Duplex and superduplex stainless steels are high-alloy engineering materials whose annealed microstructure consists of approximately equal proportions of ferrite and austenite. This phase balance may however vary by as much as ± 20% depending upon thermo-mechanical processing parameters. They are characterised by high resistance to stress corrosion cracking in Cl ion-containing environments, pitting and crevice corrosion. Their lower Ni contents and higher strength also confer economic and mechanical advantages over the super and common austenitic stainless grades, respectively.
The two-phase microstructure and high-alloy content of duplex and superduplex stainless steels however also present considerable challenges for thermo-mechanical processing including complex restorative behaviour in response to imposed deformation and the potential for the precipitation of deleterious tertiary phases. In order to optimize the performance (and address reported variability in quality) of high-integrity duplex and superduplex forged engineering components it is therefore necessary to understand the effects of thermo-mechanical process parameters on the microstructure evolution and subsequent mechanical properties of these alloys.
This work investigates the effect of variables typical of industrial forging processes on the microstructure, properties and the crystallographic texture of ZERON® 100, the first commercially available superduplex stainless steel. By contrast, much of the current research literature concerning duplex metallic materials has focused on non-industrially analogous processing parameters (heat treatment temperatures and cooling rates, for example) and unrepresentative service conditions. However, the conditions for the formation and the effects on material properties of the most common and deleterious tertiary duplex phases (e.g. V and F) are now well understood and avoided in industrial practice. Such methodologies therefore, although useful for the characterisation of tertiary precipitates and accordingly their effect on material properties, have limited applicability to the understanding of the complexities involved in thermo-mechanically processing duplex and superduplex stainless steel alloys.
A constitutive equation has been developed to describe the flow behaviour of ZERON® 100 and used as the basis of finite element simulations of the industrial forging process. These simulations have allowed for the identification of strain and temperature gradients developed throughout thermo-mechanical processing and also indicated potential areas of little-to-no grain refinement corresponding to ‘dead zones’ within forged components. High-fidelity replication of cooling curves calculated for specific locations within a thick-section forging during quenching and subsequent mechanical testing were also carried out. These mechanical tests, and EBSD analysis of commercially- and lab-processed material, confirmed the importance of the forging temperature in both the production of the starting billet material and finished product, chiefly due to its influence on the stability of grain boundary-pinning secondary austenite precipitates. Although the presence of chromium nitride tertiary precipitates have been demonstrated to embrittle the ferrite matrix, this work suggests primacy of the effect of ‘microstructure mechanics’ i.e. the refinement of the basic microstructure unit size, on impact toughness. The optimisation of the quality of duplex engineering forgings may therefore primarily be achieved through total thermo-mechanical process control in order that imposed deformation and heat treatment results in refinement of the ferrite matrix grain size.
