxidation resistance [3; 11]. Furthermore, demands concerning the fatigue resistance of structural power-plant materials are rising due to the increasing implementation of regenerative, intermittent sources of power in the grid. Current state-of-the-art advanced ferritic-martensitic (AFM) steels, with 9-12 wt. % chromium content, are limited to application temperatures below 620 \({}^{\circ}\)C because of either limitations in steam-oxidation resistance (9 wt. % Cr) or a drop in long-term creep strength (12 wt. % Cr) [12; 13; 14; 15; 16; 17]. Novel HiperFer steels potentially fulfill these mechanical strength requirements via a combination of solid-solution strengthening and the precipitation of finely dispersed intermetallic (Fe,Cr,Si)\({}_{2}\)(Nb,W) Laves-phase particles [4; 5; 6; 7; 8; 9; 18]. Furthermore, steels with high chromium content provide superior steam-oxidation resistance [2; 3]. Several variants of HiperFer steel were described by [PERSON] et al. [4; 5; 6; 7; 8; 9; 10]. As displayed in Figure 1, the phase fraction of the strengthening Laves phase increased with higher alloying contents of W andNb. While the low-alloyed HiperFer base variant 17 Cr2 (17 Cr2.6W0.6 Nb) needs precipitation annealing (PA) to exploit its maximum strength potential [6; 19], the 17 Cr5 steel (17 Cr4W1 Nb) constitutes an advanced, age-hardening variant, which was developed to be put into service in its recrystallized state without a significant loss in creep or fatigue strength [8]. Typically, the HiperFer base variant (17 Cr2) is recrystallized after forming by forging or rolling, followed by multistep processing (MSP), i.e., precipitation annealing (PA) [8; 20]. This multistep annealing must not be confused with the standard-quality heat treatment of ferritic-martensitic 9-12 Cr steels, where stress from martensitic transformation is reduced first and precipitation annealing is done in a (or multiple) consecutive step(s), typically at temperatures exceeding the envisaged application temperature range [13; 21; 22]. HiperFer is designed to be precipitation-annealed for creep strength at temperatures lower than (cf. \"Temp L\" in Figure 2a) and/or at (cf. \"Temp H\" in Figure 2a) application temperature, thus potentially enabling in situ PA during commissioning of a power plant or component [8]. Nevertheless, such multistep processes are time-consuming and thus cost-intensive if performed ex situ (i.e., prior to application). Deformation has a beneficial impact on the precipitation of Laves-phase particles in these types of steel [23] and leads to \"reactive strengthening\" [8; 24; 25]. Integrated thermomechanical processing (TMP; cf. Figure 2b), incorporating forming and PA in the same process, would therefore greatly improve cost efficiency. In such an integrated process (details given in Reference [26]) the material is homogenized after casting and thickness is reduced by forging and/or rolling outside the precipitation temperature range. Subsequently, the material is recrystallized, reduced to final thickness by controlled forging/rolling (and optional short holding) inside the precipitation temperature range, followed by rapid cooling.