Dipasri Bhattacharya

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.

Premature fatigue failure of a spring due to quench cracks

The premature fatigue failure of a compression spring during service has been analysed. Optical and scanning electron microscopy, hardness and tensile testing and X-ray diffraction techniques were employed. Quench cracks, presumably due to improper quenching, were observed across the cross-section of the spring. It is established theoretically that the diameter of the spring rod was smaller than the critical diameter for this composition of steel. X-ray diffraction showed the presence of a small amount of retained austenite (average grain size of 2 μm).

Failure analysis of coal pulverizer mill shaft

Failure of a ball and race type coal pulverizer mill shaft has been analysed. It was found that the shaft, made of EN 25 steel, failed by fatigue. The fatigue cracks originated from the keyway area, unusually from the top edge. The presence of elongated manganese sulphide inclusions due to improper heat treatment had reduced the ductility and CVN toughness of the material, and thus made the material more prone to failure.

Failure analysis of counter shafts of a centrifugal pump

An analysis of the premature failure of two counter shafts used in centrifugal pumps for lifting slurry has been carried out. Chemical analysis, microstructural characterisation, fractography, hardness measurement, tensile and Charpy impact tests were used for the analysis. The chemical compositions for the shafts were as per recommendation. The microstructure of one of the shafts was ferritic–pearlitic and its mechanical properties were inferior to the recommended values. For the other shaft the microstructure was tempered bainite; although the impact energy satisfied the specification, the other properties (hardness, UTS) were inferior. It was concluded that the improper heat treatment was the prime cause for the premature failure of the shafts.

Failure analysis of high temperature studs

Studs in the interceptor valve of a 110 MW unit failed after a service life of 148,700 h. The studs were operated under a steam pressure of 35 kg/cm2 and a temperature of 535°C. The studs were fractured at one end of the threaded end. Various techniques were employed to analyse the failure of the studs. It has been concluded that the failure of the studs was due to reverse temper embrittlement. The failure was delayed due to the presence of Mo and V. To reduce the tendency to this kind of failure, the following steps were recommended: (a) reduce the phosphorus content in the steel to a low level or (b) reduce the grain size to about 10 μm.

Studies on indentation fracture toughness on ceramic and ceramic composite using acoustic emission technique

This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SiCw) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.

Failure analysis of a weld repaired steam turbine casing

The present study was aimed at analysing the failure of a weld repaired turbine casing after 30 years of total service including 5 years after weld repair. The casing was weld repaired by a high Cr–Ni weld metal (24Cr–32Ni–4Mn–Fe) . The base metal, a low alloy ferritic steel (1Cr–0.5 Mo steel) with ferrite–pearlite structure did not show any abnormality to indicate significant degradation. Fracture surface showed voids and microcracks. The cracks might have initiated from the voids present in the weld region possibly by thermal fatigue mechanism. The high level of thermal stress appears to be operative due to thicker section of the casing and also due to high mismatch of thermal expansion coefficients of the base metal and the weld metal. Dilution of alloying elements took place and δ-ferrite was observed at the grain boundaries as predicted by the Schaeffler diagram. The δ-ferrite was transformed to σ-phase and alloy carbides during high temperature service (500°C) . The propagation of cracks generated by thermal fatigue was facilitated by the formation of embrittled σ-phase at the austenite grain boundary and this ultimately led to an early failure of the casing.

Remaining life estimation of a service exposed economiser tube

A service exposed (80,000 hr) economiser tube of a 60 MW boiler of a thermal power plant was investigated for the detailed metallurgical assessment necessary for remaining life estimation. The investigation included hot tensile tests, hardness measurement, microscopy and thickness measurement. Analysis revealed that there was no degradation of mechanical properties and microstructural features. The reduction in tube thickness was primarily due to oxidation and corrosion/erosion processes. Taking into account all the experimental data and mechanics aspects of the tube an estimated remaining life of 20,000 hrs was calculated and thus recommended for further use.

Fatigue failure of helical compression spring in coke oven batteries

The failure of a helical compression spring employed in coke oven batteries was analysed. Microstructural analysis and hardness measurements did not show any degradation of the spring material. Surface corrosion product was analysed by XRD and SEM–EDS. Sulphur and chlorine bearing compounds were detected. Macrofractography of the fracture surface revealed beach marks, indicating fatigue as the mode of fracture and surface pits. It was established that the spring failed due to corrosion fatigue.

Ultrasound versus fluoroscopy-guided caudal epidural steroid injection for the treatment of chronic low back pain with radiculopathy: A randomised, controlled clinical trial

Background and Aims: Caudal epidural steroid administration is an effective treatment for chronic low back pain (LBP). Fluoroscopy guidance is the gold standard for pain procedures. Ultrasound guidance is recently being used in pain clinic procedures. We compared the fluoroscopy guidance and ultrasound guidance for caudal epidural steroid injection with respect to the time needed for correct placement of the needle and clinical effectiveness in patients with chronic LBP. Methods: Fifty patients with chronic LBP with radiculopathy, not responding to conventional medical management, were randomly allocated to receive injection depot methyl prednisolone (40 mg) through caudal route either using ultrasound guidance (Group U, n = 25) or fluoroscopy guidance (Group F, n = 25). Pre-procedural visual analogue scale (VAS) score and Oswestry Disability Index (ODI) were noted. During the procedure, the time needed for correct placement of needle was observed. Adverse events, if any, were also noted. All patients were followed up for next 2 months to evaluate Visual Analogue Scale (VAS) score and ODI at the 2nd week and again at the end of 1st and 2nd month. Results: The needle-placement time was less using ultrasound guidance as compared to fluoroscopy guidance (119 ± 7.66 vs. 222.28 ± 29.65 s, respectively,P< 0.001). Significant reduction in VAS score and ODI (clinical improvement) was noted in the follow-up time points and comparable between the groups at all time points. Conclusion: Ultrasound guidance can be a safe alternative tool for achieving faster needle placement in caudal epidural space. Clinical effectiveness (reduction of VAS and ODI scores) remains comparable between both the techniques.