The oxidation kinetics of the intermetallic phase P-FeA1 has been investigated by means of thermogravimetry in the temperature range 800-1100°C. At temperatures higher than 900°C the initially formed metastable aluminium oxides are converted to adherent and slow growing a-A1203 scales. The rate of conversion accelerates with increasing temperature. The parabolic rate constants k, are comparable with those of P-NiAl.The influence of the A1 content on the oxidation behaviour was investigated in exposure experiments at low oxygen partial pressures for samples with 5 to 48 at.% Al. The oxide morphology changes only with the oxidation temperature, neither variations in A1 content nor in 02-pressure has any influence. In samples with higher A1 content cavities are formed beneath the oxide scale in the metallic matrix.

The high temperature oxidation of P-NiAI, undoped and doped with Ce, Y and Hf was studied in situ by thermogravimetry in He with p(0,) = 5 . W h bar at 1000°C and by high temperature X-ray diffraction at 950 and 1000°C in air. After the in situ experiments the samples were analysed by optical microscopy and SEM with EDX.It was observed by thermogravimetry that the weight gain for p-NiAl+Hf is lower than for undoped p-NiAl, whereas the weight gain for p-NiAl+Y was similar to that for 6-NiAl. For p-NiAI+Ce an enhanced increase of mass gain was observed. The in situ X-rayexperiments show that in the first hours of the oxidation process the metastable 0-Al2O3 is formed, which transforms to the protective a-A1203 after different periods of time, depending on the dopant element and the temperature. The analysis of the samples shows that the ternary phases along the grain boundaries were oxidized, whereas the ternary phase precipitates within the grains remained unoxidized. The extend of the grain boundary oxidation increases from P-NiAl+Hf and p-NiAl+Y to P-NiAl+Ce.Die Hochtemperaturoxidation von P-NiAI, undotiert und dotiert rnit Ce, Y und Hf wurde mittels Thermogravimetrie in He mit p(02) = 5.10-6 bar bei 1000°C und mittels Hochtemperatur-Rontgendiffraktrometrie bei 950 und 1000°C in Luft untersucht. Nachuntersuchungen der Proben erfolgten mit Hilfe optischer Mikroskopie sowie Rasterelektronenrnikroskopie (REM) mit EDX.Thermogravimetrische Messungen ergaben fur p-NiAl+Hf eine niedrigere und fur P-NiAl+Y eine ahnliche Massenzunahme wie fur undotiertes P-NiA1. Bei p-NiAI+Ce wurde eine deutlich hohere Massenzunahme beobachtet. Die in situ-Versuche zeigten, daB in den ersten Stunden des Oxidationsprozesses das metastabile 0-AI2O3 gebildet wird, das sich im weiteren Verlauf der Oxidation zu dem stabilen a-A1203 umwandelt. Der Zeitraum, in dem 0-A1,03 zu beobachten ist, hangt vom Dotierungselement und von der Temperatur ab. Die mikroskopische Untersuchung zeigte, daR die ternaren Phasen an den Korngrenzen oxidieren, die ternaren Phasen innerhalb der Korner aber keine Oxidation aufweisen. Die Korngrenzenoxidation nimmt in der Reihenfolge P-NiAIjHf, p-NiAl+Y und P-NiAl+Ce zu.

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1. Introduction For some years, lower alloyed austenitic-ferritic stainless steel grades have been developed, which are summarised in the „lean duplex" - family of alloys. So far, lean duplex materials have been mainly used as plate material for construction of containers and vessels in the pulp and paper business as well as in architecture and construction. As they are lower alloyed in nickel and molybdenum compared to standard duplex grades, lean duplex grades can offer economically interesting advantages combined with a higher degree of technical reliability compared to supermartensitic stainless steels. Aim of the present study was to figure out, whether lean duplex grades can close the gap between supermartensitic steels and standard duplex stainless steels with regards to weldability, structural stability, mechanical properties as well as corrosion behaviour in typical media for the oil and gas industry. Due to the fact, that there are different grades of „lean" duplex stainless steels, which significantly differ one from the other in terms of chemical composition, the investigation covered three different commercially available grades. Plasma/TIG-, Laser- and Electron Beam-Welding have been used for the industrial production of the longitudinal welded pipes made of Uranus 35N, Al 2003 and LDX 2101. For comparison the complete characterisation of longitudinally Plasma/TIG-welded pipes has been carried out according to the Shell and PDO specifications SIEP 97–5763, SP-1095, and SP-1189. The results have proven the general suitability of longitudinally welded pipes made from lean duplex grades for linepipes for sweet and slightly sour service.