Gunn Duplex Stainless Steels

Microstructure, Properties and Applications
1. Auflage 1997
ISBN: 978-1-84569-877-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 216 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Two very successful conferences - in Glasgow and Beaune - were held on duplex stainless steels during the first half of the '90s. This book takes keynote papers from each, and develops and expands them to bring the topics right up to date. There is new material to cover grades, specifications and standards, and the book is fully cross-references and indexed.The first reference book to be published on the increasingly popular duplex stainless steels, it will be widely welcomed by metallurgists, design and materials engineers, oil and gas engineers and anyone involved in materials development and properties. - The first reference book on this relatively new engineering material - Based on keynote papers from major international contributors - Covers grades, standards and specifications

Autoren/Hrsg.

Gunn, R.

Inhaltsverzeichnis

1;Front Cover;1
2;Duplex Stainless Steels: Microstructure, Properties and Applications;4
3;Copyright Page;5
4;Table of Contents;6
5;Dedication;3
6;Preface;10
7;Abbreviations and glossary;12
8;Chapter 1. Developments, grades and specifications;14
8.1;1.1 Historical developments;14
8.2;1.2 Modern developments;18
8.3;1.3 Specifications and standards;19
8.4;References;25
8.5;Bibliography;25
9;Chapter 2. Alloy design;27
9.1;2.1 Melting and refining;27
9.2;2.2 Alloying additions;27
9.3;References;34
9.4;Bibliography;36
10;Chapter 3. Microstructure;37
10.1;3.1 Commercial products;37
10.2;3.2 Element partitioning;40
10.3;3.3 Heat treatment;43
10.4;3.4 Characteristics and morphology of precipitates;48
10.5;References;60
10.6;Bibliograph;62
11;Chapter 4. Forming and machining;63
11.1;4.1 Cold forming;63
11.2;4.2 Hot forming and superplasticity;64
11.3;4.3 Machinability;65
11.4;References;67
11.5;Bibliography;68
12;Chapter 5. Physical and mechanical properties;69
12.1;5.1 Physical properties;69
12.2;5.2 Tensile characteristics;70
12.3;5.3 Hardness;76
12.4;5.4 Impact strength;76
12.5;5.5 Fatigue properties;80
12.6;References;83
12.7;Bibliography;85
13;Chapter 6. Corrosion;86
13.1;6.1 Introduction;86
13.2;6.2 General corrosion;86
13.3;6.3 Localised corrosion;94
13.4;References;103
13.5;Bibliography;104
14;Chapter 7. Stress corrosion cracking;105
14.1;7.1 Introduction;105
14.2;7.2 Chloride stress corrosion cracking;106
14.3;7.3 Sulphide stress corrosion cracking;113
14.4;7.4 Cathodic protection;119
14.5;References;121
14.6;Bibliography;122
15;Chapter 8. Welding metallurgy;123
15.1;8.1 Introduction;123
15.2;8.2 HAZ transformations;123
15.3;8.3 Weld metal;129
15.4;8.4 Fabrication cracking;138
15.5;References;143
15.6;Bibliography;145
16;Chapter 9. Welding processes;146
16.1;9.1 Introduction;146
16.2;9.2 General considerations;146
16.3;9.3 Gas shielded processes;151
16.4;9.4 Flux shielded processes;152
16.5;9.5 Solid state processes;154
16.6;9.6 Power beam processes;156
16.7;References;157
16.8;Bibliography;158
17;Chapter 10. Weldment properties;159
17.1;10.1 Tensile behaviour;159
17.2;10.2 Hardness;159
17.3;10.3 Fracture toughness;161
17.4;10.4 Fatigue;165
17.5;10.5 Corrosion properties;167
17.6;10.6 Stress corrosion cracking;175
17.7;References;176
17.8;Bibliography;179
18;Chapter 11. Non-destructive testing of welds;180
18.1;11.1 Introduction;180
18.2;11.2 Penetrant testing (PT);180
18.3;11.3 Eddy current testing;181
18.4;11.4 Radiographic testing;181
18.5;11.5 Ultrasonic testing;181
18.6;References;186
18.7;Bibliography;187
19;Chapter 12. Applications;188
19.1;12.1 Chemical and petrochemical;188
19.2;12.2 Oil and gas;190
19.3;12.3 Pulp and paper;195
19.4;12.4 Power generation;195
19.5;12.5 Marine transportation;196
19.6;12.6 Other applications;196
19.7;References;197
19.8;Bibliography;198
19.9;Design/fabrication codes;199
20;Chapter 13. Service experience;200
20.1;13.1 Cracking of seam welded pipes;200
20.2;13.2 Cracking of cold worked tubulars;200
20.3;13.3 Cracking in offshore production piping;202
20.4;13.4 The Gyda failure;203
20.5;13.5 Cathodically protected bolts;204
20.6;13.6 Acid treatment;204
20.7;13.7 Intermetallic formation in pipe weldments;205
20.8;13.8 Fatigue of pipework;205
20.9;References;206
20.10;Bibliography;206
21;Appendix A: Nominal composition of stainless steels and Ni-base alloys;207
22;Appendix B: Welding consumables for duplex and superduplex stainless steel grades;208
23;Index;213

Leseproben

2 Alloy design
2.1 Melting and refining
The melt for a duplex alloy can be produced using either high purity material in a conventional induction furnace, or high alloyed scrap in an electric arc furnace (EAF) followed by AOD, VOD or VARP converters.1 Although both these methods produce high quality materials, the latter method favours lower sulphur contents but the converters do not naturally reduce phosphorus unless reducing conditions, careful scrap selection and good base mix practice are undertaken. Overall, the aim is to reduce sulphur and phosphorus, as far as possible, in order to increase the ultimate corrosion resistance.2 The next stage of refinement involves deoxidation and inoculation by adding a combination of the following agents: SiCaMn, SiCaCe, REM, Al, Ti, Mg, Nb, Zr etc. These additions lead to the precipitation of microscopic oxides which act as multiple nucleation sites for ferrite crystals and provide grain refinement. The precise combination of agents employed depends on the (previous) oxygen and carbon contents, and is selected in view of the final corrosion and mechanical properties.3,4 Besides the deoxidants, the cooling rate has a significant effect on grain size which, in the case of castings, remains essentially unchanged during subsequent solution heat treatment and water quenching. 2.2 Alloying additions
2.2.1 Chromium
The main advantage of adding chromium to steel is to improve the localised corrosion resistance, by the formation of a passive chromium-rich oxy-hydroxide film.5 Electrochemically this is achieved by extending the passive range (see Fig. 2.1)6 and reducing the rate of general corrosion (ipass). However, there is a limit to the level of chromium that can be added to such a steel, as the beneficial effect of ever higher levels is negated by the enhanced precipitation of intermetallic phases (Fig. 2.2),7 such as sigma. These phases often lead to reduction in ductility, toughness and corrosion properties, and are covered in Section 8.3.4. 2.1 Schematic summary of the effects of alloying elements on the anodic polarisation curve (after reference 6). 2.2 Schematic summary of the effects of alloying elements on the formation of various precipitates (after reference 7). Chromium and other elements stabilise ferrite, although the effect of different elements varies. Equations have been derived to quantify elemental effects (the so-called chromium equivalents, Creq) of which the most favoured is: eq=%Cr+%Mo+0.7×%Nb   [2.1]8 2.2.2 Molybdenum
The beneficial influence of molybdenum on the pitting and crevice corrosion resistance of an alloy in chloride solutions has been recognised for many years (Fig. 2.1). As for chromium, molybdenum extends the passive potential range and reduces the corrosion current density (imax) in the active range. Molybdenum is included in both PRE relationships, Eqs 1.1 and 1.2, and is given a coefficient of 3.3 times that of chromium, while it has a similar effect on ferrite stability as chromium, Eq. 2.1. The mechanism by which molybdenum increases the pitting resistance of an alloy has been examined by a number of workers,9–12 and has been found to suppress active sites via formation of an oxy-hydroxide or molybdate ion.13 In high temperature sea water, the addition of at least 3%Mo is recommended14 to prevent crevice corrosion, while an upper limit of about 4%Mo has been quoted.15 This limit stems16 from the enhanced sigma forming tendency in the hot working temperature range, i.e. above 1000°C (Fig. 2.2). 2.2.3 Nickel
Counter to the ferrite stabilising effect of chromium (Mo and Nb), there is another group of elements which stabilise austenite: eq=%Ni+35×%C+20×%N+0.25×%Cu   [2.2] In order to maintain about 40% to 60% ferrite, balance austenite, the ferrite stabilising elements, Eq. 2.1, need to be balanced with the austenite stabilisers, Eq. 2.2. For this reason, the level of nickel addition to a given duplex alloy will depend primarily on the chromium content. At excessive Ni contents, the austenite level increases to well above 50%, with the consequence that Cr and Mo are enriched in the remaining ferrite. As a result, ferrite transformation to intermetallic phases may be enhanced16 when the alloy is exposed to temperatures in the range 650 to 950°C. Further, high Ni-contents accelerate alpha prime formation,17 an embrittling intermetallic phase, in the ferrite. In summary, nickel does have some direct effect on corrosion properties, for instance moving Ep in the noble direction and reducing ipass (Fig. 2.1), and yet it appears that the main role of nickel is to control phase balance and element partitioning. 2.2.4 Nitrogen
Nitrogen has a multiple effect on stainless steels by increasing pitting resistance, austenite content and strength. It has a similar influence on pitting as Cr and Mo, moving Ep in the noble direction and thus increasing the passive potential range, Fig. 2.1. This effect is enhanced in the presence of Mo and it has been suggested18–20 that Mo and N have a synergistic influence on pitting characteristics. The proposed factor for nitrogen in the PREN relationship varies between 13 and 30, but the most widely used value for duplex alloys is 16, Eq. 1.1. Nitrogen partitions preferentially to the austenite due to the increased solubility in the phase21 and also concentrates at the metal-passive film interface.22 During prolonged passivation of stainless steels in acid solutions, surface nitrogen enrichment has been witnessed,10,23 which explains how nitrogen can influence repassivation. However, to break down the passive film the anodic current density must be high, in the order of several A/cm2. For nitrogen bearing 316L type stainless steel in 4M HCl, nitrogen reversibly impedes active dissolution from reaching these high values, possibly by surface enrichment of nitrogen atoms.24 The inhibitive nature of nitrogen was postulated to result from its dissolution, whereby nitrogen combines with hydrogen ions to form ammonium ions. This is a cathodic reaction, which becomes too slow at high potentials to balance the anodic dissolution of the metal and allows for surface enrichment of nitrogen, leading to the blocking effect observed. Nitrogen has also been noted to increase the crevice corrosion resistance. Workers have proposed25–27 that this is due to nitrogen altering the crevice solution chemistry or by segregating to the surface, which is in keeping with the mechanism for enhanced pitting resistance.24 Another important property of nitrogen is its ability to stabilise duplex alloys against the precipitation of intermetallic phases,28 such as sigma and chi, by reducing Cr-partitioning.29 It is also reported30 that increasing the nitrogen level actually reduces the risk of nitride formation. This may appear contradictory, but is due to an increase in austenite content and so a reduction in the distance between austenite islands. The addition of C and N strengthens both ferrite and austenite by dissolving at interstitial sites in the solid solution.31,32 And yet, as carbon is undesirable in stainless steel, due to the risk of sensitisation, the addition of nitrogen is preferred. Further, as nitrogen is a strong austenite stabiliser its addition to duplex stainless steel suppresses austenite dissolution and encourages austenite reformation in the HAZ (see Chapter 9). Comparison of the nitrogen contents of different duplex stainless steels (Table 1.1), shows that the superduplex grades contain higher levels than the lower alloyed variants. This enhanced level of nitrogen is due to the higher alloy content of these steels and, in particular, enhanced chromium contents (Fig. 2.3).33 In this regard, the beneficial influence of manganese should also be noted. In other grades, with lower Cr and Mn contents, the nitrogen solubility limit can be reached, leading to severe out-gassing or porosity during solidification. 2.3 Effect of alloying elements on the solubility of nitrogen in liquid Fe-18%Cr-8%Ni alloys at 1600°C at 1 atm N2 (after reference 33). 2.2.5 Manganese
Manganese has been quoted34,35 as a austenite stabiliser for austenitic steels and yet, for duplex alloys, mixed results have been obtained.36,37 The current understanding is that manganese has little effect on duplex phase balance, especially at the levels normally encountered, and is excluded from the Creq and Nieq equations quoted above,8 i.e. Eqs 2.1 and 2.2. Nevertheless, it would appear38 that Mn can increase the temperature range and formation rate of detrimental sigma phase. Manganese additions to stainless steel increase abrasion and wear resistance39 and tensile properties without loss of ductility38. Further, Mn increases the solid solubility of nitrogen and thus allows for increased nitrogen contents to be achieved without risk of out-gassing. However, Mn-additions in excess of 3% and 6%, for nitrogen levels of 0.1% and 0.23%...