Investigation on the Micromechanisms of the Cracking of 316L Heat Exchanger Tubes

The cracking mechanisms of 316L heat exchanger tubes employed in power station were studied using optical microscope (OM) and scanning electron microscope (SEM). It is demonstrated that the hardness value, microstructure and tensile properties of selected #1 and #2 tube samples all meet the requirements of relevant standards, but the contents of Ni and Mo element of #1 tube are slightly lower than the standard requirements. The circumferential cracks on the two samples nucleate at the corrosion pits on the inner wall of the tubes, while Cl element was detected in the corrosion products of these pits. The cracks propagate from the inner wall to the outer wall along the circumferential direction of the tube, forming a dendritic crack morphology with both transgranular and intergranular propagation characteristics. Combined with the investigation of the service condition of the heat exchanger tubes and the analysis of the experimental results, it can be concluded that the main reason for cracking is the initiation of pin-corrosion when the content of chloride ion exceeds the standard during the service of the tubes, which will induce stress corrosion cracking, causing crack expansion through the wall thickness, and finally lead to leakage of the tube. In addition, from the point of view of materials, Mo is an important element to improve the pitting resistance of materials. The content of Mo element detected in the samples is lower than the standard requirement, which is also one of the reasons for the easy pitting corrosion of the inner wall of the pipe.

The Effect of Deposited Dust on SCC and Crevice Corrosion of AISI 304L Stainless Steel in Saline Environment

Crevice corrosion has become an important issue of the safety of AISI 304L austenitic stainless steel canister when exposed to the chloride environments located in coastal areas. Moreover, dust deposited on the canister surface may enhance the corrosion effect of 304L stainless steel. In this work, white emery was adopted to simulate the dust accumulated on the as-machined specimen surface. To investigate the effect of deposited white emery, chloride concentration, and relative humidity on the crevice corrosion behavior, an experiment was conducted on 304L stainless steel specimens at 45 °C with 45%, 55%, and 70% relative humidity (RH) for 7000 h. The surface features and crack morphology of the tested 304L stainless steel specimens were examined by SEM equipped with energy-dispersive spectrometry (EDS) and electron back scatter diffraction (EBSD). From the experimental results, a threshold RH for the stress corrosion cracking (SCC) initiation of AISI 304L austenitic stainless steel with different concentrations of chloride was proposed.

Effect of Titanium Addition on Cracks of Fe-Cr-C Coating by Reactive Plasma Cladding

Fe-Cr-C and Fe-Cr-C-Ti coatings were prepared by reactive plasma cladding in this paper. The crack morphology and fracture surface of the Fe-Cr-C coating were observed by SEM. The effect of titanium addition on the crack of Fe-Cr-C coating was analyzed. The results show that the coating cracks mainly consist of crack perpendicular to the fusion line, defect-induced crack and intergranular crack. The crack rate of Fe-Cr-C-Ti coating was obviously decreased after Titanium was added. When the titanium content is below 8 wt.%, with the increase of titanium content, the crack rate of Fe-Cr-C-Ti coating decreases obviously. When titanium content is between 8wt.% and 13wt.%, there are no cracks in the Fe-Cr-C-Ti coating. When the titanium content exceeds 13 wt.%, with the increase of titanium content, a small number of cracks begin to appear. The addition of titanium increases the toughness of the Fe-Cr-C-Ti coating and reduces the stress concentration.

Investigation on the rolling contact fatigue behaviours of different laser cladding materials on the damaged rail

Surface damages on rail became severe with the increasing in the axle load and speed. Laser cladding was used to repair the local damages on rail surfaces to ensure the service performance and prolong the life of rails. In the present study, five types of cladding materials (304, 314, 2Cr13, 316L and 434L) were clad at a small part (trapezoidal shape) of the rail disc to simulate the defected rail in the field. Vickers hardness tester, Optical microscopy (OM), scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDS) were employed to investigate the service properties of the repaired rail discs. Results indicated that all these five clad rail discs had lower wear rates than the un-clad rail disc. The wear rates decreased with the increasing in the initial hardness of the cladding coatings. Cracks with large angle mainly propagated along the bonding line at the front edge; while at the rear edge, the crack morphology was closely related to the hardness of clad coatings. 434L would induce the minimum wear loss and the modest rolling contact fatigue (RCF) of the repaired rail disc, which could be the potential cladding material for repairing the damaged rail.

Temperature and Concentration Dependence of Human Whole Blood and Protein Drying Droplets

The drying of bio-colloidal droplets can be used in many medical and forensic applications. The whole human blood is the most complex bio-colloid system, whereas bovine serum albumin (BSA) is the simplest. This paper focuses on the drying characteristics and the final morphology of these two bio-colloids. The experiments were conducted by varying their initial concentrations, and the solutions were dried under various controlled substrate temperatures using optical and scanning electron microscopy. The droplet parameters (the contact angle, the fluid front, and the first-order image statistics) reveal the drying process’s unique features. Interestingly, both BSA and blood drying droplets’ contact angle measurements show evidence of a concentration-driven transition as the behavior changes from non-monotonic to monotonic decrease. This result indicates that this transition behavior is not limited to multi-component bio-colloid (blood) only, but may be a phenomenon of a bio-colloidal solution containing a large number of interacting components. The high dilution of blood behaves like the BSA solution. The ring-like deposition, the crack morphology, and the microstructures suggest that the components have enough time to segregate and deposit onto the substrate under ambient conditions. However, there is insufficient time for evaporative-driven segregation to occur at elevated temperatures, as expected.