This month’s case study describes a very interesting failure of a radiant coil tube from an ethylene steam cracker.
R-Tech Materials received a section of a radiant coil tube, approximately 200 mm in length and 72 mm in diameter, with the request to conduct a root cause analysis to determine the cause of a circumferential crack (see Figure 1). R-Tech were informed that a crack was observed on the fire-side of the tube during an annual inspection. The tube had been in service for approximately 6 years. The operating temperature and pressure was 800°C and 4-6 kg/cm2 respectively. The material was advised as being G4852.
Figure 1 – Failed pipework
Examination of the inner surface of the radiant coil tube, at the position where the cracks were evident on the outer surface, revealed areas of spalling of what appeared to be a surface oxide layer, see figure 2. The crack had initiated at the inner surface of the tube (see Figure 3) and was associated with a small secondary crack. Both cracks were wide and filled with oxide. Significant oxidation, evident at both the inner and outer surfaces, appeared to have occurred preferentially at the sigma phase present within the microstructure. Transformation of the MC carbide (normally found in this material) to sigma occurs upon exposure to temperatures in excess of 550°C. Sigma phase is extremely brittle and provides easy crack propagation and thus reduces ductility and impact toughness. The depth of oxidation attack was greater in the section taken through the cracking.
Figure 2 – Inner surface of tube section
Figure 3 – Wide, oxide filled cracks
A band of carburisation was evident at the internal surface which extended around approximately 75% of the circumference, see Figure 4. Carburisation is caused by exposure to temperatures in excess of 593°C in the presence of hydrocarbons. Heavy carburisation leads to a high volume increase which can increase residual stresses within the material. In addition, carburisation leads to increases in hardness and loss in ductility. The depth of carburisation was greatest in the vicinity of the cracking observed, measuring up to 3 mm. This would be expected to exhibit the greatest depth since it corresponded to the fire-side of the tube.
The presence of both oxidation and carburisation is unusual, though it has been reported in previous literature that in hydrocarbon service, depending on CO/CO2 ratio, the gases can be both oxidising and carburising. It is reasonable to assume a similar situation with the steam/pentane mix for this tube. At high temperatures, carbon penetrates the Cr2O3 scales, causing internal carburisation of the underlying material. Previous literature has reported the presence of a subsurface internal oxidation zone together with a deeper internal carburisation zone, similar to that observed in this investigation.
Figure 4 – Wide, oxide filled cracks initiating from internal surface
The presence of circumferential cracks and their confinement to one side of the tube indicates that a bending stress had caused the failure. This can either be applied by tensile/compressive loading or thermal loading. A temperature history of the tube over the last 3 months showed that there had been 5 incidents where the temperature had fallen by 700°C and approximately 14 incidents where the temperature has dropped by over 100°C. These conditions and the morphology of the cracking suggest that failure is probably attributable to thermal-cycling i.e. thermal fatigue. A temperature swing of 93°C is sufficient to cause cracking as a result of thermal cycling.
Thermal cycling leads to spalling of the protective oxide film. Expansion and contraction, as a result of heating and cooling, will spall the oxide layer, because the base metal and the oxide expand and contract at different rates. The volume expansion associated with carburisation will also increase this affect. The more rapid the rate of expanding and contracting, (i.e. the more quickly the metal is heated and cooled), the more risk there is of the protective coating cracking and/or flaking off. As the protective scale cracks, the exposed metal surface at the root of the crack oxidises which forms a notch in the metal surface. During the next cycle, the oxide will tend to crack along this notch, causing it to deepen. As this cyclic process continues, a transversal crack propagates through the tube wall, until the tube is perforated (3). The fire-side of the tube would have been exposed to the greatest temperature fluctuation which explains the confinement of the cracks to this side of the tube.
Controlled rates of heating and cooling during start up and shutdown and controlling of temperature fluctuations during service will reduce thermal stresses and thus reduce susceptibility to thermal fatigue cracking.
A failure analysis of this type is a valuable and informative tool to reduce the likelihood of a repeat incident which improves safety and productivity, reduces the risk of unplanned outages and enables evolution towards a better product. If you would like to know more about our failure analysis services? Contact us today to find out more.