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

Plasma immersion ion implantation (PIII) is a cluster compatible doping and processing tool offering many inherent advantages over conventional beamline ion implantation. When first introduced in the late 1980s, the technique was primarily used to enhance the surface mechanical properties of metals. Recently, a substantial amount of research activities have focused on microelectronics and have led to a number of very interesting applications, such as the formation of shallow junction, synthesis of silicon-on-insulator, large area implantation, trench doping, conformal deposition, and so on. In this paper, we will review the principles of PIII, the dynamic sheath model for various kinds of plasma, reactor designs, recent applications in the area of microelectronics, as well as the future of PIII pertaining to semiconductor materials and processing.

Plasma immersion ion implantation—a fledgling technique for semiconductor processing

Stainless steel, Co-Cr and Ni-Cr alloys have played an important role in many industrial sectors to combat environmental degradation. However, low hardness and poor wear properties have impeded their tribological and tribochemical applications. Conventional thermochemical treatments can be used to significantly harden these passive alloys but at the expense of their corrosion resistance due to precipitation induced depletion of Cr in the matrix. Research in 1980s led to the discovery of a new expanded austenite phase, i.e. so called S-phase with combined improvement in wear and corrosion resistance. Recent research has revealed that S-phase can be formed not only in stainless steels but also in Co-Cr alloys and Ni-Cr alloys. It is the purpose of this paper to critically review the S-phase surface engineering of stainless steels, Co-Cr alloys and Ni-Cr alloys. Particular attention will be paid to the structure, formation conditions, supersaturation, hardening mechanisms and metastability of...

S-phase surface engineering of Fe-Cr, Co-Cr and Ni-Cr alloys

Laser nitriding of metals

The use of elevated target temperatures near 400 °C during high flux ion implantation of N2+ at energies ranging from 60 keV to 0.4 keV leads to a metastable, f.c.c. high nitrogen solid solution phase induced in austenitic (f.c.c.) Cr-containing stainless steels. This phase has not been produced in an f.c.c. Ni-Fe alloy containing no Cr. Penetration depths of the N are significantly larger than expected on the basis of known diffusion coefficients of N in Cr-containing stainless steels or in pure f.c.c.-Fe. X-ray diffraction data suggest unusual differences in N penetration and concentration depending on the f.c.c. grain orientation. Consideration is given to possible residual stresses induced by N expansion of the lattice and to the anisotropic elastic constants for austenitic stainless steels. The amount of N in interstitial solid solution approaches 40 at.% under the lower energy, higher flux conditions, but only in the (200) crystallographic planes parallel to the surface. The N-expanded f.c.c. phase with the highest amount of N is found by conversion electron Mossbauer spectroscopy to be magnetic in AISI 304 and 310 stainless steels. A comparison of our results with those from related methods such as conventional plasma ion nitriding and pulsed plasma ion implantation is made to demonstrate that the observed metastable solid solution phase is often produced by other methods provided that appropriate temperatures and processing times are used. The similar penetration depths of N in the f.c.c. Cr-containing stainless steels for the various methods are consistent with thermal diffusion and metastable solubility that are enhanced by the Cr. The metastability is associated with the low Cr mobility below about 450 °C and the strong N-Cr bond. Evidence for vacancy-enhanced diffusion is not found and we see at present no advantage to using ion energies higher than about 2 keV at these elevated temperatures for producing surfaces with optimized tribological behavior based on the extremely high strength, metastable, N solid solution phase.

Metastable phase formation and enhanced diffusion in f.c.c. alloys under high dose, high flux nitrogen implantation at high and low ion energies

The surface modification of AISI 316 stainless steel by plasma immersion ion implantation (PI3) has been investigated over a range of treatment temperatures. Below 250°C the results are similar to those obtained by conventional ion beam implantation of nitrogen, but the depth of nitrogen penetration increases dramatically with temperature. Up to 450 °C a nitrogen-expanded austenite phase is formed which is shown to have improved corrosion performance over the untreated material. At 520 °C chromium nitride is precipated and the expanded austenite transforms to martensite, leading to a reduction in corrosion resistance. Pin-on-disc testing indicates improved wear resistance at all temperatures, with reduction in the wear volume by factors of several hundred at high loads. This can be attributed to the formation of an oxide layer which prevents the initiation of severe metallic wear.

Microstructure, corrosion and tribological behaviour of plasma immersion ion-implanted austenitic stainless steel

Plasma immersion ion implantation (PI3™), in which the diffusion of nitrogen from a low pressure r.f. plasma is combined with the implantation of nitrogen ions at energies up to 45 kV, is an effective means of nitriding austenitic stainless steel. At temperatures up to 450 °C, tribological properties can be improved without loss of corrosion resistance. In common with other nitriding processes in this temperature range, a supersaturated f.c.c. phase is formed, sometimes described as expanded austenite, which is maintained to very high nitrogen concentrations. At higher temperatures, chromium nitride is precipitated and the expanded austenite decomposes, leading to a reduction in corrosion resistance. Glancing-angle X-ray diffraction (XRD) of PI3-treated AISI 316 stainless steel at temperatures between 350 and 450 °C suggests that a highly homogeneous layer of expanded austenite is produced. The expansion increases with increasing process time, but decomposition of the supersaturated phase occurs after several hours of treatment if the temperature is too close to 450 °C. For a fixed process time, the expansion appears to be greatest at the lower temperatures (350 °C), although it can also be influenced by other processing parameters such as plasma density. Microstructural examination by cross-sectional transmission electron microscopy (TEM) has challenged the identification of the supersaturated phase as expanded austenite and reveals the complexity of the modified layer not seen by glancing-angle XRD. Most striking is the formation of a thick (2–3 μm) amorphous zone which may contain nanocrystalline precipitates of CrN and α-ferrite. A highly defective layer (up to 2 μm thick) of expanded austenite has been observed to underlie the amorphous zone where nitrogen diffusion is facilitated by the high defect density. Only partial reconciliation of the TEM results with the XRD observations has been possible to date.

Nitriding of austenitic stainless steel by plasma immersion ion implantation

Plasma immersoin ion implantation (PI3) is a new technique with certain advantages over conventional ion implantation. We have developed an implanter based on an inductively coupled r.f. glow discharge plasma. Ion densities of 1010 cm−3 are obtained with filling pressures of about 10−3 mbar. Ions are accelerated from the plasma by high voltage pulses (typically – 45 kV) applied directly to the work-piece. In this paper we report on the application of PI3 to nitrogen implantation in steels. Dramatic increases in microhardness and wear resistance have been observed for a number of steels, ranging from 0.3 wt.% C mild steel to austenitic stainless steels. Despite the relatively implantation energy, the modified layer can be greater than 1 μm in thickness. The nitrogen concentration profile can be controlled by the implantation temperature and dose. Glancing-angle X-ray diffraction has been used to determine the structural changes that occur in the surface layer.

Plasma Immersion Ion-Implantation of Steels

Plasma immersion ion implantation (PI 3 ) has emerged as a viable alternative to conventional ion implantation for specific applications, such as the implantation of non-planar components and in hybrid treatments such as high energy, ion-assisted deposition and energetic ion nitriding. It is particularly suitable for the treatment of metal components where improvement in surface hardness and wear resistance is required. This paper describes the development of a PI 3TM system, aimed at treating metal components up to a few hundred square centimetres in surface area with a hybrid implantation/diffusion process. Heating is provided by high energy ion bombardment. Complete process control has been implemented using industry standard equipment familiar to the process engineering community where possible. The implanter is described and its operating capabilities and reliability in a process environment are discussed.

Development of a plasma immersion ion implanter for the surface treatment of metal components

Plasma immersion ion implantation can add high-energy ion bombardment to almost any plasma-based surface treatment. It can be used as a non-line-of-sight alternative to ion-beam implantation, for high-energy ion nitriding or high-energy ion-assisted deposition. This paper describes its use for the treatment of steels at temperatures in the range 200 to 450 °C, where it can be considered as a hybrid of ion nitriding and ion implantation. It has been particularly successful in the treatment of austenitic stainless steels, although the results presented in this paper indicate that the high-energy ion bombardment is not always essential (although often desirable) to achieve satisfactory results. While it is shown that the process can open new parts of parameter space and extend known processes to a wider range of materials, the limitations on its use are also discussed.

R. Hutchings

Papers

Microstructure, corrosion and tribological behaviour of plasma immersion ion-implanted austenitic stainless steel

Nitriding of austenitic stainless steel by plasma immersion ion implantation

Plasma Immersion Ion-Implantation of Steels

Development of a plasma immersion ion implanter for the surface treatment of metal components

Ion-assisted surface modification by plasma immersion ion implantation