Tube Metallurgical Analysis

What is normally included in a metallurgical analysis? Visual examination may be obvious, but there is a surprising amount of information that can be learned about a failure just by looking at the specimen.  Often the mode of failure can be determined or at least narrowed down through visual examination.  Macro photography and physical measurements serve to document the as-received condition of the specimen, but also may contribute information to the determination of the root cause of failure.  Radiography may be employed to determine the location of cracks, if not obvious visually.  Ultrasonic testing may be used for crack detection, thickness measurements, and profiling damage regions. Not all tests necessarily need to be performed on every specimen.  In some case, additional, more specialized, tests are needed.  For instance, if it is desirable to identified chemical compounds present on a specimen, x-ray diffraction might be used.  The choice of tests and appropriate equipment will vary with the material be examined and the potential causes of the failure.  An experienced analyst will know the appropriate techniques and their accuracies for the specific analysis required [1].

Destructive laboratory tests that are typically performed on the specimen can include:

• Optical Metallography
• Optical Fractography
• Scanning Electron Microscopy
• Energy Dispersive Spectroscopy
• Bulk Chemical Analysis
• Tensile Test
• Hardness Test

Optical Metallography

Optical metallography is used to examine the microstructure of the material.  A cross-section of the specimen is taken, mounted in bakelite, epoxy, plastic, or similar material, polished, and examined with a light microscope.  Magnifications are typically between 10 and 1000 times.  Optical metallography can reveal clues about the initial fabrication and service conditions of the material.  This includes forming processes, heat treatments, in-service temperature exposures, welding processes, and environmental degradation.  The grain structure may be distorted as a result of service-induced loads.  The path of cracks, whether intergranular or transgranular, ID or OD initiated, can be determined.  This is often an important part of the determination of the root cause of failure.

Optical Fractography

Optical Fractography is the examination of the fracture surface of a crack with an optical microscope.  Typically a crack is opened in the laboratory by bending and then the newly exposed surfaces examined with a low power stereo microscope at magnifications typically between 5 and 25 times.  The path of the crack is revealed often with the initiation site.  Additional information regarding the mode of degradation may also be present.  For instance, beach marks, which are parallel marks made on the fracture surface as a result of the progression of a fatigue crack, may be observable.

Scanning Electron Microscopy

Scanning Electron Microscopy is the examination of the surface of a specimen with a scanning electron microscope (SEM).  The sample is placed in an evacuated chamber inside the microscope and an electron beam is directed onto the surface.  The resulting interaction between the electron beam and the specimen is displayed as a digital image on a screen or captured as a digital photograph.  The SEM allows examination of the surface of the specimen at magnifications of 10,000 times or greater.  The SEM has much greater resolution and depth of field than possible with optical equipment.  For instance with the SEM, striations on a fracture surface a fatigue crack may be able to be identified.  These striations correspond to the incremental progression (each cycle) of the crack.  By counting the number of striations it may be possible to determine the number of cyclic stresses experienced by the specimen and assist in the determination of the root cause of failure.

Energy Dispersive Spectroscopy (EDS)

Energy Dispersive Spectroscopy (EDS) provides quantitative analysis of elements on a surface of a specimen.  EDS is typically used in conjunction with an SEM to determine elements present at specific microscopic locations on a fracture surface.  For instance, EDS can be used to identify elements present at the tip of a crack.  This is very useful for identifying corrosive contaminants that may be responsible for the observed degradation.

Mechanical Testing / Chemical Analysis

Mechanical tests and chemical analysis may be performed on the bulk sample material.  The primary purpose of these tests and analyses is to determine whether the sample material is consistent with specification requirements of the ASME code or other appropriate codes.  In other words to answer the question, is the sample material the material that was intended to be in-service? Chemical Analysis of the bulk sample material may be performed with EDS, x-ray diffraction, or traditional wet chemistry techniques.  Chemical analysis identifies the principle constituents as well as trace elements present in the sample. Tensile tests are performed by machining as standard specimen from the sample and then pulling it apart.  The ultimate strength, yield strength, elastic modulus, and other basic material properties are determined by this test.

Hardness Testing

Hardness tests are performed by placing an indenter on the surface at a known load and measuring the resulting mark.  Hardness can be used to identify whether a material was correctly welded and/or heat treated, and to indirectly determine the strength of a material.  

References

[1] Jackson, P. Moelling, D. Malloy, J. Taylor, M. Tube Failure Diagnostic Guide - Third Edition, 2013. ISBN 0-9719616-3-8