TWI Industrial Member Report Summary 535/1995
A M Barnes
The increased use of thermomechanically controlled processing (TMCP) in steelmaking over recent years has allowed good parent material properties (strength and toughness) to be achieved in steels with low carbon content through tight grain size control. These TMCP steels generally offer good weldability, particularly a high resistance to hydrogen cracking and superior heat affected zone toughness relative to conventional processing methods, and can also exhibit superior sour service properties.
It is widely accepted that local brittle zones (LBZs) exist within the HAZ of structural steels, most especially within those regions of the grain coarsened HAZ reheated into the intercritical regime by subsequent weld passes. This leads to formation of microphases which, depending on their nature, can significantly reduce the cleavage initiation resistance. An earlier study at TWI on normalised steels showed a marked reduction in the toughness of the intercritically reheated grain coarsened (ICGC) HAZ with the formation of M-A constituent as opposed to ferrite/carbide aggregate. The objective of the present programme was to compare samples of normalised and TMCP materials to determine how TMC processing and the associated alloy variations influenced toughness and microstructural development in both the grain coarsened (GC) and subcriticai/intercritical (SC/IC) HAZ over a range of arc energies up to 5.0kJ/mm.