IIW White Paper

9 Needs and challenges of major industry sectors for future applications

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])

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.

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]

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.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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