Stainless Steels: An Introduction and Their Recent Developments

This chapter gives an overview on the various types of stainless steels and their general classification based on microstructures. This chapter also goes through briefly the history of the discovery of ‘stainlessness’ and the status quo of stainless steels production in several major economies. Motivations for writing this book and some of the new applications that utilise stainless steels (which are covered in chapter 10) are given at the end of this chapter.

This chapter is on ferritic stainless steels. A brief introduction to the compositions and uses of the various grades of steels in the ferritic class is given. New techniques for grain refinement of the ferritic class are presented. The main problems plaguing the ferritic class, i.e., 475°C-embrittlement, formation of sigma phase and sensitisation, are discussed, with reference to the constitution diagram of the Fe-Cr system. The various theories proposed for the causes of these problems, together with the old and new methods for their alleviation/elimination, are presented in detail.

This chapter gives a brief introduction to the compositions and uses of the various grades of steels in the austenitic class. The phenomena of deformation-induced transformation (DIM) and sensitisation are then covered. The underlying mechanisms causing DIM and sensitisation, together with the factors affecting them, are discussed in detail. Mechanical stabilisation, recent research findings on and methods for detecting DIM and sensitisation are also covered.

This chapter is on martensitic stainless steels. A brief introduction to the compositions and uses of the various grades of steels in the ferritic class is given first. The purposes of and the changes in properties associated with typical heat treatments (austenisation and tempering), together with their underlying mechanisms, are then introduced. Recent results on the effects of heat-treatment parameters (temperature, cooling rate, etc.) are presented. A recently-developed tempering map for martensitic stainless steel and a new constitution diagram for ferritic-martensitic weld metals are also presented.

This chapter is devoted to duplex stainless steels (DSSs). This chapter begins with an introduction to the different types of DSSs (ordinary, lean, superduplex and hyperduplex), their applications and compositions of the commonly used grades. Their typical microstructures and methods for phase prediction via different constitution diagrams are discussed. The precipitate phases that may form in DSSs and their implications on mechanical/corrosion properties are presented (a more in-depth discussion on these precipitates is presented in Chapter 8). The superplasticity of DSSs is discussed in detail. Both the conventional method and a new one for producing the microduplex structure are introduced. Recent developments in explaining the underlying mechanisms leading to the superplastic behaviour in DSSs are discussed.

This chapter focuses on precipitation-hardening stainless steels (PHSSs). The three main types of PHSSs, i.e., the semiaustenitic class, the martensitic class and the austenitic class, are described in detail. The duplex class is also briefly discussed. The different heat treatments, together with their effects on properties, that are commonly used for attaining precipitation strengthening in PHSSs are presented. Recent developments in the understanding of the precipitation sequences and the precipitates in PHSSs are introduced.

This chapter is devoted to stainless steels containing high-levels of nitrogen. Ni-free stainless steels and those containing raised levels of manganese are also covered. The problems associated with nickel and the reasons for developing low-Ni/Ni-free stainless steels using nitrogen and manganese are introduced. New constitution diagrams for phase prediction of the new high-N (Ni-free) stainless steels are presented. Recently developed methods for introducing very high levels of nitrogen to stainless steels are discussed. The effects of nitrogen on mechanical and corrosion properties, together with the underlying mechanisms, are introduced. Although high-N stainless steels typically contain low/no Ni, this chapter also discusses briefly high-N, high-Ni grades, whose production has been suggested rather recently.

This chapter gives an overview of the various precipitate phases that can form in the different classes of stainless steels. Besides the phases that are well-studied, a newly discovered phase, called the J phase, is also introduced. The conditions under which the different phases will form, the problems they cause, the methods for alleviating these problems, and the new methods for the detection of the various phases are presented.

Although possessing superior properties, stainless steels are still afflicted by a host of problems, especially when they are exposed to elevated temperatures. Problems covered in this chapter are: hydrogen embrittlement, sensitisation, metal dusting, pitting corrosion, microbiologically-induced corrosion, high-temperature oxidation, stress-corrosion cracking, fatigue, creep, and dynamic strain ageing. The emphases of this chapter are on the recent findings on the underlying mechanisms of these problems, the main factors affecting them, new methods for their alleviation/elimination and detection, and modelling.

Stainless steels are used in many applications like heat exchangers, furnace liners, automotive components, solar panels, and kitchenware. This chapter attempts to cover new applications that have recently been reported. A few novel uses of duplex stainless steels are also covered, these are the use of duplex stainless steels for temperature sensing and magnetic drug targeting.

This chapter is on techniques that can be used to improve the properties of stainless steels. Some of these techniques improve surface properties, while some of them enhance the properties throughout the bulk of the material. Many of the techniques improve properties through grain refinement, whereas some rely on a phase transformation. Some of the techniques are specifically devised for a particular type of stainless steel (the technique that utilises reverse transformation of martensite is mainly for the austenitic type) or for steels in the liquid state.

In quite a number of applications, stainless steels are chosen for their aesthetic appeal. Besides being shiny, the fact that many colours may be imparted to stainless steels adds to their popularity, especially in the construction industry. This chapter covers the basics of colouration of stainless steels and the various techniques for achieving different colours.

This chapter is devoted to powder metallurgy (PM) of stainless steels. The main focus is on the making of PM parts and problems that are encountered during the process. The manufacture of stainless steel powders is excluded. Porous stainless steels have also been included in this chapter because powder metallurgy may be utilised for their fabrication, but other non-PM methods are also covered.

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