Becht Engineering Blog

In this section of the site contributing authors submit interesting articles relating to the various services, industries and research & development efforts of Becht Engineering.

Grade 91 Steel - How Did We Get Here? Part 1

grade91_cover-image
This blog is the first part of a 3-blog series. To view the rest of the story click HERE for Part 2 and Here for Part 3 . Part 1: History Thirty years ago, Grade 91 (9Cr-1Mo-V) steel was hailed as the savior of the power generation industry [1]; now it’s behavior has been described as too variable to ensure safe operation [2].  What happened?  At the same time Grade 91 was being developed in the late 1970’s for high temperature nuclear reactor application [3], power plants that had been designed and operated as base-loaded were suddenly cycled on a regular basis.  The standard material for high temperature steam outlet headers was first 1¼Cr–½Mo (Grade 11) and later 2¼Cr–1Mo (Grade 22); in both cases headers rapidly began to experience severe cracking in and between header penetrations.  The cracking was termed “ligament cracking” [4] and by the mid-1980’s had become a...
Continue reading
3
  197 Hits
  0 Comments
197 Hits
0 Comments

Grade 91 Steel - How Did We Get Here? Part 2

grade91_cover-image
Read Part 1 Part 2: Type IV Cracking and Inspectability The current concerns with Grade 91 are fundamentally and firmly rooted in inspectability of Type IV damage; while sensitivity of the material can be managed (see for example [1]), Type IV cracking is perhaps the Achilles heel of Grade 91.  At a high level, the thermal cycle(s) due to welding will create a thin band of material in the heat affected zone (HAZ) with properties much closer to Grade 9 than Grade 91.  While full re-normalization and tempering of the entire component after welding can greatly improve the situation, simple (subcritical) post-weld heat treatment (PWHT) does not.  Damage is overwhelmingly concentrated in this thin band of material during high temperature operation, such that when failure finally occurs, it has an almost brittle appearance since there has been little if any creep deformation or damage outside of the HAZ (see Figure...
Continue reading
3
  89 Hits
  0 Comments
89 Hits
0 Comments

Grade 91 Steel - How Did We Get Here? Part 3

grade91_cover-image
Read Part 1 or Read Part 2 Part 3: Allowable Stresses While inspectability has been at the center of recent concerns, other issues such as chemistry control, ductility and excessive oxidation have also come under scrutiny.  Perhaps most interesting is that while all of the preceding concerns have been debated, it has also been found that existing ASME allowable stresses for Grade 91 are not conservative relative to newer data techniques.  This is beautifully illustrated in Figure 5 from [2], where the continual drop in allowable stress is plotted as more and more short term test data is removed from the analysis.  The good news is that “new” material is not worse than older material (at least from a statistical and data analysis perspective).  The bad news, as discussed in [2] and can be seen in Figure 6, is that even more modern techniques like “region-splitting” do not capture the...
Continue reading
2
  77 Hits
  0 Comments
77 Hits
0 Comments