|
|
FAILURE INVESTIGATION AND ANALYSIS CASE STUDIES
|
| Primary Super Heater R-4 Zone Tube of a 140 MW Boiler | |
The MOC of tube is TU 15 CD 205 .The service life of tube is 7 years before failure.The steam temperature & pressure of tube are and 450°C and 140 kg/cm2 respectively. The tube has OD 63.5mm and ID 5.5mm. Tubes are located horizontally with flue gas passes vertically. |
 | 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:
Crack displayed inter-granular nature of propagation with many small parallel cracks adjacent to main crack is observed. This magnification was done at the TCR Engineering laboratory using a Leco Image Analyzer at 300X. |  |
| In present case the failure of tube seems to have occurred due to long term over-heating, above allowable design temperature, could be due to higher velocity of flue gas at this region or impingement of flue gases on tube surface facing flue gas or improper steam flow. |
| Inconel-600 Tubes of Salt Chest – Heat Exchanger | |
The basic function this shell and tube type salt chest heat exchanger is to increase the concentration of caustic soda. Normally when tube thinning takes place at the top, the heat exchanger is reversed and used for another three years. However, the failure took place just within three months of operation. The tube sheet plate and tubes are imported. It was informed that no stress relieving was done after fabrication. |
 | 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. |
| Condenser Tube of 500 MW Power Generator | |
The surface condenser unit is a single pressure, single-pass verticle heat-exchanger. Low pressure steam from the turbine enters into the shell through the exhaust opening located at the top. The librated air and non-condensable travel to the centrally located air removal section. The failure of tube has occurred in this air removal region. Copper colouration all over the surface is seen. There are black colour rust marks seen at several places. At this region, deeper groove and heavy corrosion is seen at baffle region. |  |
 | Scanning Electron Microscopy (SEM):
SEM analysis shows cavitation damage on the surface of tube inside the groove. |  |
Microstructure Examination:
The microstructure indicates general uneven corrosion. The surface alpha grains were deformed indicating the mechanical stress the surface has undergone. ID showed general corrosion. Here the effect of dezincification was more pronounced. The premature failures of condenser tubes are due to localized groove formation at outer surface at baffle interface regions |  |
| 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. |
| 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. |
| Bursting Problem for 204Cu Stainless Steel During Manufacturing |
|
A problem faced by SS manufacturing company whre SS 204Cu billets, it was observed that certain billets revealed bursts in transverse direction during the time of hot-rolling. The billets are continuously cast, cut to standard size. No visible cracks are seen on the cast billets. Neither any bursts are seen at rolling stage till 3rd stand. Only at 4th–6th stand about 15 to 20% of the material starts showing bursts in transverse direction. Problem is for a complete heat. In other words either the complete heat is O.K. or its starts showing defects. The bursting problem varies with different heat. |  |
 | Color Metallography:
The important clue derived from KOH etching, which indicated the solidification morphology of relative coarse dendritic structure, which could retained relative higher amount of delta ferrite within the austenitic dendritic arm. The impurities like S and P would be entrapped into the ferrite and would help to improve hot ductility behavior, which is responsible for the good performance of heat. |
Reason for Bursting:
The basic mechanism responsible for hot ductility problem is (1) Formation of low melting constitutes, which when melted at the processing temperature would tend to crack the material at high temperature, (2) Excessively coarse austenitic grains, which has lower creep strength, and (3) Excessive mechanical stresses of rolling. |  |
|
|
