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

Abstract

Future, flexible thermal energy conversion systems require new, demand-optimized highperformance materials. The High performance Ferritic (HiperFer) stainless steels, under development at the Institute of Microstructure and Properties of Materials (IEK-2) at Forschungszentrum Jülich GmbH in Germany, provide a balanced combination of fatigue, creep and corrosion resistance at reasonable price. This paper outlines the scientific background of alloy performance development, which resulted in an age-hardening ferritic, stainless steel grade. Furthermore, technological properties are addressed and the potential concerning application is estimated by benchmarking versus conventional state of the art materials.

Compositional Optimization of High-Performance Ferritic (HiperFer) Steels—Effect of Niobium and Tungsten Content

“…However, these alloys are developed to enable short quality heat treatment at the envisaged operating temperature or even age hardening by rapid precipitation kinetics. The tensile strength values achieveable in this way were significantly higher (i.e., after 1-5 h of annealing @ 650 • C: YS 0.2 /UTS: 400/660 MPa in case of the 2.4W0.6Nb and 530/820 MPa in case of the 4W1Nb alloy [9]).…”

“…Based on the HiperFer 17Cr2 (2.4W0.6Nb) prototype alloy [9][10][11][12], the chemical compositions of the trial alloys were systematically varied in thermodynamic equilibrium calculations (utilizing the commercial software package Thermocalc ® , database: TCFE7), including an increase in niobium to 1 wt.% and a variation in tungsten content from 2.4 wt.% up to 4 wt.% (Table 1). High-purity lab melts were then 2 of 9 manufactured by vacuum induction melting of high-purity raw materials, casting, soaking at 1250 • C for 2 h, and forging (3 steps from 140 × 140 mm 2 to 92 × 92 mm 2 in the temperature range from 1250 • C to 950 • C, interstage annealing: 1250 • C, 20 min.)…”

“…A proprietary development at Forschungszentrum Juelich GmbH, Germany, features a ferritic stainless steel strengthened by a combination of solid solution strengthening and precipitation of (Fe,Cr,Si) 2 (Nb,W)-Laves phase particles [5][6][7][8]. Optimization of these high-performance ferritic (HiperFer) steels so far cumulated in an alloy, which can combat the strongest low-cost, heat-resisting structural steels [9] typically applied in power engineering and the process industry up to approximately 680 • C. Alteration of the main Laves phase constituting elements W, Nb, and Si strongly influences the mechanical properties by modification of the solid solution and precipitation strengthening effects in such alloys. The relation of W and Nb contents directly affects short-term precipitation and long-term particle growth kinetics.…”

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

“…Microstructural changes, i.e., recovery of excess dislocations or partly recrystallization and consequently a drop in dislocation strengthening within the heat-affected zone, are inevitable under these prerequisites. Details on weld microstructure can be found in [44].…”

“…This results in comparatively low dislocation density (i.e., less nucleation sites for strengthening precipitates) and thus comparably low, expected mechanical strength in the as-processed state. This could be counterbalanced by tailored thermomechanical processing [44] like cold-rolling. Mechanical properties depending on thermomechanical processing might restrict utilization of the proposed steels to applications, where component production and plant construction do not cause significant microstructural alterations, e.g., to components, which are not welded.…”