Microstructure of intermetallic particle strengthened high-chromium fully ferritic steels

Abstract

An improvement of power plant efficiency necessitates an increase of the process parameters and thus enables a reduction of consumed primary resources. Furthermore more efficient, sustainable, flexible and cost-effective energy technologies are strongly needed. For this reason the current research concentrates on a new concept of high-chromium fully ferritic stainless steels which are strengthened by a combination of solid-solution and intermetallic Laves phase particles. Such steels exhibit favourable creep, thermomechanical fatigue and steam oxidation behaviour up to 650°C. Based on detailed analysis by high-resolution scanning and transmission electron microscopy the particle size evolution and compositions were studied. Variations in chemical compositions were analysed experimentally and compared with thermodynamic equilibrium composition modelling results. This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.

Science and Technology of High Performance Ferritic (HiperFer) Stainless Steels

“…Cold-rolled and recrystallization + precipitation annealed (RX+PA) 17Cr2/3 surpasses grade 92 [57] and approximately matches the performance of the novel 12 Cr steel Super VM12 [58]. While creep deformation of intermetallic particle strengthened HiperFer is controlled by growth of the strengthening Laves phase precipitates [16,47,59,60], creep damage and failure are mainly related to the accumulation of plastic deformation in PFZs (cf. Fig.…”

Science and Technology of High Performance Ferritic (HiperFer) Stainless Steels

“…Cold-rolled and recrystallization + precipitation annealed (RX + PA) 17Cr2 surpasses grade 92 [57] and approximately matches the performance of the novel 12 Cr steel Super VM12 [58]. While creep deformation of intermetallic particle strengthened HiperFer is controlled by growth of the strengthening Laves phase precipitates [16,47,59,60], creep damage and failure are mainly related to the accumulation of plastic deformation in PFZs (cf. Figure 11), evolving along high angle grain boundaries [36,47], in long-term application.…”

“…Intragranular solid solution strengthening and particle volume fraction, as well as grain boundary particle coverage, were successfully increased and PFZ width alongside grain boundaries diminished in the 17Cr5 variant. controlled by growth of the strengthening Laves phase precipitates [16,47,59,60], creep damage and failure are mainly related to the accumulation of plastic deformation in PFZs (cf. Figure 11), evolving along high angle grain boundaries [36,47], in long-term application.…”

“…Strengthening of ferritic stainless steel cannot be accomplished by MX particles, because of the lacking C and N solubility in the fully ferritic matrix. In contrast, alloying by suitable amounts of niobium and tungsten ensures a combination of solid solution and intermetallic (Fe,Cr,Si) 2 (Nb,W) Laves particle strengthening, which enables creep strength potential beyond grade 92 [14][15][16] and steam oxidation resistance superior to 12 wt.% Cr AFM steels [16]. Operational flexibility will strongly grow in importance in future thermal power conversion [13].…”

On the Relationship between the Chromium Concentration, the Z‐Phase Formation and the Creep Strength of Ferritic‐Martensitic Steels

“…The development of creep resistant steels functioning at higher temperatures required a new concept for high chromium steel, and led to the development of fully ferritic steels with a chromium content of 14% or more, not undergoing a martensitic transformation. Due to the extremely low solubility of carbon and nitrogen in ferrite, it will not be possible to produce a significant and stable dispersion of strengthening carbides and nitrides.…”