March 5, 2019
Just as the term implies, fractures are cracks. In pressure vessels, the cracks propagate because of brittleness. How maddening is this situation? It's a disturbing material defect, to say the least. Despite engineering skills and fabrication precision, plus a huge investment of production energy, a product has developed fracture points. To reduce such costly incidents in the future, we need to know more about fracture hazards.
The Seeds of Destruction
This opening subtitle does make the whole post look a little melodramatic in nature. However, there's a good reason for the overly exaggerated tone. Fractures do commonly begin as seeds, as invisible microcrystalline flaws. They can't be seen by the eye, so a visual inspection won't pick up the problem. Special, state-of-the-art, electronic apparatus is employed as a flaw detection mechanism when the human eye is no longer enough. X-Rays or radio waves sink into the metal and are reflected back at a sensor array, which graphically interprets the results. The material is hard, as hard as the job requires, but it's also lost some of its ductility, so that hardness quotient is undermined by a brittle grain structure. Strangely, when metals are brittle, even the hardest alloy can fracture.
What Causes Alloy Brittleness?
This isn't the easiest question in the world to answer. Put it this way, the fault may have been there before the vessel's fabrication process got underway. Past cold work stress or poorly applied heat treatment work could be responsible. That means nothing went wrong with the design process, every stage of the work progressed as anticipated. An annealing or normalizing phase has run afoul of an unforeseen operation, as conducted during a thermal stress reduction procedure. If it wasn't the thermal treatment, then the tempering or quenching work was substandard. Whatever the cause, it took place before the fabrication. Alternatively, the brittle transformation occurred when thermal extremes, either exceptionally low or high, impacted the low-notch toughness of the metal. Certain chemical reagents also possess alloy fatiguing properties.
Let's say a low temperature transformed the microcrystalline structure of an unfired pressure vessel. Or maybe it was an unforeseen chemical reaction. A state change occurred, and the chemically active base made the strong alloy brittle. Now, with the plastic deformation characteristics of the metal woefully leeched away, the most harmless energy exchange finds a weak link in the metal grain. A formerly concealed flaw fractures and hairline cracks propagate until they spread like the branches of a tree. The consequences are severe, with a catastrophic failure a likely outcome. As one maintenance oriented option, a hydrostatic test is an invaluable troubleshooting tool here, one that'll seek out weld and material discontinuities.
Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619
1865 Frankston Flinders Road,
Hastings, VIC 3915
Ph: (03) 5909 8218
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