Detection of microcracks in 304L austenitic steel
How can microcracks in 304L austenitic steel be detected reliably and non-destructively before they evolve into critical structural failures?
In industries where material integrity is paramount—such as energy, nuclear, or aerospace—early detection of microscopic defects is essential to ensure long-term safety, performance, and reliability of critical components.
Context
A microcrack is a very small fracture that occurs within a material, with width typically measuring less than 100 micrometers (0.1 millimeters) and of millimetric or sub-millimetric lengths. These cracks are often not visible to the naked eye and can occur in various materials. Microcracks can form due to various factors, including mechanical stress, thermal expansion, fatigue, or environmental conditions. For example, microcracks can occur in thermally affected areas in a welding process. Although microcracks are extremely small, they can strongly affect the mechanical properties of metals and compromise the structural integrity of parts or installations. Microcracks can also grow over time and cause catastrophic failures. Early detection thus allows for timely repairs, ensuring safety.
Detecting microcracks is therefore of paramount importance and a vast subject of engineering and material science. However, their size makes this detection particularly challenging. In this context, NV diamond magnetometers developed by Kwan-tek are especially interesting with their high sensitivity and extremely high resolution. Diamond-based non-destructive evaluation can complete the panel of existing tools and bring additional insight into crack detection and identification.
Microcracks in austenitic steel
In this case study, we want to illustrate the possible benefits of using Kwan-tek’s quantum NDT solutions for detecting microcracks. An interesting feature of NV diamond sensors is their ability to detect simultaneously AC and DC magnetic fields with high sensitivity. A combination of static magnetic field measurements and AC excitation was thus used to detect the microcracks, like schematized in the picture. Eddy currents were excited by an oscillating magnetic field in the vicinity of the NV fibered sensor. By analyzing the red photoluminescence of the NV diamond, we can measure both the static magnetic field (i.e. magnetic flux leakages) and the AC magnetic signature of the material (eddy current signal).
As an example, we have applied this measurement to the detection of artificial cracks fabricated by electro-erosion in austenitic stainless steel (304L), like used for the storage of liquefied natural gas.
Results
The results presented here were obtained on a 500 µm-long, 40 µm-wide and 300 µm-deep crack. The eddy current excitation was carried out at low frequencies, around 1.5 kHz, and with 10 mm-diameter coil. Nonetheless, the spatial resolution is much finer than the excitation coil, as it is limited here by the size of the diamond sensor and the stand-off distance – a few hundreds of micrometers. Although the crack is very small, it also stands out very clearly on the rest of the part. These two aspects illustrate well the potential of NV diamonds for non-destructive evaluation. Our sensors combine high resolution and high sensitivity, allowing for easy detection as well as characterization of microcracks.
Interestingly, although the sample is made of 304L austenitic steel – supposedly non-magnetic, small fringe fields exist that reveal the crack, which can be detected both with magnetic flux leakage and with the eddy current amplitude and phase signals. The measurement of the magnetic field offered by NV diamonds is also vectorial. The technique inherently provides complementary information, which opens the way not only for detecting defects, but also for identifying and sorting them. This is even reinforced by the quantitative nature of the measurement. For example, two defects of different depths will lead to proportional signal intensities, helping to sort critical microcracks from insignificant ones.
Perspectives
With this very simple example, we try to provide examples of how NV magnetometers can bring new insight for the non-destructive detection of microcracks in metals. The outstanding resolution and sensitivity offered by quantum diamonds can improve the early detection of cracks and ensure the resilience of critical infrastructures in several industries from energy to aeronautics. At Kwan-tek, we believe that our NV sensors can improve the inspection of welding in nuclear plants, in liquid gas tanks, or help characterize colonies of cracks in pipes.
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