Technology Briefing 507 - April 1995
D J Abson
Background
Steels in which high strength is achieved by low levels of alloying and the use of some thermomechanical processing have potential application in many industry sectors where lower weight or lower welding costs (from reduced section sizes and from lower preheat requirements, as a consequence of lower carbon contents) are beneficial. Increased use of such steels depends on their good weldability being established and understood, and also on the availability of suitable low hydrogen welding consumables.
In the present programme, the weldability of two Al-B low alloy steels, with minimum specified yield strengths of 550 and 690MPa, which are 60 and 50mm thick, respectively, has been investigated. Butt welds have been produced, and some hydrogen-induced cracking information has been derived. Controlled thermal severity (CTS) tests have been carried out. The relationship between cooling rate and HAZ hardness has been studied for the CTS test welds and for butt welds. The butt weld as-welded and stress-relieved HAZ toughness is being investigated, and will be reported later.
Main Conclusions
The study has demonstrated that, for the steels studied, with minimum yield strengths of 550 and 690 MPa, the BS5135 preheat requirements for the avoidance of HAZ hydrogen-induced cracking are conservative, especially for the 690 grade steel. However, weld metal hydrogen cracking was found; thus the risk of hydrogen-induced cracking in the weld metal may be greater than that in the HAZ, and welding consumables which give appropriately low weld hydrogen levels must be employed for welding high strength steels of the type investigated here, in order to exploit fully the good resistance to HAZ hydrogen-induced cracking of the steels.
Recommendations
In general, increased use of high strength steels is to be encouraged, particularly where, as for the two steels studied here the preheat requirements for avoidance of HAZ cracking are substantially below those expected from the IIW carbon equivalent. At the same time, if lower preheat levels are used; careful consideration must be given to the risk of weld metal hydrogen-induced cracking.
