Microstructural Evolution and Creep Behaviour of 2(1/4)Cr1Mo(1/4)V and 3Cr1Mo(1/4)V Modified Steels for Pressure Vessel Forged Components

摘要

The aim of this paper is to present the creep behaviours and related microstructure evolution of a new versions of Cr-Mo-V steels, i.e. 2(1/4)Cr1Mo(1/4)V modified steel (SA336, ASME Code Case2098-1), used to build the first ammonia converter in the world operating in severe hydrogen environment, and the (SA336F3V steel Code Case 2151). Creep tests have been performed in the temperature range of 450-600°C and time to rupture higher then 20000 hours for 2(1/4)Cr1Mo(1/4)V and 45000 hours have been reached for 3Cr1MoV steel. The creep results are compared with the behaviour of conventional 3Cr1Mo and 2(1/4)Cr1Mo steels, currently used to manufacture pressure vessel forged components. The creep behaviour of both new steeJs are significatively better then the reference materials. At the moment the estimated stress to have rupture in 50000 hours at 550°C for 2(1/4)Cr1Mo(1/4)V results to be 150MPa instead of 115MPa for 3Cr1Mo(1/4)V. The results of an extensive TEM analysis program to investigate microstructural evolution of both compositions, as function of temperature and time, are also described. At the same ageing conditions the 2(1/4)Cr1Mo(1/4)V reveals a relatively higher stability than 3Cr1MoV steel, also in terms of precipitation state.