TWI Industrial Member Report Summary 40/1976
By H Harasawa and P H M Hart
A programme of work has been carried out to determine the critical hardness for heat affected zone (HAZ) cracking for equal double V MMA butt welds in 25mm thick C-Mn-Nb steel as a function of Intensity of Restraint (K) . Other variables examined have been carbon equivalent level (0.39, 0. 49, 0.57) and
weld hydrogen level (about 3 and 10ml/100g deposited metal).
Compared to earlier work which showed for fillet welds, an increase in critical hardness with carbon equivalent, the present results, although also showing a reduction of reaction stress with increasing carbon equivalent, conversely showed critical hardness decreased with increasing carbon equivalent.
This is believed to be due to an increase in root strain arising from an observed increase in angular distortion and root stress concentration at the higher arc energies required to present cracking in the higher carbon equivalent steels.
For the higher weld hydrogen level and lower CE steel, weld metal hydrogen cracking became a problem at high arc energies (even after HAZ cracking had stopped) and this is also believed to arise from the increased root strain at high arc energies.