Microstructure of CB2 Steel before and after Long-term Creep Tests

A pilot 4t valve made of CB2 steel was produced in the frame of COST Action 522. Specimens for long-term creep tests were taken from positions with a wall thickness of 130 mm and 85 mm. Creep testing was carried out at 650 degrees C and a stress range from 50 MPa to 120 MPa. The longest times to rupture of specimens in individual positions were 67,857 h and 82,649 h respectively. Ruptured samples underwent fractographic and microstructural analyses. Quantitative evaluation of substructure was performed using SEM and TEM. Microstructure consisted of tempered martensite with primary boron and niobium nitrides and secondary particles of M23C6 carbide, Laves phase and vanadium nitride. During creep tests coarsening of Laves phase occurred and new particles nucleated on the site of M23C6 carbides, NbN or BN nitrides. Significant increase in volume fraction of Laves phase was detected in specimens ruptured up to 20,000 hours in comparison to as received conditions; any further increase was found out after longer creep exposures. Concerning to M23C6 carbides any important changes were found out in their size and distribution. Density of fine MX nitride was higher after creep tests than in as received conditions with the exception of two longest creep exposures. Several Z-phase particles were also identified.