Abstract:
The steam lines in the majority of power stations
operating in the UK were fabricated from kCrkMokV steam
pipe material joined using 2CrMo weld metal. Throughout
the life of these systems, which operate typically at
565°C and 165bar, weldment cracking problems have been
encountered. Many of these problems are well understood
and are managed effectively. In recent years a creep
cracking mechanism in the heat affected zone adjacent to
the fusion boundary has been identified, referred to as
Type IIIa cracking.
This project has identified significant carbon migration
across the fusion boundary of these weldments, from the
lower alloyed kCrkMokV to the more highly alloyed 2CrMo
weld metal. Carbon diffusion has been identified as
occurring during the welding cycle, however the majority
of carbon diffusion occurs during post weld heat treatment
and particularly during service exposure.
Carbon migration results in marked reduction in carbon
content in the heat affected zone adjacent to the fusion
boundary with a corresponding band of enhanced carbon
content in the weld metal adjacent to the fusion boundary.
The width of these areas increase with time at
temperature.
Micro hardness measurements have confirmed a reduction in
the hardness of the heat affected zone region as a result
of carbon diffusion. Hardness measurements have also
pointed to a step change in hardness across the fusion
boundary. This may result in a corresponding step change
in creep properties. It is suggested that this mismatch in
creep properties, coupled with a predominantly fine
grained microstructure at the fusion boundary, can result
in the fusion boundary heat affected zone region being the
weakest region of the weldment.
