Failure Analysis of Balanced Pipe Tee Cracking in 321 Stainless Steel

Literature Overview

The paper by Xi Wei, published in Petrochemical Equipment Corrosion and Protection (2011, Vol. 28, No. 3, pp. 51-54), reports a detailed failure analysis of a balanced pipe tee made of 321 stainless steel that suffered cracking in service at Sinopec Shanghai Petrochemical Company. The failure mode is identified as sulfide-induced stress corrosion cracking (SSC), and the root cause is traced to inadequate solution heat treatment, which led to carbide precipitation at grain boundaries and within grains, severely reducing the corrosion resistance of the material. This case study is highly relevant to engineers working with stainless steel pipe fittings in sulfide-containing environments, particularly in the petrochemical and oil refining industries.

Material and Service Conditions

The failed tee was made of 321 stainless steel, which is a titanium-stabilized austenitic stainless steel designed to resist intergranular corrosion. The typical chemical composition is:

The service environment contained hydrogen sulfide (H₂S), which is a well-known cause of stress corrosion cracking in stainless steels, particularly in the presence of tensile residual stresses. The tee was subjected to internal pressure, thermal cycling and mechanical loading, all of which contribute to the development of tensile stresses at the weld and heat-affected zones.

Failure Analysis Methodology

The analysis followed a systematic approach using multiple characterization techniques:

• Chemical composition analysis: confirmed the material as 321 stainless steel with adequate titanium content.
• Hardness testing: revealed elevated hardness in the affected regions, indicating carbide precipitation and possible strain hardening.
• Metallographic examination: revealed extensive carbide precipitation along grain boundaries and within grains, which is characteristic of insufficient solution heat treatment.
• Fracture surface macroscopic and microscopic examination: the fracture surface showed intergranular cracking with secondary cracking, which is typical of stress corrosion cracking. The presence of sulfide inclusions was confirmed by energy-dispersive X-ray spectroscopy (EDS).

Root Cause Analysis

The root cause of the failure is a combination of material and process factors:

1. Inadequate solution heat treatment: the solution heat treatment temperature was too low or the cooling rate was too slow, which allowed carbide precipitation at grain boundaries. In 321 stainless steel, titanium is supposed to bind with carbon to form TiC, preventing the formation of Cr-rich carbides (M₂₃C₆) that cause sensitization. However, if the heat treatment is inadequate, both TiC and Cr-rich carbides can form, leading to chromium depletion at grain boundaries.
2. Sulfide environment: the H₂S in the service environment provided the corrosive species necessary to initiate and propagate the stress corrosion cracks. The sulfide ions attack the chromium-depleted grain boundaries, initiating intergranular cracking.
3. Tensile residual stresses: the welding and forming processes introduced tensile residual stresses, particularly at the weld and heat-affected zones. These stresses, combined with the corrosive environment, provided the driving force for crack propagation.

Engineering Countermeasures

Based on this failure analysis, the following countermeasures are recommended:

• Verify solution heat treatment: implement a strict solution heat treatment procedure with a minimum temperature of 1010°C and rapid cooling (quenching) to prevent carbide precipitation. The heat treatment parameters should be verified by metallographic examination and intergranular corrosion testing (ASTM A923 Practice E).
• Post-weld heat treatment: for welded fittings, a post-weld heat treatment (PWHT) at 1010-1050°C should be performed to relieve residual stresses and restore the corrosion resistance.
• Residual stress relief: apply mechanical stress relief methods such as low-frequency vibration stress relief or thermal stress relief to reduce the tensile residual stresses in the weld and heat-affected zones.
• Material selection: for severely sulfide-containing environments, consider using duplex stainless steels (such as 2205) or nickel-based alloys (such as Hastelloy C-276) that have superior resistance to sulfide stress corrosion cracking.
• Inspection and monitoring: implement regular in-service inspection using eddy current testing (ECT) or ultrasonic testing (UT) to detect early-stage stress corrosion cracks before they lead to catastrophic failure.

Study Insights and Implications

This failure analysis case study is a valuable reminder of the importance of proper heat treatment in preventing stress corrosion cracking in stainless steel pipe fittings. The key insight is that the corrosion resistance of 321 stainless steel is highly dependent on the solution heat treatment quality, and even a small deviation in the heat treatment parameters can lead to significant carbide precipitation and intergranular sensitization. For engineers involved in the design, fabrication and inspection of stainless steel pipe fittings, this case study underscores the need for rigorous quality control of the heat treatment process, including verification by metallographic examination and intergranular corrosion testing. The failure also highlights the importance of considering the service environment in the material selection and design of pipe fittings, particularly in sulfide-containing environments where stress corrosion cracking is a known failure mode. Overall, this analysis provides a clear example of how a combination of material, process and environmental factors can lead to a catastrophic failure, and how a systematic failure analysis can identify the root cause and guide the development of effective countermeasures.