The main role of Rare Earth (RE) elements in the steelmaking industry is to affect the nature of inclusions (composition, geometry, size and volume fraction), which can potentially lead to the improvement of some mechanical properties such as the toughness in steels. In this study, different amounts of RE were added to a niobium microalloyed steel in as-cast condition to investigate its influence on: (i) type of inclusions and (ii) precipitation of niobium carbides. The characterization of the microstructure by optical, scanning and transmission electron microscopy shows that: (1) the addition of RE elements change the inclusion formation route during solidification; RE > 200 ppm promote formation of complex inclusions with a (La,Ce)(S,O) matrix instead of Al 2 O 3-MnS inclusions; (2) the roundness of inclusions increases with RE, whereas more than 200 ppm addition would increase the area fraction and size of the inclusions; (3) it was found that the presence of MnS in the base and low RE-added steel provide nucleation sites for the precipitation of coarse niobium carbides and/or carbonitrides at the matrix-MnS interface. Thermodynamic calculations show that temperatures of the order of 1200 • C would be necessary to dissolve these coarse Nb-rich carbides so as to reprecipitate them as nanoparticles in the matrix.

Abstract:The development of thermomechanical treatments (TMT) has a high potential for improving creep-strength in 9Cr-1Mo ferritic/martensitic steel (ASTM T/P91) to operate at temperatures beyond 600 • C. To maximize the number of nanoscale MX precipitates, an ausforming procedure has been used to increase the number of nucleation sites for precipitation inside the martensite lath. Relative to standard heat treatments (consisting of austenitization at about 1040 • C followed by tempering at about 730 • C) this processing concept has enabled achieving a microstructure containing approximately three orders of magnitude higher number density of MX precipitates having a size around four times smaller in ASTM T/P91 steel. On the other hand; this TMT has little effect on the size and number density of M 23 C 6 particles. The optimized microstructure produced by this TMT route proposed is expected to improve the creep strength of this steel.

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The microstructural degradation and the creep fracture behavior of conventionally and thermomechanically treated Grade 91 steel were investigated after performing Small Punch Creep Tests. A remarkable reduction in creep ductility was observed for the samples thermomechanically treated in comparison to those conventionally treated under the tested conditions of load (200 N) and temperature (700 ºC). A change in the fracture mechanism from a ductile transgranular fracture to a brittle intergranular fracture was observed when changing from the conventionally treated to the thermomechanically treated processing condition, leading to this drop in creep ductility. The change in the fracture mechanism was associated to the localized concentration of creep deformation, close to coarse M23C6 carbides, at the vicinity of prior austenite grain boundaries (PAGB) in the thermomechanically treated samples. The preferential recovery experienced at the vicinity of PAGB produced the loss of the lath structure and the coarsening of the M23C6 precipitates. The electron microscopy images provided suggest that the creep cavities nucleate in these weak recovered areas, associated to the presence of coarse M23C6. After the coalescence of the cavities the propagation of the cracks was facilitated by the large prior austenite grain size produced during the austenitization which favors the propagation of the cracks along grain boundaries triggering the intergranular brittle fracture. This fracture mechanism limits the potential use of the proposed thermomechanical processing routes.