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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. |  |
 | Microhardness Measurement:
The result of micro hardness profile is in agreement with microstructure observations for carburized layer. The effective case depth could be observed upto 750 microns after that there are a drop in hardness. The results are depicted in following table with graphical illustration. |
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 exp erienced 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. |
| Aluminum-Magnesium Alloy Sheet Without Required Strength After Welding |
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Aluminum Magnesium alloy plates were received in laboratory for mechanical testing. Tensile test results were showing deviations with regard to UTS. It was decided to carry out root cause investigation for the tensile failure. Chemical analysis of the sample did not confirm to the requirements. In fact, within the same identification samples, it sowed compositional variations indicating inhomogenous material.
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 | SEM Analysis:
SEM analysis shows oxide deposits, which may be magnesium oxide and shrinkage like porosity on the fracture surface. |
| The cause of faiulre was formation of cracks at the time of welding in the partially melted zone of parent metal.
The cracks were formed because of local segregation of impurities like sodium, sulfur, chlorine, potassium, and calcium, which did not allow proper fusion to occur between weld and parent metal. This suggests that parent metal is having a poor weldability due to inherent presence of foreign impurities at local regions and inhomogenous base metal composition. |
| Duplex Tube Used in The Fabrication of Process Plant Reactor |
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After fabrication of a heat-exchanger, the helium test at 2.5 kg/cm2 for tube and tube sheet joints of the assembly was carried out. However, the test could not be completed as the leakage in 5 nos. of tubes at the bottom tube sheet side was observed. Subsequently pneumatic test at 5 kg/cm2 was done. Here also, 5 nos. of tube were found to be leaking. The five tubes with leakage were identified as T1 to T5. |
Color Metallography:
Color metallography was resorted to confirm the presence of deleterious phases like Chi, Laves and Sigma Phases.
Plate: 58 & 59 are the etched view for Chi and Laves phase respectively. The total percentage of inter-metallic phases is within the acceptable range (<0.5). |  |
 | Optical Microscopy (SEM):
The extensive optical metallography revealed that the discontinuity was formed due to lack of fusion on ID surface all along the weld line at different locations – even on those portions, which have not undergone expansion. And, the banded pattern of ferrite and austenite both in longitudinal and transverse direction is perhaps due to plastic deformation during the time of pilgering. |
| The premature failures of condenser tubes are due to localized groove formation at outer surface at baffle interface regions (Towards inlet side of cooling water).
The localized groove formation seems to be due to initial gap between tubes OD and baffle plate hole coupled with operational induced vibrations, which destroyed the protective film at local region, and further damage occurred due to cavitation-erosion. |
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