News & View, Volume 46 | Cross-Weld Creep-Rupture Testing for Seam Weld Life Management

News & Views, Volume 46 | Cross-Weld Creep-Rupture Testing for Seam Weld Life Management

By:  Jonnathan Warwick, Terry Totemeier, and Brian Chambers, Duke Energy

News & View, Volume 46 | Cross-Weld Creep-Rupture Testing for Seam Weld Life ManagementLongitudinal seam-welded hot-reheat steam piping operating in the creep regime is a continuing life-management challenge for many older fossil-fired power plants.  In response to catastrophic seam-welded piping failures in the 1980’s, the Electric Power Research Institute (EPRI) developed a comprehensive inspection protocol to insure continued safe operation of these piping systems [1]. The protocol requires full inspection of seam-welded hot-reheat pipe once a threshold of service exposure (calculated creep life consumption) has been reached, and re-inspection at intervals after the initial inspection depending on the inspection results.  Inspection for sub-surface cracking using ultrasonic testing (conventional or advanced) is strongly recommended, in combination with checking for surface cracking using wet fluorescent magnetic particle testing (WFMT).  Initial inspection and re-inspection of these piping systems represents a large maintenance cost for utilities, especially as older plants remain in service due to the changing economics of power generation.

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News & View, Volume 46 | Turnkey Rapid-Response Plant Support Disposition of Wall Thinning in Standby Service Water Piping

News & Views, Volume 46 | Turnkey Rapid-Response Plant Support Disposition of Wall Thinning in Standby Service Water Piping

By:  Jason Van Velsor, Roger Royer, and Eric Houston

News & View, Volume 46 | Turnkey Rapid-Response Plant Support Disposition of Wall Thinning in Standby Service Water PipingStructural Integrity recently had the opportunity to support a client’s emergent needs when their Standby Service Water (SSW) piping system experienced a pinhole leak just downstream of a valve. Concerned about other locations in the piping system with similar configurations, the site asked SI to assist with the expedited development of assessment and disposition plans for these other components. In response, SI was able to lean on our core competencies in failure analysis, advanced NDE inspection, and flaw evaluation to develop and deploy a comprehensive solution that met our client’s expedited timeline and helped them to mitigate the threat of future unplanned outages. The following sections outline how SI utilized our in-depth knowledge, cutting-edge technology, and world-class engineering to meet our client’s needs.

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News & View, Volume 46 | Metallurgical Lab Case Study- Cracking of Grade 23 Steel Furnace Wall Tubes

News & Views, Volume 46 | Metallurgical Lab Case Study: Cracking of Grade 23 Steel Furnace Wall Tubes

By: Terry Totemeier

News & View, Volume 46 | Metallurgical Lab Case Study- Cracking of Grade 23 Steel Furnace Wall TubesGrade 23 is a creep strength enhanced ferritic (CSEF) steel that was designed to offer similar creep strength to Grade 91 but with lower Cr content and, in the original concept, fabrication without pre- and post-weld heat treatment making the material attractive for the furnace wall tubes of ultra-supercritical coal plants where T12 has insufficient strength and T91 would be too complex to fabricate. Experience gained with T23 has shown that pre-heat is necessary and that post-weld heat treatment should also be performed when the material is employed in “high restraint” applications such as furnace wall tubes. Like other CSEF steels, T23 is very sensitive to heat treatment, and care must be taken to ensure that hard, brittle microstructures do not enter service – particularly in high restraint applications such as furnace wall tubes.

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News & View, Volume 46 | Metallurgical Lab Featured Damage Mechanism- Waterwall Fireside Corrosion (WFSC) in Conventional Boilers

News & Views, Volume 46 | Metallurgical Lab Featured Damage Mechanism: Waterwall Fireside Corrosion (WFSC) in Conventional Boilers

By: Wendy Weiss

News & View, Volume 46 | Metallurgical Lab Featured Damage Mechanism- Waterwall Fireside Corrosion (WFSC) in Conventional BoilersIndustry experience shows that waterwall tubing in conventional boilers can be susceptible to fireside corrosion, depending on fuel type, firing practice, etc. In boilers where fireside corrosion has been identified as a maintenance issue, wastage rates of 5 to 25 mils/year are not uncommon. Since the mid 1990s, the installation of low NOx burner systems designed to lower NOx emissions has significantly increased the wastage rates in some boilers. Operators of subcritical boilers have reported wastage rates as high as 30 mils/year, while those operating supercritical boilers have reported rates exceeding 100 mils/year in the worst cases. These higher damage rates have resulted in an increase in tube failures, and operators have struggled to accurately define the extent of the damage and install the appropriate mitigating technologies.

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News & View, Volume 45 | Metallurgical Lab- Case Study – Thermowell Failure Analysis

News & Views, Volume 45 | Metallurgical Lab: Case Study – Thermowell Failure Analysis

By:  Wendy Weiss

News & View, Volume 45 | Metallurgical Lab- Case Study – Thermowell Failure AnalysisStructural Integrity (SI) was recently asked to examine a fractured thermowell and determine the damage mechanism.  The thermowell had been removed from bypass line piping in a heat-recovery steam generator (HRSG) that ran from the High Pressure (HP) bypass valve to the cold reheat section, and sent to the SI Materials Science Center. As reported by plant personnel, the fracture was located within the pipe wall. The pipe material was specified as ASME SA-335, Grade P22, and the thermowell was specified to be ASME SA-182, Grade F22.

Examination Procedure and Results

The fractured thermowell sections were visually examined and photographed in the as-received condition, as shown in Figure 1. The thermowell was comprised of two pieces: the thermowell housing itself which protruded into the steam stream, and a fitting connection to the pipe into which the thermowell housing was inserted.

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News & View, Volume 45 | Metallurgical Lab Featured Damage Mechanism Acid Dewpoint Corrosion in Conventional Fossil Boilers and Combined Cycle HRSGs

News & Views, Volume 45 | Metallurgical Lab Featured Damage Mechanism – Acid Dewpoint Corrosion in Conventional Fossil Boilers and Combined Cycle HRSGs

By:  Wendy Weiss

Acid dewpoint corrosion can occur in conventional and HRSG units in locations where temperatures fall below the sulfuric acid dewpoint temperature. This can occur when either the tube metal temperatures are below the acid dewpoint so that condensate forms on the metal surface, or when flue gas temperatures are below the acid dewpoint, so that the condensate will form on fly ash particles.

Mechanism
This type of fire-side damage occurs when sulfur dioxide (SO2) in the flue gas oxidizes to sulfur trioxide (SO3) and the SO3 combines with moisture to form sulfuric acid. If the temperatures are at or below the acid dewpoint, so that the sulfuric acid condenses, then tube metal corrosion occurs. The temperature at which condensate first forms depends on a number of factors, including the partial pressures of SO3 and water vapor in the flue gas, but is usually around 250 to 300°F.

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News & View, Volume 44 | Failed Grade 91 “Soft” Pipe Bend - A Case Study Failure Occurred With Less Than 35,000 Operating Hours

News & Views, Volume 44 | Failed Grade 91 “Soft” Pipe Bend – A Case Study – Failure Occurred With Less Than 35,000 Operating Hours

By:  Kane Riggenbach and Tony Studer

News & View, Volume 44 | Failed Grade 91 “Soft” Pipe Bend - A Case Study Failure Occurred With Less Than 35,000 Operating HoursGrade 91 steel is widely used in tubes, headers and piping of superheaters and reheaters because of its higher strength at elevated temperature compared to low alloy steels such as Grade 22.  The improved strength is a result of a tempered martensitic microstructure with a fine distribution of carbonitride precipitates.  This microstructure is achieved through careful heat treatment: normalizing, tempering, and subsequent forming and post weld heat treatments.  If these heat treatments are not performed properly, then the strength of the material essentially reverts to that of a low alloy steel like Grade 22, and is usually accompanied by a reduction in hardness, leaving the Grade 91 material in a so-called “soft” condition.

This article summarizes a case study for Grade 91 material in the “soft” condition, which was responsible for a steam leak after only 5 years of operation, illustrating how this material condition can result in forced shutdowns and safety hazards.  It is because of these consequences that it is recommended to have a Grade 91 life management program to understand if your plant may have such vulnerability.

This case study provides general background to the steam leak and describes the subsequent metallurgical evaluations performed to verify that mal-heat treatment of the Grade 91 steel was the root cause of the leak.  A follow-on article (the next issue, Volume 45, of News and Views) will provide additional insight into local stresses and analytical prediction of such failures, as well as highlighting key aspects of a Grade 91 life management program.  Suffice it to say if this plant had implemented such a program, the vulnerability of the affected spool would have been identified and mitigating actions could have been taken to avoid the leak.

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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 | 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.

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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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