Refinery furnace tubes operate at the extreme end of the temperature-pressure-corrosion spectrum. The tube metal temperature, process-side chemistry, and required service life together determine which alloy grade delivers reliable performance without premature failure from creep, oxidation, or sulfidation. Getting the grade selection right avoids both the cost of over-specification and the far greater cost of a furnace tube failure during operation.
Temperature as the Primary Driver
Below 450°C, plain carbon steel grades (ASTM A179, A192, A210) provide adequate service for refinery preheater and low-temperature exchanger applications. Between 450°C and 540°C, 1-1.25Cr alloy grades (T11/P11, T12/P12) provide the necessary creep strength and begin to offer meaningful oxidation resistance. Between 540°C and 580°C, 2.25Cr grades (T22/P22) are standard. Above 580°C, 9Cr grades (T9, T91) provide the creep strength and oxidation resistance needed for the most demanding cracker service.
Each step up in chromium content provides higher creep rupture strength at a given temperature and better resistance to high-temperature oxidation. The trade-off is increasing material cost and more demanding fabrication requirements, particularly for welding and post-weld heat treatment.
Sulphidation Resistance
Many refinery process streams contain sulfur compounds that attack steel at elevated temperatures through a mechanism called sulfidation (also known as sulphidic corrosion). The rate of sulfidation increases with temperature and sulfur content, and decreases with increasing chromium content in the steel. The modified McConomy curves provide the standard engineering tool for predicting sulfidation rates as a function of temperature, sulfur content, and alloy chromium level.
For crude distillation heaters processing high-sulfur crudes, 5Cr (T5) or 9Cr (T9) grades may be necessary even at temperatures where 1Cr or 2.25Cr grades have adequate creep strength, simply because the lower-chrome grades corrode too fast in the sulfur-bearing environment. The grade selection must therefore consider both the mechanical requirement (creep strength at temperature) and the corrosion requirement (sulfidation rate at the process-side chemistry).
Carburisation and Metal Dusting
Catalytic reformer and ethylene cracker furnace tubes face an additional degradation mechanism: carburisation. In carbon-rich atmospheres at temperatures above 550°C, carbon diffuses into the steel, forming carbides that embrittle the tube wall and progressively reduce ductility and impact toughness. Advanced alloy grades with higher chromium content resist carburisation more effectively, but even 9Cr grades can carburise over extended service periods in aggressive reformer and cracker environments.
Metal dusting — a localised, accelerated form of carburisation that produces pitting and perforation — occurs in a narrow temperature window (approximately 450–850°C) in CO-rich environments. Ethylene cracker transfer line exchangers and reformer outlet pigtails are particularly vulnerable. Where metal dusting is a known risk, nickel alloys or alumina-forming coatings may be necessary regardless of the base tube grade.
A Practical Selection Framework
The grade selection process is: determine the maximum tube metal temperature from the thermal design, determine the process-side chemistry (sulfur content, carbon activity, H₂S partial pressure), select the minimum chromium content needed for adequate sulfidation resistance using the McConomy curves, verify that the selected grade’s creep rupture strength at the design temperature satisfies the required tube life (typically 100,000 hours), and confirm the grade is weldable with available procedures and consumables at the fabrication site.
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