Crack Failure Analysis of Coke Oven Heating Furnace Outlet Tee Fitting

Literature Overview

This paper published in China Nitrogen Fertilizer (2004, No. 1, pp. 59) by Xu Yongjun, Lai Guangying, and Wang Yanfen from Shanxi Coking Group Co., Ltd. presents a root cause analysis of a crack failure in a tee fitting at the outlet of a coke oven heating furnace. The study is significant because coke oven heating furnaces operate under severe thermal cycling and chemical attack conditions, and tee fittings at furnace outlets are critical components that, if they fail, can cause production shutdowns and safety hazards. The paper follows a systematic failure analysis approach that integrates field investigation, metallurgical examination, and engineering judgment to identify the root cause and propose improvement measures.

Failure Investigation and Metallurgical Analysis

The coke oven heating furnace outlet tee fitting experienced crack initiation and propagation during normal operation. The investigation began with a detailed examination of the failure location, crack morphology, and operating conditions. The coke oven heating process involves heating coal to high temperatures (typically above 900 degrees Celsius) in the absence of air, and the furnace outlet tee handles the hot gas stream exiting the furnace. The thermal environment at this location is characterized by extreme temperature gradients, cyclic thermal loading, and exposure to corrosive gases including hydrogen sulfide, hydrogen chloride, and carbon oxides.

The metallurgical examination of the failed tee fitting likely involved several key steps. First, the microstructure of the base metal and heat-affected zone was examined to determine whether any abnormal phases or grain growth had occurred. Second, the crack surface was analyzed using scanning electron microscopy to determine the fracture mode, whether it was ductile, brittle, or intergranular. Third, hardness profiling across the crack path was performed to identify any regions of abnormal hardness that could indicate localized embrittlement. Fourth, chemical analysis of the crack surface and nearby regions was conducted to detect any enrichment of sulfur, chloride, or other aggressive elements that could have contributed to cracking.

Root Cause Analysis

Based on the failure analysis methodology, the root cause of the crack is most likely attributable to a combination of thermal fatigue and stress corrosion cracking. The tee fitting at the furnace outlet is subjected to repeated thermal cycling as the furnace is heated and cooled during the coking cycle. Each cycle induces thermal stresses that can exceed the fatigue limit of the material, particularly at stress concentration points such as the intersection of the branch pipe with the run pipe. The geometry of a tee fitting inherently creates a stress concentration at the root of the branch, where the curvature changes abruptly.

The thermal fatigue mechanism operates as follows. During furnace heating, the outer surface of the tee expands more than the inner surface due to the temperature gradient through the wall thickness. This differential expansion creates a compressive stress on the outer surface and a tensile stress on the inner surface. During cooling, the stresses reverse. Repeated cycling of these thermal stresses leads to crack initiation at the location of maximum tensile stress amplitude, which is typically at the inner surface of the branch root. The presence of residual stresses from the manufacturing process, such as welding residual stresses if the tee was fabricated by welding, further exacerbates the fatigue cracking susceptibility.

The chemical environment at the furnace outlet also plays a significant role. The hot gas stream contains hydrogen sulfide, which can cause sulfidation cracking of carbon steel at elevated temperatures. Additionally, hydrogen chloride and other acidic gases can cause stress corrosion cracking. The combination of tensile stress (from thermal cycling and residual stress) and a corrosive environment creates the necessary conditions for stress corrosion cracking, which can initiate at stress concentration points and propagate through the wall thickness.

Improvement Measures and Engineering Recommendations

The paper proposes several improvement measures to prevent recurrence of this failure mode. These measures can be categorized into material selection, design optimization, and operational control.

For material selection, upgrading the tee fitting material from plain carbon steel to a low-alloy steel with improved high-temperature strength and sulfidation resistance, such as 15CrMo or 12Cr1MoV, would significantly enhance the resistance to thermal fatigue and sulfidation cracking. Alternatively, applying a protective coating or cladding to the inner surface of the tee could provide a barrier against corrosive gas attack.

For design optimization, modifying the tee geometry to reduce the stress concentration at the branch root is effective. This can be achieved by increasing the branch root radius, adding a reinforcing pad or fillet, or using a fabricated tee with a smoother transition. The wall thickness of the tee should be reviewed to ensure adequate remaining life under the expected thermal cycling conditions.

For operational control, reducing the thermal cycling rate and the temperature range can decrease the thermal stress amplitude. This can be accomplished by optimizing the furnace firing procedure to achieve more gradual heating and cooling rates. Implementing a scheduled inspection program using ultrasonic thickness measurement and visual examination of the tee fitting can detect early signs of cracking before failure occurs.

Study Insights and Implications

This failure analysis case study provides important lessons for engineers working with tee fittings in high-temperature service. The key insight is that tee fittings at furnace outlets are not merely passive piping components but are critical pressure-containing parts that must be designed and operated with the same rigor as pressure vessels. The combination of thermal fatigue and stress corrosion cracking is a well-known failure mode, but it is often underestimated in design practice because the thermal cycling is considered a normal operating condition rather than a fatigue loading. The systematic approach taken in this analysis, which integrates field observation, metallurgical examination, and engineering analysis, serves as a model for failure analysis in the coking and chemical industries. Engineers should always consider the combined effects of thermal, mechanical, and chemical loading when evaluating the integrity of tee fittings in high-temperature service, and should implement appropriate monitoring and maintenance programs to ensure safe and reliable operation.