Papers by Author: Jaromír Sobotka

Abstract: This article is focused on optimization of fundamental utility properties and technological process of advanced low alloy steel 15NiCuMoNb5 used for power engineering mainly for production of large capacity steam boilers. Consequently problem critical locality in weld after post weld heat treatment is studied. Optimization of welding technology is proposed. Fracture behaviour of welded joint made from boiler plates 80 mm in thickness of 15NiCuMoNb5 type steel after N + T PWHT regime after welding procedure were studied. Stable crack growth measurement and evaluation of fracture toughness of base material as well as weld metal in temperature region from 0 up 350°C were carried out. Unstable cleavage fracture and fracture toughness KJc of both parts of welded joint at test temperatures cover the transition region were evaluated. The reference temperature T0 approach was used for evaluation of temperature dependence of fracture toughness in transition range.

Abstract: The long-term creep rupture tests have been carried out on three casts of a type AISI 316LN steel at 600 and 650°C. Two of the casts investigated contained additions of 0.1 and 0.3 wt.% of niobium. The growing niobium content strongly reduced the minimum creep rate and prolonged the time to the onset of the tertiary stage of creep and also shortened this stage. The enhanced creep resistance of niobium containing steels is not accompanied by the longer creep life that might have been expected. At both temperatures of creep exposure the niobium-bearing casts displayed an inferior creep ductility. Microstructural investigations revealed that niobium provoked significant grain size refinement and the formation of Z-phase. Particles of this phase were considerably dimensionally stable. Furthermore, niobium accelerated the formation and coarsening of s-phase, h-Laves and M6(C,N). The coarse intergranular particles facilitated the formation of cavities which resulted in intergranular failure mode.