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The relatively new ASTM P91 martensitic steel is highly regarded in
high temperature industry and its use as thick section components (such
as headers, steam pipes etc.) has been very beneficial to the power plant
industry in raising plant temperature and efficiency. However, limited
plant experience now indicates that unfortunately this steel, like its
predecessor low alloy ferritic steels, may be equally prone to Type IV
cracking. This can possibly result in failures relatively early in life,
as experienced by some of the plants in the UK and elsewhere. Information
from limited R&D work on cross-weld specimens and the testing of
feature specimens of 9Cr martensitic steels appears to support this view.
Some of the R & D work also raises questions about the claimed
benefits of this material in terms of creep fatigue interaction under
plant cycling conditions, especially of the welded components.
Discussions at present are underway within the European and Japanese
expert groups about reducing the allowable stress levels for the 9Cr and
12Cr martensitic steels and some adjustments have already been made at
the time of the writing this introduction (early 2006).
There are concerns about tempering temperature limits for this steel.
Similarly, there are also concerns about the criticality of the heat
treatment of this steel after welding and forming operations. Other
concerns have been raised about the time taken to do the post weld heat
treatment after welding. Indeed, a number of manufacturers and service
providers are still unsure of this aspect and some of the existing
practices have indeed been questioned. In short, unlike the low alloy
steels that the industry has had a good deal of experience with and where
the industrial heat treatment could be relaxed within the achievable
limits, the precise control of heat treatment of high alloy martensitic
steels can be critical to its satisfactory performance and much needs to
be learnt about this. Indeed, there are differences between the North
American, European and Japanese standards and codes on the allowable
limits and ASME standards, as a result, has formed appropriate
sub-committees which are re-considering the recommendations by this
standard.
With regards to the T91 tubing, steam side oxidation was reported as
the biggest concern at first by the researchers and now by the plant
operators around the world.
Furthermore, there are issues re the integrity and life assessment of
the P/T91 components. This steel does not appear to show creep voids or
easily observable and quantifiable microstructural damage until very late
in life and therefore new methodologies need to be developed to account
for this. One way of successfully managing performance and life of
components made from this steel can be the development/ use of
appropriate monitoring and inspection methods and techniques for
components made from this steel and therefore we need to learn from the
existing experience of other plant operators and researchers.
The need for this ‘International P91 Users Group' was first
discussed in December 2005 at a meeting held in London by P91 experts,
plant manufacturers and operators. The Group aims to discuss
many issues that today concern P/T91 producers, power plant/HRSG
manufacturers and plant owners/operators sing P/T91 - not just the power
plant but also the petrochemical industry where use of this steel is now
becoming better known.
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