The furnace operation conditions were a the following. The design operating pressure was 32 bars,internal temperature of tube was 800-900 C, and surface temperature of tube was 1000-1100 C. The visual inspection test (VT) was used for detecting surface discontinuities and a change of tube shape measuring, by using the Werner Caliper and Coordinate Measuring Machine (CMM). SEM-EDS and microstructure analyses were used for determining the concentration of elements in matrix of tube in different areas according to the NACE Standard TM 0498-98 [7]