Stainless steels

As chromium is added to steels, the corrosion resistance increases progressively due to the formation of a thin protective film of Cr203, the so-called passive layer. With the addition of about 12% Cr, steels have good resistance to atmo- spheric corrosion and the popular convention is that this is the minimum level of chromium that must be incorporated in an iron-based material before it can be designated a stainless steel. However, of all steel types, the stainless grades are the most diverse and complex in terms of composition, microstructure and mechanical properties. Given this situation, it is not surprising that stainless steels have found a very wide range of application, ranging from the chemical, phar- maceutical and power generation industries on the one hand to less aggressive situations in architecture, domestic appliances and street furniture on the other. By the late 1800s, iron-chromium alloys were in use throughout the world but without the realization of their potential as corrosion-resistant materials. Harry Brearley, a Sheffield metallurgist, is credited with the discovery of martensitic stainless steels in 1913 when working on the development of improved rifle barrel steels. He found that a steel containing about 0.3% C and 13% Cr was difficult to etch and also remained free from rust in a laboratory environment. Such a steel formed the basis of the cutlery industry in Sheffield and as Type 420 is still used for this purpose to the present day. During the same period, researchers in Germany were responding to pres- sures for improved steels for the chemical industry. Up until that time, steels containing high levels of nickel were in use as tarnish-resistant materials but had inadequate resistance to corrosion. Two Krupp employees, Benno Strauss and Eduard Maurer, are credited with the discovery of Cr-Ni austenitic stainless steels and patents on these materials were registered in 1912. However, workers in France and the United States are also cited as independent discoverers of these steels. During the 1920s and 1930s, rapid developments took place which led to the introduction of most of the popular grades that are still in use today, such as Type 302 (18% Cr, 8% Ni), Type 316 (18% Cr, 12% Ni, 2.5% Mo), Type 410 (12% Cr) and Type 430 (17% Cr). However, even in the 1950s, stainless steels were still regarded as semi-precious metals and were priced accordingly. Up until the 1960s, these steels were still produced in small electric arc furnaces, sometimes of less than 10 tonnes capacity. The process was carried out in a single stage, involving the melting of scrap, nickel and ferro-chrome, with production times in excess of 389 hours. However, substantial gains were achieved with the installation of larger furnaces with capacities greater than 100 tonnes and the introduction of oxygen refining techniques also increased productivity very substantially. Since the early 1970s, the production of stainless steels has been based on a two-stage process, the first employing a conventional electric arc furnace for the rapid melting of scrap and ferro-alloys but using cheap, high- carbon ferro-chrome as the main source of chromium units. The high-carbon melt is then refined in a second stage, using either an argon-oxygen decarburizer (AOD) or by blowing with oxygen under vacuum (VOD). The AOD process is now employed for over 80% of the world's production of stainless steel and produces 100 tonnes of material in less than one hour. However, in addition to achieving faster production rates, the intimate mixing with special slags results in very efficient desulphurization. Other benefits also accrue from the facility to produce carbon contents of less than 0.01% and hydrogen levels of 2-3 pm.

Underlying metallurgical principles

As indicated in the Overview, stainless steel grades cover a wide range of compositions which results in the generation of a variety of microstructures and mechanical properties.

Reference will be made in this chapter to the composition-structure relationships which show that alloying elements in stainless steels can be divided into two groups, namely those that promote the formation of an austenitic structure at hot rolling or solution treatment temperatures and those that promote the formation of delta ferrite. Chromium is the principal alloying element in stainless steels and this promotes the formation of delta ferrite at high temperature. However, iron can accommodate up to about 13% Cr at a temperature of around 1050"C and still remain completely austenitic at that temperature. On the other hand, the Ms-Me temperature range of a 12% Cr steel is sufficiently high to allow this material to transform completely to martensite on cooling to ambient temperature. An increase in chromium from 12 to 17% brings about a progressive change from austenite to delta ferfite at high temperature and the ferrite remains unchanged on cooling to ambient temperature. Nickel is a strong austenite-forming element and is added to stainless steels in order to preserve an austenitic structure in the presence of high chromium contents. Thus a steel containing 18% Cr 9% Ni is completely austenitic at a temperature of 1050"C but the overall alloy content now depresses the Ms-Mf temperature range to sub-zero temperatures. Therefore this material retains its austenitic structure on cooling to ambient temperature, providing a relatively low strength but a high level of formability.

Elements such as silicon, molybdenum and titanium also promote the formation of delta ferrite at high temperature, whereas carbon, nitrogen, manganese and copper promote the formation of austenite. Therefore consideration must also be given to the presence of these elements, in addition to the balance between chromium and nickel, in determining the structure of stainless steels at elevated temperatures. However, both austenite- and ferrite-forming elements will depress the Ms-Mf range and influence the microstructure formed on cooling to ambient temperature. Therefore the constitution of stainless steels is governed by:

(i) the balance between austenite- and ferdte-forming elements which controls the structure at hot rolling and solution treatment temperatures,

(ii) the overall alloy content which controls the Ms-Me temperature range and therefore the structure and properties at ambient temperature.

Austenitic stainless steels can be subjected to severe cold-forming operations, for example in the cold rolling of hot band to strip gauges and also in the production of domestic sinks and tableware from annealed strip. This introduces the topic of strain-induced martensite, whereby a material which is austenitic in the solution-treated condition can transform partially or completely to martensite with the application of cold work at ambient temperature. Detailed consideration is given to this topic later in the chapter, but in essence it relates to the stability of the austenitic structure, as influenced by the overall alloy content, and the destabilizing effects due to the magnitude and temperature of cold deformation.

The metallurgy of the 12% Cr martensitic grades is similar to that involved in the engineering grades, although the presence of such a large amount of chromium induces a very high degree of hardenability and these steels are capable of devel- oping a martensitic structure in substantial section sizes, even in the aft-cooled condition. However, like their low-alloy counterparts, the 12% Cr grades must be tempered to produce a good combination of strength and ductility/toughness and both types of steel often incorporate additions of molybdenum and vanadium in order to improve the tempering resistance through the formation of stable carbides.

Whereas austenitic stainless steels are used in domestic or architectural applica- tions, where corrosion resistance and aesthetic appeal are the main requirements, they are also employed in pressure vessels where both corrosion resistance and strength are important considerations. As indicated in the Overview, solid solution strengthening with additions of nitrogen is the main avenue for the production of higher-strength austenitic stainless steels. Precipitation-strengthening reactions can also be induced in these grades through the precipitation of carbides and inter- metallic compounds based on nickel, aluminium and titanium. However, such materials have found little commercial application, due possibly to weldability problems and poor corrosion properties. Stainless steels resist corrosion through the formation of a thin passive film of Cr203 and, very broadly, the corrosion resistance of these materials increases with chromium content. However, as illustrated by the previous remarks, marked variations in microstructure are introduced with the addition of alloying elements with a marked effect on mechanical properties. Thus the 12% Cr martensitic grades are capable of developing high levels of strength but with only moderate resistance to corrosion. In contrast, an austenitic grade, based on 18% Cr 9% Ni, has a low strength but a significantly higher resistance to corrosion, the latter being enhanced by the addition of molybdenum. Therefore throughout the broad range of stainless steels, there will be a compromise between corrosion resistance and other properties, such as strength, formability and weldability. Additionally, it is necessary to differentiate between the various types of corrosion in stainless steels, notably:

·       general corrosion

·       intergranular corrosion

·       pitting corrosion

·       stress corrosion

However, whereas these types of corrosion will be discussed at a later stage, the authors are conscious of the fact that the treatment of corrosion in this text is superficial and reflects their limited knowledge of the subject.

Composition-structure relationships

Iron-chromium alloys

The simplest stainless steels consist of iron-chromium alloys but in fact the binary iron-chromium system can give rise to a wide variety of microstructures with markedly different mechanical properties. The Fe-Cr equilibrium diagram is shown in Figure 4.1 and is characterized by two distinctive features, namely:

1. The presence of sigma phase at about 50% Cr.

 2. The restricted austenite phase field,often called the gamma-loop.

Sigma phase is an intermetallic compound, which is hard and brittle and can be produced in alloys containing substantially less than 50% Cr. It also has an adverse effect on the corrosion resistance of stainless steels and therefore care should be taken to avoid extended exposure in the temperature range 750-820~ which favours its formation.

From a commercial standpoint, the area of the Fe-Cr diagram of greatest importance is that containing up to about 25% Cr, and a simplified illustration of that region for alloys containing 0.1% C is shown in Figure 4.2. Because chromium is a mild carbide former, many types of stainless steel are solution treated at temperatures significantly higher than those used for low-alloy steels in order to dissolve the chromium carbides. A solution treatment temperature of 10500C is typical of a variety of stainless steel grades and this will be used as a reference temperature in relation to the microstructure at high temperature. As illustrated in Figure 4.2, 0.1% C steels can accommodate up to about 13.5% Cr at 10500C and still remain austenitic with a face-centred-cubic structure. As the chromium content of the steels is increased within this range, the hardenability also increases very substantially such that large section sizes can be through- hardened to martensite on cooling to room temperature. For example, a steel containing 12% Cr and 0.12% C will form martensite at the centre of a 100 mm bar on air cooling from 10500C and the limiting section can be increased to about 500 mm by oil quenching from this temperature. It should also be noted that the Ms-Mf transformation range is depressed significantly with large additions of chromium. However, for most commercial grades of 11-13% Cr steels, the transformation range is above room temperature and therefore the formation of retained austenite is not a major problem.

Iron-chromium-nickel alloys

Whereas chromium restricts the formation of austenite, nickel has the opposite effect and, as illustrated in Figure 4.4, the Fe-Ni equilibrium diagram displays an expanded austenite phase field. In the context of stainless steels, chromium is therefore termed a ferrite former and nickel an austenite former. Thus having created a substantially ferritic microstructure with a large addition of chromium, it is possible to reverse the process and re-establish an austenitic structure by adding a large amount of nickel to a high-chromium steel. As indicated in Figure 4.3, a steel containing 17% Cr and 0.1% C will have a microstructure of about 65% delta ferrite-35% austenite at a solution treatment temperature of I050~ The various changes that then occur with the addition of nickel to a base steel of this composition are illustrated in Figure 4.5. This shows that the delta ferrite content is steadily reduced and at 1050~ the steels become fully austenitic with the addition of about 5% nickel. On cooling to room temperature, the austenite in these low-nickel steels transforms to marten- site and therefore there is initially a progressive increase in hardness with the addition of nickel as martensite replaces delta ferrite. However, the addition of nickel also depresses the Ms-Mf transformation range and at nickel contents greater than about 4% the M f temperature is depressed below room tempera- ture. Further additions of nickel therefore lead to a decrease in hardness due to incomplete transformation to martensite and the formation of retained austenite. As indicated by the hardness data in Figure 4.5, refrigeration at -78~ causes the retained austenite to transform to martensite over a limited composition range until the Ms temperature coincides with the refrigeration temperature. In commer- cial 18% Cr 9% Ni austenitic stainless steels, the Ms has been depressed to very low temperatures and little transformation to martensite can be induced, even at the liquid nitrogen temperature.