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When Fire Destroys

In our industry, fire is most often used to refine, purify or harden. The key is control and using temperatures appropriate for the specific processes. When fire is out of control, however, destruction ensues. Such was the case when fire destroyed a large home in rural Illinois this past winter. A study of the metal gusset plates in the rafters gives us some idea just how hot a housr fire gets and what it can do to the metal. Before discussing the metallurgical detail, however, looking at the cause of the fire may prevent another in the future.

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Visual Examination and Light Microscopy

THE VISUAL EXAMINATION of fractures is deeply rooted in the history of metals production and usage, as discussed in the article “History of Fractography” in this Volume. This important subject, referred to as macrofractography, or the examination of fracture surfaces with the unaided human eye or at low magnifications (–<50), is the cornerstone of failure analysis. In addition, a number of quality control procedures rely on visual fracture examinations. For failure analysis, visual inspection is performed to gain an overall understanding of the fracture, to determine the fracture sequence, to locate the fracture origin or origins, and to detect any macroscopic features relevant to fracture initiation or propagation. For quality control purposes, the fracture features are correlated to processing variables. In this article, examples of visual fracture examination will be given to illustrate the procedure as it applies to failure analysis and quality determination.

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Use of Light Microscopy in Failure Analysis

The light microscope is one of the most important tools used by metallurgists in failure analysis. Development of electron metallographic instruments, such as the scanning electron microscope, has not diminished the importance of light microscopy. Basically, the light microscope is used to define the relationship of the fracture path to microstructural features and to assess the nature of the microstructure and its influence on the failure mechanism. Microscopic examination is used to detect abnormalities due to material quality, fabrication, heat treatment and service conditions. Proper implementation of light microscopy is of critical importance in failure analysis.

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Use of Failure Analysis in Materials Selection

Materials selection for parts or components usually occurs under two conditions. The first is when a new part or component is designed while the second is when an existing part or component needs to be upgraded. Upgrading may be needed due to a variety of reasons. If the part or component is relatively new, upgrading may beneeded if part life is marginal or inadequate. Or, a competitor’s newly improved part is performing better and sales of your part are droping. Or operating conditions may be changed and your part is no longer performing acceptably. Of course, it is possible that parts have failed and the failure study recommends upgrading the material being used, perhaps also with other suggestions regarding the design and manufacturing process.

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Microstructural Analysis of Steel from the World Trade Center Buildings 7, and 1 or 2

Important Observations by FEMA Concerning WTC 71.

  1. The collapse of WTC 7 is of significant interest because it appears that the collapse was due primarily to fire, rather than any impact damage from the collapsing WTC 1 tower. There is no history or record of a fire induced collapse in a fire protected steel building prior to this event.
  2. Collapse is consistent with an initial failure that occurredinternally in the lower floors toward the east side of the building. Fire ignition likely started as a result of falling debris from WTC 1 damaging the south face of WTC 7 (Floors 6,7, 8, 10, 11, 19). The building main structural support members are located in the 5th to the 7th floors.
  3. The fire progressed throughout the day, virtually unimpeded by manual or automatic fire suppression systems. The fire got hotter as time progressed. There is no physical, photographic or other evidence to either substantiate or refute the discharge of fuel oil from the pipingsystem. “Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis has only a low probability of occurrence”

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Metallography and Failure Analysis

Catastrophic failures of all welded T-2 Tankers and Liberty Ships in WW2 focused attention on the field of failure analysis and led to the development of fractography and fracture mechanics. Bridge failures have also focused attention on fractography, fracture mechanics and failure analysis.

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Metallographic Techniques in Failure Analysis

METALLOGRAPHIC EXAMINATION is one of the most important procedures used by metallurgists in failure analysis. Development of powerful electron metallographic instruments, such as the scanning electron microscope, has not diminished the importance of light microscopy. Basically, the light microscope is used to
assess the nature of the microstructure and its influence on the failure mechanism. The purpose in using the light microscope may be twofold. One purpose may be to determine the relationship between the microstructure and the crack path (in failures involving fracture) and/or the nature of corrosion or wear damage. The second purpose is to determine whether processing or service conditions have produced undesirable microstructural conditions that have contributed to the failure, such as abnormalities due to material quality, fabrication, heat treatment, and service conditions. Examples are given in this
article to demonstrate such analytical work.

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Metallographic Examination of Heavily Eroded Structural Steel from World Trade Center Buildings 1, 2 and 7

Steel samples from Buildings 1, 2 and 7 of the World Trade Center were collected during the Federal Emergency Management Agency forensic investigation shortly after the September 11, 2001 incident. Macroscopically, the steel samples supplied exhibited severe “erosion” with plate thickness varying from 12.7mm to a total loss of metal in many areas. Also, some localized plastic deformation was observed. A determination of the cause of this unexpected erosion and an estimate of the maximum temperature that this steel likely experienced are the subjects of this paper.

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