Influence of Thermal Exposure on the Structure and Phase Composition of Aluminized Layer Obtained by Explosion Welding and Hot-Dipping Process

It is shown that for the formation of Fe2Al5 intermetallic coatings on the steel surface the aluminized layer obtained by explosion welding must be subjected to a double heat treatment (660 °C, 3 h + 640 °C, 3 h). The first heat treatment ensures the necessary size of the diffusion zone, and the second leads to the formation of a main crack at the interface of Fe2Al5 and FeAl3 intermetallides, which allows to separate the unreacted layer of aluminum and form a coating with hardness of 10 GPa. To form Fe2Al5 intermetallic coating on the steel surface, the aluminized layer, which is immersed in the melt, must be heat treated at 800 °C. The alloying of the diffusion zone by Si and Cu with the replacement of aluminum by the AK12M2 alloy leads to a decrease in the thickness of the diffusion zone and the appearance of additional phases of Al7Fe2Si and (Al,Si)5Fe3 in its composition. The duration of the subsequent heat treatment at 800 °C for complete dissolution of the surface layer increases the hardness of the resulting coating on the basis of a solid solution of Si in Fe2Al5 is 7.5-8 GPa.

Investigation into the Effects of High Temperature on Mechanical Properties of Thermal Spray Coatings Applied by Plasma Spraying

C. C. Berndt advanced investigations of mechanical properties of thermal spray coatings under 4-point bending. He found that this investigation method is sensitive to the mechanical properties of thermal spray coatings.This paper contains the detailed investigation results for thermal spray coatings of zirconium dioxide under 4-point bending, i.e. tests of the specimens subjected to spraying at varying conditions and pre-test soaking with the various duration at 1100 °С.It was established how the mechanical properties of thermal spray coatings changed depending on the spraying mode and high temperature soaking. The test results show that the double heat treatment of coatings is more preferable than one-time heat treatment as it make the properties change linearly. It is more easily controllable during operation of the components with thermal spray coating.

Promoting Bacillus cereus Spore Germination for Subsequent Inactivation by Mild Heat Treatment

Sublethal heat treatment may activate dormant spores and thereby potentiate the conversion of spores to vegetative cells. As the germinated spore is known to possess lower heat resistance than its dormant counterpart, it has been postulated that double heat treatment, i.e., spore heat activation followed by germination and then by heat inactivation, can be used to control spores in foods. Production of refrigerated processed foods of extended durability often includes more than one heat treatment of the food components. This work simulates conventional heat treatment procedures and evaluates double heat treatment as a method to improve spore control in model food matrixes of meat broth and cream sauce. Bacillus cereus NVH 1230-88 spores were supplemented in food model matrixes and heat activated at 70°C and then heat inactivated at 80 or 90°C. The samples were held at 29 to 30°C for 1 h between primary and secondary heat treatments, to allow spore germination. Nutrients naturally present in the food matrixes, e.g., amino acids and inosine, could act as germinants that induce germination. The levels of germinants could be too low to produce effective germination within 1 h. Following primary heat treatment, some samples were therefore supplemented with a combination of L-alanine and inosine, a germinant mixture known to be effective for B. cereus spores. In both matrixes, a combination of double heat treatment (heat activation, germination, and inactivation) and addition of germinants gave a reduction in spore counts equivalent to or greater than that obtained with a single heat treatment for 12 min at 90°C. Addition of germinants was essential to induce effective germination in cream sauce during 1 h at 29 to 30°C, and germinants were therefore a crucial supplement to obtain an effect of double heat treatment in this matrix. These data will be valuable when setting up temperature-time-germinant combinations for an optimized spore reduction in mild-heat–treated foods.