Obtención y tratamiento de los datos

Steels, which contain at least 10% chromium, are usually referred to as “stainless steels” because of their resistance to the formation of visible corrosion products when left exposed to the atmosphere. The addition of strong oxide-forming element such as chromium replaces the oxide on the surface of alloys by a tenacious film, which confers corrosion resistance to the alloy [35]. Needless to say that if the passive film is destroyed, corrosion resistance approaches that of low alloy steels.

Olsson recently reviewed results obtained over the last two decades regarding the investigation of ‘passive’ films on stainless steels [36]. Firstly, it must be noted that it is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen [37]. The passive film is typically l-3nm thick [37], Once a film is formed, the reaction rate between the metal and the environment is reduced by several orders of magnitude [36]. However, the passive film can change with the environment. It can grow or dissolve, and may adsorb or incorporate anions such as the chloride anion, Cl' [36].

With regard to the actual composition of the passive film, a model has been suggested for passive films formed on austenitic stainless steels [36], The outer part of the film consists of a hydroxide (compound containing OH") film. Indeed an analytical study on the austenitic stainless steel 316L has shown that during immersion in artificial seawater the passive film formed is enriched in chromium hydroxides (CrOH2) [38].

Chromium is the most abundant cation in the passive film as iron is selectively dissolved in the production of OH" anions. The hydroxide film forms on top of an oxide layer. The oxy-hydroxide film is formed on top of a nickel-enriched layer, which is next to the bulk metal.

2 6

-Oxy-hydroxide film

Bulk metal

Figure 2.1 Schematic of passive film prevalent in austenitic stainless steels [36].

Evident from the model depicted in figure 2.1 is the fact that the composition and properties of the passive film depend on the alloy composition. In addition, factors such as the ECP, the presence of anions such as Cl", pH and temperature influence the passive film [36].

A wide range of stainless steels have been developed during the last 80 years to meet the demands of service in highly corrosive media, and these alloys also offer attractive mechanical properties at temperatures from absolute zero to above 800°C.

Consequently, the term “stainless steel” now applies to several different classes of material with entirely different metallurgical characteristics and chemical compositions, which are used in diverse applications. For example, stainless steels are used in the offshore industry for their corrosion resistance and low maintenance needs [39]. On the negative side are the higher investment costs. However, the initial cost may be offset by the longer lifetimes attainable. It is interesting to note that the cleanability and corrosion resistance of stainless steels makes them ideal for use where stringent sanitary and health conditions are of utmost importance.

Stainless steels have been developed using alloying additions and processing routes designed to impart specific chemical, physical and mechanical properties; effectively through microstructural control [40].

The information in the following review is drawn from standard mechanical engineering reference books [35, 41].

There are more than 200 different grades of stainless steel. Stainless steels may be divided into five families (figure 2.2): (1) austenitic, (2) ferritic, (3) martensitic, (4) precipitation hardened and (5) duplex. Each family has some unique physical and mechanical properties; however, there is within each family a range of corrosion resistance that is achieved by varying the alloying elements. This allows for the substitution of an alloy from one family for that of another and thus advantage can be taken of particular physical/mechanical properties. Some general characteristics are noted in figure 2.2.

The Stainless Steel Family

Superferritic stainless steels

*

Add Cr, Mo

Ni-Cr-Fc

alloys

t

303, 303 Se

Add Ni for corrosion resistance in high

temperature environments Add S or Sc for machinabiJity

Figure 2.2 Schematic diagram showing compositional and property linkages in the stainless steel family of alloys [cited in 42J.

Austenitics ( y)

Referred to by its unified numbering system designation, UNS S30400 (type 304), or

“ 18-8” (18Cr, 8Ni) of the 300 series alloys, is probably the most widely used stainless

steel. When improved corrosion resistance is required, alloys with higher chromium, nickel or molybdenum content, or both, such as UNS S31600 and S31700 are used.

Compositions lie typically in the range of 18-25% Cr, 8-20% Ni, with low C and some Mo, Nb or Ti. Yield strengths range between 170MPa and 260 MPa.

Austenitic stainless steels are chosen because of their resistance to general corrosion, which is superior to that of a ferritic steel of similar chromium content. In addition, the face-centered cubic (FCC) structure implies good ductility.

Austenitic grades can be hardened by cold work but not by heat treatment; cold work increases strength with an accompanying decrease in ductility. They are relatively easy to form and are readily welded. Austenitic stainless steels may contain some ferrite, generally in castings and welds. Two significant advantages are conferred by the presence of a proportion of ferrite; the prevention of fissuring on solidification and resistance to intergranular corrosion. Compared to carbon steel, austenitics have higher coefficients of thermal expansion and lower thermal conductivities.

Applications range from domestic to architectural to the petrochemical and chemical processing industries. As a result, they are the most commonly used of all stainless steels. At the high end of the spectrum are the relatively high alloy 6% Mo superaustenitic stainless steels, which have accumulated many years of service experience in piping systems on offshore oil and gas platforms.

Ferritics ( a)

Ferritic stainless steels are iron-chromium alloys with 11 to 30 percent chromium.

The largest quantity produced is Type 409 (11 Cr), which is used extensively for automobile catalytic converters, mufflers, and exhaust system components.

The ferritic alloys have a body centered cubic structure (BCC). Grain growth is rapid due to the high atomic mobility in the BCC lattice, resulting in a coarse-grained microstructure and consequently poor toughness. As a result, ferritic stainless steels

3 0

-are not readily weldable. The coefficient of thermal expansion is similar to that of carbon steel. Yield strengths range between 170MPa and 275 MPa.

Ferrtic stainless steels can be strengthened slightly by cold working but have poor formability compared to austenitics. They are difficult to produce in plate thicknesses, but are readily available in sheet and bar form.

Some contain molybdenum for improved corrosion resistance in chloride environments. In contrast to austenitic stainless steels, they exhibit excellent resistance to chloride stress corrosion cracking due to an absence of nickel. Resistance to pitting and crevice corrosion is a function of the total chromium and molybdenum content. Superferritics contain 28% Cr and 4% or more Mo and have exceptional resistance to general, stress and pitting corrosion.

In general, ferritic stainless steels are significantly cheaper than austenitic steels due to the absence of nickel and are used for chemical plant components, domestic and catering equipment, automobile trim, domestic and industrial heater parts, exhaust systems and fasteners. The most highly alloyed ferritic stainless steels have a long service history in seawater-cooled utility condensers.

Martensitics

The alloys in this family have a body centered tetragonal structure. Martensitic stainless steels are alloyed with chromium and relatively high levels of carbon. The most common martensitic alloy is type 410 (UNS S41000) and contains about 11.5%

Cr. The high carbon contents help to produce and strengthen the martensitic structure.

Martensitics are typically difficult to form and weld. However, high strength (yield strength range 205-275MPa) and hardness are prevalent. In addition, they have excellent wear/abrasion resistance but limited corrosion resistance. In most environments, the martensitic grades have less corrosion resistance than austenitic and ferritic grades.

The martensitic grades can be strengthened by heat treatment. Hardness levels up to 60HRC can be achieved in those grades with high carbon contents. The steels are

hardened by quenching from above 950°C to form a hard and brittle structure, which must be tempered. To achieve the best combination of strength, corrosion resistance, ductility, and impact toughness, they are tempered in the range of 150 to 370°C.

Martensitics are particularly suited to operations requiring a cutting edge and their applications include valves, tools, cutlery, scissors, turbine blades, coalmining equipment and surgical instruments.

Precipitation Hardening

Alloys in this group such as UNS S 17700 are used in applications requiring high strength and a moderate degree of corrosion resistance. While some precipitation- hardening stainless steels have a 600-series designation, they are most frequently known by names, which suggest their chemical composition, for example, 17-4PH (17Cr, 4Ni).

Precipitation-hardening stainless steels can be strengthened by a relatively low- temperature heat treatment. The low-temperature heat treatment minimizes distortion and oxidation associated with higher-temperature heat treatments. They can be heat- treated to strengths greater than heat-treated martensitic grades.

The range of corrosion resistance achievable is not as comprehensive as either the austenitics or ferritics. However, most exhibit corrosion resistance superior to the martensitics and approach that of the austenitic grade 304.

Their excellent mechanical properties and corrosion resistance has caused precipitation-hardened stainless steels to be used for gears, fasteners, cutlery and aircraft and steam turbine parts. Extremely practical is the fact that they can be machined to finished size in the soft condition and precipitation hardened later. Their most significant drawback is the complex heat treatment required which, if not properly carried out, may result in extreme brittleness.

The basic mechanical properties and general corrosion resistance of the stainless steel family are summarized in table 2.1 and figure 2.3 respectively.

3 2

-Alloy type UTS Yield strength Elongation Hardness

MPa MPa % HRB

Austenitic

200-300 series 485-655 170-260 40 88-96

Ferritic

400 series 380-515 170-275 21 88-96

Martensitic 415-485 205-275 21 89-96

Precipitation

Hardening 895-1620 655-1520 6-16 105-116

Austenitic/ferritic

(duplex/Superduplex) 600-750 450-550 15-25 105

Table 2.1 Mechanical property ranges for various stainless steel classes.

Figure 2.3 Schematic of general corrosion resistance for various stainless steel classes.