There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Recent developments in stainless steels

Of late, nanotechnology seems to be rapidly thrusting its applications in all aspects of life including engineering and medicine. Materials science and engineering has experienced a tremendous growth in the field of nanocomposite development with enhanced chemical, mechanical, and physical properties. A wide array of research has been conducted in the processing of nanocomposites. Consolidation of these systems from loose particles to bulk free form entities has always been a challenge. To name a few, traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to the excessive grain growth during processing. This article reviews in detail the results from numerous studies on various methods for manufacturing nanocomposites with improved properties and retained nanostructures. Both challenges and recent advances are discussed in detail in this review.

Challenges and advances in nanocomposite processing techniques

The evolution of microstructure and texture during cold rolling of commercial-purity titanium (CP-Ti) was studied with particular reference to deformation twinning and dislocation slip. For low to intermediate deformation up to 40% in thickness reduction, the external strain was accommodated by slip and deformation twinning. In this stage, both compressive ( { 1 1 2 ¯ 2 } 〈 1 1 2 ¯ 3 ¯ 〉 ) and tensile ( { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 ¯ 〉 ) twins, as well as, secondary twins and tertiary twins were activated in the grains of favorable orientation, and this resulted in a heterogeneous microstructure in which grains were refined in local areas. For heavy deformation, between 60 and 90%, slip overrode twinning and shear bands developed. The crystal texture of deformed specimens was weakened by twinning but was strengthened by slip, resulting in a split-basal texture in heavily deformed specimens.

Effect of deformation twinning on microstructure and texture evolution during cold rolling of CP-titanium

Thermomechanical processing of advanced high strength steels

The dynamic recrystallization (DRX) of AISI 304 stainless steel was studied with torsion test in the temperature range of 900–1100°C and the strain rate range of 5.0×10 −2 –5.0×10 0 s −1 . The initiation and evolution of DRX were investigated with microstructural analysis and then the critical strain for DRX initiation could be confirmed by analysis of flow stress. The volume fraction of DRX ( X DRX ) as a function of processing variables, such as strain rate ( e ), temperature ( T ), and strain ( e ) were established. For the exact prediction of X DRX , the variation of flow stress with strain was analyzed in detail. The calculation of grain size was based on X DRX and the investigated microstructure at a few points. It was found that the calculated results agreed with the microstructure of the alloy at any deformation conditions.

Dynamic recrystallization behavior of AISI 304 stainless steel

In this paper, we have developed the equations for predicting austenite grain size (AGS) of microalloyed medium carbon steel during hot rolling Dynamic recrystallization, which plays a major role in reducing the flow stress and AGS, was described by modifying Avrami's equation In addition, for better AGS prediction of material during rolling, the fraction of dynamic recrystallization was expressed as a function of strain together with the Zener–Hollomon parameter Torsion tests were performed in the temperature range of 900–1100 °C and the strain rate range of 50×10 −2 –50×10 0 s −1 to study the softening behavior To examine the validity of the developed equation, we have then applied it to a four-pass continuous deformation process simulated by hot torsion test For the exact prediction of microstructural evolution during hot torsion test, the change of temperature was calculated using finite difference method It was found that the calculated grain size of the alloy is in good agreement with the observed grain size It is expected that a fine grained steel showing high strength and high toughness can be obtained by controlling deformation conditions on the basis of the newly developed equation

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.

Hot rolling simulations of austenitic stainless steel

Static restoration mechanism during hot interrupted deformation of 304 stainless steel was studied in the temperature range from 900 to 1100°C, various strain rate from 0.05 to 5/sec and pass strain of 0.25–3 times peak strain. It was clarified that the static recrystallization was happened after 3–10 seconds at first deformation. The static restoration was depended on the pass strain, deformation temperature and strain rate and fractional softening (FS) values increased with increasing strain rate, deformation temperature and pass strain. Recystallization kinetics was explained with Avrami equation and Avrami constant was 1.113. This value was independent of deformation variables significantly. The time of 5, 50, 95% recrystallization was evaluated using such equations: t0.05 = 2.9 × 10−12 ∈−1.17\({\dot \varepsilon }\)−0.94D exp(222000 J/mol/RT), t0.5 = 2.0 × 10−10 ∈−1.56\({\dot \varepsilon }\)−0.81D exp(197000 J/mol/RT), t0.95 = 1.9 × 10−8∈−1.63\({\dot \varepsilon }\)−0.76D exp(173000J/mol/RT). The predicted values by use of upper equations had a good agreement with a measurement.

Static recrystallization kinetics of 304 stainless steels

The high-temperature deformation behavior of AISI 430 ferritic stainless steel was studied by torsion tests. The deformation tests were performed in the temperature range of 900–1100°C and strain rate range of 5.0×10−2 −5.0/sec. The evolutions of flow stress and microstructure show the characteristics of continuous dynamic recrystallization (CDRX). The flow stress gradually decreased with strain over the peak stress without steady state. Below the 100% effective strain, grains appeared in small angle grain boundaries with a misorientation of 3–9°. In addition, when heavy deformation (>300%) was applied, higher misorientation (∼15°) was achieved. The tendency of CDRX increased with increasing strain rate and decreasing temperature. The dependence of CDRX grain size on the strain rate and temperature was discussed.

Yeon-Chul Yoo

Papers

Dynamic recrystallization behavior of AISI 304 stainless steel

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

Hot rolling simulations of austenitic stainless steel

Static recrystallization kinetics of 304 stainless steels

Continuous dynamic recrystallization of AISI 430 ferritic stainless steel