Tag Archive for: Dissimilar Metal Weld (DMW)

News & View, Volume 44 | A First-of-a-Kind NDE Innovation from SI The first PDI qualified manually-encoded DM Weld Procedure

News & Views, Volume 44 | A First-of-a-Kind NDE Innovation from SI – The first PDI qualified manually-encoded DM weld procedure

By:  Jason Van Velsor, Joe Agnew, and Owen Malinowski

News & View, Volume 44 | A First-of-a-Kind NDE Innovation from SI The first PDI qualified manually-encoded DM Weld ProcedureDetermining 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.

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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.

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News & View, Volume 44 | Dissimilar Metal Welds in Grade 91 Steel

News & Views, Volume 44 | Dissimilar Metal Welds in Grade 91 Steel

By:  Terry Totemeier

Introduction
News & View, Volume 44 | Dissimilar Metal Welds in Grade 91 SteelA dissimilar metal weld (DMW) is created whenever alloys with substantially different chemical compositions are welded together – for example, when a low-alloy steel such as Grade 22 (2¼ Cr-1Mo) is welded to an austenitic stainless steel such as TP304H (18Cr-8Ni).  Many DMWs are commonly present in fossil-fired power plants, examples being material transitions in boiler furnace tubes, stainless steel attachments welded onto ferritic steel tubes or pipes, and stainless steel thermowells or steam sampling lines in ferritic steel pipes.  The chemical composition gradients associated with DMWs present unique issues relative to their design, in-service behavior, and life management, particularly for those DMWs operating at elevated temperatures where solid-state diffusion and cyclic thermal stresses are factors, which was previously presented in News and Views (Volume 43, page 19).

With the now widespread use of Grade 91 steel (9Cr-1Mo-V-Nb) for elevated-temperature applications in modern power plants, DMWs involving this material have become common, and increasing service experience has revealed some unique characteristics and failure mechanisms, especially in thicker-section DMWs with austenitic materials.  This article presents a short overview of Grade 91 DMWs:  their design, fabrication, and failure, with emphasis on current industry issues.

There are two basic classes of DMWs in Grade 91 steel:  ferritic-to-ferritic and ferritic-to-austenitic.  The first type corresponds to Grade 91 welded to another ferritic steel with a lower chromium content, such as Grade 22; the second type corresponds to Grade 91 welded to an austenitic stainless steel such as TP304H.  Each of these types has unique concerns and considerations.

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