Thermomechanically Induced Precipitation in High-Performance Ferritic (HiperFer) Stainless Steels

Abstract

Novel high-performance fully ferritic (HiperFer) stainless steels were developed to meet the demands of next-generation thermal power-conversion equipment and to feature a uniquely balanced combination of resistance to fatigue, creep, and corrosion. Typical conventional multistep processing and heat treatment were applied to achieve optimized mechanical properties for this alloy. This paper outlines the feasibility of thermomechanical processing for goal-oriented alteration of the mechanical properties of this new type of steel, applying an economically more efficient approach. The impact of treatment parameter variation on alloy microstructure and the resulting mechanical properties were investigated in detail. Furthermore, initial optimization steps were undertaken.

Impact of Thermomechanical Fatigue on Microstructure Evolution of a Ferritic-Martensitic 9 Cr and a Ferritic, Stainless 22 Cr Steel

“…This further increases the stress response (Figure 3d). Third, the thermomechanically induced precipitates [28][29][30][31][32] pin the dislocations and through this make (at least part of the) dislocation-induced strain hardening quasi permanent. With the applied heating/cooling rate of 10 Ks −1 the first 20 cycles accumulate up to a dwell time of just 200 s (3, 33 min.)…”

“…It is produced by VDM Metals GmbH by vacuum induction melting (VIM) and delivered in the solution in an annealed (1050 • C/5 + minutes, depending on section thickness/rapid air-cooling) state. It precipitates small (Fe, Cr, Si) 2 (Nb, W)-Laves phase particles ( Figure 2) [23,24], which effectively strengthen the material [20,[25][26][27], during application in the temperature range from 600-900 • C. Thermomechanically induced precipitation of the Laves phase particles [28][29][30][31][32] plays a significant role in the fatigue performance of this type of steel, too. Furthermore, Crofer ® 22 H offers excellent resistance to corrosion in steam and CO/CO 2 -containing atmospheres, based on the formation of a protective, duplex Cr 2 O 3 /(Mn, Cr) 3 O 4 layer at the component surface [23].…”

Up-Scaling of Thermomechanically Induced Laves Phase Precipitation in High Performance Ferritic (HiperFer) Stainless Steels

“…The present work is part of a project that aimed to further optimize the chemical composition [ 7 ] and development of a new thermomechanical processing route [ 8 , 9 ] of HiperFer stainless steels. This article focusses on the thermomechanically induced precipitation (during final forging) of the Laves phase as part of the complete thermomechanical processing chain [ 8 ], as schematically shown in Figure 1 .…”

“…The present work is part of a project that aimed to further optimize the chemical composition [ 7 ] and development of a new thermomechanical processing route [ 8 , 9 ] of HiperFer stainless steels. This article focusses on the thermomechanically induced precipitation (during final forging) of the Laves phase as part of the complete thermomechanical processing chain [ 8 ], as schematically shown in Figure 1 . A controlled thermomechanically induced precipitation of Laves phase particles, in a desired morphology and quantity, as well as an optimization of corresponding forging parameters, was achieved on a laboratory scale [ 8 , 9 ].…”

“…This article focusses on the thermomechanically induced precipitation (during final forging) of the Laves phase as part of the complete thermomechanical processing chain [ 8 ], as schematically shown in Figure 1 . A controlled thermomechanically induced precipitation of Laves phase particles, in a desired morphology and quantity, as well as an optimization of corresponding forging parameters, was achieved on a laboratory scale [ 8 , 9 ]. The present work specifically deals with the very first up-scaling experiments of the optimized forging parameters, from a successful laboratory scale to an industrial scale, to enable a proper standard characterization of the achieved mechanical properties.…”