Failure has been observed to occur on grade P91 alloy flat head closures in Heat Recovery Steam Generators (HRSG), operating at high temperature (>538°C) and high pressure (>125 bar), after a relatively low in-service life. Since creep is expected to be the major failure mechanism, creep analysis and creep life prediction were conducted on four commonly used flat head designs. We have previously shown that failure of welded P91 material is highly dependent on the material properties in the Heat Affected Zone (HAZ). As such we modeled a common multilayer welding procedure to determine the local material properties of both the Coarse Grain and Fine Grain HAZ for each flat head design. Once the final material properties were obtained, we exposed each simulated flat head to typical operational conditions and calculated the total stress (pressure + thermal) and temperature at each node. With this we then used a Deformation Mechanism Map (DMM) for the P91 alloy to determine the creep rate at each node. This allowed us to identify areas that are accumulating creep strain from different creep mechanisms and therefore susceptible to creep and to approximate the creep life.
Finally, a comparison made between the four commonly used flat head designs by ASME to show which design is more desired with respect to welding process.