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WA - 2016 - Ron Cecil Lecture: Welding of quenched and tempered steels

Ron Cecil Lecture: Welding of quenched and tempered steels

West Australian Branch Meeting

8 August 2016

Ron Cecil Lecture: Welding of quenched and tempered steels

Eddy Derwort, Welding Engineer, Bradken Ltd

This year the annual Ron Cecil Lecture was a joint meeting with the Australian Foundry Institute, and drew an audience of more than 40 members and visitors to hear Eddy Derwort speak on welding quenched and tempered (Q&T) steels.

Eddy holds a Post Graduate Diploma Materials Welding and Joining, and a Master’s degree in Engineering Practice from the University of Wollongong. He has been involved in welding for nearly 40 years, and has been with companies in the Bradken group since 1996. He has extensive experience in welding structural steel in the mining and oil & gas industries, including pressure equipment and repair of Q&T steels.

Eddy started his presentation by noting that even though the principles of welding Q&T steels are well established, he is still often called on to analyse welding failures in Q&T steels. It is still not always appreciated that welding is, in effect, a localised quench and temper process, to which such steels respond, usually unfavourably. It is all too common to hear questions such as “I only want to weld on a little attachment; is it really worth worrying about?” With Q&T steels the answer is “Yes, it is!”

Hydrogen assisted cold cracking (HACC, under-bead cracking) is a very common cause of failure in welding Q&T steels. Three conditions have to be met for HACC to occur: a susceptible microstructure, stress, and dissolved hydrogen, and eliminating any one of these three can avoid HACC. The most important control on microstructure is pre-heating to avoid too-rapid cooling, however, the most controllable factor is hydrogen. Liquid iron has a much higher solubility for hydrogen than solid iron, and key to avoiding hydrogen are to eliminate sources such as surface contamination and welding consumables, and to accelerate diffusion of hydrogen out of the solid steel by prolonged post-weld heating.

This has been known for many years, as Eddy demonstrated with a video made at Delft Technological University in the 1950s. Few in the audience had seen this video, and sight of hydrogen bubbles being released from a submerged weld certainly made theory appear much more real. He reinforced this with a more recent video from Canadian company BMT Fleet (both videos are on YouTube).

In practice, the most common source of hydrogen is moisture in the coatings on welding rods used in manual metal arc welding. These are available in controlled-hydrogen specifications, but avoiding moisture pick-up requires rigid control over storage and prompt use once exposed to air. Hydrogen levels are indicated in the flux-coated rod specification e.g. the suffix H4 means 4 mL hydrogen per 100g. However, 4 hours at 80 % RH is enough to change H4 to H10. This is enough to make the rod unsuitable for welding Bisplate80, which should be limited to H6. Eddy’s experience is that outside the oil & gas industry the hydrogen levels in such flux-coated consumables are effectively uncontrolled.

Failure by HACC can also be avoided by reducing stress. This can be achieved by joint design, reduction of constraint and post-weld heating for stress relief. Eddy showed examples of welding set-ups in which the external heating provided both pre-heat and post-heat. He also made a number of references to AS1554 (Structural steel welding), and pointed out that the standard 50 degree Vee joint is based on the arc cone to allow root fusion.

Residual stress can also be reduced by peening. Eddy explained that to be effective peening has to plastically deform the surface; it requires serious energy input.

Eddy concluded his presentation with some examples of complex welding procedures (e.g. chain links) in which the design involved finite element analysis and under-matched (lower strength) consumables. Compressive stresses from controlled shrinkage were factors in the weld design.

Eddy then took questions from the audience, which led to a discussion on the circumstances in which cellulosic-coated rods can, and cannot, be used for root runs in pipeline welding.

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