Recent Completed Project on Metallography, Accelerated Rupture Testing and Remaining Life Assessment of Superheater & Reheater Tubes
ETD is pleased to announce the successful recent completion of a metallography, accelerated rupture testing and remaining life assessment project of superheater & reheater tubes.
It is reported by the client that they had hired a sub-contractor to perform accelerated rupture testing and remnant life assessment work for 1 superheater and 1 reheater tubes (of T91 materials). This contractor had completed the creep test and RLA based on an iso-thermal basis with reference to Manson-Haferd curve. Recently, the client hired a second contractor to do the same job. This second contractor had done the creep test and RLA based on an iso-stress basis. The actual service stress in this case is relatively low. The client had found that the results from these two contractors differed greatly; the results of second contractor showed unrealistically high remaining life and they did not consider progressive wastage.
Therefore, ETD was approached with a request to perform the same work (accelerated rupture testing and RLA) again for these two coal-fired tubes to provide an estimate for the remaining life of the components.
The scope of the work carried out included the following tasks:
i. Metallography - Metallography of two tube samples were carried out to establish the creep damage (i.e. creep cavitation & microstructure degradation) levels. The microstructure and any degradation characterised using damage classification for creep damage of the relevant materials. Specific scope in this task included:
Visual examination of the tube samples including internal and external appearance;
Assessment of condition of internal and external surfaces including identification of inclusions/ contaminations, scale/ deposits, welding defects etc (if any exists) and obtain photos of evidence;
Dimensional examination - OD measurement at 12-6 and 3-9 O’clock positions at fixed length intervals throughout the entire tube length.
Tube wall thickness measurements for all aforementioned clock positions.
Metallographic examination of the tube samples using optical microscopy to assess the condition of internal and external surfaces, to identify phase changes in microstructure / material degradation/ precipitations and also to identify the damage mechanism(s);
Estimation of tube operating temperature based on measurements of steam-side oxide thickness;
Hardness testing and evaluation to determine the equivalent tensile strength values (using published hardness/ UTS correlation tables) to assess the mechanical strength or softening of the materials.
ii. Accelerated Stress Rupture Testing - A programme of isothermal rupture testing was then performed comprising three specimens from each of the two tube samples to be tested. The tests were performed at conditions designed to achieve target durations of 500, 1000 and 3000 hours as proposed by ETD. After reviewing the design and operating parameters, the test conditions (i.e. temperature, stress etc) was selected by ETD based on previous experience and reference database to give failures at the above number of hours.
All these tests will be conducted by ETD’s collaborative accredited laboratory in Europe and according to the international and ECCC specifications.
iii. Analysis of Creep Test Results and Remaining Life Calculations - The results from the above rupture testing programme were compared with standard data for the relevant steel grade to establish where, within the upper and lower bounds, the material creep properties lie. Hence an equation describing the actual creep strength of the ex-service material being modelled. This equation was then used to estimate the creep life at the service conditions.
Provided that the as-installed tube wall thickness and service hours to date are known, then the corrosion rate (wall thinning rate) can be calculated using the thickness measurements from the first task. This corrosion rate was used to determine the combined effect of creep and corrosion on the component remaining life at the effective operating temperature estimated in the first task.
A final report, detailing the findings of the tasks undertaken, was issued for comments by the client. It will include photographic records from the metallographic study and provide details of the remaining life estimate based on accelerated rupture testing. Any possible causes for concern will be highlighted. The influence of material degradation and other microstructural changes were also discussed in this report.
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