Stainless Steels

In 1821, the French engineer Pierre Berthier observed that a certain amount of chromium added to iron alloys, in addition to enhancing their stiffness, also improved remarkably their corrosion resistance to acids. Almost a century later, in 1909, Leon Guillet and Albert Portevin in France studied independently the microstructure of Fe-Cr and Fe-Cr-Ni alloys. In 1911, the German metallurgist P. Monnartz, following the pioneering activity of his predecessors, explained the passivation mechanism and determined the lowest percentage of chromium required to impart a rustless ability to steels. The new alloy did not corrode or rust when exposed to weather, and the new iron alloy was then simply referred to as rostfrei Stahl in Germany, rustless or rustproof iron in Great Britain, and acier inoxydable in France. However, in the United States and United Kingdom, it was later denoted by the more modern designation still used today, stainless steel. In 1913, the first casting of a stainless steel was performed at Sheffield in the United Kingdom.

 

Stainless steels are a large family of iron-chromium-based alloys (Fe-Cr) that are essentially low-carbon steels containing a high percentage of chromium, at least above 12 wt.% Cr, to impart the same corrosion resistance conferred by pure chromium in chrome plate. This addition of chromium gives the steel its unique corrosion-resistance properties denoted as stainless or rustproof. The chromium content of the steel allows the formation on the steel surface of a passivating layer of chromium oxide. This protective oxide film is impervious, adherent, transparent, and corrosion resistant in many chemical environments. If damaged mechanically or chemically, this film is self-healing when small traces of oxygen are present in the corrosive medium. It is important to note that in order to be corrosion resistant, the Fe-Cr alloy must contain at least 12 wt.% Cr and that when this percentage is decreased, for instance by precipitation of chromium carbide during heating, the protection is lost and the rusting process occurs. Moreover, the corrosion resistance and other useful properties of stainless steels are largely enhanced by increasing the chromium content usually well above 12 wt.% Cr. Hence the chromium content is usually 15 wt.%, 18 wt.%, 20 wt.%, and even up to 27 wt.% Cr in certain grades. In addition, further alloy additions (e.g., Mo, Ti, S, Cu) can be made to tailor the chemical composition in order to meet the needs of different corrosion conditions, operating temperature ranges, and strength requirements or to improve weldability, machinability, and work hardening. Generally, the corrosion resistance of stainless steels is, as a rule, improved by increasing the alloy content. The terminology heat resistant and corrosion resistant is highly subjective and somewhat arbitrary.  The term heat-resistant alloy commonly refers to oxidation-resistant metals and alloys (see Ni-Cr-Fe alloys in the nickel and nickel alloys section), while corrosion resistant is commonly applied only to metals and alloys that are capable of sustained operation when exposed to attack by corrosive media at service temperatures below 315°C. They are normally Fe-Cr or Fe-Cr-Ni ferrous alloys and can normally be classified as stainless steels. There are roughly more than 60 commercial grades of stainless steel available, and the global annual production was roughly 25 million tonnes in 2004.

 

 

Following a classification introduced by Zapffe and later modernized to accommodate new grades, stainless steels can be divided into five distinct classes (see Table 1). Each class is identified by the alloying elements that affect their microstructure and for which each is named. These classes are as follows:

(i) austenitic stainless steels;

(ii) ferritic stainless steels;

(iii) martensitic stainless steels;

(iv) duplex or austenoferritic stainless steels; and

(v) precipitation-hardened (P-H) stainless steels.

In practice, empirical parameters called nickel and chromium equivalents can be utilized to assess the relative stability of austenite and ferrite, respectively. These equivalents are defined as follows:

 

Eq(Ni) = wNi + 30 × wC + 0.5 wMn,

Eq(Cr) = wCr + wMo + 1.5 wSi + 0.5 wNb,

where wi denotes the mass fraction of the chemical element indicated by the subscript. The Ni and Cr equivalents are usually used to assess the phase formation in weldments. Hence modifying the chemistry of the weld metal can ensure a better result by avoiding hot cracking.