Ethylene pyrolysis furnace tubes with a high-Cr, high-Ni alloy, weld overlaid by
Plasma Powder Welding (PPW) technique, were evaluated in a laboratory and in commercial
furnaces. These tubes have a thick layer (approx. 2.0mm) of 45 % Cr, 50 % Ni and 1 % Mo
alloy welded on the entire inner surface of HP-mod or 35Cr/45Ni base tubes.
The carburization behavior of these furnace tubes has been evaluated in the
commercial furnaces for over 6 years as well as under accelerated conditions in a laboratory.
The results show that if the PPW overlay was applied, no carburization was observed in the
base tube under real-life operation conditions where severe carburization occurred in tubes
made of conventional materials. This significant tube life improvement can be explained by the
mechanism of carbon diffusion through the boundary of PPW overlay and the base tube metal.
In addition, a continuous, self healing, and catalytic coke inhibiting chromium oxide layer,
recognized on the PPW overlaid surface, results in longer run length, higher ethylene yield,
and higher conversion.