News & Views, Volume 44 | Example Grade 91 High Energy Piping DMW Joint Stress and Metallurgical Analysis
By: Ben Ruchte
Determining 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.
While the 2018 Spring outage season is mostly behind us, we all know a key element in being able to provide safe, reliable, clean and economic power to energy consumers is how successfully plant outages are accomplished. 
A March 16, 2017, advisory bulletin (Docket No. PHMSA-2016-0131 – “Pipeline Safety: Deactivation of Threats”) gave guidance on the deactivation of pipeline threats, including the threat of internal corrosion.
A 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).
Structural Integrity and GP Strategies recently announced an agreement to bring SI’s technology for calculating, tracking, and trending life consumption of piping and boiler components to GP Strategies EtaPRO real-time monitoring platform (Press release here).
Nuclear plant workers accrue most of their radiation exposure during refueling outages, when many plant systems are opened for corrective and preventive maintenance. The total refueling outage radiation exposure can be 100-200 person-Rem at a typical Boiling Water Reactor (BWR), and 30-100 person-Rem at a typical Pressurized Water Reactor (PWR). Accrued refueling outage radiation exposure values can be significantly greater than these values depending upon radiation fields, outage work scope, and emergent work. Outage radiation exposure is one metric used by a plant to determine outage success and by industry regulators in assessing the overall performance of a plant. Plants with high personnel radiation exposure tend to be those plants with more equipment problems and more unscheduled shutdowns; consequently, they may be subjected to increased regulatory oversight. 
Things change, that’s just a fact of life. But when it comes to engineering codes and standards, change can be confusing, frustrating and expensive. As it relates to seismic design and certification of equipment, it is beneficial to understand the impact of code changes early to begin incorporating requirements in new equipment design, product updates and in the certification process.
Structural Integrity 