Welding: just because you can, doesn’t mean you should!
Louise Petrick, Technology Manager, Welding Technology Institute of Australia
Louise is an International Welding Engineer, with a foundation in the study of metallurgy at the University of Pretoria and metallurgy and welding at the University of the Witwatersrand. She is a Certified Materials Professional with MA and a member of Engineers Australia. The focus of her presentation was welding process selection for the required service and environment, emphasising that the three critical considerations are (1) application, (2) application, and (3) application!
Louise described welding as an art, a skill, a trade, an engineering discipline and a science, involving more sciences and variables than those involved
in any other industrial process. In general, the aim of welding is the make two pieces act as one, and when correctly selected for the application,
it is the most economical and efficient way to join materials permanently. The general principle of welding is to apply energy to cause coalescence
of materials either in liquid or solid state. As well as applying to metals, welding can be used to join plastics and ceramics, but this presentation
was concerned mainly with steel and corrosion resistant and creep resistant alloys.
There are more than 50 welding processes for metals and the first consideration in selection is the service requirement. There are many aspects to service
requirement including mechanical, structural, creep, cryogenic service and corrosion. Structural aspects can relate to static performance, creep under
sustained load, fatigue under fluctuation loads, and fabrication, where failures such as lamellar tearing can occur during the welding operation. These
considerations lead to selection of an appropriate welding process, procedure, welding consumables, and methodology.
Louise went further into structural considerations, describing how joint and weld geometry, for example, complex nodes, can influence loading. Susceptibility
to general corrosion should also be a concern, leading to consideration of inspectability during the life of the structure. Creep resistance is a concern
since welding changes the state of precipitates in the microstructure, and hence creep resistance. Creep ductility can be correlated with the impact
toughness of the heat affected zone around the weld, but ASTM standards for welding do not specify impact toughness, and this can have unanticipated
consequences when creep properties are important. Performance in cryogenic service similarly requires care in weld specification and testing.
During welding, intense heat destroys the effects of mechanical processing of the base metal, while the filler metal changes the composition and the microstructural
phase distribution. High temperature in the heat affected zone also causes grain growth, and changes the distribution of precipitates, notably carbides.
Heat input rate in arc welding is proportional to arc power (current and voltage) and inversely proportional to travel speed. The two immediate effects
are on weld bead size and on weld cooling rate. Consideration of the relevant continuous cooling transformation (CCT) curves shows that with austenitic
alloys fast cooling is desirable, but martensite formation in ferritic steels is avoided by slow cooling.
Louise then turned to examples showing how the welding procedure has direct effects on the nature of the weld. This covered manual metal arc, submerged
arc and gas shielded TIG welding. Considerations here include anode polarity, which affects penetration, and oxygen pickup or removal. Alternating
current welding is another variation, which can be used in welding aluminium to help clean the metal surface. High arc currents increase efficiency,
but can cause arc turbulence, which can result in entrainment of atmospheric gases.
Returning to her opening points, Louise concluded with the reminder that the application affects the material selection, and material and application together
should determine the appropriate welding process, and procedure.
In answering questions from the audience Louise described the role of the WTIA as a not-for-profit industry body concerned with certification of individual welders and companies, training, and industry advocacy. Knowledge transfer is a major focus and includes input to standards, and the publication of guidance notes and technology notes, the latest of which apply to AS1554 (2014).