Indranil Chattoraj

An investigation of the failure of low pressure steam turbine blades

Abstract An analysis of the failure of LP turbine blades of a 210 MW thermal power plant has been presented in this paper. The blade material is of 12% Cr steel with tempered martensitic microstructure. Microstructural analysis as well as hardness and tensile tests did not indicate any degradation in terms of microstructure and mechanical properties. Physical discontinuities were observed in the braze joint which might have been formed due to improper brazing operation. Failure of the brazed joints between the blade and lacing rod was found to be due to improper brazing operations and corrosion effects during service. Fractographic evidence showed that the cracks were initiated from various points on the blade surface, which were at the interface with the lacing rod. Striations and beach marks were also observed which indicated the occurrence of high cyclic loading on the blades. Frequency data obtained from plant indicated the possibility of excessive vibration generated due to fluctuation in grid frequency during operation. Thus, the situation was aggravated due to a resonant condition of vibration, facilitating the propagation of cracks which were initiated earlier.

Pitting Enhanced Fatigue in 7075 Aluminium Alloy

Pitting corrosion of two tempers (T6 and T73) of 7075 Aluminium alloy were quantified by digital image processing. The effect of pitting corrosion on fatigue life in 7075 Aluminium alloy in two aging conditions was studied. Pitting was found to be slightly more severe for T6 tempers compared to T73 tempers for different time of exposures. The fatigue lives of two tempers of 7075 Aluminium alloy at high peak stresses were compared in the uncorroded and pre-corroded (pitted) states. Absolute fatigue lives of T73 samples were much higher than that of T6 in the uncorroded as well as pre-corroded condition. The various crack initiation methodologies observed through fractographic analysis is discussed.

Stress Corrosion Cracking of Duplex Stainless Steels

The relative superiority of duplex stainless steels (DSS) over austenitic grades with regards to stress corrosion cracking (SCC) is discussed. The benefits of N to SCC resistance of DSS are provided. The selective dissolution of phases and its impact on corrosion SCC is reviewed. The hydrogen embrittlement of DSS is reviewed with emphasis on the ferrite participation, the role of environments and fracture morphology. The evolution of secondary phases and precipitates and the resultant change in corrosion resistance and SCC in DSS is discussed.

Erratum to: Intergranular Corrosion Behavior of 304LN Stainless Steel Heat Treated at 623 K (350 °C)

RAGHUVIR SINGH, Principal Scientist, MUKESH KUMAR, Project Staff, MAINAK GHOSH, Senior Scientist, GAUTAM DAS, Sr. Principal Scientist, and I. CHATTORAJ, Chief Scientist, are with the CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. Contact e-mail: raghujog@yahoo.co.in; rsr@nmlindia.org P. K. SINGH, Scientific Officer G, is with Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai, India. The online version of the original article can be found under doi:10.1007/s11661-012-1519-4. Article published online April 9, 2013

Study on Sensitization Susceptibility and Texture of Cold Rolled AISI 304LN Stainless Steel

Attempt has been made to correlate the cold rolling (CR) texture with the degree of sensitization (DOS) in rolled 304LN after ageing at 500-600oC for various durations. The susceptibility of sensitization is observed to increase with %CR cold rolling from 5-25% and aged at 500-600oC. No one to one correlation between texture and DOS is observed. Brass component, overall, seems better for lower DOS while Goss component led to relatively higher DOS values. It is observed that the ratio of Brass to Copper may have a critical value for lower DOS.

AN INVESTIGATION OF THE FAILURE OF LOW PRESSURE STEAM TURBINE BLADES

An analysis of the failure of LP turbine blades of a 210 MW thermal power plant has been presented in this paper. The blade material is of 12% Cr steel with tempered martensitic microstructure. Microstructural analysis as well as hardness and tensile tests did not indicate any degradation in terms of microstructure and mechanical properties. Physical discontinuities were observed in the braze joint which might have been formed due to improper brazing operation. Failure of the brazed joints between the blade and lacing rod was found to be due to improper brazing operations and corrosion effects during service. Fractographic evidence showed that the cracks were initiated from various points on the blade surface, which were at the interface with the lacing rod. Striations and beach marks were also observed which indicated the occurrence of high cyclic loading on the blades. Frequency data obtained from plant indicated the possibility of excessive vibration generated due to fluctuation in grid frequency during operation. Thus, the situation was aggravated due to a resonant condition of vibration, facilitating the propagation of cracks which were initiated earlier.