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FAILURE INVESTIGATION AND ANALYSIS CASE STUDIES
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| Crown Wheel (Bevel Gear) of A Rotary Trailer |
 Premature failures were reported in the crown wheel and pinion of a rotary trailer used for agricultural purpose. The crown wheel and pinion is a part of gear box assembly and rotates at 540 RPM. Due to service demands of rotary trailer it is expected to under go shock loads. The gears are manufactured by forging, normalizing, machining and carburizing to a depth of 1.5 mm case from EN-36C material. |
Fractography:
Fracture surface reveled presence of coarse retched marks with predominately brittle type of failure. At initial stage of crack propagation beach marks are present, which suggest fatigue cracks and the mode of fracture is initiated by fatigue. |  |
 | Microstructure Examination:
Extensive microstructure examination reveled presence of fatigue cracks in the plate of maximum stress below the outer edge of teeth. Carburized layer displayed fine tempered martensite structure. However, basic structure of gear reveled coarse under tempered martensite structure & which is not desirable. Such microstructure have poor “Shock Resistance” property & lead to failure under shock loads. |
| Based on above summary & failure mechanism, it is evidentiary that pitting due to wear & initiation of fatigue cracks due to lower case of hardened depth are responsible factor for initiation of damage. The rotary trailer is expected undergo shock loads, which could not be taken by poor microstructure at core & failed under brittle mode from prior austenitic grain boundaries. Coarse under tampered martensite structure has inherent residual stresses, which will be released under shock load & give micro-cracks & lead to premature failure. |
| Radiant Coil of a Cracker Furnace H-130 Refinery | |
In a bottom fired furnace tube failure have experienced service of 14 months against the normal life of 6 to 7 yrs. MOC of tube is 25 Cr/35 Ni. The average tube metal temperature remains between 1000 to 1100 ºC temperatures. As per the manufacturer data, these tubes are designed for 1150ºC. The pressure inside the tube is 1 kg/cm2 g. |
 | Scanning Electron Microscopy (SEM):
Upon SEM examination conducted by engineers at TCR, it revealed presence of inter-granular cracks and presence of numerous creep cavities at grain boundary. Presence of micro-cracks are observed more towards outer surface and near by crack region. Severity of cracks and cavity reduces when we move away from the main crack. |
Microstructure Examination:
The crack is associated with carburizing more so at outer surface with decreasing the depth of carburizing toward ID. Another important evidence of crack originating outer diameter and progressing towards ID. This magnification was done at the TCR Engineering laboratory using a Leco Image Analyzer at 300X. |  |
| In present case the failure of tube has occurred due to localized overheating, which reduced ductility and failed under operational vibrations. TCR recommends looking in to the possibility of development of high temperature at the time of decoking operation. |
| 8th Stage Blade of a Steam Turbine | |
After 8-years of useful service life, a steam turbine was reported to have been working with abnormal vibrations. When turbine was opened five blades of 8th stage were found in broken condition from the root. Steam turbine operates with steam temperature of 770°F & working pressure at 568.3 Psi. |  |
 | Scanning Electron Microscopy (SEM):
Fracture surface kept under SEM show multiple origins of the fracture and clearly shows progressive mode of failure. Fig. suggest rubbing of the metal surface where the failure had occurred. |
Microstructure Examination:
Microstructure on cross section of blade and showing the defect of deformation. At higher magnification crack shows branching nature progressing in the forwarded direction i.e. perpendicular to the central axis seems to have followed trans-granular path. |  |
| Failure of 8th stage blade has occurred due to corrosion fatigue, initiated at most stressed area. Only one blade was submitted for investigation. It is difficult to pin point which blade failed first. |
| Sac Plant Piping Going to V-801 |
In a Sulphuric acid concentration plant, as a part of process, condensate is chilled in a heat exchanger. The line, which is connected from heat exchanger (E08-3) to vacuum pump, one elbow was reported to have leaked and needed replacement.
Severe corrosion was reported inside the replaced pipeline within 10 days of operation. The extent of corrosion was so severe that entire replaced pipeline reduced to paper thickness with punctures. The pipeline is operating with 1 to 2% H2SO4, 0.5% HNO2 and 0.6 to 1.0 % HNO3 at 10 to 20°C temperatures. |
 | Low Magnification Examination:
Low magnification examination was done by the failure Analysis and Investigation team from TCR Engineering to find out the corrosion characteristics. Internal surface of pipe, weld and elbow showed severe corrosion on pipe. The close-up view of corroded surface inside the pipe show effect of general corrosion and flow pattern. Leakages observed in the form of openings between weld and pipe. |
Microstructure Examination:
Uniform dissolution at ID is observed under microstructure examination at a magnification of 300x at the TCR Engineering laboratory in India. |  |
| The fluctuation in Nitric acid concentration did not allow to stabilize passivity on newly fabricated pipeline resulted into severe corrosion |
| Integral Pinion Shaft of a Cement Mill | |
Premature failure of integral pinion shaft was reported a cement mill. The shaft failed after service life of approximately 15,000 hours (625 days) against intended design life of 30 years. The shaft is made from EN 10083-1 (1991) 30CrNiMo8 with through hardened and tempered to achieve 310-335 BHN. The shaft rotates at 133 to 134 RPM. The failure of the shaft noticed in form of cracks. Cracks were observed at 45° to the longitudinal axis of shaft. |
Low Magnification Examination:
Fracture surface at thread region shows relatively flat fracture whereas further fracture shows brittle nature with chevron marks. Fracture surface below thread region at keyway disclosed multiple ridges with relatively coarse fatigue striations. |  |
 | Scanning Electron Microscopy (SEM):
SEM done by TCR Engineering reveal inter-granular fracture with inter-granular cracks. A fracture is brittle and shows inter-granular mode. Presence of fine cracks is observed. |
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