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09156 An Investigation of Failed Burner Tubes Used for Silicon Steel Strip Annealing – Damage Assessment and Possible Measures to Improve Performance

Picture for 09156 An Investigation of Failed Burner Tubes Used for Silicon Steel Strip Annealing – Damage Assess
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Product Number: 51300-09156-SG
ISBN: 09156 2009 CP
Author: Robert Salchinger, Walter Hartnagel, Henrik Asteman and Werner Schwarz
Publication Date: 2009
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The production of high quality silicon steel strip requires high annealing temperatures, and at present the burners used are operating close to their upper temperature limit. This shortens their life-time considerably. The main failure of the burners is caused by a sharp breakage of the inner nozzle tube. This in turn diverts the flame and causes flame impingent on the outer tube casing.

A failure investigation was performed in order to decide the exact cause and suggest measures in order to allow a life-time improvement and/or increase the temperature. Three burners at different stages of degradation were used for the investigation.

The alloy used for the inner nozzle and the outer tube casing was EN: G-NiCr28W (a Cr2O3 former). SEM/EDX analysis showed that Cr disappears from the steel in an alarming rate, most probably by water vapor enhanced evaporation. The protective Cr2O3 decomposes with catastrophic oxidation and spallation as a result. The thinning of the tube wall and the appearance of holes eventually causes the break.

This type of failure might be avoided by choosing an alloy that forms Al2O3 instead of Cr2O3. One such candidate would be Alloy 60 HT R that forms Al2O3 and has good mechanical strength up to 1250°C.

Keywords: High temperature corrosion, Gas-burners, Flue gas, Water vapor, Cr-evaporation, Al2O3
The production of high quality silicon steel strip requires high annealing temperatures, and at present the burners used are operating close to their upper temperature limit. This shortens their life-time considerably. The main failure of the burners is caused by a sharp breakage of the inner nozzle tube. This in turn diverts the flame and causes flame impingent on the outer tube casing.

A failure investigation was performed in order to decide the exact cause and suggest measures in order to allow a life-time improvement and/or increase the temperature. Three burners at different stages of degradation were used for the investigation.

The alloy used for the inner nozzle and the outer tube casing was EN: G-NiCr28W (a Cr2O3 former). SEM/EDX analysis showed that Cr disappears from the steel in an alarming rate, most probably by water vapor enhanced evaporation. The protective Cr2O3 decomposes with catastrophic oxidation and spallation as a result. The thinning of the tube wall and the appearance of holes eventually causes the break.

This type of failure might be avoided by choosing an alloy that forms Al2O3 instead of Cr2O3. One such candidate would be Alloy 60 HT R that forms Al2O3 and has good mechanical strength up to 1250°C.

Keywords: High temperature corrosion, Gas-burners, Flue gas, Water vapor, Cr-evaporation, Al2O3
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