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

Influence of Ti, C and N concentration on the intergranular corrosion behaviour of AISI 316Ti and 321 stainless steels

Effect of aging at 700 °C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds

Low cycle fatigue (LCF) tests were performed for a type 316Ti stainless steel (SS) in high temperature water. Fatigue crack initiation behaviors in high temperature water were investigated. It was found that there existed several kinds of Ti-bearing precipitates, consisting of isolated TiN or duplex (Al, Mg)O/TiN, Mo-rich (Ti, Mo)C and Ti(N,C) in the steel. Fatigue cracks were mainly initiated at Ti-bearing precipitates, phase boundaries of austenite/α-ferrite phases and persistent slip bands (PSBs) in austenite. It is believed that synergism between the mechanical factors and electrochemical reactions played a key role in the process of fatigue crack initiation in high temperature water. Related fatigue crack initiation mechanisms for the 316Ti SS are discussed.

Crack initiation mechanisms for low cycle fatigue of type 316Ti stainless steel in high temperature water

The dynamic impact response of AISI 321 steel at strain rate of 4000, 5500, 6500 and 7500 s−1 was investigated using the split Hopkinson pressure bar system. The alloy samples processed to have grain size of 0.24, 3, 13 and 34 µm were studied. While the yield strength and hardness increases with decrease in grain size, strain hardening rate is comparable for all grain sizes. Microstructural evaluation of the impacted specimens using high-resolution electron backscattered diffraction (HR-EBSD) technique showed grain boundary strengthening, deformation twinning, deformation-induced martensitic transformation, dislocation multiplication during slip and precipitation of carbides that act as barriers to dislocation motion as additional source of strengthening. Slip and twinning were the dominant deformation mechanisms observed in the steel. Twinning, dislocation multiplication during slip and carbide precipitation contributed more strongly to strain-hardening in coarse-grained (CG) specimen while stain-induced martensite and grain boundary strengthening are the most beneficial to strengthening in the ultra-fine-grained (UFG) specimens. The temperature rise in the specimens during impact increases with strain rate. This slowed the kinetics of twinning, phase transformation and dislocation interaction especially in CG structure. Both XRD and HR-EBSD texture results confirmed the development of {110}||CD (CD: compression direction) texture in the austenite phase at the expense of {100}||CD and {111}||CD fibres. Thermomechanical instability leading to the formation of adiabatic shear band (ASB) occurred in the test specimens as they deformed at high strain rates. While the amount of deformation twinning and αʹ-martensite decreases towards the ASB, only the carbides and small fraction of αʹ-martensite are observed inside the ASB. Grain refinement via rotational dynamic recrystallization occurred within the ASB. The extent of grain refinement increased with increase in initial grain size of the test specimen.

Multiple strengthening sources and adiabatic shear banding during high strain-rate deformation of AISI 321 austenitic stainless steel: effects of grain size and strain rate

Mechanisms of stress relief cracking in titanium stabilised austenitic stainless steel

During service at high temperature, stabilized stainless steels are well known to present a serious form of intergranular cracking in the Heat Affected Zones (HAZ) referred to as the reheat cracking phenomenon which is very deleterious for the structural integrity of welds made in this steel. The purpose of the present study is to present quantitative results on mechanical behaviour and the damage at high temperature of simulated HAZ which show different sensitivities to intergranular cracking. Creep laws are determined and an intergranular damage model is identified using the local approach to fracture based on mechanical tests performed at 600 °C and metallurgical observations on smooth specimens and two types of notched specimens.

Modelling creep damage in heat affected zone in 321 stainless steel. Part I: Quantitative study of intergranular damage

Stabilized stainless steels like AISI 321 are well known to present a serious form of intergranular cracking in the HAZ referred to as reheat cracking. The purpose of the present part is to validate the creep damage model identified using the local approach methodology on notched bar creep tests (Part I) by checking if these damage laws are able to give accurate fracture predictions. The comparison between experimental data obtained at 600°C and numerical simulations is made for five different conditions with three different simulated HAZ and on two specimen geometries including CT specimen tested under stress relaxation conditions and sharply notched thin wall tubular specimens tested in torsion (Mode II loading). Both of these specimens have not been used for the identification of the damage model coefficients.

M. Chabaud-Reytier

Papers

Mechanisms of stress relief cracking in titanium stabilised austenitic stainless steel

Modelling creep damage in heat affected zone in 321 stainless steel. Part I: Quantitative study of intergranular damage

Modelling creep damage in heat affected zone in 321 stainless steel. Part II: Application to creep crack initiation simulations