Welding of High Strength Steels
Report 2006.TL.7045, Transport Engineering and Logistics.
High strength steels (HSS) will be used more often to solve increasingly
demanding engineering problems. This investigation was setup to view the
possibilities of welding high strength steel, determine the fatigue
properties, examine fatigue-enhancing procedures and view the possibilities
of welding high strength steels to other steels.
The high strength steels viewed comply with standard NEN-EN 1011. This standard
approves all common welding procedures, with which the HSS can be welded. The
demands on the welding environment are listed in part 2 of the standard. Weld
metals should be used which have at least the same toughness as the HSS and the
strength recommend by the manufacture. To prevent cold cracking of the weld,
weld metals should not have hydrogen contents above 5 ml/100g.
When welding with a low-hydrogen content consumable and sufficient heat
input, no preheating is needed for high strength steels with yield strengths
up to 690 MPa. Only thin plate steel (ÿ 10mm) of the steels with higher
yield strengths will need no preheating.
The fatigue strength of unwelded HSS is considerably higher than the fatigue
strength of mild steels. However welded HSS has defects from welding where
fatigue cracks can easily initiate. After testing it was found that the fatigue
strength of welded HSS is no better than that of welded mild steel. Therefore t
he fatigue strength can be estimated using the same standards as used for mild
Improving the quality of the weld utilizes some of the potential fatigue
strength which has been lost due to welding. The welds are improved by
either removing residual stresses or improving the weld geometry. The
greater the fatigue strength of the unwelded material, the greater the
fatigue improvement will be after weld treatment. The improvement techniques
have the possibility to double the fatigue strength of the weld. The
strongest technique seems to be Ultrasonic Impact treatment with is a form
of residual stress treatment. Using an ultrasonic hammer the weld is beaten
introducing compressive stresses in the weld area. Unlike other residual
stress treatments the Ultrasonic hammer is easy to operate and is not
uncomfortable for the operator. New standards on improvement techniques
would greatly improve the use of these techniques as to few are motioned in
current standards and do not reflect the same amount of improvement as has
been gained in testing.
High strength steels can be welded to mild steels as they are both described
in the same welding standard. However the weld is as strong as the weakest
link, in this case the mild steel. Due to the shortage of available of
information no answer can be formed here. Further testing in the joining of
HSS other materials is needed as this seems yet to be performed.
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