A Method Of Calorising Steel Pipes/Sections Surfaces

Abstract: A method of calorising of pipes on steel section comprising the steps of: a. checking the dimensions and chemistry of the pipes based on the supplies test certificate, b. degreasing the said pipes using sodium hydroxide and potassium per magnate solution heated between 70 to 85 deg. c, c. water washing the degreased pipe twice at room temperature, d. picking the washed pipe using a light HCI solution, e. water washing the said pickled pipes twice, f. flux cleaning the thus pickled and washed pipe at 70 to 80 deg. c using a solution of KCI, NaCI, and ZnCI2. g. Dipping the said flux cleaned pipes in hot flux made of KCI, Na3AIF6, NaCI, and ZnCI2 for 6 minutes with the temperature of the bath being maintained between 740 to 770 deg c. h. Transferring the pipes within 6 minutes to molten aluminium - silicon bath containing 9 to 10% silicon at 760 to 790 deg c. i. Allowing the pipes to be in this bath for 6 to 8 minutes and removing the same after wiping the excess aluminium using air wipers. j. Cooling the said pipes and keeping ready for the diffusion process as enumerated further. k. Bundling the hot dip aluminium coated pipes and filling in to a retort, l. Evacuating the air within the retort with a vacuum pump, m. Lowering the retort a vertical furnace and starting the heating cycle, n. Allowing the hot air along with the residual moisture inside the retort to escape upto 125 to 130 deg c, o. Evacuating the residual air once again at this point and introducing nitrogen and maintaining a positive pressure of nitrogen, p. Heating the retort with the pipes further upto 560 to 570 deg c and maintaining the temperature at that point for 30 minutes, q. Subsequently raising the temperature upto 850 to 900 deg c, maintaining this temperature for 2 to 3 hrs and then cutting off the heating and cooling the retort with nitrogen to around 280 to 300 deg c within 1 to 1 1/2 hour, r. Removing the retort from the furnace and allowing it to cool naturally to room temperature before the pipes are unloaded. s. Immediately introducing a second retort which has been kept ready with the similarly aluminium coated pipes, during the heating of the first retort and on removal of the first retort in order to keep the process continuous. t. Further cleaning and straightening the pipes is done. u. Further a small section of the pipe is sectioned and checked in the laboratory for microstructure analysis, to determine the layer of the diffusion.

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