Tag Archive for: Creep Damage

News & View, Volume 44 | Example Grade 91 High Energy Piping DMW Joint Stress and Metallurgical Analysis

News & Views, Volume 44 | Example Grade 91 High Energy Piping DMW Joint Stress and Metallurgical Analysis

By:  Ben Ruchte

News & View, Volume 44 | Example Grade 91 High Energy Piping DMW Joint Stress and Metallurgical AnalysisDetermining a course of action once in-service damage is discovered often requires applying a multi-disciplinary approach that utilizes Nondestructive Examination (NDE), analytical techniques such as stress analysis, and metallurgical lab examination.  Such was the case recently for a combined cycle plant where indications were found through NDE on the inlet sides of two identical main steam stop/control valves but were not seen on the outlet side.  In this case, Structural Integrity (SI) did not perform the field NDE but was requested to perform analytical and metallurgical assessments of the welds.  The welds in question joined the 1Cr-1Mo-1/2V (SA-356 Grade 9) main stop/control valve body castings to Grade 91 piping, so the welds represent a ferritic-to-ferritic dissimilar metal weld (DMW).  See the Dissimilar Metal Welds in Grade 91 Steel, (page 15) for further information. The welds were made using a 1Cr-1/2Mo (AWS type B2) filler metal, which matches the chromium content of the valve body, but is significantly undermatching in strength to both the valve body material and the Grade 91 piping. 

The course of action taken was to perform local stress analysis and remaining life estimates for the downstream (outlet) connections of the valves to assess likelihood of future damage and establish an appropriate re-inspection interval.  Detailed metallurgical analysis was also performed on a ring (entire circumference) section removed from one of the upstream welds (which exhibited both surface and volumetric indications in the weld metal) in order to provide insight into the damage mechanism and inform the stress analysis and remaining life estimates.

READ MORE

News & View, Volume 44 | Metallurgical Lab Featured Damage Mechanism Long-Term Overheating:Creep (LTOC) in Steam-Cooled Boiler Tubes

News & Views, Volume 44 | Metallurgical Lab Featured Damage Mechanism – Long-Term Overheating/Creep (LTOC) in Steam-Cooled Boiler Tubes

By:  Terry Totemeier

News & View, Volume 44 | Metallurgical Lab Featured Damage Mechanism Long-Term Overheating:Creep (LTOC) in Steam-Cooled Boiler TubesLong-term overheating and creep damage are often the damage mechanisms associated with the normal or expected end of life of steam-touched tubes, generally occurring after 100,000 hours or more of service life at elevated temperatures and pressures. Long-term overheating and creep can also occur when the rate or accumulation of creep damage is moderately higher than anticipated by original design. There are a number of possible reasons for this, but in general the problem can be attributed to one of the following: a non-conservative original design, higher-than-anticipated heat absorption, lower-than-anticipated steam flow, or wall loss caused by external wastage.

Mechanism
The mechanism of failure for LTOC is simply the accelerated accumulation of creep damage in the component over a span of time that is well short of the anticipated design life, but sufficiently long that creep is the dominant damage mode. This damage is typically associated with the operation of the tube above the oxidation limit for the material involved.  This has two effects, which both contribute to long-term creep failure: reduction in wall thickness due to oxidation loss, and build-up of oxide on the tube internal surface, which insulates the tube from the cooling effect of the steam, leading to increasing tube metal temperatures over time.

READ MORE