Metals Science Research and Consulting


by Dr. Stan T. Mandziej

 

Stress-relief embrittlement of

microalloyed HSLA steel weld metal


HSLA steel weld metal, microalloyed with ~400ppm Ti and ~40ppm B can embrittle during stress-relief treatment. Such treatment is however compulsory for multi-bead welds on thick plates. Thus a test is needed to determine when this weld metal would not embrittle while still retaining its high strength. An adequate three-cycle test was developed on Gleeble physical simulator with use of crosswise laser dilatometer. 
After the stress-relieving treatment this weld metal has tempered acicular ferrite microstructure with pronounced prior austenite grain boundaries (left picture) while a sample of it when fractured at room temperature shows numerous "smooth" intergranular facets (right picture).
Dislocation substructure in ferrite of Ti+B microalloyed HSLA steel weld metal before stress-relief annealing, with dominating criss-crossing a/2<111> screw dislocations (left picture) and the substructure near to prior austenite grain boundary (PAGB) after the stress-relief annealing (right picture); note the ultra-high dislocation density within the PAGB, making it prone to cracking.
The cylindrical sample of non-embrittling weld metal during simulated three-cycle stress-relief treatment in Gleeble's vacuum chamber expands its diameter thus accommodating the imposed stress (left picture) while the embrittling sample shows in the third cycle noticeable decrease of diameter due to annihilation of dislocations (right picture).