Weld Seam Failure Analysis of Coke Tower Bottom Feed Pipeline Elbow

Literature Overview

This paper by You Bilong and Yi Tao, published in Petrochemical Corrosion and Protection (Vol. 29, No. 5, 2012), presents a detailed failure analysis of a coke tower bottom feed pipeline elbow weld seam. The failure occurred after 10 years of operation in a coking unit at Sinopec Jingmen Branch. The analysis employed chemical composition analysis, metallographic examination, thermal stress analysis, and hardness testing to determine the root cause. The study identifies a complex failure mechanism involving high-temperature sulfidation corrosion combined with thermal stresses from dissimilar metal welding between 309 and 321 stainless steels. This case study is highly relevant to engineers working on high-temperature coking unit piping systems where sulfidation corrosion and dissimilar metal welds are common challenges.

Failure Description and Observation

The failure occurred at the circumferential weld seam of a coke tower bottom feed pipeline elbow. Key observations from the failure investigation:

Observation Category	Finding	Interpretation
Crack location	Near circumferential weld seam	Weld HAZ is the critical damage zone
Crack orientation	Circumferential (hoop direction)	Axial tensile stress is the driving force
Crack morphology	Straight, flat crack surface	Suggests slow, stable crack growth
Crack propagation direction	Consistent with weld cross-section	Fatigue or creep-assisted crack growth
Service history	10 years of operation	Long-term degradation mechanism

The circumferential crack orientation is particularly significant. In a pipeline elbow, circumferential cracks are driven by axial tensile stresses, which in this case arose from differential thermal expansion between the 309 and 321 stainless steel components joined by the dissimilar metal weld.

Root Cause Analysis

The failure mechanism is a synergistic combination of two primary factors:

Factor 1: High-Temperature Sulfidation Corrosion

With the progressive deterioration of feedstock quality over the 10-year service period, the sulfur content in the process stream increased, intensifying high-temperature sulfidation corrosion. This corrosion mechanism:

Reduces the effective wall thickness of the pipeline, decreasing structural load-bearing capacity.
Creates a weakened microstructure at the metal-sulfide interface, providing crack initiation sites.
Accelerates material degradation at elevated temperatures where sulfide scale formation is thermodynamically favored.

Factor 2: Thermal Stress from Dissimilar Metal Welding

The weld joint between 309 stainless steel (weld filler) and 321 stainless steel (base metal) introduces a thermal mismatch problem:

Property	309 Stainless Steel	321 Stainless Steel	Mismatch Effect
Thermal conductivity	~14.9 W/m·K	~16.3 W/m·K	309 retains more heat
Outer wall temperature	Lower (due to lower conductivity)	Higher	Temperature gradient across weld
Thermal expansion	Similar (austenitic grades)	Similar	But differential temperature causes differential expansion
Axial stress at weld	Compressive (lower temperature side)	Tensile (higher temperature side)	309 experiences axial tensile pull

The thermal conductivity difference between 309 and 321 stainless steels, combined with the temperature gradient across the weld, creates a significant axial tensile stress in the 309 weld metal. This thermal stress, superimposed on the welding residual tensile stress, creates a sustained tensile stress state at the weld that drives crack initiation and growth.

Combined Failure Mechanism

The high-temperature sulfidation corrosion weakens the material, while the thermal and residual stresses provide the driving force for crack initiation and propagation. The result is a failure that would not have occurred under either mechanism alone—this is a classic example of a multi-mechanism failure where the interaction between corrosion and mechanical stress accelerates damage beyond what either mechanism would cause independently.

Standards and Material Selection Considerations

The dissimilar metal weld between 309 and 321 stainless steels is a common practice in high-temperature service, but it requires careful consideration:

Welding Standard	Recommended Filler for 321 SS	Notes
ASME IX	Type 309 or 347	309 is commonly used but creates thermal mismatch
AWS D1.6	Type 309L or 347L	Low-carbon variants reduce sensitization risk
EN 12070	Type 309 or 347	European standard for stainless steel welding
API 570	309 acceptable with stress relief	Requires post-weld heat treatment

The selection of 309 stainless steel as the weld filler for a 321 base metal is technically acceptable per most welding codes, but the thermal conductivity mismatch creates a long-term integrity concern that is not always adequately addressed in design. Engineers should consider the following mitigations:

Post-weld heat treatment (PWHT): Stress relief at 1050–1100 °C followed by rapid cooling can reduce residual stresses and restore full stabilization.
Thermal barrier coating: Applying a thermal barrier coating on the 309 side can reduce the temperature gradient across the weld.
Alternative filler selection: Type 347 (Ni-stabilized) or Type 321 (Ti-stabilized) filler metals may reduce the thermal mismatch, though they have lower melting points.
Weld geometry optimization: Using a wider weld cap or multiple weld passes can distribute thermal stresses more evenly.

Integration with Engineering Practice

This failure case provides critical lessons for engineers working on high-temperature coking unit piping systems:

Feedstock quality monitoring: Implement continuous monitoring of sulfur content in process streams to detect feedstock deterioration early and adjust operating parameters accordingly.
Dissimilar metal weld management: Develop a specific integrity management plan for all dissimilar metal welds in high-temperature service, including regular inspection and condition assessment.
Corrosion protection: Consider corrosion-resistant alloy (CRA) upgrades for critical piping sections exposed to high-temperature sulfidation environments.
Inspection strategy: Focus ultrasonic testing (UT) and radiographic testing (RT) on circumferential welds in high-temperature sulfidation service, with particular attention to the weld HAZ.

FMEA Analysis for Dissimilar Metal Weld Integrity

System Element	Failure Mode	Cause	Effect	Severity	Occurrence	Detection	RPN
309/321 weld	Circumferential cracking	Thermal stress + sulfidation corrosion	Pipeline leak, production loss	10	4	3	120
Weld HAZ	Sensitization	Thermal cycling	Reduced corrosion resistance	8	5	2	80
Base metal	Wall thinning	Sulfidation corrosion	Reduced pressure capacity	10	6	4	240
Operating system	Feedstock deterioration	Uncontrolled feed quality	Accelerated corrosion	8	5	3	120

Key Questions and Reflections

This case raises an important question about the adequacy of current welding code requirements for dissimilar metal welds in high-temperature corrosive service. While ASME IX and other welding codes specify acceptable filler metal combinations, they do not always address the long-term thermal mismatch effects that can develop over years of operation. Engineers must supplement code compliance with additional assessment of thermal stress effects and corrosion-accelerated degradation.

Another reflection concerns the role of feedstock quality management. The progressive deterioration of feedstock quality over 10 years was a significant contributing factor to the failure. This highlights the importance of establishing feedstock quality thresholds and implementing process adjustments when feedstock quality degrades, rather than allowing the system to operate under increasingly severe conditions.

Study Insights and Implications

This failure analysis provides a comprehensive example of how multiple degradation mechanisms—high-temperature sulfidation corrosion and thermal stress from dissimilar metal welding—can interact to produce a failure that would not have occurred under either mechanism alone. The key engineering takeaway is that dissimilar metal welds in high-temperature corrosive service require a holistic integrity management approach that considers both mechanical and chemical degradation mechanisms. Engineers should implement a combination of material selection, welding procedure optimization, corrosion monitoring, and targeted inspection to ensure the long-term integrity of such critical welds.