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Metallurgical and Corrosion Failures
There are numerous ways in which a component may fail in service. Often, failure involves fracture. There are six recognized failure modes of metallic materials, as follows:
- Ductile overload
- Brittle failure
- Fatigue
- Corrosion
- Erosion
- Wear
A failure analysis investigation starts by gathering background information. This includes material, manufacturing process, circumstances surrounding the incident, review of engineering drawings, material specifications, previous problems, review of maintenance and inspection reports. In metallurgical and corrosion failure investigations it is helpful to have an intact component that has never been used/placed in service, called “exemplar”, for comparative examination.
When possible, we always conduct a site examination to document the evidence in situ, to observe its interaction with the surrounding equipment/media/environment, to decide which part/section of the evidence needs to be further examined in a laboratory setting. We always suggest that other interested parties be allowed to conduct a joint site examination with us before the evidence is removed. In cases where the removal of samples is not permitted, or it is not possible, we obtain accurate replicas of the fracture surface which can be used later for fracture surface examination.
Following the notification of all interested parties and following the preparation of a Testing Protocol agreed upon by all the experts, a joint destructive examination can commence. A metallurgical and corrosion failure analysis investigation continues by examining the fracture surface under a stereo and an optical microscope followed by examination under a more powerful scanning electron microscope. There are relevant features on fracture surfaces which are not visible but at higher magnifications (i.e., x200 to x6000 or more, depending on the situation).
Samples of larger component that do not fit under the microscope are cut and examined under the microscope for relevant features, followed by metallurgical evaluation of microstructures (a procedure which will indicate whether there are metallurgical deficiencies i.e., pores, voids, cracks, inclusions, segregations at grain boundaries) in the material that could have contributed to the failure. Metallurgical evaluation of microstructures will yield information regarding the heat treatment history of the part. Bulk chemical analysis would indicate whether the material follows the manufacturer specifications. Mechanical testing will determine the properties of a material when force/stress is applied and the results will indicate whether it meets the manufacturer specifications in terms of strength, ductility, hardness, toughness, etc.
In recent years, computer tomography (CT scanning) is increasingly used in failure analysis. CT scanning is an x-ray methodology that yields 3-dimensional results. In other words, CT scanning is a powerful non-destructive and non-physical contact methodology that allows seeing inside a product, beyond the surface, without the need to cut and/or slice it.
In case of corrosion failures, we conduct surface elemental chemical analysis on the fracture surfaces in as-is condition to identify possible contaminants, and traces of aggressive chemicals against the base material.
The experts at Origin and Cause apply a rigorous scientific approach to determine the failure mode and cause of a component and whether the failure was due to a manufacturing deficiency/defect, installation, maintenance (or lack of thereof), usage, or wear and tear. We discuss the cases with our clients at the onset of the investigation and advise on the best course of action. We keep our clients updated through the entire process of investigation and we discuss our findings.
Types of investigation
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