CASE STUDIES

     ABSTRACT

Engineering components are designed to perform a particular function for a certain minimum period of your time . Failures therefore, means its inability to perform its intended function. In spite of best effort, thanks to inherent uncertainty in design parameters and material behavior it's certainly impossible to avoid failure. a correct analysis of such case provides valuable insight into the mechanics of the method, identify the factors responsible for failure and suggest necessary steps to be undertaken to beat  such incidents in future. Such studies have led to several innovations in design,
development of recent materials and process. The paper presents a broad overview of the tools and techniques used for such analysis. 

 The best way to illustrate how failure analysis should be carried out is to go

through a four case studies

• Bulging of wire rods (processing failure)

• Fatigue failure of pulverizer shaft of a bowl mill

• Development of cracks in Horton Sphere

• Failure of boiler tubes

    Bulging of wire rods (processing failure) -: High carbon steel wire rods of a particular size (5.5 mm dia) made from 110 x110 mm CC billets developed bulging at several isolated locations. Wires made
from wire rods of all sizes were frequently failing by rupture during wire drawing process. Initial microstructural examination and EDS analysis indicated that the steel was dirty and failure was because of low melting inclusions having Si, Al, Cl because the major elements. However, examination of freshly fractured surfaces revealed that the steel was fairly clean but contained countless cavities and pores of assorted sizes.  All indirect evidences showed that the high pressure of trapped hydrogen gas in cavities did not allow them to collapse and get welded during hot rolling. A simple model calculation showed that prime pressure of trapped hydrogen gas may end up in bulging in wire rods of 5.5 mm dia and not in other diameter - higher or lower than 5.5 mm dia. On the basis of these findings it was suggested that hot rolling temperature should be increased from 1050°C to 1150°C so that the gases could diffuse out. Once this was implemented the problem disappeared.

    Fatigue failure of pulverizer shaft of a bowl mill -:  This was made of the En-24 grade of steel. A preliminary examination showed that failure was due to fatigue and the crack initiated from a keyhole. Microstructural examination revealed the presence of sulfide inclusions. Steels having such inclusions are likely to have poor resistance to crack growth. Therefore initially the reason was thought to be due to these inclusions. Mechanical testing also showed that toughness was indeed very poor. Some of the test pieces got suitable heat treatment to seek out whether the optimum combination of strength and toughness may well be developed. It was found that toughness can be improved several folds by annealing. Rough stress analysis showed that the necessity of high strength isn't so critical. It was concluded that premature failure was mainly due to improper heat treatment and not due to inclusions. As expected properly annealed pulverizer shafts subsequently was found to last much longer.

    Development of cracks in Horton Sphere -: These are large spherical pressure vessels typically used for the storage of LPG. These are made of welded steel plates. The case investigated had developed long shallow cracks along the weldment and a few short deep cracks close to the heat-affected zone. Whilst the previous may well be removed by grinding, the latter was of great concern. Examinations showed that the microstructure near the deep crack was acicular (martensite) which could have formed as a result of arc strike during welding. A simulated test was devised to generate similar cracks near the weldment. This showed that innumerable sites were there where such cracks could nucleate in future.  To overcome such problems in future post-weld heat treatment was recommended even though it's not mandatory for vessels having but 25 mm thickness

    Bulging of wire rods (processing failure) -: Boiler tubes are subjected to high pressure at elevated temperature. These are mostly made of either plain carbon or low alloy steel. At high temperature steel
creeps. It elongates with time even if the stress is significantly lower than its yield strength. Failure occurs when the accumulated strain reaches the creep ductility. Therefore major consideration for the designer is to see that (i) creep strain that accumulates over the design life is acceptable (ii) accumulated strain is lower than creep ductility (iii) design life is significantly lower than the time to failure.

        The way creeping materials fail can often give us information about the conditions under which failure took place. There are three basic mechanism of fracture. Intergranular fracture takes place at low stresses. It is associated with thick lip rupture and crack originates from the outer surface. Transgranular fracture takes place at higher stresses. Tensile ductility and reduction in area near rupture is quite high.
At higher temperature and stresses dynamic recrystallization operates leading to tensile rupture. The case studies presented would highlight how to establish the conditions under which failure has taken place and also suggest how to plan a simulated test in the laboratory to reproduce such failure.

        CONCLUDING REMARKS -: The consequence of failure can be tragic and expensive. There are innumerable cases of engineering disasters leading to loss of life and property. They are certainly
unwelcome. However, for an engineer, it is a source of learning. Engineers learn their most important lessons when things go wrong. Failure often provides an experimental test of realism which is rarely possible to achieve in the laboratory or on the computer. Therefore, utmost care should be taken while conducting a failure analysis to preserve and not destroy any evidence. It is in fact a learning process. A few of the causes presented would reveal that often the most obvious thing is not the real cases of failure. A failure analyst must have an open mind and be able to examine and evaluate the views of others involved in the work. The report should be convincing and easily understood and acceptable by those involved in its design/fabrication /operation, only then the recommendation is likely