## 2 Materials  
The advanced ferritic-martensitic (AFM) steel P91, belonging to the 9 Cr-1 Mo family of steels, is widely applied in current thermal power plants [23; 24]. P91 has a tempered martensite structure after austenitization, air cooling, and tempering. As shown in Figure 1, the tempered martensite is characterized by prior austenite grain boundaries (PAGBs), as well as packet and lath boundaries [3]. The stability is achieved by precipitation (predominantly Cr and Mo-rich M\({}_{23}\)C\({}_{6}\), M\({}_{7}\)C\({}_{3}\), M\({}_{3}\)C, M\({}_{2}\)X, Cr\({}_{2}\)N), solid solution, and sub-grain hardening [3]. M\({}_{23}\)C\({}_{6}\) and MX precipitate along PAGBs lath boundaries, packet boundaries (see Figure 1), as well as inside intra-lath regions [3]. However, long-term stability during creep stress is deteriorated by the formation of the intermetallic Laves phase and Z-phase [3].  
The P91 investigated in the present study was extracted from a pipe section with an outer diameter of 160 mm and a wall thickness of 20 mm. Austenitization was carried out at 1050 \({}^{\circ}\)C for 0.5 h, followed by air cooling. Afterwards, tempering at 750 \({}^{\circ}\)C for 1 h and subsequent air cooling were performed.  
The additionally investigated Crofer(r)22 H is a 22 wt.% Cr, fully ferritic stainless steel and was developed by a collaboration between Forschungszentrum Juelich GmbH, Germany, and VDM Metals GmbH, Germany [25]. It serves as a low-cost, metallic interconnect material for the light-weight design of solid oxide fuel cell stacks [26]. Figure 2 depicts a typical microstructure of recrystallized (1075 \({}^{\circ}\)C/22 min/air cooling), and subsequently, precipitation annealed (650 \({}^{\circ}\)C/2 h/water quenching) Crofer(r)22 H. Fine Laves phase particles are uniformly dispersed [27; 28] in the matrix and additionally, the high angle grain boundaries (HAGBs) are covered by Laves phase. Furthermore, particle-free zones (PFZs) are formed along the HAGBs. The strength of the material is based on a combination of solid solution hardening and intermetallic (Fe,Cr,Si)\({}_{2}\)(Nb,W) Laves phase particle precipitation [4]. Moreover, thermomechanically triggered precipitation of the Laves phase particles [29; 30] has a significant impact on TMF-behavior [5], and fatigue performance [6] of such Laves phase strengthened, ferritic steels. In addition to that, an excellent creep behavior in the temperature range of 600-650 \({}^{\circ}\)C has been demonstrated by Reference [4] for this type of steels.  
Figure 1: The characteristic microstructure of tempered ferritic-martensitic steel P91 [5]. PAGB, prior austenite grain boundaries.  
In the presented work, recrystallized (1050 \({}^{\circ}\)C/5+ minutes, depending on section thickness/rapid air-cooling) Crofer(r)22 H from VDM Metals, Germany was investigated.  
The chemical compositions of Crofer(r)22 H were determined with inductively coupled plasma optical emission spectrometry and are given in Table 1, which also contains the chemical composition from the manufacturer's datasheet of P91. In Table 2, the typical mechanical properties at an ambient temperature of the as-delivered state are shown, respectively.