The DISMEW project has developed a life assessment routine for dissimilar welds in steam piping, which functions as a post-processor program for the output of a component finite element analysis. The development is based on a targeted program of experimental and analysis work. Two material dissimilar weld combinations were considered: 2.1/4 Cr1Mo to Modified 9Cr1Mo and Modified 9Cr1Mo to Esshete 1250. Results from high temperature creep and low cycle fatigue tests on crossweld specimens machined from the welds have been used to rank the weld procedures and calibrate Linear Damage Summation and Continuum Damage Mechanics lifting models.
Two component tests (one for the P91-Esshete 1250 combination and one for the P91-P22 combination) have been carried out on butt-welded pipe sections. These provided benchmark data for verifying the life assessment prediction. The tests were conducted under isothermal conditions with complex loading involving sucessive periods of creep (internal pressure plus axial bending) and fatigue (reversed bending) loading using an experimental facility specially adapted for the project. Comprehensive deformation and replica measurements were made to have a clear picture of the damage development.
A series of finite element analysis were carried out to simulate the behaviour of the two components under the test conditions, paying particular attention to the thermal stresses which arise due to differences in thermal expansion coefficient between the base and filler materials. The problem is obviously most severe for the ferritic- austenitic joint and it was found necessary to consider the post-weld heat treatment phase to arrive at usable results. It is acknowledged that further work is needed in this area.
The life assessment procedures have been implemented as the "POST-D" post processor. The damage location and life estimates for the P22-P91 component agreed with the benchmark component test results. In the case of the P91-Esshete component, the location of maximum damge was correctly predicted. On the other hands, analysis of the FE output for models including thermal mismatch produced severe overestimations of the creep damage.
The objective of the project is to set up a procedure for the design and life prediction of welded parts of high temperature components subject to creep and low cycle fatigue damage. In this area knowledge is presently lacking and procedures used in design codes are too approximate to be used for estimates of lives of welded pats (by utilities) and for life optimisation of welded components (by constructors). Due to the growing importance of dissimilar metal welds between advanced chromium rich steels, candidates for the hottest parts of power plant, and the traditional steels of the colder parts, the proposed activity will be focused on the creep and fatigue behaviour of dissimilar metal welds. A three year programme is proposed, comprising of:
- formulation of a damage evaluation procedure, based on models calibrated over data from creep and low cycle fatigue tests on specimens taken from welds,
- application of the procedure to "post-process" the output from a thermo-mechanical finite element analysis of a selected component,
- verification of the life assessment procedure via actual component tests.
The output of the project will be a self-contained manual and software package for the design and life assessment of components containing dissimilar metal welds. Its distribution will be restricted to the partners in the initial post-project phase, in which further validation of the procedure will be considered.
The project brings together six partners from five EC countries (two R&D companies in the plant life diagnosis field, a welding institute, two component producers and one power utility). Benefits are expected with regard to plant efficiency (more reliable knowledge of dissimilar weld properties favours the adoption of the advanced steels considered here, allowing increased operation temperatures), optimisation of welding procedures and of design involving dissimilar metal welds, and improved (residual) life diagnosis of service exposed welded components.
Funding SchemeCSC - Cost-sharing contracts
PA4 8UW Renfrew
7300 AM Apeldoorn