Effect of Graphitization on the Strength of Cast-Iron Tubes of Economizers of Hot Water Boilers

The article contains the results of examination of cast-iron pipes of water economizers of water-heating boilers, which were in operation in the period of 1986–1991 and which have no external visual defects. We have determined a dependence of wear-out in the form of graphitic corrosion of the surface layer of inner walls of economizer pipes. Researches have been conducted with the use of an JSM-6390LV scanning electron microscope equipped with an attachment for conduct of energy-dispersive analysis, and with the use of a DuraScan 20 microhardness tester. There are images of areas given, where microhardness testing was performed. The areas were exposed to graphitic corrosion. The dimensions of the layer completely transformed as a result of graphitization and the transition zone were determined. The microhardness of the layer completely transformed as a result of graphitization and the transition zone was measured. It was found that the microhardness of these zones is significantly lower than the microhardness of gray cast iron. We have also presented the results of energy-dispersive analysis and specific diagnostic features identifying the graphitization process. In the article we propose a method of assessment of supporting capacity of cast-iron pipes of water economizers based on the rate of their graphitization. A mathematical model takes into account all main properties, features, physical and geometric characteristic of the real structure, which has an analytical model represented by a beam rigidly restrained on both heads, and which is bent by well-distributed dead weight load arising from the pipe’s own weight and the weight of the water flowing through it. The calculated residual wall thickness of 4.53 mm after wear of its internal cylindric surface is comparable with the known empirical data of statical analysis of emergency situations at economizers. The data given in the article could be used by specialists for expert examination of cast-iron pipes of economizers, as well as for justification of their sorting or life extension.