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

https://biblio.ugent.be/publication/818298/file/818329.pdf

Hydrogen-fueled internal combustion engines

Four types of hydrogen detectors are used by researchers, engineers, and manufacturers 
today, and if hydrogen continues to play a role in emerging alternative energy sources, 
there will be exponential growth in the use and need for more advanced and more robust 
devices in the future. The types of sensors reviewed in this chapter are (1) room-temperature hydrogen leak sensors; (2) thermometers, particularly useful at low temperature; 
(3) liquid hydrogen volume and mass gauges; and (4) para/ortho hydrogen ratiometers.

In recent years, processing maps are being used to design hot working schedules for making near-net shapes in a wide variety of materials. In this paper, the results obtained on the characterization of hot working behavior of titanium and its alloys using the approach of processing maps are described. In commercial purity α titanium, dynamic recrystallization (DRX) domain occurs at 775°C and 0.001 s−1 with an efficiency of power dissipation [2m/(m+1) where m is the strain rate sensitivity of flow stress] of 43%. The DRX domain shifts to higher strain rates when the interstitial impurity content is lowered. In the near-α and α-β alloys like IMI 685, Ti–6Al–4V, the preform microstructure has a significant influence on the processing maps. For example, in the transformed β (Widmanstatten) preform microstructures, these alloys exhibit a domain of spheroidization at lower temperature and a domain of β superplasticity at higher temperatures, both occurring at slow strain rates. These domains merge at the β transus because of the occurrence of damage processes which lower the tensile ductility. On the other hand, processing maps on alloys with equiaxed preform microstructure exhibit a clear superplasticity domain in the α-β range and the β phase undergoes DRX with a power dissipation efficiency of ≈45–55%. Titanium materials in general, exhibit wide flow instability regimes due to adiabatic shear bands formation at higher strain rates and hence careful process design has to be adopted for successful forging and microstructural control.

Processing maps for hot working of titanium alloys

In recent years, much progress has been made in the studies of nanostructured Al alloys for advanced structural and functional use associated both with the development of novel routes for the fabrication of bulk nanostructured materials using severe plastic deformation (SPD) techniques and with investigation of fundamental mechanisms leading to improved properties. This review paper discusses new concepts and principles in application of SPD processing to fabricate bulk nanostructured Al alloys with advanced properties. Special emphasis is placed on the relationship between microstructural features, mechanical, chemical, and physical properties, as well as the innovation potential of the SPD-produced nanostructured Al alloys.

Nanostructured aluminium alloys produced by severe plastic deformation: New horizons in development

A novel procedure for hydrogen charging and studying the Internal Hydrogen Embrittlement (IHE) of steels is described here. A cylindrical notched tensile sample with an extended end is employed for hydrogen charging. The extended portion of the sample forms the cathode in an alkaline bath and a constant uni-axial tensile load is applied during hydrogen charging. The stress gradient set up by the notch, which is not in contact with the electrolyte, enhances the hydrogen concentration at various trapping sites of the matrix beyond the solubility limit. Subsequent to charging, the specimen is kept under the same load as that during charging, for another 24 h to stabilize the population of hydrogen within the specimen matrix. At the end of this stage, the specimen is tensile tested to failure at room temperature. Two different steels namely maraging and mild steels have been chosen to study the effect of hydrogen ingress on mechanical properties. While an increase in tangent modulus (linear portion of the stress–strain diagram), yield strength, work hardening rate and ultimate tensile stress (UTS) has been observed on hydrogenation, a decrease in total elongation has been noticed for both the steels studied. Fractographic investigation has revealed that the fracture mode is predominantly ductile dimple (failure by micro-void coalescence) in both the materials and that the hydrogen reduces the size of the dimples. The observations of this investigation are significant in two respects: firstly, it demonstrates the efficacy of a hydrogen charging method for steels which can introduce hydrogen to a level much higher than its solubility limit and secondly, it reports for the first time enhancement of modulus and work hardening by hydrogen charging. These observations have been rationalized on the basis of current understanding on the effect of hydrogen on plastic properties and hypothesis of the models of IHE. It is suggested that the trapping of hydrogen by dislocations and other structural features of the matrix and the mutual interactions of their strain fields can account for the observed effects on yield strength, tangent modulus, work hardening rate, UTS and ductility.

A study of internal hydrogen embrittlement of steels

Evolution of microstructure and deformation resistance in creep of tempered martensitic 9-12%Cr-2%W-5%Co steels

The hot deformation characteristics of alpha-zirconium in the temperature range of 650 °C to 850 °C and in the strain-rate range of 10-3 to 102 s-1 are studied with the help of a power dissipation map developed on the basis of the Dynamic Materials Model.[7,8,9] The processing map describes the variation of the efficiency of power dissipation (η =2m/m + 1) calculated on the basis of the strain-rate sensitivity parameter (m), which partitions power dissipation between thermal and microstructural means. The processing map reveals a domain of dynamic recrystallization in the range of 730 °C to 850 °C and 10−2 to 1−1 with its peak efficiency of 40 pct at 800 °C and 0.1 s-1 which may be considered as optimum hot-working parameters. The characteristics of dynamic recrystallization are similar to those of static recrystallization regarding the sigmoidal variation of grain size (or hardness) with temperature, although the dynamic recrystallization temperature is much higher. When deformed at 650 °C and 10-3 s-1 texture-induced dynamic recovery occurred, while at strain rates higher than 1 s-1, alpha-zirconium exhibits microstructural instabilities in the form of localized shear bands which are to be avoided in processing.

/pdf/processing-map-for-hot-working-of-alpha-zirconium-47gpemyrvn.pdf

Processing map for hot working of alpha-zirconium

Austenitic stainless steel welds in components used in light water reactors are susceptible to thermal aging embrittlement at reactor operating temperature of around 300 °C after a long service life. In this study, low temperature aging embrittlement of types 304L and 316L stainless steel welds with 10% ferrite was investigated on the basis of changes in mechanical properties and microstructure after aging up to 20,000 h at 335, 365 and 400 °C. Spinodal decomposition and G-phase precipitation in the ferrite was observed after aging which lead to embrittlement in the material. In contrast to the small effect on tensile properties, the impact toughness was significantly degraded after aging. Charpy impact test of the aged samples showed decrease in upper-shelf and lower-shelf energy and increase in ductile brittle transition temperature. Large increase in the microhardness of ferrite phase was observed with no change in austenite hardness. The embrittlement in 316L weld was higher compared to 304L weld for similar aging condition. The kinetics of aging embrittlement was established based on Arrhenius relationship. A constant activation energy was determined for 304L weld in the temperature range 335–400 °C, however, 316L weld showed different activation energy values in each temperature range.

Low temperature thermal aging of austenitic stainless steel welds: Kinetics and effects on mechanical properties

Multi-axial forging of Al was carried out at room temperature in a confined channel die as a means of achieving severe plastic deformation (SPD). The microstructures of the 3, 6 and 9 pass samples were quantified using electron back scattered diffraction to obtain the distribution of boundary spacing and fraction of high angle boundaries. Subsequent compression tests at room temperature were carried out to determine their strength and strain rate sensitivity. Softening was observed beyond 6 passes both in terms of the pressing stress as well as the subsequent compression tests. Beyond 6 passes the fraction of high angle boundaries increased and the geometrically necessary dislocations (GND) density decreased. It was estimated that during SPD, Al had a higher hardening contribution due to GND and subgrain walls than that due to Hall–Petch type grain boundary strengthening. It was shown that as the fraction of high angle boundaries increased with increasing deformation, the dislocation strengthening term decreased and the Hall–Petch term increased, resulting in a net softening effect.

J.K. Chakravartty

Papers

A study of internal hydrogen embrittlement of steels

Evolution of microstructure and deformation resistance in creep of tempered martensitic 9-12%Cr-2%W-5%Co steels

Processing map for hot working of alpha-zirconium

Low temperature thermal aging of austenitic stainless steel welds: Kinetics and effects on mechanical properties

Softening of Al during multi-axial forging in a channel die