Attività

Failure analysis of a commercially pure titanium tube in an air conditioner condenser

Joining of titanium and stainless steel is challenging due to the formation of hard, brittle intermetallics. This study focuses on engineering ductile materials for joining transition metals. Friction welding of tube to tube-plate by an external tool, a novel solid state welding process was employed to join titanium tube and stainless steel tube plate. The interlayers engineered were copper, silver and Cu–Zn alloy. The micrographs revealed phase transformations in titanium tube and unaffected stainless steel base. Interface peak microhardness of 458 HV was observed for Ti/Cu–Zn/SS welded sample. The intermetallics formed were characterized by X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy. A novel shear test procedure was developed to evaluate the maximum shear load. It was found that joints with silver as interlayer withstood the maximum shear load of 56 kN. The shear surfaces were further analyzed and investigated for fracture study.

Titanium has today replaced copper alloys as the most favoured tube material for salt water cooled condensers. Main reason is the excellent corrosion resistance of titanium in chloride containing environments. The experience of titanium bar condensers is usually more than satisfactory, even if a few tube leaks have occurred. Possible damage mechanisms by high cycle fatigue, galvanic corrosion, water-droplet erosion and by flow-assisted corrosion are discussed. These perils can be handled by a number of adequate countermeasures analysed in laboratory work and meanwhile proven by plant service.

The corrosion resistance of titanium in sea water is extremely excellent, but titanium 、nickel 、zirconium tube are expensive, and the copper alloy tubes resistant in polluted sea water were developed, therefore they were not used practically. In 1970, ammonia attack was found on the copper alloy tubes in the air-cooled portion of condensers, and titanium tubes have been used as the countermeasure. As the result of the use, the galvanic attack on copper alloy tube plates with titanium tubes as cathode and the hydrogen absorption at titanium tube ends owing to excess electrolytic protection was observed, but the corrosion resistance of titanium tubes was perfect. These problems can be controlled by the application of proper electrolytic protection. The condensers with all titanium tubes adopted recently in USA are intended to realize perfectly no-leak condensers as the countermeasure to the corrosion in steam generators of PWR plants. Regarding large condensers of nowadays, three problems are pointed out, namely the vibration of condenser tubes, the method of joining tubes and tube plates, and the tubes of no coolant leak. These three problems in case of titanium tubes were studied, and the problem of the fouling of tubes was also examined. The intervals of supporting plates for titanium tubes should be narrowed. The joining of titanium tubes and titanium tube plates by welding is feasible and promising. The cleaning with sponge balls is effective to control fouling.
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