The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.

Bainite in Steels

This paper presents an overview of duplex stainless steels (DSS) with particular emphasis on super DSS, i.e. steels containing sufficient amounts of chromium, molybdenum, and nitrogen to produce a pitting resistance equivalent greater than 40. Duplex stainless steels have an attractive combination of mechanical and corrosion properties and are thus suitable for many marine and petrochemical applications, particularly where chlorides are present. The paper covers the following aspects of DSS: physical metallurgy, mechanical properties, corrosion properties, metallurgy of welding, machinability, and applications. A large number of references to the literature are given to aid the reader who is interested in acquiring a deeper understanding of the behaviour of this family of steels.MST/1685

Super duplex stainless steels

A commercial 2205 duplex stainless steel with three different solution treatments (at 1020, 1080 and 1200°C for 3 min) followed by continuous cooling at four respective cooling rates (1, 0.5, 0.25 and 0.1°C s −1 ) has been investigated by means of optical metallography, transmission electron microscopy, electron probe microanalysis, X-ray diffraction and differential thermal analysis. It is found that the lower solution treatment temperature with the lower cooling rate significantly enhanced the σ phase transformation. The σ precipitate could be detected when the specimens were solution-treated at 1020 and 1080°C and cooled at the rate of 0.25°C s −1 ; substantial amounts of σ phase formed when the specimens were cooled at 0.1°C s −1 . The precipitation of σ phase was considerably retarded as the solution temperature increased from 1080 to 1200°C. The precipitation behaviors of σ phase and M 23 C 6 carbide were characterized by transmission electron microscopy. The results indicated that the Cr- and Mo-rich σ phase preferentially nucleated at the pre-formed M 23 C 6 carbide particles, which were located at δ / γ interface or within δ ferrite grain. The selected area diffraction patterns displayed the complicated crystallography of σ phase, and revealed the orientation relationships between the interfacial σ precipitate and adjacent matrix phases.

Effects of solution treatment and continuous cooling on σ-phase precipitation in a 2205 duplex stainless steel

The morphology of continuously cooled bainite in steels is examined in this review and compared with that of isothermally transformed bainite. Some experimental observations of microstructures in continuously cooled commercial steels are also presented; these results demonstrate the presence of microstructures which are not easily defined in terms of any of the “classical” bainitic morphologies. The numerous terms created over the last 50 years to describe specific bainite morphologies have led to some confusion, and it is suggested that the commonly used terminologies do not adequately describe the full range of bainitic microstructures which are observed. A general definition for the bainite transformation is proposed. A classification system for bainitic microstructures is also developed, encompassing both isothermally transformed and continuously cooled bainites. It is suggested that primary bainite morphologies be classified as B1, B2, or B3, depending on whether the acicular ferrite is associated with (1) intralath precipitates, (2) interlath particles/films, or (3) discrete regions of retained austenite and/or secondary transformation product(e.g., martensite or pearlite), respectively. Possible ambiguities with the descriptions of autotempered martensite and Widmanstatten ferrite are also discussed, and a number of areas are identified where further work would be helpful.

A perspective on the morphology of bainite

The Influence of Microstructure on Brittle Fracture Toughness

The Bainite in Low Carbon Low Alloy High Strength Steels

Precipitation of 6 phase in an Fe-25Cr-7Ni-3Mo alloy with α/γ duplex structure has been studied systematically and the following results were obtained:(1) TTP diagram for the a phase precipitation was a C-type with a nose at 820°C.(2) σ phase nucleated on the α/γ interfaces and grew into α grains as a eutectoid reaction: α→σ+γ. Enrichments of Cr, Mo and W atoms into σ phase were remarkable.(3) Hardness increase by the precipitation of σ was not so large, while impact value markedly decreased even at a few percent precipitation. σ phase was also found to decrease hot ductility.(4) The precipitation range during cooling was 700-900°C and σ phase precipitation could be avoided if cooling rate was larger than about 20°C/ min after solution treatment.

Precipitation of σ Phase in a 25Cr-7Ni-3Mo Duplex Phase Stainless Steel

The Microstructure and Toughness of High Tensile Strength Steels

Effects of hot or cold deformation and thermal history on theσ phase0 precipitation in (A) a 25Cr-7Ni-3Mo and (B) a 23Cr-6Ni steels with ferrite/austenite (α/γ) duplex structure have been studied. Results obtained are as follows:(1) The incubation time for the σ phase precipitation in Steel B was a couple of orders of magnitude longer than that in Steel A. This should be attributed to the difference in chemical composition between two steels, especially in a phase, where the majority of σ phase is known to precipitate.(2) σ phase precipitation is enhanced by hot work or thermal strain as well as cold work. The enhancing effect of cold work is predominant, and the precipitation of σ particles is observed in γ grains as well as in α grains.(3) The precipitation of σ phase is retarded by the prior heating history before aging; (a) heating at the α single phase region or (b) remelting. Especially the effect of the treatment (b) is remarkable. These retardation effects can be ascribed to homogenization of alloying elements by these heat treatments. The difference between the cases of (a) and (b) is thought to result from the degrees of their homogeneities.

Tatsuro Kunitake

Papers

The Bainite in Low Carbon Low Alloy High Strength Steels

The Bainite in Low Carbon Low Alloy High Strength Steels

Precipitation of σ Phase in a 25Cr-7Ni-3Mo Duplex Phase Stainless Steel

The Microstructure and Toughness of High Tensile Strength Steels

Effects of Plastic Deformation and Thermal History on σ Phase Precipitation in Duplex Phase Stainless Steels