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
Grade 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.
suction pipe-to-elbow stainless steel weld was identified in both units of a nuclear power plant as depicted in Figure 1.
Structural Integrity (SI) has significant depth and expertise in current pipeline safety regulations and dedicates substantial resources to ensure a comprehensive understanding of proposed pipeline safety regulations.
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.
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.
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.