ASTM/ASME A/SA387 Grade 91 is a modified 9Cr-1Mo steel composed of nitrogen, niobium and vanadium. Although the most common form of Grade 91 material is a plate form, the steel is incorporated into other ASME materials specifications for castings, forgings, fittings, pipes and tubes.
Applications of ASTM/ASME A/SA387 Grade 91 Material
The most common application of ASTM/ASME A/SA387 Grade 91 material is as a high temperature structure material in the fabrication of intermediate heat exchangers, steam generators, secondary piping of a liquid metal reactor that operates at around 550°C, and boiler components used in ultra-supercritical thermal power plants that operate at around 600°C.
The high creep strength of ASTM/ASME A/SA387 Grade 91 material makes it suitable for use in pressure vessels and in some interior structures of gas cooled reactors. The material is the subject of intense research in a boiler service. It demonstrates a distinct strength advantage at 566°C as opposed to 304H stainless steel and Grade 22 steel. This grade has a superior thermal fatigue and creep resistance over Grade 22 (2¼Cr-1Mo) steel, making it the material of choice for thick-section vessel applications.
Grade 91 has a better fireside corrosion resistance than Grade 22, but inferior to 300 series stainless steels. The variations in operating and service conditions between one section and another, for instance in a boiler plant, demand the use of different materials for some components for improved structural integrity of the plants. One of the ways to meet this demand is joining two or more materials by welding.
Research on Dissimilar Metal Welds
The School of Engineering at the University of Portsmouth along with Masteel UK Limited is working on the possibility of welding ASTM/ASME A/SA387 Grade 91 material to other steels. A group of researchers and students at the school are currently exploring the possibility of a dissimilar metal weld, such as Grade 91 and Duplex/Stainless steel welds. The group will study the process control and parameters in welding that will have an impact on the mechanical performances of components of a power plant.
The welding of Grade 91 to lower Chromium (Cr) content materials (Grade 22) resulted in the formation of a carbide depletion zone in the lower Cr content materials in the proximity of the fusion line of the dissimilar metal weld. This causes the reduced strength or overall performance of the weld. Hence, welding is then considered for two more comparable Cr content materials, such as Grade 91 and Duplex/stainless steel.
According to a research conducted by a group of Korean researchers, the severity of damage at the dissimilar metal welded joints (between Grade 91 and 316L stainless steel) was more when compared to similar metal welded joints (i.e., between 316L and 316L or Grade 91 and Grade 91). Hence, the school has proposed the study to gain insights into whether process control and parameters (before and after welding) significantly affect the final performance of the weld. The chemical compositions of specimen materials are listed in the following table:
Chemical Compositions of Specimen Materials
Foret. R, Zlamal.B, and Sopousek J. Structural Stability of Dissimilar Weld between Two Cr-Mo-V Steels. Supplement to Welding Journal, American Welding Society and Welding Research Council, October 2006.
Thomas A, Pathiraj.B, Veron. P. Feature tests on welded components at higher temperatures—Material performance and residual stress evaluation. Engineering Fracture Mechanics 74 (2007), pp. 969–979.
Hyeong-Yeon Lee, Se-Hwan Lee, Jong-Bum Kim, Jae-Han Lee. Creep–fatigue damage for a structure with dissimilar metal welds of modified 9Cr–1Mo steel and 316L stainless steel. International Journal of Fatigue (2007), Vol. 29, pp. 1868-1879.
Supporting material / External Links
- Elsevier - A provider of scientific, technical and medical information products and services.
- International Journal of Fatigue - Dedicated to providing information on a full range of scientific and technological issues associated with fatigue.
- ScienceDirect - Source of scientific, technical and medical full text research.
- International Journal of of Pressure Vessels and Piping - Publishes the latest research results and related information on all its associated aspects.
- Swindeman R.W. et al. Issues in replacing Cr-Mo steels and stainless steels with 9Cr-1Mo-V steel. International Journal of Pressure Vessels and Piping. 2004;507-512.
- Lee H-Y, et al. Creep-fatigue damage for a structure with dissimilar metal welds of modified 9Cr-1Mo steel and 316L stainless steel. International Journal of Fatigue. 2007;1868-1879.
- University of Portsmouth’s school of engineering
- Masteel UK Limited’s ASTM/ASME A/SA387 Grade 91
- Masteel UK Limited’s 316L stainless
- Dr Sarinova Simandjuntak’s profile
Masteel UK Limited would like to extend a special thanks to Dr Sarinova Simandjuntak for providing a tour of the facilities at the University of Portsmouth’s School of Engineering. Dr Sarinova Simandjuntak also provided excellent feedback on how the material the University purchased would be used for their research project and was an integral contributor to the publication of this article.
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