Zhuojin Pipe Fitting Co., LtdCrack Analysis and Microstructure Characterization of Low-Carbon Steel Pipe in Air Cooler Service
Literature Overview
The research by Wang Huiqiang and colleagues from Hebei Agricultural University, published in the Transactions of Materials and Heat Treatment in 2014, addresses a critical failure mode encountered in the fabrication of air cooler bundles using low-carbon steel pipe (Grade 10, corresponding to ASTM A106 Gr. A or similar low-carbon grades). The study investigates the cracking phenomenon observed during the hole expansion (reaming) process of these pipes, employing a comprehensive metallurgical analysis approach including optical emission spectroscopy (OES), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and optical microscopy (OM).
This work is of significant practical importance because air cooler tubes, typically made from seamless steel pipes conforming to standards such as GB/T 8163 or ASTM A179, undergo various forming operations during manufacturing. Cracking during forming operations represents a serious quality issue that can lead to costly rework, material waste, and potential field failures.
Metallurgical Analysis Results
The investigation revealed two primary metallurgical factors contributing to the cracking susceptibility of the affected steel pipe material:
| Characterization Method | Finding | Implication |
|---|---|---|
| SEM/EDS on fracture surface | Large quantity of non-metallic inclusions near the crack initiation zone | Inclusions act as stress concentrators and crack initiation sites |
| Optical microscopy (OM) | Undissolved cementite (Fe3C) particles present at ferrite grain boundaries | Boundary cementite reduces ductility and increases brittleness |
| OES chemical analysis | Composition within specification but potentially marginal for forming | Carbon content and manganese level influence cementite stability |
| Mechanical testing | Reduced elongation and fracture toughness | Confirms embrittlement due to microstructural factors |
The presence of undissolved cementite at ferrite grain boundaries is particularly concerning. During the normalizing and heat treatment process of low-carbon steel pipe, the cooling rate determines the extent of cementite dissolution and redistribution. An excessively rapid cooling rate, or a cooling rate that does not allow sufficient time for carbon diffusion from the ferrite matrix to the grain boundaries, results in retained cementite films. These boundary cementite networks create preferential paths for intergranular crack propagation under tensile or form-induced stresses.
Failure Mechanism and Countermeasures
The cracking mechanism can be understood through the following sequence: during the hole expansion process, the inner surface of the pipe undergoes significant plastic deformation, generating substantial tensile and shear strains. The non-metallic inclusions, which are typically oxides, sulfides, or silicates entrapped during the steelmaking process, create local stress concentrations. When the applied strain exceeds the local yield strength at these inclusion sites, microvoids nucleate and coalesce, initiating cracks. The intergranular cementite films then provide a low-energy path for crack propagation, leading to macroscopic failure.
The authors recommend two primary countermeasures:
- Strict control of raw material quality, specifically targeting inclusion content through improved steelmaking practices such as calcium treatment to modify inclusion morphology from angular to spherical shapes.
- Reducing the cooling rate during the normalizing heat treatment process to allow sufficient time for cementite dissolution and carbon redistribution, thereby eliminating boundary cementite films.
From a quality assurance perspective, these recommendations align with the requirements of standards such as ASTM A106 and GB/T 8163, which specify limits on non-metallic inclusions (per ASTM E45 or GB/T 1979) and prescribe minimum heat treatment conditions. Engineers specifying steel pipe for forming applications should explicitly require inclusion rating limits (e.g., ASTM E45 Type 1 ratings no worse than 1-1-1-1 for total inclusions) and verify the heat treatment cooling rate through hardness profile testing across the pipe wall thickness.
Practical Recommendations for Pipe Forming Operations
For practitioners involved in pipe fitting fabrication, including the manufacture of air cooler bundles, heat exchanger tubes, and pipe fittings requiring expansion forming, the following quality control measures are recommended:
- Incoming inspection should include metallographic examination of transverse sections to assess inclusion content and distribution, as well as grain boundary condition.
- Hardness testing across the full wall thickness should be performed to verify uniform heat treatment response; a hardness variation exceeding 30 HV across the wall thickness may indicate inadequate austenitization or non-uniform cooling.
- Forming trials should be conducted on representative material lots before full-scale production, with forming limit diagrams (FLD) established for the specific material grade and condition.
- When forming defects are observed, a systematic root cause analysis following the PDCA cycle should be initiated, examining material condition, forming parameters, and tooling geometry.
The study underscores the critical importance of material metallurgy in forming operations. Even when the chemical composition falls within specification limits, microstructural factors such as inclusion morphology and boundary phase distribution can dominate the forming performance. This insight reinforces the principle that steel pipe quality is determined not only by composition and mechanical properties but also by microstructural integrity, which must be verified through appropriate non-destructive and destructive testing methods.