Progressive Deformation Modes of G91 Steel Subject to Stress–Strain Cycles: Viscoplastic Simulation Towards Developing High-Temperature Design Rules

• Published in: Transactions of the Indian National Academy of Engineering (Online) Vol. 7; no. 2; pp. 463 - 474
• Main Authors: Chellapandi, P., Rao, C. Lakshamana
• Format: Journal Article
• Language: English
• Published: Singapore Springer Nature Singapore 01.06.2022
• Subjects: Engineering; Original Article; Assessment of Bree and New Efficiency Index diagrams; Ratcheting design rules; Simulation of cyclic deformation modes; Constitutive model for G91
• ISSN: 2662-5415; 2662-5423
• DOI: 10.1007/s41403-021-00309-9

Development of ratcheting design rules for the modified 9Cr–1Mo steel (G91) is the international effort of designers and code developers for the Generation IV and fusion reactors. This is because the ratcheting design rules specified in the ASME and RCC-MR are originally developed focusing on austenitic stainless steels which harden under both monotonic and cyclic loadings, quite different from G91. In the case of G91, the number of load cycles to attain a saturation condition is very high and, moreover, never gets saturated in some specific cases. While the ASME ratcheting design rules (Bree and O’Donnell–Porowski diagrams), were established basically from the analytical routes, RCC-MR (efficiency index diagram) adopts experimental route. Both the codes conceived robust benchmarks, viz. (1) pressurized elasto-plastic thin shell subject to cyclic thru-the-wall temperature gradients by ASME and (2) tension–torsion bars by RCC-MR. In the present paper, a numerical simulation of ratcheting has been reported on these two benchmarks, but employing a new ‘20-Parameter Chaboche Model’ for G91. The prediction ability of the model on uniaxial material properties that govern the ratcheting phenomenon are demonstrated. Further, various cyclic deformation modes as depicted in Bree diagram are simulated, followed by prediction of the accumulated ratcheting strains long the ratcheting boundary. Based on several data derived from numerical simulations at various temperatures, a typical O’Donnell–Porowski diagram is plotted. Besides, the numerically simulated points along with a few more data generated from the tension–torsion tests are depicted in a new efficiency diagram proposed by CEA, France. It is concluded that the Bree diagram can be applied for the base metal portions of components to protect against ratcheting. Further R&D required to improve the proposed efficiency index diagram for the inclusion in RCC-MRx code, with adequate conservatism.