Defects In Stainless Steel Research Articles

Unsupervised learning-enabled pulsed infrared thermographic microscopy of subsurface defects in stainless steel

Metallic structures produced with laser powder bed fusion (LPBF) additive manufacturing method (AM) frequently contain microscopic porosity defects, with typical approximate size distribution from one to 100 microns. Presence of such defects could lead to premature failure of the structure. In principle, structural integrity assessment of LPBF metals can be accomplished with nondestructive evaluation (NDE). Pulsed infrared thermography (PIT) is a non-contact, one-sided NDE method that allows for imaging of internal defects in arbitrary size and shape metallic structures using heat transfer. PIT imaging is performed using compact instrumentation consisting of a flash lamp for deposition of a heat pulse, and a fast frame infrared (IR) camera for measuring surface temperature transients. However, limitations of imaging resolution with PIT include blurring due to heat diffusion, sensitivity limit of the IR camera. We demonstrate enhancement of PIT imaging capability with unsupervised learning (UL), which enables PIT microscopy of subsurface defects in high strength corrosion resistant stainless steel 316 alloy. PIT images were processed with UL spatial–temporal separation-based clustering segmentation (STSCS) algorithm, refined by morphology image processing methods to enhance visibility of defects. The STSCS algorithm starts with wavelet decomposition to spatially de-noise thermograms, followed by UL principal component analysis (PCA), fine-tuning optimization, and neural learning-based independent component analysis (ICA) algorithms to temporally compress de-noised thermograms. The compressed thermograms were further processed with UL-based graph thresholding K-means clustering algorithm for defects segmentation. The STSCS algorithm also includes online learning feature for efficient re-training of the model with new data. For this study, metallic specimens with calibrated microscopic flat bottom hole defects, with diameters in the range from 203 to 76 µm, were produced using electro discharge machining (EDM) drilling. While the raw thermograms do not show any material defects, using STSCS algorithm to process PIT images reveals defects as small as 101 µm in diameter. To the best of our knowledge, this is the smallest reported size of a sub-surface defect in a metal imaged with PIT, which demonstrates the PIT capability of detecting defects in the size range relevant to quality control requirements of LPBF-printed high-strength metals.

Dynamic Interaction between Dislocation and Irradiation-Induced Defects in Stainless Steels during Tensile Deformation

A series of in-situ transmission electron microscopy (TEM) observations during tensile deformation were conducted on the ion-irradiated stainless steel. The jerky motion of dislocations appeared, and dislocations were pinned and depinned at the defects through the in-situ TEM observation. The jump distance traveled by dislocation was measured and discussed as the mean interval of defects interacting with the dislocation motion. Microstructural information of irradiation defects such as obstacle interval was obtained by TEM and atom probe tomography (APT), and the type of pinning site was identified. It was found that Frank loops and black dots were irradiation defects that strongly interacted with dislocations. It was suggested that solute atom clusters act as weak obstacles for dislocations in the dynamic interaction behavior with dislocation motion.

Vacuum Intensification of Filling Defects with Penetrants under Liquid-Penetrant Testing

The vacuum intensification of the process of filling defects in stainless steel and glass samples with penetrants under liquid-penetrant testing has been studied experimentally. Results indicate a high efficiency of vacuum impregnation with small amounts of penetrants on tested surfaces, especially when detecting flaws with openings of 1–3 μm. It has been established that the sensitivity of testing is increased by 30–100% compared with the traditional technique.

Development of a Bidirectional-Excitation Eddy-Current Sensor With Magnetic Shielding: Detection of Subsurface Defects in Stainless Steel

To reduce the effects of surface characteristics of materials and background noise of a magnetic excitation field, a new sensor with magnetic shielding was developed for eddy-current (EC) testing. Based on the analysis of EC theory, the performances of different cores, including air core, solid cylindrical ferrite core, hollow cylindrical ferrite core, and magnetic shielding ferrite core, were analyzed. The simulation results show that the magnetic shielding core can increase the EC density in the test material. External magnetic field shielding can effectively improve the defects’ detection sensitivity and the detection depth of subsurface defects. According to the simulation results of finite-element excitation, a bidirectional-excitation EC sensor with magnetic shielding was prepared and optimized according to the orthogonal experiment. Finally, the experimental results show that it can effectively detect the notch defects of a 10-mm-thick stainless steel plate surface at a distance of 7 mm. A linear relationship was observed between the change of impedance amplitude, phase difference, and depth of subsurface defects.

스테인레스 316L강의 배관용접결함에 대한 유도초음파 특성 평가

Abstract Stainless steel is a popular structural materials for liquid-hydrogen storage containers and pipingcomponents for transporting high-temperature fluids because of its superior material properties such as high strengthand high corrosion resistance at elevated temperatures. In general, tungsten inert gas (TIG) arc welding is used forbonding stainless steel. However, it is often reported that the thermal fatigue cracks or initial defects in stainlesssteel after welding decreases the reliability of the material. The objective of this paper is to clarify the characteristicsof ultrasonic guided wave propagation in relation to a change in the initial crack length in the welding zone ofstainless steel. For this purpose, three specimens with different artificial defects of 5 mm, 10 mm, and 20 mm instainless steel welds were prepared. By considering the thickness of s stainless steel pipe, special attention was givento both the L(0,1) mode and L(0,2) mode in this study. It was clearly found that the L(0,2) mode was moresensitive to defects than the L(0,1) mode. Based on the results of the L(0,1) and L(0,2) mode analyses, themagnitude ratio of the two modes was more effective than studying each mode when evaluating defects near thewelded zone of stainless steel because of its linear relationship with the length of the artificial defect.

Depth profiling of defects in stainless steel using electric potential sensors and a non-contact AC potential drop method

A new technique is described a non-contact version of the well-known AC potential drop method. Electric potential sensor technology is capable of measuring, non-contact, the spatial electric potential associated with a sample via the displacement current, through capacitive coupling. This can be achieved due to the extremely high input impedance of the sensors, resulting in many cases in no loss of signal when compared with a contact voltage measurement. For laboratory samples, an AC current is passed through the sample and the spatial potential above the surface monitored as a function of position. For a full non-contact implementation this current would be induced inductively as in eddy current testing systems. Previous work on stainless steel using this new technique was able to detect 1 mm of material loss from the inside of a 24 mm-thick sample. The work described here extends this by applying multiple frequencies simultaneously to a sample and analysing the data for each frequency component separately. In this way, depth information about faults within a material may be obtained, due to the skin depth effect. This enables the location and characterisation of faults in three dimensions to be achieved while only performing a single scan.

Fundamental experiment for inspection of cooling pipes in operation by using ultrasonic technique

Shear horizontal (SH) wave propagation characteristics were evaluated as a method for identifying defects in stainless steel, with the potential application of developing technology for inspecting defects during operation of nuclear reactors. Defects were positioned between a transmitter and receiver, and the propagation angle of the ultrasonic wave was varied by changing the transmission frequency. Next, the waveform from a defect was compared to reference waveform data for the case of no defects. The results showed that the differential signal had maximum amplitude at 500 and 700 kHz in the case of defects in the front surface, and at 560 kHz for defects at the bottom surface. We found that the transmission frequency yielding maximum signal amplitude was equal to the propagation direction of the ultrasonic wave, that is, the frequency determined from the direction angle, acoustic velocity and width of the permanent magnet. Further, from a study of the transmission and received frequencies, we found that in the case where the received frequencies were different from the transmission frequencies, the received frequency varied with the depth of defects.

Radiation defects in stainless steel and high-nickel alloy at hydrogen and helium ion bombardment

Radiation defects in a stainless steel and a high-Ni alloy are investigated after D + and He + ion bombardment at 400°C. Development of dislocation loops and pores with ion fluence is analyzed. Unusual fractal-like dislocation loops were observed after D + bombardment. Influence of gaseous impurities in defect structure formation is discussed.

Radiation defects in stainless steel and high-nickel alloys irradiated by deuterium ions at 400�C

Radiation defects in stainless steel and high-nickel alloys irradiated by deuterium ions at 400�C

Rare earths counteract hot rolling defects in stainless steel

Rare earths counteract hot rolling defects in stainless steel