Metals Science Research and Consulting


by Dr. Stan T. Mandziej

 

Creep life of weld HAZ


Weld’s HAZ is a critical site in which prematurely type-IV cracks nucleate during exposure to creep conditions. The accelerated creep test (ACT) allows to determine the creep life of the HAZ on cross-weld sample. Combination of the weld HAZ simulation on Gleeble followed by ACT is intended to optimize welding conditions in order to improve the creep life of HAZ.

 

Schematic of heat-affected zone of the electric arc weld; the electric current flows through fusion plane and heat flows out from the fusion plane.


By heating the central portion of bar-like sample in Gleeble; with a short span between mounting jaws, double width of weld's HAZ can be simulated

 

The table shows ACTest results on cross-weld samples of two creep resisting steels, having very similar creep life at 6500C however showing different properties of HAZ — the HAZ of F steel at  6000C is weaker than HAZ of the FB2 steel at 6250C.

 

By combining the weld’s HAZ simulation on Gleeble (thermal cycles at short i.e. 12mm span between Cu-jaws) with the ACT (Cu-jaws span 28-32mm) the creep lifetime can be studied directly after the HAZ simulation, on the same sample.

 

When comparing the ACT results on two samples of the same steel, the first in as-delivered state and the second with middle portion in HAZ simulated state, one may see that the HAZ simulated (without PWHT) sample is initially stronger than the as-delivered one, however with progress of test its strength visibly decreases and it fails earlier. When the HAZ simulation is followed by PWHT, the time to failure in ACT increases, but this does not yet tell the creep life. To determine the creep life detailed analysis of ACT graphs is needed. 

 

To be continued