Influence of Chemical Composition and Thermo-Mechanical Processing on the Formation of Delta Ferrite in 10.7CrMoVNbN Steel

M/S Toshiba JSW Turbine & Generator Pvt. Ltd. reported the failure of 10.7CrMoVNbN steel turbine blades used in its power plant for the production of electricity. The basic reason for its failure was the higher content of delta ferrite. The objective of the present study is to reduce the failure of 10.7CrMoVNbN steel caused by delta-ferrite inclusion. 10.7CrMoVNbN steel is used for the fabrication of steam turbine blades by M/S Toshiba JSW Turbine & Generator Pvt. Ltd. Four Heats with chemical composition as well as thermo-mechanical processing and heat treatment were prepared. The main variation in all Heats was the variation in Creq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Cr}}_{\text{eq}}$$\end{document}. Samples were prepared as per standard metallographic technique. Samples etched with kalling’s-1 agent revealed a tempered martensite morphology. The morphology of tempered martensite got refined with the reduction in Creq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Cr}}_{\text{eq}}$$\end{document}. Further, the samples when etched with sulfuric acid etchant revealed the delta-ferrite morphology. Delta ferrite decreased with a decrease in Creq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Cr}}_{\text{eq}}$$\end{document}. FESEM-EDS analysis revealed the presence of carbides and various phases in the steel. Further, to confirm the presence of different phases in the steels, microhardness analysis was performed. Microhardness analysis revealed the presence of tempered martensite matrix, different carbides, and delta ferrite. Further, the decrease in Creq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Cr}}_{\text{eq}}$$\end{document} led to a marginal increase in yield strength, ultimate tensile strength, whereas a significant increase in impact strength was observed. Overall, Heat 4 with Creq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Cr}}_{\text{eq}}$$\end{document} of 11.59 showed the maximum improvement in the mechanical properties. Further, the δ-ferrite content of Heat 4 was 0.3% which was lower than the permissible limit of 2%. Hence, Heat 4 fulfilled the material specifications specified by Toshiba and reduced the material failure caused by δ-ferrite inclusions.