IIW White Paper

4 Needs and challenges in welding and joining technologies

High strength structural steels with strength levels of up to 1,100 MPa are increasingly used for fabrication of mobile cranes and bridges. Here, matching the ultra high strength of the base materials can only be achieved by very fast cooling rates of the joints providing additional needs regarding tolerable toughness levels. As an application with respect to welded pipelines, such materials having a strength level of up 900 MPa are increasingly applied to hydro power plants. Since the high strength levels in such materials are usually associated with a ductility reduction, minimisation of the risk of weld failure by hydrogen assisted cold cracking will represent a major challenge in the near future. Furthermore, potential use of high strength steel grades (X80 and up to X120) for seam and girth welded pipeline applications create serious challenges with respect to the control and prediction of unstable ductile fracture. Regarding stainless steels, four major driving forces can be identified in development: industry needs for improved performance, improvements in steel making technology, weldability aspects and last but not least costs. With greater attention being paid to achieving low long-term maintenance costs, increasing environmental awareness and greater concern with life cycle costs, the market for stainless steel continues to improve rapidly. The cost relative to alternative materials, however, will definitively continue to be an important factor in particular in order to find new markets in the rapidly developing regions. It is difficult to identify one main line in stainless steel development and resulting challenges to weld these materials, since the group as such is so diversified. Quite a number of different stainless steel types, as for instance duplex stainless steels, have first been introduced into offshore technology before they were applied to other industrial branches. The materials used in offshore technology, in particular for subsea applications, generally provide the highest innovation grades, since increasing strength levels can only be utilised by a persistent corrosion resistance under the harsh and aggressive conditions to avoid any failure cases which are under no circumstances tolerable in the very sensitive environments. In recent years, martensitic, duplex and high-nitrogen austenitic stainless steels have been introduced into the offshore industry, mainly for flowlines as well as for downhole and top-site equipment. The introduction and increased use of leaner less expensive grades, such as lean duplex and 11-13Cr ferritic- martensitic grades, will contribute to pressure on a reduction of prices and also to finding new applications where currently mild steel is used. On the other hand, there is also continuous development of new specialised highly alloyed grades intended for very corrosive environments and high temperatures. Nitrogen is definitely increasing in popularity, being probably the least expensive of all alloying elements, and is not unlikely to be introduced to a larger extent also in standard grades in an attempt to improve properties and decrease alloying costs. The success of new grades is inevitably connected to weldability issues. With the continuous striving towards more efficient, higher productivity welding procedures and processes, some old topics remain important and some new ones arise. Hot cracking, in particular in fully austenitic materials needs to be studied further. Due to cases of failures in the past, hydrogen effects during welding and in-service remain an issue for martensitic, duplex and ferritic grades. Effects of high heat input on highly alloyed grades (precipitation of deleterious phases) and ferritic materials (HAZ grain growth) need to be well understood to define weldability limits. With newer processes such as laser and laser-arc-hybrid methods there are also less possibilities to modify weld metal composition through addition of filler metal making it more difficult to optimise, for example, phase balance and corrosion resistance.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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