High austenitic-ferritic duplex steels

Fig. 5. Metastable Fe—Ni—Cr "temary"-pliase diagram where C content is 0.1 wt % and for alloys cooled rapidly from 1000°C showing the locations of austenitic, duplex, ferritic, and martensitic stainless steels with respect to the metastable-phase boundaries. For carbon contents higher than 0.1 wt %, martensite lines occur at lower ahoy contents (43). A is duplex stainless steel, eg. Type 329, 327 B, ferritic stainless steels, eg. Type 446 C, 5 ferrite + martensite D, martensitic stainless steels, eg. Type 410 E, ferrite + martensite F, ferrite + pearlite G, high nickel ahoys, eg, ahoy 800 H,...

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

High-alloy pipeline steels (e.g. austenitic-ferritic or duplex) have been used where the product stream demands materials with better corrosion resistance than carbon steel. In practice the external corrosion resistance of these materials cannot be guaranteed, so cathodic protection is employed to protect areas which may be subject to corrosion. 

Duplex, and super-duplex stainless steels, contain high percentages of chromium. They are called duplex because their structure is a mixture of the austenitic and ferritic phases. They have a better corrosion resistance than the austenitic stainless steels and are less susceptible to stress corrosion cracking. The chromium content of duplex stainless steels is around 20 per cent, and around 25 per cent in the super-duplex grades. The super-duplex steels where developed for use in aggressive off-shore environments. 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

In recent deeades, also relatively high alloy ferritic-austenitic steels (duplex steels) have been developed. Earlier, these steels were considered immune to chloride SCC. However, both laboratory experiments and experience of fracture on installations in the North Sea have shown that SCC can occur on duplex steels under unfavourable eonditions developed beneath the thermal insulation on hot steel surfaces. The insulation is used to reduce heat loss and to protect people. If seawater penetrates the insulation and reaches the steel surface, the water will evaporate and this causes chloride enrichment. Maximum unfavourable conditions can be established high chloride content, efficient oxygen access (there is a thin water film at the most critical sites), and temperatures above 100°C. Ferritic-austenitic stainless steels should not be used under these conditions. 

The next-higher grade of materials that are used in oil and gas industries are stainless steels. Almost all kinds of stainless steels are used in oil and gas industrial applications. Stainless steels are classified as ferritic, austenitic, duplex, and martensitic stainless steels. The main advantage of stainless steels is their high corrosion resistance. The most primitive stainless steel is ferritic stainless steel. 

High-temperature and pressure tests (weight loss, electrochemical measurements, and U-bends) were conducted by Honda et al. [152] with austenitic, ferritic, and duplex stainless steel alloys in alkaline sulfide solutions at 150°C. Dupo-iron [160] reported on SCC tests conducted in white liquors using the constant load test. 

Use of CRA is competitive with inhibition in deep, high-pressure wells, particularly in those locations where inhibitor supply may be a space and logistic problem. CRA includes stainless steels (austenitic, ferritic, martensitic, and duplex), nickel-based alloys, cobalt-based alloys, and titanium alloys. Economics is a major factor in alloy selection. The 13 Cr tubing has often been used in gas wells for low H2S wells. Tubing materials selection for a deep well could involve price increments between alloys of 1 to 3 million. High-strength CRA is used to minimize costs. SMYS values of 150 ksi (1000 MPa) are common. The CRA is often cold-worked to achieve the required yield strength. 

Thus it will be seen that design and the way in which an artefact is used will largely influence which steel may prove successful. Simple precautions such as the flushing out of pumps and pipework with fresh water after use can mean the diflerence between satisfactory behaviour and limited life. The test data in Table 3.23" illustrate the marked eflect of water velocity. Those applications where prolonged contact with static sea-water cannot be avoided have provided some of the impetus for the development of the more highly alloyed austenitic, ferritic and duplex steels which are finding increasing favour for arduous sea-water service. 

High-alloy multiphase steels Ferritic/pearlitic-martensitic steels Ferritic-austenitic steels/duplex steels... 

The ferritic-austenitic steels are passive in seawater due to their high chromium content and do not suffer from general corrosion. Their resistance to pitting and crevice corrosion is raised by the molybdenum and nitrogen components. The duplex steels have therefore proved their worth well in a wide range of marine engineering apphcations [131-134]. 

Unalloyed and low-alloyed steels/cast steel 193 Unalloyed cast iron and low-alloy cast iron 224 High-alloy cast iron 226 Ferritic chromium steels with < 13% Cr 228 Ferritic chromium steels with >13% Cr 229 High-alloy multiphase steels 235 Ferritic/pearlitic-martensitic steels 235 Ferritic-austenitic steels/duplex steels 235 Austenitic CrNi steels 237 Austenitic CrNiMo(N) steels 239 Austenitic CrNiMoCu(N) steels 244 Nickel 260... 

Ferritic chromium steels with < 13 % Cr 320 Ferritic chromium steels with > 13 % Cr 320 High-alloy multiphase steels 320 Ferritic/pearlitic-martensitic steels 320 Ferritic-austenitic steels/duplex steels 320 Austenitic CrNi steels 323 Austenitic CrNiMo(N) steels 323 Austenitic CrNiMoCu(N) steels 323 Nickel-chromium alloys 339 Nickel-chromium-iron alloys (without Mo) 339 Nickel-chromium-molybdenum alloys 339 Nickel-copper alloys 339 Zinc 343 Bibliography 344... 

Duplex steels Alloys containing both the body centered cubic alpha-phase and the face centered gamma-phase are known as duplex steels. They may be low carbon (less than 0.03%) low chromium alloys (less than 20% Cr) with a continuous austenitic matrix or high chromium (>20%) alloys with a ferrite matrix. Dislocation A crystalline imperfection in which a lattice distortion is centered around a line. 

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