IIW White Paper

Needs for weight reduction in aircraft and automotive transport to achieve higher vehicle fuel efficiency are only partially being met by metal matrix composites and switching from steel to aluminium and magnesium alloys. Matching filler metals for metal matrix composites are lacking, as are suitable filler metals and welding systems for high production joining of steel to aluminium. The IIW pioneered both the measurement of diffusible hydrogen in welds and the understanding of the phenomenon of hydrogen induced cracking. Indeed, the international standard for measurement of diffusible hydrogen, ISO 3690, is a direct product of the collaboration of experts within IIW Commission II. Likewise, ISO TR 17844 Welding – Comparison of standardised methods for the avoidance of cold cracks provides guidance on selection of pre-heat temperatures for various conditions of restraint and various levels of diffusible hydrogen. This Technical Report was developed by collaboration among the experts in IIW Commission IX. Taken together, these two documents contribute to the solutions of welding of high strength steels. These are only two examples of how the work of the IIW has contributed to the global quality of life over many years. Ongoing and future work in IIW with welding consumables will continue to focus on hydrogen induced cracking issues in high strength steels. These include differentiating the contributions to diffusible hydrogen from moisture and other hydrogenous compounds in the consumable as-manufactured, from adsorbed moisture in the consumable due to exposure to humid air, and from incursion of the humid air into the arc independent of the consumable employed. As part of this work, modernisation of the ISO standards for diffusible hydrogen (ISO 3690) and electrode exposure (ISO 14372) can be expected to take place, along with updating of the guidance documents for avoiding cold cracking. Hydrogen limits and corresponding preheat requirements for crack free welding with very high strength steels remain to be defined. Continued sharing of approaches to high strength steel consumable development can be expected. Traditional approaches to consumable design employing acicular ferrite microstructures have advanced the state-of-the-art consumables for low and intermediate strength steels to the point where they can largely match the properties of high quality steels in these strength ranges. Higher strength filler metals apparently cannot be achieved with acicular ferrite microstructure, forcing further development in the direction of low carbon bainite/martensite. Complex roles of traditional macro-alloying elements, along with the even more complex roles of non-traditional micro-alloying elements and tramp elements will continue to be explored. Many modern high performance alloys, of either body-centred cubic crystal structure or face-centred cubic crystal structure, rely upon extensive controlled precipitation of carbides, carbo-nitrides, nitrides, borides and/or intermetallic compounds to achieve outstanding creep resistance. Optimisation of these precipitates in the base metal alloy can be achieved by controlled mechanical and heat treatment. Possibilities for controlled heat treatment of weld metal and the HAZ are more limited, and possibilities for their mechanical treatment are nearly non-existent. It therefore often becomes necessary to design welding consumables for achieving near-optimum precipitates in the as-welded condition, or after a limited PWHT, which is a much more daunting challenge. Metal matrix composites, such as the aluminium alloys containing silicon carbide whisker crystals or aluminium oxide dispersions, continue to challenge the filler metal designer. Means of producing weld metal with an appropriate dispersion of strengthening particles remain to be discovered. All of the above concerns, and many more, related to welding consumables, can and are being addressed by the IIW.

4.1.3 Testing Testing for the evaluation of the weldability in combination with respective joint design and the service behaviour of welded structures needs to be carried out separately. Weldability tests are generally targeted at the avoidance of joint defects during the fabrication phase of a component. Since cracks represent the

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life