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

As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.

INCONEL 718: A solidification diagram

Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start of Nb (C, N) precipitation as a function of temperature and composition. The predicted curves have been compared with the experimental observations of several authors to determine the effects of thermomechanical processing variables on the density of preferred nucleation sites and to incorporate these in the equations. Good agreement between the predicted and observed forms of precipitation curve is obtained with consistent constants in the equations when account is taken of the influence of different methods of detecting the onset of precipitation. Combining the calculated precipitation start curves with the dependence of recrystallization kinetics on composition and thermomechanical process variables when all niobium is in solution leads to prediction of the lower temperature limit for complete recrystallization and of the upper temperature limit for effecti...

Effect of composition and process variables on Nb(C, N) precipitation in niobium microalloyed austenite

Damage characterization of polymer-based composite materials: Multivariable analysis and wavelet transform for clustering acoustic emission data

Advanced structural health monitoring of concrete structures with the aid of acoustic emission

Tool wear evaluation in drilling by acoustic emission

The diffusion coefficients of Nb have been studied in unalloyed γ Fe over the temperature range 948–1201°C and in Fe–1·5Mn, Fe–0·6Si, and Fe–1·5Mn–0·6Si alloys over the range 1081–1201°C by the direct sectioning method using the isotopes 94Nb and 95Nb. The Arrhenius curves for diffusion into pure γ Fe and γ Fe alloyed with Mn and/or Si are linear in the temperature range 1081–1201°C, and give approximately the same activation energy Q = 267·4 kJ mol−1. Below these temperatures lnD v. 1/T, where D is the volume diffusion coefficient and T the temperature, is non-linear for diffusion into γ Fe. The values of Q and D 0 (the frequency factor) obtained in pure γ Fe are more representative than the values Q = 344·5 kJ mol−1 and D 0 = 5·30 × 10−2 m2 s−1 found in the literature. It was possible to examine the influence of the substitutional alloying elements Mn and Si on the behaviour of Nb diffusion in γ Fe and the following effects were observed: the diffusivity of Nb decreases with Mn content but incre...

Diffusion of Nb in Fe and Fe alloys

Wavelet Transform (WT) was applied to Acoustic Emission (AE) signals from scratch tests on stainless steel samples with Cr or Ti nitride coatings obtained by physical vapor deposition, in order to characterize the coating adherence failure mechanisms. The WT, employed to analize the essentially non-stationary AE signals, was adequate to identify different stages of failure (transversal coating microfractures and coating/matrix debonding microfractures), which could be precisely delimited by a unique parameter.

José Ruzzante

Papers

Tool wear evaluation in drilling by acoustic emission

Diffusion of Nb in Fe and Fe alloys

Ti and Cr nitride coating/steel adherence assessed by acoustic emission wavelet analysis

Coating adherence in galvanized steel assessed by acoustic emission wavelet analysis

Localization algorithm for acoustic emission