26
Improving Global Quality of Life
Through Optimum Use and Innovation of Welding and Joining Technologies
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
