Traditional high strength aluminium alloys have yield strength about 500–600 MPa, which can be considered as really high strength for aluminium alloy. About decade ago Islamgaliev et al. (2001) produced an aluminium alloy with really high strength and good total elongation through severe plastic deformation. This leaded to structure with grain size less than 100 nanometers and it also contained nanoparticles with size less than 50 nanometers. Fabricated alloy (original alloy was Russian V96Z1, which quite good corresponds to the alloy EN AW-7149) had yield point of 750 MPa and elongation of 20 %. (Islamgaliev et al., 2001, p.878–881)
Recent nanoparticle experiments with aluminium are often linked to friction stir processes. For example Sharifitabar et al. (2011) studied the effect of aluminium oxide particles on the alloy 5052 with temper H32. They added nanosize particles as a powder to the base material via friction stir process. Particle size was about 50 nanometers.
Friction stir treatment with powder increased the tensile strength about 20 % (from about 220 MPa to 260 MPa). Yield strength was originally 150 MPa and it decreased to about 130 MPa while elongation increased from 11 % to 18 %. (Sharifitabar et al., 2011, p.4169–4171)
7 RESULTS AND DISCUSSION
Arctic area is a harsh environment, where large amount of natural resources are located.
Applications need to be designed properly and material behavior has to be fully understood, because conditions there are extreme. Used materials have to stand at least –40 °C, but also almost –70 °C have been measured in Arctic. For onshore application the melting permafrost has shown to cause serious problems for pipelines. The most important and interesting property, when materials are chosen to cold environment, is the toughness. Toughness is wide concept and it can be separated to different parts;
fracture toughness, impact toughness and crack arrest toughness.
At the moment there are standardized and certified carbon steels, which can be used in Arctic areas. Shipbuilding steels have minimum testing temperature –60 °C, class F and –40°C, class E. Highest certified strength in shipbuilding steels is 690 MPa. Some steels for fixed offshore applications and also general structural steels have to be tested at temperatures of –40 °C, –50 °C or –60 °C. Highest standardized strength of steel for fixed offshore applications is 500 MPa and for structural uses 700 MPa. In fixed offshore use for major primary structures these testing temperatures means that these steels can be used at temperature of –10 °C or minimum –30 °C (LAST - 30 °C demand), so virtually it is not enough for Arctic environment, where temperatures are usually below –40 °C. Based on different studies, pipeline steels can be manufactured to be suitable for Arctic environment. Highest standardized yield strength of pipeline steel is 830 MPa.
Availability of carbon steels differs in different classes. Class with strength under and including 235 MPa (NS) does not have any structural steel and class over 235 up to and including 400 MPa (HS) does not have structural steels with improved toughness at –60
°C. Class 400 up to and including 700 MPa (EHS) have lots of different kinds of steels, which are suitable for Arctic conditions. Class over 700 MPa (UHS) have only structural steels and one pipeline steel, from which the structural steels are not standardized in Eurocode. That means that there is no designing guidance or rules for those ultra high strength steels.
New steels can be manufactured to be really tough, but usually joining (for example welding) of these steels makes their properties worse and the joining technology is
usually the limiting factor. The most common carbon steels, which are in use at subzero temperatures at the moment, have yield strength from 355 to 500 MPa and weldability of these steels is good. Large pipelines, which are built through Arctic areas, have yield strength about 500MPa and also slightly over.
Aluminium alloys or stainless steels are not usually used in primary structures for offshore or onshore applications in cold environment, even if they are well suitable for subzero temperatures (they are not susceptible for brittle fracture). Modern high-strength aluminium alloys have yield high-strength over 500 MPa and they are readily available. Some properties of aluminium limit the use; for example the elastic modulus is quite small and their weldability is not so good compared to steels.
Recent studies have shown that austenitic and austenitic-ferritic stainless steels can be manufactured without nickel and that those steels have higher strength than traditional ones; austenitic steels with yield strength about 500 MPa and duplex grades over 800 MPa. Even other properties – for example toughness, hardness, corrosion resistance – do not get worse compared to some traditional grades. In ferritic grades huge development has been done in recent years – some are tough even at for example –60
°C and have yield strength about 1000 MPa. Super martensitic grades are also really tough and have superior properties, for example yield strength over 1000 MPa. The use of SMSSs have been increased in recent years. Stainless steels, even if they are nickel-free, might be too expensive to be used for primary structures, but in other aspects they are really good choice for cold environment.
Nanotechnology has made it possible to manufacture steels to have really high strength without losing good ductility. New materials and manufacturing methods are being developed in fast cycle but standardization of them is not so fast. It is interesting to see, how and when nanotechnology fully penetrates to metallic materials industry – tests have shown that nanostructured materials have really good toughness, elongation and ultra high strength. Some recently developed nanostructured carbon steels have yield strength even over 1500 MPa and they have really high impact toughness and also elongation has been rather high, about 10 %. In category of stainless steels scientists have developed grades with yield strength of 1000 MPa with 40 % elongation and even 1500 MPa with 33 % elongation. Nanostructured aluminium alloys have been manufactured to have yield strength of 750 MPa with 20 % elongation.
Fracture behavior of new really high strength steels is not fully understood. This has been noticed from different kind of tests, where pipeline steels have been tested for example in full-scale lines; even if steels have been accepted through Charpy impact tests, they have broken up brittle in test. Rather new testing methods, CTOD and CTOA, have showed up to be good choices to estimate fracture behavior of new steels, but these methods are not so simple compared to Charpy impact test and the results are not so unambiguous or comparable. It seems that materials have been developed to manage well in Charpy impact test and now some unknown properties, which affect on fracture properties of ultra high strength steels, have not been noticed. A lot of discussion is linked to usability of Charpy testing method and how testing results should be plotted, especially with new steel grades.
As was mentioned, writing of standards or other rules is not so fast than development of new materials. This is one reason, why class 500 MPa steels are in use at the moment, even if class 690 MPa steels are certified for shipbuilding. In general, there are not yet proper standards for structures, which are located in Arctic area. Normal standards for petrochemical industry have different rules for designing offshore application. One rule is that certain structures have to be tested 30 °C below lowest anticipated service temperature (LAST). This means Charpy testing temperature below –80 °C for some steels in certain places. At the moment only nickel alloyed steels can manage through this kind of demand. General opinion from different references keeps the demand “30
°C below LAST” too hard or senseless for Arctic applications. There are some conflicts between standards and rules from classification societies. They are usually linked to stainless steels or aluminium alloys and for carbon steels, the demands between each other are very similar.
For future development, it is important to examine how well CTOA and CTOD tests follow the fracture behavior of new ultra high strength steels, stainless steels and aluminium alloys. Also other possible testing methods should be considered. New standards for applications for Arctic areas are really needed and they are already under development. Joining methods for new materials (especially steels), which have nanosize crystal structure, needs to be examined. Reason for this is that the toughness and strength of new metallic materials are usually linked to really small, even nanosize,
crystal structure, and welding (or more accurately, heat) can cause some serious changes to properties of these materials.
One important goal is to clarify the designing rules and regulations in steel building industry, especially for applications in really cold environment and/or offshore. One question is that which standard or classification has to be used, when different kind of structures are constructed to Arctic areas – wind mills, onshore oilrigs and other onshore buildings (situation is quite clear for offshore applications). Future developments and research recommendations are listed in table 32.
Table 32. Revealed future developments and research recommendations based on this thesis.
Future developments Research recommendations
- Clarification of designing rules and regulations
- New standards for cold environment
- Joining methods and technologies for new materials
- Comparing of different test methods (CTOD, CTOA, DWTT, CVN, etc.)
- Development of database on materials and technologies
8 SUMMARY
This Master’s Thesis is part of an Arctic Materials Technologies Development –project,
which concerns South-East Finland-Russia ENPI CBC programme 2007-2013 –program. The main target of this study is to clarify what kind of metallic materials are
used or can be used in Arctic areas. Carbon steels, stainless steels and aluminium and its alloys were under examination. These metallic materials were studied in three categories: materials in offshore use, general structural use and as a pipeline material.
Also possibilities of nanotechnology for improvement of properties of metallic materials were studied, mainly through examples.
Arctic has really harsh conditions and temperature drops down to about –40 °C in any area in Arctic. This gives the base demand for examination of suitable materials: they have to be ductile at least in this temperature. Different testing methods measure the behavior of materials, from which the Charpy impact test is maybe the most important at the moment. It reveals the temperature, where material´s fracture mode changes from ductile to brittle. Also other newer testing methods are in use, for example crack tip opening displacement (CTOD), crack tip opening angle (CTOA) and drop-weight tear test (DWTT). New methods are more practical, because usually they are executed with full-size (actual size or thickness) test piece, but the results are difficult to compare and there are no standardized demands for these tests in Europe.
At the moment carbon steels are manufactured to respond the demands of different industries and for example extra high strength shipbuilding steels (class 690 MPa) are available with approved impact properties at –60 °C. These steels are not yet in use, but classes 500 MPa and 355 MPa are used in ice breakers, oil rigs and so on. Austenitic stainless steels and aluminium alloys are also suitable for cold environment, because they are not susceptible to brittle failure, but they are not economical choice. Also some duplex grades, new super martensitic and super ferritic grades might be used at low temperatures (–40…–80 °C).
Recently developed manufacturing methods, which include also nanotechnology, make materials even tougher and more durable. This sets up new challenges for manufacturing methods, like welding or cold forming.
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