Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
F.R. East Welding of High Strength Steels
Literature survey, 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 steels.
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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