High temperature alloys tubes are used in the hottest section in the radiant box of petrochemical furnaces commonly known as ethylene furnaces or fired heaters. Many metallurgical improvements to these alloys are intended so the material can withstand the elevated temperature and aggressive environment to obtain a longer operation time. This paper explains the most common damage mechanisms of these alloys in radiant section such as creep/carburization, thermal fatigue/carburization, and thermal shock. State of the art analytical techniques such as macro-etching of carburization depth, optical microscope, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF) were utilized in analyzing these failures. Preventive solutions were recommended to overcome such failures in the future and ultimately increase the operation time and most importantly increase the production rate.
The secret of the use of high temperature alloys in the aggressive environment of furnaces is the addition of the alloying elements. These alloying elements diffuse interstitially and substitutionally into the structure. Pure metals tend to be very soft because the crystal structure is not distorted and the atoms can move relatively easily. So the addition of the alloying elements will create different phases and that will enhance the thermos-mechanical and thermo-chemical properties.
Numerous factors are considered during the design and material selection of the furnace. This paper discusses only the metallurgical aspects of furnace tubes. The focus during the design of the furnace tubes is always to improve the resistance to high temperature attack. Some metals, such as aluminum and chromium are so reactive that they react with oxygen at room temperature to form an oxide layer which is so stable that it becomes an impenetrable oxide barrier preventing the diffusing of hydrocarbon into the parent tube metal at high temperature service. This is the reason of having the chromium as a basic component of most high temperature alloys. Nickel and silicon are also important because they are major contributors to improve the resistant to oxidation. Other elements such as molybdenum, titanium and nitrogen are added to modify the metallurgical and mechanical properties.
The furnace in general consists of two sections as shown in Figure 1. The two sections are radiant where the tubes are exposed to extremely high temperature and the second section is the convection section where the heat is less than radiant section. The tubes in the radiant section are heated by a direct radiant heat by the burners. The excessive heat from the radiant section move to the convection section to heat the tubes. The gas feed enter the convection section and then is heated up gradually till reaches the radiant section. The material selection of the tubes are varied based on the exposed temperature from carbon steel at the beginning of the feed to high grade austenitic alloy in the radiant section.