Failure Analysis of Cracking in Cracking Furnace Dilution Steam Pipe Tee

Literature Overview

This 2024 paper by Chen Jing and colleagues from the Institute of Metal Research, Chinese Academy of Sciences, and North Huajin Chemical Industry Co., Ltd., presents a comprehensive failure analysis of 304H austenitic stainless steel tees in a dilution steam pipeline of an ethylene cracking unit. The tees failed after only one year of service, and the investigation encompassed material chemistry, mechanical properties, metallographic examination, residual stress measurement, and fracture surface analysis. The work provides valuable lessons for the selection and treatment of stainless steel fittings in high-temperature, corrosive service environments.

Service Conditions and Failure Description

The dilution steam pipeline in an ethylene cracking unit operates under demanding conditions:

The failure manifested as multiple cracks originating from both the inner and outer surfaces of the tee, with most crack initiation sites exhibiting intergranular corrosion features. The crack propagation mode was predominantly transgranular with intergranular character.

Systematic Failure Analysis Approach

The investigation followed a structured analytical approach:

Material Chemistry and Microstructure

The analysis of the material composition and microstructure revealed evidence of sensitization - the precipitation of chromium-depleted carbides at grain boundaries during prolonged exposure to high-temperature service. This chromium depletion at grain boundaries reduces the local corrosion resistance and creates preferential paths for intergranular attack.

Residual Stress Assessment

The measured residual stress state was significant:

• Maximum axial stress at tee run pipe surface: 147 MPa
• Maximum circumferential stress near intersection line (20 mm from crack): 114 MPa

These stress levels reached approximately half of the material yield strength, indicating substantial residual stresses from the manufacturing process (likely welding and/or forming). The high residual stress state is a critical contributing factor to stress corrosion cracking (SCC).

Fracture Surface Analysis

The fracture surface examination revealed:

• Crack initiation at both inner and outer surfaces
• Predominantly transgranular crack propagation with intergranular features
• Presence of corrosion products containing O, S, and Cl at the crack vicinity
• These elements indicate exposure to acidic corrosive species in the service environment

Failure Mechanism Determination

The combined evidence points to stress corrosion cracking (SCC) as the primary failure mechanism, with the following contributing factors:

The failure mechanism can be summarized as: sensitized 304H material, subjected to high residual stresses from manufacturing, was exposed to a high-temperature corrosive environment containing acidic species. The combination of these three factors - susceptible microstructure, tensile stress, and corrosive environment - created the necessary conditions for SCC initiation and propagation.

Remediation and Engineering Recommendations

Based on the failure analysis, the following remedial actions are recommended:

1. Material upgrade: Replace 304H with a more resistant grade such as 310H, 347H, or a super-austenitic stainless steel with higher chromium and nitrogen content, which provide greater resistance to sensitization and SCC.
2. Stress relief treatment: Apply appropriate solution heat treatment (austenitization at 1050-1100°C followed by rapid cooling) to the tees to dissolve any precipitated carbides and restore full corrosion resistance. Alternatively, a lower-temperature stress relief treatment may be applied to reduce residual stresses without causing further sensitization.
3. Residual stress control: Implement welding procedures that minimize residual stresses, such as using back-purging, controlling interpass temperatures, and applying post-weld heat treatment.
4. Environmental control: Where possible, reduce the concentration of aggressive species (particularly chlorides) in the steam system through improved water treatment.
5. Inspection program: Implement a periodic inspection program using ultrasonic testing (UT) or acoustic emission (AE) to detect early-stage SCC before catastrophic failure occurs.

Study Insights and Broader Implications

This failure case study is a powerful reminder of the importance of considering the interaction between material, stress, and environment in high-temperature service applications. The 304H grade, while generally considered suitable for high-temperature service, is susceptible to sensitization and SCC when exposed to specific environmental conditions. The one-year failure life is alarmingly short and highlights the consequences of inadequate material selection for the specific service environment.

From a quality control perspective, this case underscores the importance of:

• Residual stress measurement as a mandatory inspection step for critical fittings in corrosive service
• Material certification that includes verification of solution heat treatment compliance
• Service-specific material selection that accounts for the actual environmental conditions, not just the nominal temperature and pressure

The work also demonstrates the value of a systematic failure analysis approach that integrates multiple analytical techniques. No single examination method would have provided a complete understanding of the failure mechanism; it was the convergence of evidence from chemistry, metallography, stress measurement, and fracture analysis that enabled the definitive identification of SCC as the root cause.

For engineers involved in the design and maintenance of high-temperature process piping, this case study provides a clear lesson: the combination of sensitized stainless steel, high residual stresses, and a chlorinated environment is a recipe for premature failure, and all three factors must be addressed simultaneously to achieve reliable long-term service.