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Carbon Spheroidization of Carbon Steel Heating Element Sheath

ENVIRONMENT:

Air, Alumina Desiccant Beads

EQUIPMENT:

Heating Element Sheath

MATERIAL:

A106, Grade B, Carbon Steel

OPERATING TEMPERATURE:

275 to 300F

FAILURE:

Long Term Overheating - Carbon Spheroidization

 

The dryer is 36" in diameter with 48 vertical bayonet heating elements 68" long. The dryer is filled with granules of loosely packed desiccant. The operation of the dryer is cyclical, with a 10-hour desiccation leg followed by subsequent 5-hour regeneration of the desiccant. Heating to 275 to 300F renews the desiccant with electrical resistance heating elements inside the carbon steel sheaths. The process is controlled utilizing a timer and a thermostat that is set to monitor the temperature of the exiting air flow.

A visual evaluation of the dryer interior was made three years prior to the failure. After failure, a total of 27 tubes were discovered to be corroded to varying degrees primarily in their midsections, Figure 1.

Cross-sections were made of corroded and non-corroded material. The microstructure of the non-corroded material consisted of ferrite and pearlite grains characteristic of carbon steel, Figure 2. However, the microstructure observed in the corroded material was very different. The carbon had undergone the process of spheroidization, usually resulting from prolonged exposure to temperatures in excess of 1000F, Figure 3.

Elemental analysis did not reveal any indication of corrosive compounds in the scale or on the desiccant beads. Prolonged overheating of the heating elements due to a lack of over temperature protection on the heating elements caused this failure.

 
Figure 1. Corroded sheath exterior (0.85X Original Magnification) Figure 2. Etched sample of a section of non-corroded material (200X Original Magnification with Nital Etch)
 
Figure 3. Etched sample of corroded material observe carbon spheres (200X Original Magnification with Nital Etch)  

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