The effect of molybdenum on high-temperature properties of 9 pct Cr steels

Heat-resistant 9 Cr steels with 1, 2, and 3 pct Mo were tested for mechanical properties, weldability, and creep-rupture properties. The elevated-temperature and rupture strengths increase with increasing molybdenum content. While the 9 Cr-1 Mo steel is martensitic and is precipitation strengthened with carbides, the 9 Cr-2 Mo and 9 Cr-3 Mo steels receive added benefits from precipitation of Laves phase and solid-solution strengthening. The latter cause little decrease in ductility and impact resistance. The 9 Cr-2 Mo and 9 Cr-3 Mo steels are characterized by a duplex microstructure which aids weldability. Weld cracking tests show no need for preheating the latter steels, although the martensitic 9 Cr-1 Mo steel is known to be susceptible to weld cracking if not preheated. Both duplex-structure steels have good resistance to stress-relief cracking. Anisotropy of mechanical properties, due to the orientation of the duplex structure in the rolling direction, is less than that observed in the fully martensitic 9 Cr-1 Mo steel.