Failure Analysis of 20 Steel Equal Tee Due to Brittle Fracture

Literature Overview

This paper, published in Heat Treatment of Metals in 2022 by Li Dejun and colleagues from the State Key Laboratory of Metal Materials for Oil and Gas Equipment and PetroChina Coalbed Methane Co., Ltd., investigates the brittle fracture failure of a 20 steel equal tee used in a metering skid. The study combines fractographic analysis, chemical composition verification, microstructural examination, hardness testing, and electrochemical corrosion performance evaluation to identify the root cause of the failure. The work is supported by funding from PetroChina's basic research and strategic reserve program and technology development program.

Failure Investigation Methodology

The investigation employed a systematic approach combining multiple analytical techniques to establish the failure mechanism:

Root Cause Analysis

Manufacturing Process Deficiency

The root cause of the failure was identified as the omission of normalizing or annealing heat treatment after hydraulic cold expansion forming of the 20 steel seamless pipe tee. The cold expansion process introduces significant plastic deformation in the pipe wall, which results in:

• Work hardening that increases hardness and strength but reduces ductility and toughness.
• Accumulation of dislocation density that creates internal stresses and reduces the material's ability to accommodate further deformation.
• Degradation of the microstructure from a balanced ferrite-pearlite structure to a cold-worked condition with high dislocation density and potential for strain-induced martensite formation.

Consequences of Missing Heat Treatment

The absence of post-forming heat treatment had multiple detrimental effects:

• The cold-worked microstructure exhibited significantly reduced fracture toughness, making the tee susceptible to brittle fracture under normal operating stresses.
• The elevated hardness indicated by Brinell testing confirmed the presence of cold work hardening that should have been relieved by normalizing or annealing.
• The electrochemical corrosion testing revealed that the cold deformation significantly degraded the corrosion resistance of the tee material, accelerating corrosion of the tee body.
• Corrosion pits on the inner wall of the tee served as crack initiation sites for the brittle fracture, creating a synergistic effect between mechanical degradation and environmental attack.

Standards Compliance Issues

The failure highlights a critical gap in quality control and standards compliance. According to relevant standards for butt-weld fittings, including ASME B16.9 and ASTM A234, fittings formed by cold working processes such as hydraulic expansion must be heat treated to restore mechanical properties to the specified minimum values. The omission of this required heat treatment step represents a fundamental violation of the manufacturing specification.

Engineering Practice Implications

This failure analysis has several important implications for engineering practice:

• Post-forming heat treatment is not optional; it is a mandatory requirement for fittings manufactured by cold working processes. The mechanical properties and corrosion resistance of the fitting are directly dependent on proper heat treatment.
• Incoming inspection of purchased fittings should include verification of heat treatment records and, where possible, hardness testing to confirm that the material is in the proper heat-treated condition.
• For 20 steel tees, hardness values significantly above the expected range for the annealed or normalized condition should trigger further investigation, including microstructural examination and, if necessary, fracture toughness testing.
• Corrosion inspection of tee internals should be performed with particular attention to areas of high stress concentration, where corrosion pits are most likely to initiate cracks.

Key Questions and Reflections

This case raises the question of how widespread the practice of omitting post-forming heat treatment may be in the industry. If even one manufacturer of a high-profile company such as PetroChina failed to follow the required heat treatment procedure, it suggests that this may be a systemic issue in the fitting supply chain. The economic incentive to skip heat treatment, which adds cost and processing time, may be significant for low-value fittings such as 20 steel tees, even though the consequence of failure can be catastrophic. The paper also highlights the importance of electrochemical corrosion testing as a quality control tool, as the degradation of corrosion resistance due to cold work is not immediately apparent from conventional mechanical testing.

Summary

This failure analysis provides a clear and instructive case study of how the omission of a single mandatory manufacturing step—post-forming heat treatment—can lead to brittle fracture failure of a pipe tee in service. The combination of cold work hardening, reduced fracture toughness, and accelerated corrosion created a synergistic failure mechanism that would not have been predicted by any single test method alone. The study reinforces the fundamental principle that manufacturing process compliance is not merely a regulatory formality but a critical determinant of component reliability and safety. Engineers involved in the procurement and inspection of pipe fittings must ensure that all required heat treatment steps are performed and documented, and that incoming inspection protocols are designed to detect non-compliant material condition.