89
Through Optimum Use and Innovation of Welding and Joining Technologies
Improving Global Quality of Life
Long lasting experience with creep exposed welded structures has shown, that the HAZ is, because of the
mechanism of “Type IV Cracking”, often regarded as the weakest link, in respect of creep strength, in welded
constructions.
Type IV cracking is defined as the formation and propagation of failures in the fine-grained HAZ and the
intercritically heated region of the HAZ. Type IV cracking has been reported in low alloyed ferritic/bainitic
steels (½Cr½Mo¼V, 1CrMo, 1CrMoV, 1¼Cr½Mo, 2CrMo, T/P22, T/P23, T/P24), aswell as in ferritic/martensitic
9-12%
Cr steels (P91, X20CrMoV121, P92, P122, E911). Type IV cracking is considered as the major “
end
of life
”
failure mechanism for ferritic creep resistant steel weldments in the power generating industry.
Figure 9.8
shows the appearance of Type IV cracking in an E911 cross-weld.
General acceptance of the necessity for long term creep testing data of cross-welds, weld metal and base
metal for reliable material selection is inevitable. The awareness of designers, engineers and operators of
the risk of extrapolating results of short term creep tests to longer times has already contributed to an
increase in safety. In Japan and Europe a 9Cr-3W-3Co steel with controlled addition of boron has been
investigated. Contrary to the creep resistant steels recently used, this steel does not show the formation
of a fine-grained region within the HAZ. By the elimination of fine grains in the HAZ the formation of creep
damage by Type IV mechanism, which is strictly limited to fine-grained regions, should be avoided. Although
the mechanisms active in this steel are still under investigation, this might be a possible approach for the
prevention of Type IV cracking in advanced creep resistant 9-12% Cr steels. This will help to increase the
efficiency of advanced fossil power plants for sustainable energy supply and to contribute effectively to the
reduction of CO
2
emissions.
Figure 9.7
Weld strength factors (WSF) for the
100,000
hours creep rupture strength of different
steel grades (Reproduced courtesy P. Mayr,
based on data from M. Tabuchi and Y. Takahashi
[2006
ASME Pressure Vessels and Piping Division
Conference], and J. Schubert, A. Klenk and K. Maile
[2005
International Conference Creep and Fracture
in High Temperature Components – Design and Life
Assessment issues])
Figure 9.8
Cross-weld sample prepared from E911 pipe welded with matching filler and creep tested at
600
°C for 14,000 hours. Very localised formation of voids and their coalescence to macro cracks at the
outer region of the HAZ observed by SEM led to final fracture [Reproduced courtesy: P. Mayr]
9
Needs and challenges of major industry sectors for future applications
