Development of New Heat Treatment Method to Impart High Creep Strength and High Toughness to Rotor Material for Condensing Steam Turbine

Abstract

Higher temperatures in the HP section of the condensing Turbine requires rotor with high creep strength while low temperatures in the LP section require the rotor to have high toughness. The design basis for current rotor material, Ni-1.25Cr-Mo-V forged steel was high toughness at low temperatures. This led to rotor having insufficient creep strength in very high temperature region around control stage which limited the maximum allowable temperature of inlet steam thereby limiting the efficiency of the Turbine. This paper highlights the development of new heat treatment method to improve Ni-2.25Cr-Mo-V forged steel (10325MTE) as rotor material. To improve the mechanical properties of the material, heat treatment simulations were performed. A series of tests were done on the rotor. These tests evaluated the creep strength at high temperatures and toughness low temperatures. SCC (Stress Corrosion Cracking) susceptibility was evaluated in an accumulated corrosive environment using SSRT (Slow Strain Rate Test).