Study On Failure Mechanisms And A New Type Protective Coating Of Convertor Water-wall Tubes

The chimney of converter is one of the main equipment in steel-making factories. The boiloff and cooling of cooling water in the chimney tubes change with smelting process rapidly and periodically. In the runtime, the chimney flues suffer a very brutal condition, and bring a severe damage which is mainly the explosion of the tubes and water leakage. At the end of the chimney near the converter, the work condition is much worse. As a result, the life time of the movable chimney is just one year, and in the life time there are so many accidents that examining and repair must be done regularly. The normal order and step of the production are broken down and this results in a great economic loss. Depending on the failure mechanisms, this paper develops some new type thermal spray protection coatings, and investigates the properties through a series of experiments. The aim of investigation is to find out a new coating, and solve the problem of the explosion of the tubes and prolong the life time of movable chimney successfully.The failure causes of the movable chimney are analyzed through metallographic microscope, Electron Probe X-ray Microanalyzer(EPMA) and other apparatus. And there comes a conclusion that the failure of the tubes is due to the following causes: during the service, the tubes are under a condition of high temperature which is far out of the normal requirements, the difference of temperature causes heat stresses, and then causes transverse cracks. Through the cracks oxygen penetrates into the tube. The oxidation of the tube in the cracks decreases the strength of the place where cracks locates, so cracks extend forward, and the tube can't stand the water pressure and breaks.According to the failure mechanisms of the tubes, and the requirements to the work conditions, the performance of mechanism and practice, the deposit process of the coating, three metallic coatings which have a high contain of nickel and chromium are developed, and the coatings are deposited by subsonic flame thermal spray. A lot of experiments such as high temperature oxidation and corrosion, erosion, bonding strength and hardness of coating, thermal cycling and thermalshock tests are carried out in this study. The mechanisms of hot corrosion and high temperature oxidation resistance of these alloy coatings are explored and their performance are compared with 20g which is now widely used as a kind of boiler tube materials by analyzing the microstructure and distributing of element of coatings with the techniques of metallographic analysis, Scanning Electron Microscope (SEM) and Electron Probe Micro-Analysis (EPMA) in this paper.Considering the results of the above tests all-around, NiCr-3 is the best coating of the three coatings, because it has better bonding strength, excellent thermal cycling and thermal shock resistance, normal erosion resistance in room temperature, and excellent high temperature oxidation and hot corrosion resistance, at the same time, depositing such coating consume a little raw and processed materials, the process in simple, and depositing can be carried out on the spot. It is a economical, simple and super protection coating for the chimney of converter.