Watching Material Composition

It seems that every so often a problem surfaces that is the result of losing control of the composition of metal used to create an important component. When this occurs, it often leads to a long discussion over who should be responsible for testing the metal composition to insure it is correct.

In a recent discussion of this kind, the participants seemed to miss the point. Responsibility rests with the individual who understands the relationship between the properties of the material that was selected and the features that allow that material to continue to operate in a safe and reliable manner. In order to have a complete understanding of the issues involved, it makes sense to go back and describe how components are designed and how materials are originally selected.

In the design process, few instances allow the designer the luxury of selecting a specific alloy or blend of elements that best fits all the needs. More likely, the designer has to select from a number of commonly available materials with characteristics that generally fit the all the needs of the application. The alloy that is finally selected will have specific corrosion characteristics, strength properties, durability and other properties. In order to obtain something close to the required corrosion capabilities, the designer may have had to sacrifice some strength. As a result, he may need to make the component more robust and heavier. In order to obtain the appropriate strength, he may have had to sacrifice some corrosion resistance. As a result, he may need to specify the device be inspected for corrosion more frequently and replaced on a given interval.

Since the adaptations the designer made (more robust or more frequent inspections) were dependent upon the specific characteristics of the metal that was used, it is likely those adaptations will need to change if the composition of the material is changed.

Taking things a bit further, it is possible that the application in which the device is used may change from the original assumptions. In a refinery or chemical plant, if the composition of the process stream changes, the assumptions that the initial designer made are no longer applicable. In this case, the inspection and maintenance requirements may need to change to address the way the alloy will behave in its altered environment. In this situation, the information assembled by the original designer will no longer be relevant. As a result the individuals who understands the current conditions and the new requirements is responsible for ensuring that the material composition is consistent with the service, the inspection and the maintenance practices.

In any case, history serves as a good source of warning. Few things go on forever without a slip-up at some point. The composition of metal alloys in critical services are no exception. Sooner or later, the maker will lose control of the composition of an alloy. If the quality control and quality assurance processes do not exist, that mistake will make it all the way to the application. If the application is highly dependent upon the properties provided by the metal composition, deterioration and failures are likely to occur.

In those circumstances where failure would produce an unacceptable impact, it is absolutely critical that tight controls be placed on material composition control. Many plants choose to apply Positive Material Identification to all incoming materials. Others depend on their suppliers to verify material composition. Still others (and maybe even more) simply depend on the world around them being a kind and forgiving place to exist. These people have never asked the question or followed up on the responses to ensure that guards against loss of composition control exist. As long as the world is kind and forgiving, nothing bad will happen.

Maybe everything will be fine.

This article introduces the following subjects:
• Material composition control.
• Selection of materials during design.
• Changes in material requirements post-application.
• Positive Material Identification.

If readers would like to comment on these or other closely related subjects, they are invited to do so.